Electrical services

Health Technical Memorandum

06-01: Electrical services supply
and distribution
Part A: Design considerations

SOURCE OF ELECTRICAL SUPPLY

PRIMARY SOURCE SECONDARY SOURCE

PUBLIC ELECTRICAL SUPPLY HEALTHCARE PREMISES
(PES) DISTRIBUTION INTERNAL ELECTRICAL INFRASTRUCTURE
NETWORK OPERATOR (DNO)
Alternative Combined
Emergency

Electrical services – Health Technical Memorandum 06-01 Electrical services supply and distribution: Part A – Design considerations
Energy Heat and
LV (400 V) HV (11 kV) Power Plant
Plant Power (CHP)

High Voltage
Substation
HIGH
VOLTAGE
NETWORK(S)

Low Voltage
Distribution Switch Panel

ISBN 0-11-322755-8
Low Voltage Low Voltage Low Voltage
Sub Main (Unified) Distribution Sub Main (Segregated) Sub Main (Dual Unified)
Board Distribution Board Distribution Board

TERTIARY POWER
www.tso.co.uk SUPPLY
9 780113 227556
UNINTERRUPTIBLE
POWER SUPPLY
THE ELECTRICAL SYSTEM

LTAGE NETWORK(S)
DH INFORMATION READER BOX

Policy Estates
HR / Workforce Performance
Management IM & T
Planning Finance
Clinical Partnership Working

Document Purpose Best practice guidance

ROCR Ref: 0 Gateway Ref: 7165
Title Health Technical Memorandum 06-01, Part A: 'Electrical services
supply and distribution'
Author Department of Health / Estates and Facilities Division
Publication Date Oct
Feb 2006
2007
Target Audience PCT CEs, NHS Trust CEs, SHA CEs, Care Trust CEs, Foundation
Trust CEs , PCT PEC Chairs, NHS Trust Board Chairs, Special HA
CEs

Circulation List Department of Health libraries, House of Commons library,

Strategic Health Authorities, UK Health Departments, Directors of
Estates and Facilities,

Description Health Technical Memorandum 06-01, Part A, provides guidance

on the design, installation and testing of all fixed wiring and
integral electrical equipment used for electrical services within
healthcare premises.

Cross Ref HTM 06-01, Part A

0
Superseded Docs HTMs 2007, 2011 and 2014
0
Action Required 0
0
Timing N/A
Contact Details Chris Holme
Department of Health/Estates and Facilities Division
Quarry House, Quarry Hill
Leeds
LS2 7UE
0113 254 5932
0
0
For Recipient's Use
Electrical services
Health Technical Memorandum
06-01: Electrical services supply and
distribution

Part A: Design considerations

London: The Stationery Office

Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Published by TSO (The Stationery Office) and available from:

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© Crown copyright 2007

Published with the permission of the Estates and
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on behalf of the Controller of Her Majesty’s Stationery Office. The paper used in the printing of this document (Revive Silk)
is 75% made from 100% de-inked post-consumer waste, the
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ISBN 0-11-322755-8
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First published 2007
Printed in the United Kingdom for The Stationery Office

ii
Preface

About Health Technical Memoranda main source of specific healthcare-related guidance for
estates and facilities professionals.
Engineering Health Technical Memoranda (Health
Technical Memoranda) The core suite of nine subject areas provides access to
give comprehensive advice and guidance on the design, guidance which:
installation and operation of specialised building and • is more streamlined and accessible;
engineering technology used in the delivery of healthcare.
• encapsulates the latest standards and best practice in
The focus of Health Technical Memorandum guidance healthcare engineering;
remains on healthcare-specific elements of standards,
policies and up-to-date established best practice. They are • provides a structured reference for healthcare
applicable to new and existing sites, and are for use at engineering.
various stages during the whole building lifecycle:
Figure 1 Healthcare building life-cycle

DISPOSAL CONCEPT

RE-USE
DESIGN & IDENTIFY
OPERATIONAL OPERATIONAL
MANAGEMENT REQUIREMENTS

Ongoing SPECIFICATIONS
MAINTENANCE TECHNICAL & OUTPUT
Review

PROCUREMENT
COMMISSIONING

CONSTRUCTION
INSTALLATION

Healthcare providers have a duty of care to ensure that Structure of the Health Technical
appropriate engineering governance arrangements are in
Memorandum suite
place and are managed effectively. The Engineering
Health Technical Memorandum series provides best The series of engineering-specific guidance contains a
practice engineering standards and policy to enable suite of nine core subjects:
management of this duty of care.
Health Technical Memorandum 00
It is not the intention within this suite of documents to Policies and principles (applicable to all Health
unnecessarily repeat international or European standards, Technical Memoranda in this series)
industry standards or UK Government legislation. Where
appropriate, these will be referenced. Health Technical Memorandum 01
Decontamination
Healthcare-specific technical engineering guidance is a
vital tool in the safe and efficient operation of healthcare Health Technical Memorandum 02
facilities. Health Technical Memorandum guidance is the Medical gases

iii
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Health Technical Memorandum 03 Electrical Services – Electrical safety guidance for low
Heating and ventilation systems voltage systems
Health Technical Memorandum 04 In a similar way Health Technical Memorandum 07-02
Water systems will simply represent:
Health Technical Memorandum 05 Environment and Sustainability – EnCO2de.
Fire safety
All Health Technical Memoranda are supported by the
Health Technical Memorandum 06 initial document Health Technical Memorandum 00
Electrical services which embraces the management and operational policies
from previous documents and explores risk management
Health Technical Memorandum 07
issues.
Environment and sustainability
Some variation in style and structure is reflected by the
Health Technical Memorandum 08
topic and approach of the different review working
Specialist services
groups.
Some subject areas may be further developed into topics
DH Estates and Facilities Division wishes to acknowledge
shown as -01, -02 etc and further referenced into Parts A,
the contribution made by professional bodies,
B etc.
engineering consultants, healthcare specialists and
Example: Health Technical Memorandum 06-02 Part A NHS staff who have contributed to the review.
will represent:

Figure 2 Engineering guidance

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iv
Executive summary

Health Technical Memorandum 06-01 – ‘Electrical This document should be used for all forms of electrical
services supply and distribution’ replaces Health design work ranging from a new greenfield site to
Technical Memorandum 2007 – ‘Electrical services modifying an existing final subcircuit. The relevance of
supply and distribution’ and Health Technical each section will depend on the extent of the design
Memorandum 2011 – ‘Emergency electrical services’, works.
and absorbs Health Technical Memorandum 2014 –
It provides guidance to managers of healthcare premises
‘Abatement of electrical interference’.
on how European and British Standards relating to
This part (Part A) provides guidance for all works on the electrical safety such as the IEE Wiring Regulations
fixed wiring and integral electrical equipment used for BS 7671, the Building Regulations 2000 and the
electrical services within healthcare premises. Electricity at Work Regulations 1989 can be used to fulfil
their duty of care in relation to the Health and Safety at
Work etc Act 1974.

v
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Acknowledgements

Main working group

Owen Cusack Northumbria Healthcare NHS Trust
Peter Desforges Develop
Alan Gascoine Mayday Hospital
Ian Hawthorn Sandwell General Hospital
Chris Holme Department of Health Estates and Facilities
Jim Mellish Skanska UK PLC
John Murray Bender UK
Nigel Porter Welsh Health Estates
Mike Ralph Great Ormond Street Hospital
Steve Wilson Faber Maunsell AECOM

Subject working groups

Switchgear and Protection Schneider Electric
Containment Systems
Dick Shelly Skanska UK PLC
Andrew Galloway Great Ormond Street Hospital
Isolated Power Supplies
John Murray Bender UK
Electromagnetic Compatibility
Alwyn Finney ERA Technology

vi
Contents

Preface
Executive summary
Acknowledgements
Chapter 1 Scope of Health Technical Memorandum 06-01 1
Abbreviations and definitions
Chapter 2 Introduction 5
Overview
How to read this Health Technical Memorandum
Chapter 3 Initial considerations 7
Sources of supply
Resilience
Essential/non-essential supplies
Primary sources of supply
Secondary main sources of supply – generation
Tariff negotiations and private generation
Supply voltages
Design of installations for growth and change
Assessment of existing electrical systems
Greenfield site
New-build on existing site
New equipment on existing site
Load profile
Diversity factors
Consideration for EMC requirements
Roles and responsibilities
Access for maintenance
Commissioning procedures
Connection to the DNO
Connection to the healthcare site network
Supply from the DNO
Supply from internal connection point
Chapter 4 Understanding risk and ownership 17
Introduction
Need for risk assessment
Ownership and design
Risk profile
Clinical risk
Category 1 – Support service circulation
Category 2 – Ambulant care and diagnostics
Category 3 – Emergency care and diagnostics
Category 4 – Patients in special medical locations
Category 5 – Life support or complex surgery
Non-clinical and business continuity risk
Category 1 – Business support services

vii
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Category 2 – Building services safety and security

Category 3 – Building services environmental control
Category 4 – Medical support services
Electrical infrastructure
Resilience
Electrical infrastructure system selection
Chapter 5 Power quality 25
Power factor correction
Located at the intake point
Located at sub-main distribution boards
Located on the electrical equipment
Harmonics
Located at the intake point
Located at sub-main distribution boards
Located on the electrical equipment
Voltage surge protection
Chapter 6 Distribution strategy 28
Design for resilience
Supply connections
Risk of transformer failures
Risk of generator failures
Other reasons for failure of electricity supply
Distribution system – high voltage
HV network – one radial circuit with one substation only
HV network – one radial circuit with three substations
HV network – three radial circuits each with one substation
HV ring network – ring with four substations
Primary and secondary distribution systems
Primary supply – unified infrastructure
Primary and secondary supply – unified and segregated infrastructure
Primary and secondary supply – unified and dual-unified infrastructure
Dual primary and dual secondary supply – unified and dual-unified infrastructure
Dual primary and dual HV secondary supply – dual-unified infrastructure
Dual primary and dual LV secondary supply – dual-unified infrastructure
Final circuits
Fixed equipment
Power outlets
Lighting circuits
Chapter 7 Primary power – distribution centres 41
HV substations
Location
Construction
Access and egress
Layout
Fire precautions
Environmental requirements
Equipment and notices provided
Transformer enclosures
Location
Construction
Access and egress
Fire precautions
Environmental requirements

viii
 Contents

LV switchrooms
Location
Construction
Access and egress
Layout
Fire precautions
Environmental requirements
Equipment and notices required to be provided
Chapter 8 Secondary power centres and plant 48
Secondary power general arrangements
Photovoltaic power secondary power source
Wind turbine power source
General – secondary power plant location
Essential power capacity
Essential and emergency power provision
Standby generators
Design criteria
Component parts
Generator configuration
Mobile plug-in generator island operation
Generator(s) in island operation
Generator(s) operating in parallel with PES
LV generators feeding HV ring main
Generator control
Generating set management
Multi-set operation
Mains return
Computerised load management of generators
Standards and references
Generator engines
Batteries and battery charging
Fuel and fuel storage
Exhaust systems
Environmental considerations
Chapter 9 Protection and switchgear 62
High-voltage switchgear
Withdrawable units
Semi-withdrawable units
Fixed-pattern units
High-voltage busbar sections
High-voltage protection devices
High-rupture-capacity (HRC) fuse links to BS 2692, IEC 60298
Time fuse links to ESIS 12.6
Inverse definite minimum time (IDMT) relays
Bias differential relays
Earth fault passage indicators
Grading of protection systems
Network reconfiguration after a fault or outage
Distribution transformer types
Fluid-type transformers
Dry-type transformers
Package substation
Transformer protection

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Generator protection
Low-voltage switchboards
Form 2
Form 3
Form 4
Motor control centre (MCC)
Final distribution boards and consumer units
Low-voltage protection devices
Switch
Disconnector
Fuse
Circuit breaker
Low-voltage busbar sections
Discrimination of protective devices
Discrimination with HBC/HBC fuses
Discrimination with MCB/MCCB
Discrimination with MCB/fuse
Discrimination with RCDs
Automatic load management of switchgear (HV, LV)
Chapter 10 Tertiary power supplies 74
Batteries for uninterruptible power supplies
Battery type
Battery life
Battery arrangements
Battery autonomy
Batteries for inverter units
Battery type
Battery life
Battery arrangements
Battery autonomy
Generator batteries
Battery type
Battery life
Battery autonomy
Chapter 11 Electromagnetic compatibility 77
Standards
Procurement requirements
EMC phenomena
Standards and levels
Electromagnetic environment
Designing systems for EMI control
EMC control for power systems
EMC control for cables and cable-containment systems
EMC control for general systems
Cable segregation and separation
Cable screening, trunking and trays
Crosstalk characteristics
Using conductive structural supports as runs for cables
Identification of critical systems
Earthing and bonding
Chapter 12 Wiring systems 87
High voltage
Low voltage

x
 Contents

Medical IT
Protected extra low voltage systems
Separated extra low voltage systems
Chapter 13 Earthing 88
High-voltage earthing methods
High-voltage network cables
High-voltage generator earths
Low-voltage main earthing methods
Low-voltage generator earths
Switchroom earths
Earths for radiographic rooms
Medical IT or isolated power supply earths
Microshock
Circuit protective conductors
Functional earth
Monitored earthing systems
Lightning protection
Lightning protection system components
Ionised fields
Chapter 14 Containment 97
Trenches, service tunnels and ducts
Ladder rack – tray – basketry
Trunking – conduits
Preformed wiring containment
Layout considerations
Fire precautions
Remodelling and extensions
Circuit segregation
Access for maintenance
Suitable locations
Chapter 15 Cable and busbar types 101
High-voltage distribution
Low-voltage distribution
Cable identification
Busbar distribution
Control alarm and communication cables
Control communication and non-fire-alarm cables
Information technology cables
Fire alarm cables
Chapter 16 Final circuits 105
Uninterruptible power supplies
Standards
Rating
UPS environment
UPS description and configurations
UPS fault condition design
UPS power quality
UPS resilience
Inverter units
Central battery units
Rectifier units for theatre operating lamps
Isolated power supplies (IPS)
IPS environment

xi
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

IPS communication
Resilience
The patient environment
IPS low-power circuits
General low-power circuits
Socket-outlets/connection units
Sockets for special locations
Sockets for operating theatre suites
Socket for mobile X-ray units
Spark-proof sockets
Number of outlets per final circuit
Fixed equipment
Supplies to external buildings
Temporary supplies
Connections for mobile trailer units
General lighting
Theatre operating lamps
Examination lamps lighting
Emergency escape lighting
Standby lighting
Fire alarm, security circuits and critical alarms
BEMS communication and control wiring systems
Chapter 17 Validation and commissioning 118
Validation of specific plant
Generators and CHP plant
Uninterruptible power supplies
Isolated power supplies
Fixed wiring distribution, switchgear and protection
Records to be kept
As-installed drawings
Building logbook
Appendix 1 Maximum interruption times to the primary supply 123
Appendix 2 Sample test record sheets 124
Appendix 3 Drawing symbols 134
References 135
Acts and Regulations
British, European and International Standards
Department of Health publications
Miscellaneous publications

xii
1 Scope of Health Technical Memorandum
06-01

1.1 Health Technical Memorandum 06-01 – ‘Electrical BSRIA: Building Services Research and Information
services supply and distribution’ replaces Health Association
Technical Memorandum 2007 – ‘Electrical services
CCM: CIBSE commissioning manual
supply and distribution’ and Health Technical
Memorandum 2011 – ‘Emergency electrical CCTV: closed-circuit television
services’, and absorbs Health Technical CDM: Construction Design and Management
Memorandum 2014 – ‘Abatement of electrical Regulations
interference’.
CE: European Conformity
1.2 This part (Part A) provides guidance for all
works on the fixed wiring and integral electrical CENELEC: The European Committee for
equipment used for electrical services within Electrotechnical Standardization
healthcare premises. The document should be used CHP: combined heat and power
for all forms of electrical design work ranging from
a new greenfield site to modifying an existing final CIBSE: Chartered Institute of Building Services
subcircuit. Engineers
1.3 This document provides guidance to managers of CPC: circuit protective conductor
healthcare premises on how European and British CT: current transformer
Standards relating to electrical safety such as the
IEE Wiring Regulations BS 7671, the Building DB: distribution board
Regulations 2000 and the Electricity at Work DBU: distribution unit
Regulations 1989 can be used to fulfil their duty of
care in relation to the Health and Safety at Work dc: direct current
etc Act 1974. DNO: distribution network operator
1.4 The policies and principles of all engineering DRUPS: diesel rotary uninterruptible power supplies
services are described in Health Technical
DTC: diagnostic and treatment centres
Memorandum 00 – ‘Policies and principles’, which
should be read in conjunction with this document. EC: European Community
ECG: electrocardiogram
Abbreviations and definitions
EEA: European Economic Area
24 x 7: 24 hours a day, 7 days a week
EEC: European Economic Community
ac: alternating current
EI: extreme inverse
ACB: air circuit breaker
EMC: electromagnetic compatibility
AMD: assumed maximum demand
EMCD: electromagnetic compatibility directives
AVR: automatic voltage regulator
EMI: electromagnetic interference
BASEC: British Approvals Services for Electrical Cables
EPR: electronic patient records
BEMS: building energy management system
ERB: earth reference bar
BMS: building management system
ERIC: Estates Return Information Collection
BS: British Standards

1
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

ESD: electrostatic discharge MCCB: moulded-case circuit breaker

ETSI: European Telecommunications Standards Institute MD: maximum demand
EU: European Union Medical IT: medical impedance terra (earthed) also
known as IPS
FL: full load
MEIGaN: Medical Electrical Installation Guidance
GRP: Glass-reinforced plastic
Notes
GSM: global system for mobile communication
MET: main earth terminal
HBC: high breaking capacity
MRI: magnetic resonance imaging
HBN: Health Building Note
NEAT: NHS Environmental Assessment Tool
HDU: high dependency unit
NHS: National Health Service
HFN: Health Facilities Note
OCB: oil circuit breaker
HGN: Health Guidance Note
OJEC/OJEU: Official Journal of the European
HRC: high rupturing capacity Community/Union
HV: high voltage (11 kV) ONAN: oil natural circulation, air natural flow
HVAC: heating ventilation and air-conditioning PEC: protective earth conductor
ICU: intensive care unit PEI: primary electrical infrastructure
IDMT: inverse definite minimum time PELV: protected extra LV
IEC: International Electrotechnical Commission PES: public electrical supply
IEE: Institute of Electrical Engineering PET: protective earth terminal
IGBT: Insulated-gate bipolar transistor PF: power factor
IM&T: information management and technology PFC: power factor correction
IMD: insulation monitoring device PFI: Private Finance Initiative
IP: ingress protection (rating) PPE: personal protective equipment
IPS: isolated power supplies (also known as medical IT) PPS: primary power source
ISO: International Standards Organisation PSCC: prospective short-circuit current
ISS: intake substation PV: photovoltaic cell
IT: impedance terra earthed (derived from an isolated PVC: polyvinyl chloride
power supply)
RCBO: residual current breaker with overcurrent
ITU: intensive therapy unit
RCD: residual current device
IV: intravenous
REF: restricted earth fault
LBTC: logbook template customisable
RMU: ring main unit
LBTS: logbook template standard
SCADA: supervisory control and data acquisition
LDRP: labour delivery room and post partum
SCBU: special care babies unit
LPS: lightning protection system
SELV: separated extra LV
LV: low voltage
SF6: sulphur hexafluoride
M&E: mechanical and electrical
SI: Système Internationale
MCB: miniature circuit breaker
SP & N: single phase and neutral
MCC: motor control centre

2
 1 Scope of Health Technical Memorandum 06-01

SPS: secondary power source correct and in a safe manner. The designer need not be a
direct employee of the healthcare organisation.
TETRA: trans-European trunked radio access
Essential: any part of the electrical distribution and/or
TFL: time fuses links may also be referred to as tlf time-
final circuits that can be automatically transferred
lag fuses
between either the primary or secondary supply circuits.
THD: total harmonic distortion
Medical electrical equipment: electrical equipment
TN-C: combined neutral and earth throughout the intended to diagnose, treat or monitor a patient under
electrical distribution system medical supervision which will make physical or electrical
TN-C-S: neutral and earth is combined at point of contact with the patient, transfer energy to and from the
supply and separate throughout the electrical installation patient, or detect such energy flows.
Note: equipment is not covered by this Health Technical
TN-S: separate neutral and earth throughout the Memorandum. The Medicines and Healthcare products
electrical system Regulatory Agency (MHRA) is the government agency
TP & N: three-phase and neutral which is responsible for ensuring that medical equipment
works and is acceptably safe.
TPS: tertiary power supply
Medical IT (IPS): IT electrical system having specific
UMTS: universal mobile telecommunications service requirements for medical installations. The system will
UPS: uninterruptible power supply include a monitoring device to provide an alarm on loss
of IMD connections, insulation failure, overload and
VRLA: valve regulated lead acid (battery) high temperature.
VT: voltage transformer Medical location: location intended for the purpose of
XLPE: cross-linked polyethylene diagnosic treatment (including cosmetic) or monitoring a
patient under medical supervision.
Applied part: part of a medical electrical device which in
normal use necessarily comes into physical contact with • Group 0 Medical location where no applied parts are
the patient for the device to perform its function intended to be used.
• Group 1 Medical location where discontinuity of the
or electrical supply is not a risk to human life (unless the
location is part of a Group 2 location).
can be brought into contact with the patient
• Group 2 Medical location where discontinuity of the
or electrical supply can cause danger to life.
Note: “discontinuity” means any unplanned loss of the
needs to be touched by the patient. power supply (see MEIGaN).
Authorised Person (HV): a person appointed to take MEIGaN: Medical Electrical Installations Guidance
responsibility for the effective management of the safety Note published by MHRA.
guidance given in Health Technical Memorandum 06-03
– ‘Electrical safety guidance for high voltage systems’. Normal (non-essential): any part of the electrical
distribution and/or final circuits connected only to
Authorised Person (LV): a person appointed to take the primary distribution and with no means of being
responsibility for the effective management of the safety connected to the essential (secondary) distribution.
guidance given in Health Technical Memorandum 06-02 Note: in some distributions, manual reconfiguration may
– ‘Electrical safety guidance for low voltage systems’. allow the normal circuits to be temporarily connected to
Dual-unified distribution: separate primary and the essential (secondary) distribution.
secondary circuits collectively forming the electrical Patient: living person undergoing healthcare, therapy or
distribution of the healthcare facility. The secondary diagnostic investigation (including dental and cosmetic).
supply is equal to the primary supply; that is, both
primary and secondary circuits are fully rated and provide Patient environment: any area in which intentional or
a resilient distribution. unintentional contact can occur between the patient and
parts of the electrical system or between the patient and
Designer: a person (or organisation) with the other persons in contact with parts of the system (see
responsibility to design the electrical services technically Figure 44).

3
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Point of use: Electrical distribution points where Stakeholder: a person (or organisation) with vested
electrical equipment may be connected. This may be an interest (not necessary pecuniary) in the electrical services
accessory or isolator etc. quality and provision at healthcare premises. The
stakeholders will normally be an employee of the
Protected extra LV: a SELV system that is earthed at one
healthcare organisation.
point only. Additional protection against direct contact is
achieved by barriers and/or enclosures. Alternatively the Tertiary power supply: a third supply that supplements
insulation will have a “withstand” test voltage of 500 V the primary and secondary supplies, usually in the form
dc for 60 seconds. of an UPS or battery system.
Residual risk: a risk that has not been fully mitigated by Unified distribution: primary (normal) and secondary
the design process. (essential) circuits combined as one circuit to form a
common electrical distribution of the healthcare site.
Segregated distribution: an electrical distribution that
Where the secondary power source does not provide
includes separated primary and secondary circuits not of
100% capacity of the primary power source, local
equal size or capacity. The secondary circuits are the only
automatic devices will be required to isolate the non-
circuits that are supported by the standby power system.
essential circuits whenever the primary power source is
Separated extra LV: an LV system (normally not not available.
exceeding 50 V ac or 120 V ripple-free dc) derived from a
safety source such as an isolating transformer to BS EN
61558-1:1998 and BS EN 61558-2.
Single point of failure: a connection point (other than a
point of use) where any upstream single fault will cause
the loss of supply to the downstream parts of the
distribution.

4
2 Introduction

Overview from the various distribution centres to the final

circuits and point of use locations.
2.1 Health Technical Memoranda 2007, 2011 and
2014 have been superseded and combined into one Chapter 3 provides guidance on what needs to be
document: Health Technical Memorandum 06-01. considered when planning a new development in terms
of the electrical services.
2.2 Health Technical Memorandum 06-01 Part A
addresses design considerations for the electrical Chapter 4 explains how the design options should
services supply and distribution within any be assessed to minimise the risk of system failure and
healthcare facility. Part B addresses the operational the consequential impact to patients and users of the
management and maintenance of the electrical healthcare premises. The chapter introduces the
services supply and distribution within any concept of clinical risk in terms of the clinical function
healthcare facility. Both parts provide best of a department within the healthcare facility. The
practice guidance on the design and operational chapter also considers the business continuity risks in a
management of electrical services within healthcare similar manner. The chapter provides a risk matrix that
premises. may be used in the evaluation of the distribution
strategy.
2.3 Throughout this document, the following voltages
Chapter 5 defines the required standard of the
are used (see BS 7671:2001 for the defined voltage
electrical supply.
bands):
Chapter 6 describes the techniques to be used that will
Extra low voltage 50 V ac or 120 V ripple-free dc provide an appropriate resilient electrical system for any
Separated extra low 50 V ac or 120 V ripple-free dc healthcare facility (or part within a healthcare facility).
voltage (SELV) Chapter 7 describes the physical electrical
Protected extra low 50 V ac or 120 V ripple-free dc switchrooms, their construction, location, and
voltage (PELV) environmental requirements.
Low voltage 230 V phase to neutral, phase to earth Chapter 8 continues the descriptions of switchrooms
or line-to-line (medical IT) that may be used for alternative, including emergency,
400 V phase to phase power supplies.
High voltage 11,000 V phase to phase Chapter 9 provides the details of the electrical
6350 V phase to neutral
equipment that will be found in the primary and
secondary distribution centres. The chapter provides
2.4 This document has been divided into 18 chapters: information on the protection and isolation methods of
• Chapters 1 and 2 set out the structure of the the electrical services and distribution.
Health Technical Memorandum; Chapter 10 provides details of battery units that may
be used to start standby generator plant or provide
• Chapters 3 to 11 deal with design issues. more local power to items such as uninterruptible
Although this Health Technical Memorandum power supplies (UPS) or inverter units.
is written for new works and developments on a
greenfield site, it should be used for all works Chapter 11 provides details on how the electrical
and adaptations to the fixed wiring of any distribution system may radiate or absorb
healthcare facility; electromagnetic interference and how these problems
may be mitigated.
• Chapters 12 to 17 describe how the electrical Chapter 12 describes the general wiring formats in
services should be installed and put together relation to earthing and insulation.

5
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Chapter 13 describes the full earth systems to be used. How to read this Health Technical
The chapter has a section for HV, LV, and switchroom Memorandum
earths. In addition, the specific requirements for
2.5 This document has been written in a top-down
isolated power supplies, earthing radiographic rooms
format. The design process for new builds is
and circuits with high leakage currents are discussed.
likely to follow a similar planning, design and
The method of providing earth monitoring systems to
construction order to that of this document.
BS 4444:1989 for mobile trailer units is also considered
in the guidance. The chapter concludes with the 2.6 It is recommended that designers and stakeholders
requirements for lightning protection systems. review Chapter 4 as well as Chapter 6 for all
Chapters 14 and 15 describe the various cable types projects.
and the respective methods of installing them in 2.7 Designers and stakeholders should review any other
healthcare premises. chapter in relation to the nature of the particular
Chapter 16 describes the various final circuits, project.
including uninterruptible power supplies, isolated
power supplies, fixed equipment temporary supplies
and alarm circuits.
Chapter 17 provides designers and stakeholders with
an insight into the validation and commissioning tests
required before a new installation may be signed off
and formally accepted.
Appendices and References can be found at the end.

6
3 Initial considerations

3.1 This chapter introduces the design element of the b. tertiary source of supply:
document. The intent is to assist designers and
(v) uninterruptible power supplies (UPS),
stakeholders to develop the design of electrical
rotary or silent;
networks for new builds, but equally it applies
to modifications to existing installations. Some (vi) UPS static;
sections of this chapter can be addressed prior to or (vii) battery packs.
during the outline design stage, but all sections can
be addressed before the detailed electrical design 3.4 Adequate stocks of the fuel used by standby
stage. generators and/or CHP will need to be maintained.
The fuel for wind turbines (wind) and/or
Sources of supply photovoltaic systems (solar) may not always be
readily available at the optimum volumes, and
3.2 All healthcare premises require an electrical therefore these sources of power should not be
connection to the public electrical supply (PES), considered as essential power sources. UPSs use
which will be provided and operated by the batteries as a power source, which have a definitive
distribution network operator (DNO). Electrical autonomy dependent on the connected load. UPS
supplies to large healthcare premises are mainly and battery supplies should only be considered as a
at 11 kV (high voltage), while smaller healthcare short-term measure. The PES connection may be
premises may be supplied at 400 V (low voltage). used as the secondary source of supply where
The supply frequency at both voltage levels will appropriate measures for capacity, resilience,
be 50 Hz (see the Electricity Safety, Quality and maintenance and safety have been included with
Continuity Regulations 2002 for further details). the embedded sources of electrical power. However,
Within this Health Technical Memorandum, the it may be less viable to provide a dual PES
connection to the PES will be referred to as the connection than it is to provide additional on-site
“primary source of supply”. Any embedded secondary power supplies, for example standby
generating plant and/or combined heat and power generators.
(CHP) plant can be used as the primary source
of supply, provided appropriate measures for Resilience
resilience, maintenance and safety have been
included. 3.5 Large healthcare premises should generally be
supplied by a dual PES (ideally both at 100%
3.3 Many healthcare premises will require resilience of fully rated) arranged with either an automatic or a
the internally distributed electrical installation, manual change-over system. In order to maximise
which should be provided according to the clinical the resilience of dual supply arrangements and
risk assessments (see Chapter 4). The resilience may minimise the actual single point of failure, the
be provided by embedded sources of electrical supplies should be diverse. Where possible, they
power from plant such as: should originate from separate DNO substations,
a. secondary source of supply: in turn ideally fed from separate parts of the
National Grid, with independent cable routes to
(i) standby generators;
and across the healthcare site to the substations.
(ii) CHP systems;
3.6 Having two separate HV supply feeders is an
(iii) wind turbines; additional safeguard for larger premises; whether
this is practicable largely depends on the local
(iv) photovoltaic systems.
distribution system and the DNO.

7
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Essential/non-essential supplies damaging both circuit cables should be minimised

as much as possible.
3.7 When planning for new installations, the option
of segregated non-essential and essential electrical 3.14 In areas where there are many essential subcircuits,
systems or a unified electrical system (that is, separate cable trays should be used for the routing
duplex or simplex) should be evaluated. of duplex essential and non-essential circuit cables.
3.8 The need for more essential supplies increases the 3.15 Consideration should be given to the requirements
demand on the standby system. Electrical supplies of BS 5588 and fire-protected cables or cable
in the healthcare sector are growing at a rate of routes.
between 3% and 6% year on year. A suitable
philosophy should be agreed with estates staff to Primary sources of supply
reflect this growth before sizing the standby plant 3.16 This section deals with the primary supply,
and distribution strategy. distribution and sub-distribution of the primary
3.9 A risk-orientated approach should be adopted, electrical infrastructure (PEI), and presents best
and different categories of the essential and non- practice configurations for both HV and LV DNO
essential load identified to assign appropriate supplies (see Figure 1). The configurations are
standby provision (historically, it has been presented generally in order of resilience, from low
customary to have a discrete segregated essential/ to high. The selection of a particular configuration
non-essential service combination). For clinical will be dependent on the specific factors of each
areas, there should be 100% essential load individual design in terms of available DNO
provision. A segregated duplicated essential system supply, type of healthcare facility, category of
could be used to overcome the inherent single-fault patient etc. Whichever configuration is selected, it
breakdown potential. This would also facilitate the should be based on a risk analysis to determine the
operational opportunity to test and periodically appropriate level of resilience (see Chapter 7).
validate electrical installations. 3.17 The configurations presented in this section should
3.10 The provision of two segregated systems, each of not be taken as being definitive, prescriptive or
smaller power capacity, should be balanced against restrictive. They are intended as a guide to best
having one larger unified power system in terms practice and not intended to restrict innovation in
of economics and reliability in emergencies. The any design.
availability of the distribution and final subcircuits
for testing, validation and upgrading of systems Secondary main sources of supply
should also be taken into consideration. – generation
3.11 In systems that employ a segregated duplicated 3.18 This section deals with secondary supplies of the
essential service, thought should given to the space PEI and presents best practice configurations for
needed for separate feeders, independent risers and both HV and LV standby supplies (see Figure 2).
stand-alone distribution board (DB) cupboards so The configurations are presented generally in order
as to reinforce the system resilience. This will help of resilience, from low to high. The selection of a
to ensure that a local failure will not compromise particular configuration will be dependent on the
the entire system. specific factors of each individual design.
3.12 Even when two segregated systems are provided Whichever configuration is selected, it should be
(essential/non-essential), an emergency coupling based on a risk analysis to determine the
should be normally locked open between them. appropriate level of resilience (see Chapter 8).
This allows the standby generator to be connected
to both systems if necessary; for example, during a Tariff negotiations and private
prolonged outage, some normally non-essential generation
services may become essential, such as catering and
laundry. In addition, with the coupling it is 3.19 At an early stage of the design process, designers
possible to provide a larger test load. and stakeholders should assess the capacity of the
new electrical load. Negotiations with an electrical
3.13 Where essential, non-essential and duplex essential energy supplier should be initiated at this early
circuits are installed, diverse cable routes should be stage. Where the building services operator is
provided, and the possibility of a single cable fault

8
 3 Initial considerations

Figure 1 Primary electrical infrastructure for healthcare premises

SOURCE OF ELECTRICAL SUPPLY

PRIMARY SOURCE
PUBLIC ELECTRICAL SUPPLY
(PES) DISTRIBUTION
NETWORK OPERATOR
(DNO)
LV (400 V) HV (11 kV)
NETWORK(S)
VOLTAGE

High Voltage
HIGH

Substation

Low Voltage
Distribution Switch Panel

THE ELECTRICAL SYSTEM

LOW VOLTAGE NETWORK(S)

Low Voltage Low Voltage Low Voltage

Sub Main (Unified) Sub Main (Segregated) Sub Main (Dual Unified)
Distribution Board Distribution Board Distribution Board

TERTIARY POWER
SUPPLY
UNINTERRUPTIBLE
POWER SUPPLY

ISOLATED
POWER SUPPLY

FINAL CIRCUITS

CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK
CLINICAL RISK
CATEGORY 1 CATEGORY 2 CATEGORY 3 CATEGORY 4 CATEGORY 5
CATEGORY 1

9
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 2 Secondary electrical infrastructures for healthcare premises

SOURCE OF ELECTRICAL SUPPLY

SECONDARY SOURCE
HEALTHCARE PREMISES
INTERNAL ELECTRICAL INFRASTRUCTURE

Alternative Combined
Standby
Energy Heat and
Power Plant
Plant Power (CHP)
NETWORK(S)

High Voltage
VOLTAGE

Substation
HIGH

Low Voltage
Distribution Switch Panel

THE ELECTRICAL SYSTEM

LOW VOLTAGE NETWORK(S)

Low Voltage Low Voltage Low Voltage

Sub Main (Unified) Sub Main (Segregated) Sub Main (Dual Unified)
Distribution Board Distribution Board Distribution Board

TERTIARY POWER
SUPPLY
UNINTERRUPTIBLE
POWER SUPPLY

ISOLATED
POWER SUPPLY

FINAL CIRCUITS

CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK
CLINICAL RISK
CATEGORY 1 CATEGORY 2 CATEGORY 3 CATEGORY 4 CATEGORY 5
CATEGORY 1

10
 3 Initial considerations

responsible for the purchase of electrical energy, healthcare facility will have an electrical supply at
they will also be responsible for the negotiations. one of the following voltages:
However, in the more usual case where the
electrical energy is a pass-through cost, or where 11 kV Large acute hospital, typical
the building services operator is the healthcare floor area greater than 8500 m2
organisation, the healthcare organisation will be 11 kV/400 V Medium-sized acute hospital,
responsible for the above negotiations. typical floor area 5500 m2 to
8500 m2
3.20 The opportunities for alternative energy sources
should be explored wherever practical. For example, 400 V TP & N General/community hospitals,
sources such as CHP or wind power will reduce the health centres, large off-site
net carbon emissions and potentially provide an clinics, off-site administrative
improved economic solution. Where alternative buildings, stores and
energy sources are used, the resilience of such plant decontamination facilities
should be considered. This may be in the form of 230 V SP & N GP and dental practices, small
“N+1” CHP plant or suitable alternative supply off-site clinics
from the DNO. The types of healthcare facility in the above list
3.21 Healthcare organisations should use the NHS are for illustration and are not definitive. For
Environmental Assessment Tool (NEAT) both voltage tolerances in the above list, see the latest
to help find out how their facilities and services issue of the IEE Regulations (BS 7671:2001),
impact on the environment and to estimate the and the Electricity Safety, Quality and Continuity
level of environmental impact taking place (http:// Regulations 2002.
www.dh.gov.uk). 3.25 Some larger sites may have multiple feeds (at the
3.22 Where the proposed alterations are for intake point) with an internal distribution network
modification and/or adaptations to the internal (see Chapters 6–8). In such cases, the declared
electrical infrastructure, tariff negotiations may not voltage will be either 11 kV or 400 V. Such
be required. Nevertheless, the use of alternative connection arrangements provide an improved
power sources should still be considered to offset resilience of supply.
the increased electrical demand. 3.26 Some healthcare sites, particularly older sites that
3.23 Guidance on the environmental benefits of have expanded over a number of years, may have
alternative energy sources can be obtained from: multiple intake points (which may not all be at the
same supply voltage). The various intake points
• Building Services Research and Information should be consolidated to a single or multiple feeds
Association (BSRIA) (http://www.bsria.co.uk); at a common point. Such arrangements will
• Chartered Institute of Building Services provide economies with tariff and standing charges.
(CIBSE) (http://www.cibse.org); 3.27 The DNO may arrange with the healthcare facility
• Combined Heat and Power Association (under a wayleave agreement) to have their own
(CHPA) (http://www.chpa.co.uk); electrical equipment, including transformer, on-
site. This arrangement is frequently used in rural
• Department for Environmental Food and Rural areas where the healthcare facility is some distance
Affairs (http://www.defra.gov.uk); from the nearest DNO substation. In such cases,
• The Carbon Trust (http://www.carbontrust.co. the DNO’s electrical equipment will be at a higher
uk). voltage (11 kV) to that supplied at the healthcare
site’s intake terminals (400 V).
See also Health Technical Memorandum 07-02 –
‘Encode’.
Design of installations for growth and
Supply voltages change
3.28 Changes in medical technology and healthcare
3.24 The DNO will deliver the PES at the customer’s
intake terminals at a declared voltage in accordance practice have had an effect on the requirements for
with the requirements of the Electrical Safety, electrical power in healthcare. Examples include:
Continuity and Quality Regulations 2002. Each

11
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

• cook-chill: the introduction of cook-chill has Greenfield site

meant more meals are cooked electrically and
3.35 Where the proposed works is a new building site
ward-based “regeneration ovens” have been
and not part of an existing hospital complex, the
introduced;
assessment of existing electrical systems may be
• electronic patient records (EPR) and patient limited to an understanding of the DNO’s
entertainment systems: although these have a infrastructure in the area. This knowledge will
very low electrical power requirement, such help in determining the cost associated with any
systems require a significant increase in reinforcements.
containment and space.
New-build on existing site
3.29 Alterations to existing installations, unless planned
and allowed for during the original construction, 3.36 Where the proposed works are a new building site
can be costly, particularly when structural changes within part of an existing complex, the assessment
are involved. of existing electrical systems will determine the
extent of any spare capacity at the proposed
3.30 Healthcare premises are frequently remodelled
connection point. The connection point may be
within the economic life of an electrical
at the site intake or embedded in the internal
installation. Designers and stakeholders should
distribution network. This knowledge will help to
identify means of remodelling the electrical
determine the practicalities and cost associated with
distribution and determine to what extent any
any reinforcements of switchgear, protection
flexibility for remodelling should be incorporated
devices and cables.
with the initial design.
3.31 Each electrical distribution centre should include New equipment on existing site
an element of equipped and unequipped enclosures 3.37 Where the proposed work is limited to the
for retrofitting of switches, protective devices and installation of new equipment or the modifications
so on. of sub-distribution and final circuits, the power
3.32 The designers should evaluate, by risk assessment, requirements and an understanding of the existing
the degree of remodelling and natural expansion distribution (including final subcircuits) will
that will be incorporated into the initial determine the most appropriate connection point
installation. The risk assessment should reflect both and any reinforcements of switchgear, protection
clinical and commercial risks. devices and cables.
3.33 This allowance for growth and remodelling should
be incorporated into the adopted distribution Load profile
strategy (see Chapter 6). 3.38 Designers and stakeholders should understand the
electrical profile of the healthcare facility at an early
Assessment of existing electrical stage. This will prove invaluable in assessing the
systems viability of any secondary and/or tertiary energy
sources (for example CHP plant). Where the
3.34 Designers will need to make a reasonable healthcare facility is an existing site, electrical
assessment of any existing electrical services that demand data records should be available. The
are to be modified or connected to as part of the data may be available from the building energy
proposed works. For existing sites, the building management system (BEMS), the utility supplier
logbook (see Chapter 17) will provide details of (in the form of digital pulse metering) or from the
the existing electrical systems, periodic test results Estates Return Information Collection (ERIC).
(including fault levels), applied diversity, load
profile and schematic drawings of the electrical 3.39 Table 1 indicates a typical range of power densities
system and/or network. Examination of the settings found in healthcare premises over a five-year
on all adjustable protection devices will identify the period. The actual figures will reflect the
extent of any tolerance within the grading and technology used and the particular department.
discrimination of such protective devices. Power densities at the lower end of the range
reflect average power densities of large healthcare
premises. Power densities at the upper end of the
range reflect power densities for smaller healthcare

12
 3 Initial considerations

premises such as GP surgeries. The figures do not 3.40 Figure 3 shows a typical daily electrical profile for a
take account of any growth. Power densities at the hospital. The annual profile is shown in Figure 4.
lower end do not necessarily imply any improved The actual shape and kVA values are unique to the
efficiency over power densities at the higher end. site and a function of the size and clinical facilities
However, power densities outside the range may provided.
give cause for further investigation.
3.41 The various options of providing the electrical
energy required to satisfy the load profile should be
Table 1 Typical range of power densities for a
evaluated. Where the profile indicates a relatively
healthcare facility over a five-year period
short peak maximum demand, which is high in
Power W/m2 GJ/100 m3 relation to the average demand, the cost of the
General power 9–25 3.7–10.3 DNO connection will be disproportionately
expensive, as the cost of the connection is only
IM&T power 3–6 1.2–2.5
optimised at the maximum demand period. In such
Medical power 5–15 2.0–6.2 cases, the healthcare organisation should consider
Lighting developing some of the electrical energy from on-
General 9–15 3.7–6.2 site alternative energy sources such as CHP or wind
Special 0.9–1.5 0.4–0.6 power. Where alternative energy sources are used,
Task 1.35–2 0.5–0.8 the resilience of such plant should be considered.
Medical 0–1 0–0.4

Figure 3 Typical diurnal electrical profiles for a hospital

kVA

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time of day

Figure 4 Typical annual electrical profiles for a hospital

kVA

Jan

Feb

March

April

May

June

July

Aug

Sep

Nov

Dec
Oct

13
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Diversity factors radiography) may be higher on weekday mornings

than in the afternoon, while the accident and
3.42 The electrical diversity factor is the ratio of emergency department may peak in the early
instantaneous power to the total installed power. evenings.
Diversity factors can be applied to each element of
the electrical service (for example the lighting load 3.43 The above variations in actual diversity will reflect
or low-power load) or to a whole department. the true load profile as identified in paragraphs
Where the healthcare site is large and has multiple 3.38–3.41 above.
substations, the diversity factors can be calculated 3.44 Designers should assess the actual diversity factors
for each individual substation. It should be clear for each service and department to make a value
that the diversity factor for a particular service (for judgement of the site-wide normalised diversity.
example lighting) may differ during the day and This figure can then be applied to the total installed
year, while the diversity of, say, the chiller plant power and the allowance for growth to determine
may have a different cycle. Similarly, the diversity the electrical capacity of the DNO connection.
variation for one department (for example Table 2 shows some typical figures.

Table 2 Typical electrical diversity factors for healthcare premises

Power density PF Building Substation Connected Off-peak Growth
(W/m2) diversity diversity load diversity diversity factor
AHU fans 15–25 0.95 0.7–0.90 1.00 0.7–0.90 0.68 0.10
Building services pump 1–3 0.95 0.7–0.90 1.00 0.7–0.90 0.68 0.10
Lifts 5–8 0.95 1.35–0.23 0.50 0.3–0.5 0.6 0.05
Chiller 25–35 0.95 0.9–0.95 1.00 0.9–0.95 0.8 0.05
General low power 9–25 0.95 0.49–0.60 0.70 0.7–0.85 0.65 0.20
Information systems 3–6 0.95 0.80 0.80 1 0.65 0.20
Medical equipment 6–16 0.95 0.35–0.49 0.70 0.5–0.7 0.2 0.20
General lighting 9–15 1.00 0.49– 0.63 0.70 0.7–0.9 0.8 0.15
Specialist lighting 0.9–1.5 1.00 0.35–0.63 0.70 0.5–0.9 0.5 0.15
Task lighting 1–2 1.00 0.45–0.54 0.60 0.75–0.9 0.4 0.15
Medical lighting 0–1 1.00 0.42–0.54 0.60 0.7–0.9 0.7 0.15
Notes:
Power density: The power density relates to the relevant internal floor area of the healthcare premises.
Power factor (PF): Power factor is assumed to be the corrected power factor at each substation.
Building diversity: The building diversity reflects that not all substations within the healthcare premises will have the same
operating profile. The building diversity is the product of the connected load and substation diversity. The building diversity is
the actual demand seen at the point of common coupling with the PES.
Substation diversity: Substation diversity reflects that not all areas, of any one substation, will have the same operating profile,
for example clinics and 24-hour areas. The substation diversity is multiplied by the connected load diversity to produce the
building diversity.
Connected load diversity: Connected load diversity reflects that any electrical system (fixed medical equipment etc) will not be
operating at full demand or used to maximum capacity at all times of the day.
Off-peak diversity: Off-peak diversity reflects that not all equipment will be used (to the same profile) at night as in the day (for
example 12-hour clinics etc). The off-peak diversity is not used in these calculations, but will be an element used in the energy
calculations.
Growth factor: Growth factor is an allowance for the natural expansion in electrical equipment used, and potential remodelling
of the hospital. Growth factor is applied to switchgear, cable sizes, and transformer sizes etc. The function of the growth factor is
to ensure that the electrical network will not need premature replacement.

14
 3 Initial considerations

Consideration for EMC requirements It will not always be possible to design-in

equipment that is CE-marked to show compliance
3.45 Electrical installations must be compliant with the with the EMCD. An installation may comprise
requirements of the Electromagnetic Compatibility CE-marked and non-CE-marked equipment.
Regulations 2005. The regulations describe the Supply of non-CE-marked equipment is acceptable
electrical installation as a manufactured item, and providing the installer who is integrating them into
therefore require the installation to be tested for the system can be sure that they will not cause
electromagnetic radiation and absorption. undue interference to the installation or be overly
Procurement contracts for electrical equipment sensitive to the electromagnetic environment where
associated with the distribution of the electrical the equipment will be used.
installation should stipulate that the equipment
must be compliant with the Electromagnetic 3.47 The designer should obtain a technical file from
Compatibility Regulations 2005. the installer demonstrating that good engineering
practice has been applied, and should show details
Roles and responsibilities of any concessions granted to items that are not
compliant with the installation specification, but
3.46 The Electromagnetic Compatibility Directive
which may be used without detrimental effects.
(EMCD) and the Electromagnetic Compatibility
Regulations 2005 describe who is responsible for 3.48 Designers and stakeholders may assume that any
meeting compliance. There are only two parties equipment which is supplied for operation in the
involved – the manufacturer and the operator. medical environment will have sufficient immunity
In a healthcare building project: to operate successfully in that environment, and it
should not emit excessive radiation. The above
• the designer and installer of an integrated statement assumes that the healthcare organisation
heating, ventilation and air-conditioning has an approved policy for the procurement of
(HVAC) or power distribution control system medical equipment which does not involve the
are the “manufacturer”; designers or stakeholders of the electrical
• the manager of the healthcare premises and the installation.
building services maintenance organisation are
the “operators”.

Figure 5 Roles and responsibilities

Building owner/ Ensures system legally Requires documentary
designer complies with EMC proof of compliance
legislation

Designer Designs using compliant Specifies documents to be

systems. Defines EMC provided
specifications and good
EMC practices to be used

Integrator Provides compliant Specifies documents to be

integrated installation provided

Contractor Purchases and installs Supplies evidence of

systems that meet the compliance to the
design specification. operator
Installs using good EMC
practices

15
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Access for maintenance 3.55 In cases where the new installation includes
embedded generator plant (including CHP or
3.49 In order to comply with the Construction, Design PV cells), the DNO will need to understand the
and Management Regulations (CDM) 1994, methods used to clear any faults from the internal
designers need to give due account for access and energy sources in order that they are not reflected
maintenance at a very early stage of the design, onto the DNO’s network.
irrespective of the size, complexity and extent of the
proposed installation. 3.56 For greenfield sites, it may be necessary to
coordinate the activities of the DNO, meter
3.50 Electrical services should not compromise the operator and energy supplier before a connection
space and access routes for other services such can be completed (see paragraphs 8.29–8.47).
as mechanical and public health. Maintenance
tasks should be carried out with the minimum 3.57 The DNO will reserve the right not to connect a
disruption to continuity of supply and business. new installation where the installation fails to
comply with its criteria.
3.51 The following documents provide additional
information: Connection to the healthcare site network
• Health Technical Memorandum 06-01 Part B – 3.58 Where the new installation will be connected into
‘Operational management’; part of the healthcare site’s existing network, the
• Health Technical Memorandum 00 – ‘Policies application to connect will take the form of a
and principles’; “completion certificate” as identified in
BS 7671:2001. The healthcare facility will be
• Defence Works Functional Standard DMG 08 entitled to conduct a range of tests to be satisfied
– ‘Access and accommodation for engineering that the installation is safe for energising and that
services: space requirements for plant access, any fault currents which may arise are cleared
operation and maintenance’; before reflecting onto the remaining part of the
• manufacturers’ operational and maintenance healthcare site’s network.
manuals. 3.59 The site engineer should reserve the right not to
connect a new installation where the installation
Commissioning procedures does not comply with the guidance given in this
3.52 Designers should consider how the installation Health Technical Memorandum and local electrical
will be commissioned and how the required test safety guidance. The site engineer may also reserve
measurements will be made. This will include the the right not to connect a new installation where
inspection of services that may be hidden at the the installation compromises the distribution
time of handover. It will also include the strategy of any other part of the electrical network.
implications of any phased occupation (see 3.60 At a very early stage of the design (or if appropriate,
Chapter 17 for more information). procurement planning), the designer should assess
3.53 The design team should make an application to the power requirement and subsequently make a
connect the new works to the DNO or healthcare request for a suitable supply.
site prior to doing so. At this point, the installation
should be suitably safe and ready to be energised. Supply from the DNO
3.61 Where the new supply will be direct from the
Connection to the DNO DNO, the designer should contact the DNO for
3.54 Where the new installation will be connected the appropriate forms. The cost of the supply will
directly to the DNO’s network (PES), the reflect the level of infrastructure reinforcement.
application to connect will take the form of
a “completion certificate” as identified in Supply from internal connection point
BS 7671:2001. The DNO will be entitled to 3.62 Where the new supply will be connected to
conduct a range of tests to be satisfied that the part of the healthcare facility’s existing internal
installation is safe for energising and that any fault distribution, the designer should liaise with the
currents which may arise are cleared before site engineer to determine the most appropriate
injecting into the DNO’s network. connection point, and the required method and
degree of reinforcement of the internal distribution.

16
4 Understanding risk and ownership

4.1 This chapter deals with the assessment of risk and 4.5 The design process should ensure that single
the need to ensure that the design of the primary points of failure are minimised by providing the
electrical infrastructure (PEI) adequately protects appropriate level of resilience at the point of use.
the end-user, and in particular patients, from Risk management carefully balances the approach
electrical failures. It promotes multidisciplinary to a design strategy with the cost/benefit
design-team and stakeholder involvement relationships, where cost represents investment,
throughout the design process to ensure an business continuity and operational risk.
appropriate distribution strategy (see Chapter 6), 4.6 Failures of the PEI system are commonly
incorporating resilience, redundancy and considered as a consequential effect of the failure
duplication as necessary. This should identify any of the incoming DNO supply, main transformer,
“residual risks” from the design. The identification main switchboard etc. In these cases, it is assumed
of the residual risks will enable the healthcare that the emergency power system (secondary and/
organisation to manage their collective ownership or tertiary power supply) is available. However,
of risk management and hence make appropriate failure of the PEI itself is also possible. All potential
non-electrical and/or fixed wiring operational and points of failure should therefore be considered
emergency contingency plans in accordance with during the design process. The emergency supply
DH Emeregency Planning Guidance. system design may be different for each type of
failure.
Introduction
4.2 A failure can occur at any point or at any time Note
in any electrical system, regardless of the design An inappropriate level of resilience or response to a
standards employed. The design and installation of failure may compromise patient safety.
PEI systems inherently allows failure (by operation
of a protective device) to minimise the risk of
danger and/or risk of injury. This is true of internal Ownership and design
PEI systems as well as the wider PES network
4.7 The duties of the stakeholders involved in the
delivered by the DNO. The effects of accidental
design, or assessment and operation, of an existing
damage and the need for maintenance and training
infrastructure should ensure (as far as reasonably
should not be overlooked. It is essential that an
practicable) that all risk levels and the likelihood
appropriate level of risk management is considered
of an electrical failure are balanced against the
and practical emergency contingency plans are
consequences of such failures. All stakeholders
always available and ready to implement.
and designers should understand and accept the
intended operation, limitations and inherent
Need for risk assessment possible failure scenarios of the electrical system
4.3 Appropriate controls should be put in place to and, where necessary, implement contingency
reduce any risk to an acceptable manageable level. arrangements where risks of electrical failures
It is essential that, at the very least, legislative cannot be, or are not, mitigated within or by the
requirements be met and risk be managed electrical system itself. These risks will include
proactively. those inherent and residual from the design strategy
(see Chapter 6). For example, the stakeholders and
4.4 A complete risk assessment for a sustainable designers may agree that it is an acceptable risk that
electrical supply is a key duty of care owed to clinical risk Category 1 and 2 areas (see paragraphs
patients, staff and visitors. 4.12–4.24) need not have any embedded secondary

17
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

power source (SPS) to cover an outage of the 4.13 The consequence of a power failure is assessed and
electrical system. The management of such residual categorised against some broad clinical patient
risks may therefore be to reorganise any postponed groups and patient care plans. This is on a
elective consultations. scale from ambulant through to critical care.
Consequence is also related to the organisation in
Risk profile terms of contingency arrangements, emergency
preparedness and business continuity, all of which
4.8 This Health Technical Memorandum divides risk have a financial implication. There is also the
into two core elements: clinical risk (subdivided operational consequence of the electrical system
into patient and non-patient areas) and non-clinical in terms of the operation and maintenance of the
business continuity risks (subdivided into medical infrastructure from the point of view of both its
services and engineering services). Designers and physical construction and installation, and the
stakeholders should consult with medical and managerial and technical staff structure in place to
technical staff to evaluate the overall risk and operate the electrical infrastructure.
the measures proposed to address the perceived
outcomes. Most critical within this assessment is 4.14 The level of consequence of a power failure may
the mobility and degree of healthcare support be evaluated as increasing with patients’ clinical
provided to the patient, including medical category, for instance, and the level of consequence
procedures, critical care and continuity of per category will equally be dependent on the
treatment. duration and extent of the failure.
4.10 Small healthcare premises such as GP practices and 4.15 IEE Guidance Note 7 (also IEC 60364-7-710)
health clinics/centres may have areas that fall into addresses “special locations”. Chapter 10 of the
Category 1 and possibly Category 2. Community IEE guide deals with specific in-patient areas
hospitals may have departments in Categories 1, 2 and classifies the dependence certain healthcare
and 3, but unlikely in Category 4 or 5. Large acute departments have on a sustainable electrical supply.
healthcare premises and above may well have The guide relates in part to the reliability of
departments in all categories. There is no rule that electrical supplies and their subsequent safety
definitively places healthcare premises in any one requirements. These classifications are ranked in
category, or defines one category for a particular time performance (seconds) to re-establishing a
healthcare site. supply following an interruption, whether
controlled or otherwise.
4.11 Each healthcare facility will have a mixture of
categories (clinical risks and non-clinical business 4.16 Within a GP practice or health centre, it may
continuity risks) in varying ratios. The assessed be assessed as acceptable to have single points of
higher clinical or non-clinical and business failure in a system, given that patients are likely
continuity risk for any particular area will to be more mobile than patients in critical care
determine the adopted electrical infrastructure areas, and staff will be able to move away from the
strategy for that area. Designers and stakeholders affected area in the event of a power failure. At the
should evaluate the economics of providing other end of the scale, for example in critical care
different distribution strategies (see Chapter 6) areas, the consequence of a prolonged, or even a
for each risk category area, or of applying an very short, power failure could be serious health
appropriate distribution strategy for the highest- disabilities or, in the worst cases, fatality. In this
order risk category to many or all areas. instance, a more resilient infrastructure with
additional levels of secondary and/or tertiary power
Clinical risk supplies would be appropriate.

4.12 Within any healthcare environment, there are wide 4.17 While it is not intended to be absolute, this section
ranges of departments with complex requirements should be sufficient to prompt the necessary
and potential risks. The risk management process discussion at all stages of the design process. The
will categorise each department in terms of categories given are intended to demonstrate a
susceptibility to risk from total (or partial) loss of range of patient risk from an electrical fault or loss
electrical supply. of electrical supply.
4.18 Consideration of the categories in Figure 6 should
establish a minimum acceptable risk option at the

18
 4 Understanding risk and ownership

point of treatment or care. For the purpose of this

business continuity risk if these areas are not
guidance, the patient levels described are not
connected to the SPS for supply failures that last
intended to be exhaustive, but rather an aid to
greater than three hours (notwithstanding the
consider the issues.
requirements of the escape lighting etc that may
be provided from a local tertiary power source).
Category 1 – Support service circulation

4.19 These areas do not directly relate to the patient

Category 3 – Emergency care and
environment of any group under Chapter 10 diagnostics
in ‘IEE Guidance Note 7’. The areas include
circulation spaces, waiting areas, offices and non- 4.21 These areas relate to the patient environment
patient care areas such as laboratories or finance of group 0 under Chapter 10 in ‘IEE Guidance
departments. Consequently, engineering services Note 7’. The areas will include mental health
do not have an immediate effect on the clinical wards and some maternity areas. Patients are
treatment or safety of patients (notwithstanding not generally connected to any electro-medical
the requirements of the escape lighting and so on equipment. However, medical monitoring or
that may be provided from a local tertiary power medical test equipment may occasionally be used
source). and connected externally to the patient’s body
for a short or intermittent time (for example
Category 2 – Ambulant care and patient monitors or ultrasound machines).
Clinical treatment and patient safety will not be
diagnostics compromised by an interruption of electrical
4.20 These areas do not directly relate to the patient power. However, the interruption of electrical
environment of any group under Chapter 10 in power should be limited to less than 15 seconds
‘IEE Guidance Note 7’. The areas may include for other engineering services used in the
patients in consultation (excluding examination) support of the clinical treatment such as
or general out-patient areas. Loss of supply may medical gases, hot and cold water, HVAC,
give rise to disruption, inconvenience and a communication etc (notwithstanding the
reduced environmental quality but would not requirements of the escape lighting etc that may
directly compromise patient clinical treatment be provided from a local tertiary power source).
and safety. The loss of electrical power to other
engineering services (for example ventilation or
medical gases) equally will not cause concern for
the safety of the patient or staff. There may be a

Figure 6 Patient clinical risk categories

PATIENT CLINICAL RISK CATEGORY

1 2 3 4 5
Support service Ambulant care Emergency care Special medical Life support/
circulation and diagnostics and diagnostics locations complex surgery

19
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Category 4 – Patients in special medical for a prolonged period. Clinical treatment and
locations patient safety may be compromised by any
interruption of electrical supply. A patient’s
4.22 These areas will relate to the patient natural electrical resistance is significantly
environment of group 1 under Chapter 10 in reduced when electro-medical conductive
‘IEE Guidance Note 7’. The areas may include parts are placed in the body. Supplementary
LDRP (labour, delivery, recovery, post-partum) equipotential bonding (within the patient
areas (maternity), endoscopy rooms, accident environment) should be provided for patient
and emergency general/minors, haemodialysis safety. Best practice provision should include,
areas, ECG areas, nuclear medicine, radiography but not be limited to, IPS systems and the
diagnostic, magnetic resonance imaging (MRI), provision of an alternative electrical supply
endoscopic examination rooms, urology (tertiary power supplies) within 0.5 seconds
treatment areas, or therapy rooms and of any interruption of the electrical supply if
ultrasound. Patients may have electro-medical required by the medical equipment. Other
equipment, medical monitoring or medical test engineering services used in support
equipment connected externally to their body of the patient clinical treatment should be
for a prolonged period. Clinical treatment and connected to the secondary power source (SPS)
patient safety may be compromised (but not within 15 seconds of any interruption of the
endangered) by any interruption of electrical electrical supply (notwithstanding the
supply. Electrical protective devices should requirements of the escape lighting etc that may
include an RCD, but may not require an IPS be provided from a local tertiary power source).
circuit. Supplementary equipotential bonding 4.24 Tertiary power supplies such as a UPS (see
will be required in the patient environment. Any paragraphs 16.3–16.19) or a battery, within the
interruption of the electrical supply to medical equipment, may be considered as a method to
equipment should be limited to within limit the interruption of electrical supply to less
15 seconds. Consideration may be given to than 0.5 seconds. Standby generator(s) (see
providing an alternative electrical supply (tertiary Chapter 8) may be considered as a method of
power supplies) within 0.5 seconds, subject limiting the interruption of electrical supply
to the range of patient treatment. Other between 0.5 seconds and 15 seconds. In
engineering services used in support of clinical Category 4 or Category 5 areas, a patient may be
treatment should be connected to the SPS at risk from both a general loss of supply and a
within 15 seconds of any interruption of local final subcircuit fault. In Category 5 areas,
the electrical supply (notwithstanding the enhanced levels of resilience for the provision of
requirements of the escape lighting and so on patient therapies may be required. This may be
that may be provided from a local tertiary power provided by interleaved circuits at the bed-head
source). or pendant. Such arrangements will assist in the
ability to perform maintenance with minimal
Category 5 – Life support or complex disruption.
surgery
Non-clinical and business continuity
4.23 These areas will relate to the patient
environment of group 2 under Chapter 10 in
risk
‘IEE Guidance Note 7’. The areas are defined 4.25 While clinical risk is the important factor in the
as operating theatre suites, critical care areas, design of PEI in healthcare premises, it does not
cardiac wards, catheterising rooms, accident just relate to the criticality of patients. There are
& emergency resuscitation units, MRI, numerous supporting elements and departments
angiographic rooms, PET and CT scanner essential to continuity of care and business
rooms. Patients may have electro-medical continuity.
equipment, medical monitoring or medical test 4.26 The failure of these services should be assessed on
equipment (for example intracardiac procedures) the same basis as the clinical risk. The increasing
connected externally or internally to their bodies reliance on information technology and electronic
medical records is an obvious example of this,

20
 4 Understanding risk and ownership

where the loss of electrical power could affect on the SPS, where the sum of their respective
essential diagnosis of a patient or the ability to loads would only represent a very small percentage
operate a clinic. Essential items of building services of the SPS. An understanding of the need for
and plant may also necessitate the closure of maintenance and the capacities of any such battery
departments in the event of a power failure where packs employed on these facilities will be required
these services are not adequately protected by a (notwithstanding the requirements of the escape
resilient electrical system. lighting etc that may be provided from a local
tertiary power source).
4.27 Consideration of the categories in Figure 7 should
establish a minimum acceptable risk option at the
Category 3 – Building services environmental
point of treatment or care. For the purpose of this
control
guidance, the non-clinical and business continuity
described is not intended to be exhaustive, but an 4.30 The building services environmental control
aid to consider the issues. systems will include HVAC systems, hot water
services, energy centres and building energy
Category 1 – Business support services management systems. In general, an interruption of
the electrical supply could represent a compromise
4.28 The business support areas are departments such
to the treatment or welfare of patients. A single-
as finance, stores, laundries and workshop areas.
conversion UPS should be provided to allow
In general, an interruption of the electrical supply
certain systems such as computer applications to
may not compromise the treatment or welfare of
be shut down safely. Electrical load management
patients. It may be appropriate to provide a single-
systems should be considered where the
conversion UPS (see paragraphs 16.3–16.19) to
interruption to the electrical supply (to, say, chilled
allow certain systems such as computer applications
water systems) gives an unacceptable rise in the
to shut down safely. Electrical load management
internal space temperature by internal heat gains
systems may prove useful where the interruption to
(see paragraphs 8.17–8.19; notwithstanding the
the electrical supply (for these areas) is for more
requirements of the escape lighting etc that may be
than four hours (see paragraphs 8.17–8.19;
provided from a local tertiary power source).
notwithstanding non-patient safety measures with
the provision of a tertiary power source).
Category 4 – Medical support services
Category 2 – Building services safety and security 4.31 The medical support areas are departments such
as disinfection units, laboratories, medical records,
4.29 The requirements for these facilities are covered by
and physiotherapy. An interruption of the electrical
various British and European legislative documents,
supply may represent a slight disruption to the
for example BS 5839-1:2002. Typically, battery
treatment or welfare of patients. A single-
packs or single-conversion UPS systems will
conversion UPS (see paragraphs 16.3–16.19)
support such requirements. An interruption of the
should be provided to allow certain systems such as
electrical supply could compromise the safety and
computer applications to be shut down safely.
welfare of patients. These facilities may be included

Figure 97 Non-clinical and business continuity risks

Figure

NON-CLINICAL AND BUSINESS CONTINUITY RISK CATEGORY

1 2 3 4
Business support Building services Building services Medical support
services safety and security environmental services
control

Figure 10

21
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Electrical load management systems may prove 4.34 Cause-and-effect risk models are used to analyse
useful where the interruption to the electrical the global electrical infrastructure from intake to
supply (for these areas) is for long periods, say point-of-use equipment. The risk evaluation should
more than two hours (see paragraphs 8.17–8.20; consider single electrical faults that cascade into
notwithstanding the requirements of the escape multiple electrical faults and unrelated
lighting etc that may be provided from a local simultaneous multiple faults (see Chapters 6
tertiary power source). and 9).
4.35 A distribution strategy should be developed that
Electrical infrastructure drives the risk of failure of an electrical supply (at
4.32 This Health Technical Memorandum divides the point of use) to a low or residual risk (Figure 8).
electrical infrastructure into two core sections: A lower distribution strategy with increased
primary and secondary (see Chapter 6): redundancy of primary and/or secondary power
supply plant – which still achieves the same risk
• the two types of distribution may share the level indicated above – should be adopted. Further
same cables, which defines a unified electrical consideration may be given to accepting a slightly
infrastructure; higher risk factor than the indicated lowest possible
• where there are two sets of cables common to for a particular clinical area.
the primary and secondary distribution, the
network is said to be a dual-unified distribution; Resilience
• where the primary and secondary distribution 4.36 Overall, the PEI can be considered in three main
has entirely separate cables, the distribution sections:
strategy is said to be a segregated distribution.
• supply;
The most resilient distribution strategy will have
• distribution;
both dual-unified distribution and primary and
secondary power sources. • point of use.
4.33 Business continuity risk assessments evaluated by 4.37 The essential elements of a resilient infrastructure
“cause-and-effect” models may be used to analyse are:
the impact of electrical failures on departments
• redundancy;
which are reliant on the services provided. Within
the integrated departmental model, consideration • moving the first “single points of failure” as near
should be given to the cause and effect of electrical to the point of use as possible;
failures which escalate exponentially with time.

Figure 8 Electrical failure risks evaluation to clinical categories

RISK OF ELECTRICAL FAILURE BY INFRASTRUCTURE
Distribution strategy (refer to Chapter 6)
Dual primary
(refer to Chapter 4 under ‘Clinical risk’)

Dual supply Dual supply and secondary

Unified and unified and unified and supply unified
Risk by clinical category

Unified segregated dual unified dual unified and dual unified

distribution distribution distribution distribution distribution
Life support complex
HIGH HIGH SIGNIFICANT MODERATE LOW
surgery
Special medical locations SIGNIFICANT SIGNIFICANT MODERATE MODERATE LOW
Emergency care and
MODERATE MODERATE MODERATE LOW RESIDUAL
diagnostic
Ambulant care and
MODERATE MODERATE LOW LOW RESIDUAL
diagnostic
Support services and
LOW LOW RESIDUAL RESIDUAL RESIDUAL
circulation

22
 4 Understanding risk and ownership

• appropriate access for practical maintenance and resilience. The distribution strategy should include
testing procedures. adequate resilience and access space so that routine
testing and maintenance can be carried out safely,
4.38 The resilience required to maintain essential supply
without placing patients, staff and users at
in the event of not only primary failures but also
unnecessary risk. Such strategies may call for a
secondary failures should be considered. A suitable
redundancy in certain electrical equipment, for
assessment of the likelihood of concurrent failures
example generators, UPSs and IPSs. The provision
occurring within a foreseeable period should be
of resilience for maintenance is considered best
made, and therefore the operation and inter-
practice.
relationship of the system and its component parts
should be fully understood. With regard to the
consequence and risk, any reasonably foreseeable Electrical infrastructure system selection
secondary failures should be appropriately 4.42 A number of different elements link together to
protected against. An example here may be a form the primary and secondary infrastructure
standby generator failing to start (second-line fault) system (see Figure 9). Some of the elements will
on a supply blackout (first-line fault). be optional dependent on design strategy (see
4.39 Incoming electrical supplies may be constrained by Chapter 6) and required resilience issues based on
what the DNO is able to provide, or what has been assessed risk from power failures. All possible
assessed as cost-effective for the type of healthcare configurations of the electrical infrastructure
facility. The distribution strategy should maintain elements will have risk-mitigation strategies
the minimum acceptable resilience level throughout associated with the possibility of power failures
the internal electrical system. occurring. Similarly, each element will link to other
elements, and the links and interactions between
4.40 An iterative design process will help stakeholders to them will present additional risk minimisation. The
assess the distribution strategy. The process may be overall risk of a power failure occurring can be
used to determine the location of the first single mitigated by the correct selection of element
point of failure in addition to the method used to configurations and interconnections. Standard
mitigate the risks on the distribution downstream system and component configurations at
of that point. The provision of tertiary supplies appropriate infrastructure sections can be broadly
(UPS) on final distribution boards or the ability to categorised in terms of their resilience and therefore
manually reconfigure the distribution may be residual risks. Evaluating the cause and effect can
suitable risk mitigation. make selection of the appropriate configurations
4.41 The effects of electrical power failures due to apparent at the point of use (deepest part) of the
faults at any level within the infrastructure can be infrastructure.
designed out by the robustness of the network

23
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 9 Electrical infrastructure generic flow diagram

SOURCE OF ELECTRICAL SUPPLY

PRIMARY SOURCE SECONDARY SOURCE

PUBLIC ELECTRICAL SUPPLY HEALTHCARE PREMISES
(PES) DISTRIBUTION INTERNAL ELECTRICAL INFRASTRUCTURE
NETWORK OPERATOR (DNO)
Alternative Combined
Standby
Energy Heat and
LV (400 V) HV (11 kV) Power Plant
Plant Power (CHP)
NETWORK(S)

High Voltage
VOLTAGE

Substation
HIGH

Low Voltage
Distribution Switch Panel

THE ELECTRICAL SYSTEM

LOW VOLTAGE NETWORK(S)

Low Voltage Low Voltage Low Voltage

Sub Main (Unified) Distribution Sub Main (Segregated) Sub Main (Dual Unified)
Board Distribution Board Distribution Board

TERTIARY POWER
SUPPLY
UNINTERRUPTIBLE
POWER SUPPLY

ISOLATED
POWER SUPPLY

FINAL CIRCUITS

CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK
CLINICAL RISK
CATEGORY 1 CATEGORY 2 CATEGORY 3 CATEGORY 4 CATEGORY 5
CATEGORY 1

24
5 Power quality

5.1 The quality of the electrical supply is the electrical system will be corrected. The power factor
responsibility of the DNO, which will comply with correction equipment should be automatically
the requirements of the Electrical Safety, Quality disconnected if the primary supply is interrupted,
and Continuity Regulations. However, the use of and if used in conjunction with the standby
the electrical energy within healthcare premises can generator plant (or CHP plant) adjusted to suit
affect the quality of the internal distribution in the generator (or CHP plant) manufacturer’s
terms of power factor and harmonics. recommendations. Where the only power factor
correction equipment is located at the intake, the
5.2 Healthcare premises have numerous switch mode
appropriate cable sizes for the higher currents
power supplies and inherently high inductive
should be used.
electrical loads and unless corrected, the power
factor will be poor, requiring large transformers,
Located at sub-main distribution boards
cables and high-energy cost. Improving a typical
poor power factor from say 0.75 lagging to an 5.6 Power factor correction can be installed at the sub-
appropriate power factor of say 0.95 lagging will main distribution board, in which case only the
reduce the kVA demand by 20% and will be outgoing circuits will be corrected. The advantage
reflected as a utility cost savings. The actual of power factor correction units installed at sub-
inductive load will vary throughout the day and main distribution boards is that several inductive
year. The inductive load will also vary across the loads can be corrected with one common unit. This
site according to the location of mechanical plant, will save on the capital cost and space required.
lifts and to some extent the clinical departments. Where the power factor correction equipment is
located at the intake and sub-main distribution
5.3 The normal supply frequency is 50 Hz with a
boards, the appropriate subcircuit cable sizes for the
tolerance of ±1%. The nature of the electrical
higher currents should be used. Similarly, the rating
equipment used throughout the site can inject
of the sub-main distribution board and protective
secondary frequencies that cause significant
equipment may need oversizing.
disturbances to the internal distribution and the
supply. The majority of secondary frequencies, Located on the electrical equipment
known as harmonics, are generated from switch-
mode power supplies found in a wide range of 5.7 Where individual pieces of equipment generate a
electrical and electronic equipment. high inductive reactance such as large motors, it
is advisable to install the power factor correction
Power factor correction direct to the motor. This arrangement has the
advantage of reducing the voltage drop on the
5.4 The usual method of correcting a low power factor motor supply cable(s) and hence smaller
uses capacitive reactance to oppose the inductive distribution cables can be used.
reactance. Using capacitor banks with automatic
step changes will ensure that the net reactance does 5.8 Power factor correction equipment may generate
not produce a leading power factor. There are three harmonic currents as well as allowing the
basic locations for power factor correction downstream harmonics to pass through. Therefore
equipment. consideration should be given to the use of
detuning inductors within the power factor
Located at the intake point correction equipment.

5.5 Power factor correction can be installed at the main 5.9 Power factor correction equipment can be installed
distribution intake point, in which case the entire as an integral part of a switchboard, or as a

25
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

freestanding unit. Where a cubicle of the which then should be sized for such currents and
switchboard is used to house the power factor heat. Consideration should also be given to the use
correction equipment, consideration should be of clean-earth conductors for power supplies to
given to the effect this will have on future flexibility items such as IM&T computer server and hub
and remodelling of the power factor correction rooms. A single-phase third-harmonic current on a
equipment and/or the switchboard circuits. balanced three-phase circuit can produce currents
at 70% of the fundamental current and flow within
5.10 Power factor correction equipment requires natural
the neutral conductor. If the harmonic currents
ventilation to remove the small amount of heat it
reach the distribution transformer they will be
generates. Further information on the amount of
reflected into the primary delta winding and
natural ventilation should be available from the
circulate. The unchecked harmonic current in the
manufacturer.
primary winding of a distribution transformer will
be dissipated as unwanted heat and noise.
Harmonics
5.15 The network analysis of harmonic currents
5.11 The normal supply frequency is 50 Hz with a propagated within the electrical systems should be
tolerance of ±1%. The nature of the electrical made and communicated to the supplier of any
equipment used throughout the site can create standby generator. The generator design will need
secondary frequencies that cause significant to reflect the anticipated harmonic currents.
disturbances to the internal distribution and the Harmonic currents not allowed for within the
supply. The majority of the secondary frequencies, generator design may cause excessive heating, high
known as harmonics, are generated from short torque loads and consequently vibration within the
surge currents and transient currents arising from generator while running. Only active harmonic
non-linear electrical loads such as switch-mode filters or isolating passive harmonic filters should be
power supplies and rotating machinery, variable used while the generator is online (see paragraph
speed drives and so on found in a wide range of 5.18).
electrical and electronic equipment.
5.16 The electrical load of a typical healthcare facility
5.12 Surge transient currents are also caused by lightning with many modern medical facilities and support
strikes. Further details on the lightning strikes and services may have non-linear loads (propagating
surge arrestors can be found in paragraphs 13.39– harmonic disturbances) at 40% of the overall load.
13.49. Unless the harmonics are controlled and eliminated
5.13 Even-order harmonic frequencies are self-negating downstream from the transformer primary
and do not cause a real disturbance to the electrical winding, the transformer may need to be derated
distribution. Odd-order harmonics with zero by the “factor-K method” (as defined in BS 7821),
rotation effect, known as “Triplen” harmonic which may typically be 70% of the transformer
frequencies (3rd-, 9th- and 15th-order), and odd- nameplate kVA rating to avoid transformer
order harmonics (5th, 7th and 11th) can cause damage.
significant disturbances leading to high currents 5.17 Alternative methods may include using oversized
and voltages, and overheating of cables and neutral conductors, using separate transformers
equipment, particularly transformers. for linear circuits and using inductive loads or
5.14 Electrical systems must comply with the preferably harmonic filters. Active or passive
Energy Networks Association’s Engineering harmonic filters should be used, which can be
Recommendations G.5/4, which limits the located in one of three locations depending on the
reflected total harmonic distortion (THD) at point source and severity of the disturbance.
of common coupling. For voltages up to 0.4 kV,
the THD is 5%, whereas at 11 kV the THD is 4%. Located at the intake point
Designers should evaluate the sources of harmonic 5.18 Harmonic filters can be installed at the main
disturbances within the healthcare site’s electrical distribution intake point, in which case the entire
network and the methods to mitigate the effects. electrical system will be corrected. Any passive
Harmonics can be controlled by the use of harmonic filters should be automatically
harmonic filters (active or passive). The use of disconnected when any standby generator plant is
oversized neutral conductors (200% to 300%) will supplying the load. Harmonic filters should not be
carry the harmonic current back to the transformer,

26
 5 Power quality

located here, as large harmonic currents may Located on the electrical equipment
require neutrals oversized by as much as 300%.
5.21 Where an individual piece of equipment generates
5.19 Note – for the purpose of this section of the a high transient current or voltage from a switch-
guidelines, the intake point means the HV/LV mode power supply, it is advisable to install active
substation, LV switchboard. Alternatively, where harmonic filters direct to the equipment. This
the site has an internal HV network, the intake arrangement has the advantage of reducing the
point means each such substation LV distribution harmonic disturbances on the final subcircuit
board. cables.

Located at sub-main distribution boards Voltage surge protection

5.20 Harmonic filters may be installed at the sub-main 5.22 Consideration should be given to the provision
distribution board, in which case only the outgoing of voltage surge protection at the LV intake point,
circuits will be corrected. The advantage of where equipment which may be sensitive to such
harmonic filters installed at sub-main distribution voltage surges is connected to the distributed
boards is that several sources of harmonic inductive installation from the intake.
loads can be corrected with one common unit. This
will minimise the harmonic disturbance reflected
on the sub-main and main distribution cables and
save on the capital cost and space required.

27
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

6 Distribution strategy

6.1 For the purpose of this Health Technical switchboards. While this chapter describes the
Memorandum, it is assumed that the highest resilience provided by generators, it does not
distributed voltage in any healthcare facility will be describe the starting or control methods, which
11 kV. DNO connections to healthcare facilities can be found in Chapter 8.
may be at elevated voltages such as 33 kV, but it is
6.5 The resilience can be enhanced at the final
considered that such voltages are not distributed
distribution board with the use of tertiary power
within the healthcare site. Where healthcare
supplies such as UPS (see paragraphs 16.3–16.19).
facilities do have a DNO connection at voltages
Alternatively, the resilience can be enhanced with
above 11 kV, the strategy for such connections
battery packs fitted to medical equipment such as
(and if appropriate, distribution) should follow the
intravenous (IV) pumps. Further guidance on
distribution philosophy described in this chapter.
alternative supplies can be found in Chapter 8.
This Health Technical Memorandum only
This section deals with the strategy and design of
considers any electrical energy used at low voltage
the fixed wiring network to achieve the desired
as a three-phase or single-phase connection. This
resilience.
Health Technical Memorandum does recognise that
some electrical energy used in healthcare premises 6.6 To achieve this resilience, all stakeholders and
will be at SELV or PELV, but is only concerned designers should contribute to the risk assessment
with the fixed wiring at low voltage. In a similar debate (see Chapter 4).
way, electrical energy used at high voltage, for some 6.7 The available electrical supply rating should be
large vapour compression chillers for example, is verified with the DNO. Most DNOs will provide
acknowledged, but again this Health Technical between 500 kVA and 800 kVA as a single LV
Memorandum only addresses the fixed wiring at (0.4 kV) connection. Supply ratings between
HV. 750 kVA and 12 MVA should be supplied at high
6.2 When designing the strategy for the electrical voltage (11 kV). Where a healthcare facility has an
network(s), it is essential to take a holistic assumed maximum demand (AMD) greater than
approach. The electrical system may include 12 MVA, the DNO connection should be at 33 kV
HV and LV distribution networks, or just LV or above. Clearly, where the healthcare facility has
distribution networks, depending on the size of the an AMD no more than 500 kVA, the internal
site. electrical infrastructure will only be at low voltage;
other AMDs will require an HV and LV internal
6.3 The topology of the LV network(s) can provide the
infrastructure.
most resilient service at the point of use. The cost
of such security of supply may be compromised if
the HV system is not equally resilient. Best practice Design for resilience
is achieved when the distribution strategy places 6.8 Throughout this Health Technical Memorandum
the first single point of failure as close to the final (and in common parlance) resilience is expressed
subcircuits as practical to satisfy the critical nature in terms of “N+1”. This Health Technical
of the healthcare facility while remaining financially Memorandum considers N+1 to mean the normal
viable. total requirement plus one resilient unit. For
6.4 The required system resilience can be achieved in example, where the electrical demand is 1000 kVA,
two basic ways, first by having dual circuits and two transformers at 1000 kVA would satisfy the
secondly by having an alternative power supply. N+1 definition. However, three 500 kVA
Standby generators can be connected at the intake transformers would also be defined as N+1, as the
point or may be connected at specific LV normal element comprises two units. When

28
 6 Distribution strategy

calculating the resilience of a system it is important 99.999% (or 0.001% unreliable), which could
to consider elements that are mutually inclusive mean 5.25 minutes unavailability per year.
and not mutually exclusive. For example, the However, the issue is the time to repair or replace a
standby generator complement (with a common faulty transformer, which may be at least one week.
point of coupling) would be a mutually inclusive Distribution strategies that have two transformers
system. The standby generator complement and with a common primary supply but have their
primary electrical infrastructure at a common point secondary linked by a normally open bus coupler
of coupling are mutually exclusive. Best-practice would provide a transformer system resilience of
electrical distribution strategy solutions provide N+1. While both transformers are on duty, they
resilience to the first-fault conditions at a common would share the instantaneous load at 50% each.
point of coupling. Distribution strategies that Opportunities for transformer and busbar
provide resilience above the first-fault condition maintenance are improved, as well as improved
(at a common point of coupling) are unlikely to continuity of supply following a transformer
be economically viable. Distribution strategies that outage. Such arrangements are shown for the
provide N+1 resilience at several different points of intake substation (ISS) in Figure 13. Transformers
common coupling are more robust and connected in parallel or operating transformers on
economically viable. no load are not recommended.

Supply connections Risk of generator failures

6.9 In electrical supplies to large healthcare premises 6.12 Generators have many moving parts requiring
(in electrical terms this means greater than lubrication, cooling and control. The standby
2 MVA), a general arrangement with a dual-PES generator controls are electronic and
connection should be adopted, arranged with either electromechanical devices used to modulate the
an auto-changeover or a manual-selection switch. output in response to the demand inputs. Standby
Dual supplies with diverse routes may be generators should be maintained in an operational
considered an economic strategy to maximise the readiness state in order to provide their principal
resilience and minimise the actual single point of function of standby supply. Generator reliability
failure. They should originate, if possible, from may be of the order of 99.95% (or 0.05%
separate DNO substations, ideally fed from unreliable), which equates to 4.5 hours
separate parts of the National Grid, with unavailability per year. The majority of generator
independent cable routes to the healthcare site’s failures are a result of the generator not starting or
substations. However, the origin and nature of the occur during the first five minutes after starting.
PES supply routes will largely be beyond the
6.13 Distribution strategies that have two standby
control of the designer or stakeholders.
generators (each rated at full load) with a common
6.10 Two separate HV supply feeders (or LV if point of coupling with the distribution network
appropriate) may provide an additional safeguard would provide generator system resilience of N+1.
(for the healthcare site) against a PES connection Three standby generators, similarly connected, all
failure. Whether this is practicable largely depends rated at 50% of the connected design load, would
on the local distribution system and the DNO. The also provide a generator system resilience of N+1.
healthcare organisation will not be in control of the Three generators each rated at 100% of the
PES network and therefore cannot influence the connected design load would provide N+2, but
value of a second diverse routed connection. The may be difficult to justify economically.
healthcare organisation has more control over any Opportunities for standby generator and busbar
embedded resilience and internal backup power maintenance are improved, as well as improved
sources, which may provide more robust security continuity of supply following a generator outage
of electrical power. (while the sets are on line). Such arrangements are
shown in Figure 17.
Risk of transformer failures
Other reasons for failure of electricity supply
6.11 The only moving part of a transformer is the tap-
changing mechanism, which may be discounted 6.14 The following list provides further reasons for
when considering transformer reliability. As a failure of the main internal electrical distribution
result, transformer reliability can be as high as systems, all of which are minimised by adopting

29
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

the guidance within this Health Technical 6.17 A single HV supply from the DNO feeds onto
Memorandum (Parts A and B) and the guidance the healthcare site’s HV-switchboard part of the
given in Health Technical Memorandum 06-02 – substation. This would typically be up to, say,
‘Electrical safety systems’: 1500 kVA.
• cable faults within the private network; 6.18 This type of distribution strategy is appropriate for
a small to medium-sized acute hospital. As there is
• inappropriate grading of protection devices;
only a primary source of supply, this system creates
• poorly designed network with common points a single point of failure right through to the LV
of failure; distribution switchboard(s). Designers and
• reliance on one form of standby protection; stakeholders should consider the benefits afforded
by the additional resilience that may be required,
• accidental isolation. dependent on the clinical risks (see Chapter 4).
6.19 The resilience of the HV supply of Figure 10 may
Distribution system – high voltage be enhanced by including a secondary source of
6.15 Healthcare premises with an AMD greater than supply at the intake, as a second DNO connection,
800 kVA will require an internal HV network. or an LV standby generator at the LV switchpanel.
There are two basic forms, radial networks (for Clearly, generators at the LV switchpanels would
AMDs up to say 3.5 MVA) and ring networks (for add resilience to the internal distribution. Having
AMDs above 3.5 MVA). multiple LV standby generators is unlikely to
provide any economic benefit.
6.16 The following simplified schematics are provided to
show the main HV supply arrangements. They are
HV network – one radial circuit with three
arranged generally in order of resilience from low to
substations
high, but their selection as a design solution will be
dependent on the supply arrangement available 6.20 In Figure 11, a single HV supply from the DNO
from the DNO, the type of healthcare facility, and feeds onto the healthcare site’s HV-switchboard
the level of assessed risk with regard to end-users. part of the substation. From the intake substation,
Where typical HV distribution arrangements are a single HV radial circuit connects up to two more
shown connected to the LV distribution, these are HV substations. Each of the substations would
included only to assist in the understanding of the typically be up to say 1500 kVA; however, the
LV arrangements. LV distribution arrangements are AMD for the healthcare site would be between
considered more fully in paragraphs 6.31–6.59. 800 kVA and 3.5 MVA.
This section does not describe the control of any 6.21 This type of distribution strategy may be
standby generator system (see Chapter 8 for appropriate for a healthcare site with many
details). detached buildings. The areas served by any single
substation do not exceed a clinical risk assessment
HV network – one radial circuit with one
of Category 2. As there is only a primary source
substation only
of supply, this system creates a single point of
failure right through to the LV distribution
Figure 10 HV network – one radial circuit, one
switchboard(s). The benefits afforded by additional
substation
resilience that may be required are dependent on
the clinical risks (see paragraphs 4.12–4.24).
DNO or healthcare premises
connection 6.22 The resilience of the HV supply may be enhanced
by including a secondary source of supply at the
intake as a second DNO connection. Additional
transformers at each substation (all 100% rated
and not connected in parallel), or LV generator(s)
connected at the LV switchpanels, may also
enhance the infrastructure resilience. The second
Substation ISS transformers at each substation will provide
additional resilience and reduce the impact of
transformer failure and maintenance. Standby

30
 6 Distribution strategy

Figure 11 HV network – one radial circuit, three substations

DNO connection

Substation 1 Substation 2 Substation 3

The healthcare site

generators could be local to each substation or in a HV network – three radial circuits each with one
common central facility, depending on the spread substation
of the site. Clearly, the generator at LV switchpanels
6.23 In Figure 12, a single HV supply from the DNO
would add resilience to the internal distribution.
feeds onto the healthcare site’s HV-switchboard
Having multiple LV generators at a common
part of the substation. From the intake substation,
central facility may provide economic benefit to the
three HV radial circuits, each with one substation,
healthcare facility during maintenance periods.
are connected. Each of the substations would
typically be up to 1500 kVA; however, the AMD
for the healthcare site would be between 800 kVA

Figure 12 HV network – three radial circuits each with one substation

DNO or site connection

Substation ISS

Substation 1 Substation 3

Substation 2

The healthcare site

31
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

and 3.5 MVA. This distribution network is intake substation should consist of circuit breakers,
an enhancement of that in Figure 11 above, as and all field substations should have a common
failure of any part of the internal HV electrical switch type. See Chapter 9 for additional details of
infrastructure will affect a smaller area. The dual HV protection and switchgear details.
100%-rated transformers (not operated in parallel)
6.28 This type of distribution strategy may be
of substation 2 will provide improved transformer
appropriate for a large acute hospital with several
and switchgear maintenance opportunities for that
other support facilities on the same site. The areas
area.
served by the ISS substation and substation 3 may
6.24 This type of distribution strategy may be include clinical risk assessments of Category 4 or
appropriate for a healthcare premises with many Category 5. The areas served by substation 2 and
detached buildings. The areas served by any one substation 4 may include clinical risk assessments
substation do not exceed a clinical risk assessment of Category 3. As there is only a primary source
of Category 3. As there is only a primary source of supply, this system creates a single point of
of supply, this system creates a single point of failure right through to the LV distribution
failure right through to the LV distribution switchboard(s).
switchboard(s).
6.29 The resilience of the HV supply of Figure 13 may
6.25 The resilience of the HV supply of Figure 12 may be enhanced by including a secondary source of
be enhanced by including a secondary source of supply at the intake, as a second DNO connection.
supply at the intake as a second DNO connection. Additional transformers at each substation (all
Standby LV generator(s) connected at the LV 100% rated and not connected in parallel), or
switchpanels will enhance the infrastructure standby LV generator(s) connected
resilience and facilitate improved transformer at the LV switchpanels, may also enhance the
maintenance opportunities. Standby generators infrastructure resilience. The transformers at each
could be local to each substation or in a common substation may provide resilience and assist in
central facility, depending on the spread of the site. transformer failure and/or maintenance. Standby
Clearly, the standby generator at LV switchpanels generators could be local to each substation or in a
would add resilience to the internal distribution. common central facility, depending on the spread
Having multiple LV generators at a common of the site. Clearly, the generator at LV switchpanels
central facility may provide economic benefit to the would add resilience to the internal distribution.
healthcare facility during maintenance periods. Having multiple LV standby generators at a
common central facility may provide economic
HV ring network – ring with four substations benefit to the healthcare facility during
6.26 In Figure 13, a single HV supply from the DNO
maintenance periods.
feeds onto the healthcare site’s HV-switchboard 6.30 The resilience of the HV ring network can also be
part of the substation. From the intake substation enhanced if the ring normally operates in a “closed
one HV ring-circuit connects to all other internal ring” control. Electrical faults may then be isolated
HV substations (which may be more than the four to a discrete section of the ring (typically one
shown here). Each of the substations would substation leg or between two substations) and
typically be up to say 1500 kVA; however, the hence not affect the supply to any part of the
AMD for the healthcare site would be greater healthcare site. An alternative level of resilience may
than, say, 3.5 MVA. This distribution network is an be available by the introduction of a switch control
enhancement of that in paragraphs 6.23–6.25, as monitoring and management system to the HV
failure of any part of the internal HV ring electrical ring switch. Such a system can automatically
infrastructure will affect a smaller area. Following a reconfigure the HV network open position of the
ring distribution fault, the manual or automatic closed ring, and hence restore power to all areas,
operation (where the switchgear has suitable well within in a few minutes (see Chapter 9).
controls) of network ring switch positions will
restore the inherent resilience. Primary and secondary distribution
6.27 The HV ring network of Figure 13 indicates the systems
four basic types of HV substation: single and dual
6.31 All LV distributions should be configured as
ring main units, single or dual circuit breakers. The
TN-S systems as defined by the IEE Regulations

32
 6 Distribution strategy

Figure 13 HV ring network – ring with four substations

PES
DNO connection

Substation ISS

Substation 2 Substation 3 Substation 4

The healthcare site

BS 7671:2001. Within special areas, medical 6.33 The method of fire protection should be considered
risk Categories 4 and 5 and wet areas such as for essential distribution circuits, and the
post-mortem rooms, the wiring system would opportunities for flexible remodelling. For
be configured as a medical IT system with compliance with BS 5588, essential circuits
non-tripping earth fault for patient areas and associated with life-support services should be
tripping for wet areas by insulation monitors to either fire-rated or fire-protected.
IEC 61557-8 (see paragraphs 13.6–13.14 for more
6.34 Single line representation is used in the diagrams
details). Consideration may be given to the use of a
for single- and three-phase distribution. Where
protected extra LV (PELV) system or a separated
typical HV distribution arrangements are shown
extra LV (SELV) system.
connected to the LV distribution, these are
6.32 The following simplified schematics are provided to included only to assist in the understanding of the
show the main LV supply arrangements. They are LV arrangements. HV distribution arrangements
arranged in order of resilience from low to high, are considered more fully in paragraphs 6.15–6.30.
but their selection as a design solution will be
dependent on the supply arrangement available
from the DNO, the type of healthcare facility, and
the level of assessed risk with regard to end-users.

33
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Primary supply – unified infrastructure 6.39 This simple arrangement may be appropriate
for GP practices, health centres and office
Figure 14 Primary supply – unified distribution accommodation, dependent on the assessed level
system of risk posed by a failure.
Substation ISS
Primary and secondary supply – unified and
segregated infrastructure
6.40 This form of LV infrastructure has a primary
supply, connected directly to either the DNO
or the healthcare site’s transformer, and a non-
distributed secondary supply connected at an
internal LV switchboard. The transformer,
switchgear and main cables should be rated to
take the AMD and an allowance for growth (see
paragraphs 3.28–3.33). However, the standby
generator is rated only for the segregated essential
Risk Risk Risk part of the healthcare site’s electrical demand.
category 1 category 1 category 2
6.41 The first single point of failure will be the
connection point (to the DNO healthcare site’s
transformer) for the unified non-essential circuits.
However, the segregated essential circuits have a
6.35 This is the simplest form of LV infrastructure with
single point of failure much nearer the point of
a primary supply only, direct either from the DNO
use. This infrastructure does not fully provide for
or from a single HV transformer arrangement,
inconvenience-free maintenance opportunities to
feeding a unified LV distribution network. The
all areas. Therefore, the infrastructure lends itself to
transformer switchgear and main cables should be
healthcare premises that have part 24/7 facilities
rated to take the AMD and an allowance for
and part non-24/7 facilities. Enhancing the
growth (see paragraphs 3.28–3.33).
infrastructure resilience and adding additional
6.36 The first single point of failure will be the standby generator units to the essential circuits may
connection point (to the DNO healthcare site’s improve the maintenance opportunities and
transformer). A LV infrastructure of this kind is business continuity. Alternatively, a manual load
appropriate for clinical risk Categories 1 or 2 (see management system coupled with the facility to
paragraphs 4.12–4.24). This infrastructure does interconnect the essential and non-essential circuit
not provide for inconvenience-free maintenance (via cables or a manual bus coupler) may offer a
opportunities. Therefore, the infrastructure lends similar increased resilience (see Chapter 9).
itself to healthcare premises that do not operate
6.42 An assessment of the potential for expansion and/or
24 hours a day/7 days a week (24/7) facilities, and
remodelling the healthcare premises should be
hence give rise to operational windows in business
made, to understand how the LV distribution of
continuity.
Figure 15 could accommodate such adaptations.
6.37 Enhancing the infrastructure resilience and adding
a facility to connect a mobile generator plant at the Primary and secondary supply – unified and dual-
intake may improve the maintenance opportunities unified infrastructure
and business continuity. Alternatively, a tertiary 6.43 This form of LV infrastructure (see Figure 16) has
power source, single-conversion UPS (see a primary supply connected directly to either the
paragraphs 6.3–6.19) may be connected to DNO or the healthcare site’s transformer, and a
dedicated equipment such as computer systems. distributed SPS also connected at the intake LV
6.38 An assessment of the potential for expansion and/or switchboard. The transformer, standby generator,
remodelling of the healthcare premises should be switchgear, and main cables should all be rated to
made, to understand how the LV distribution of take the full assumed maximum demand and an
Figure 14 could accommodate such adaptations. allowance for growth (see paragraphs 3.28–3.33).

34
 6 Distribution strategy

6.44 The first single point of failure will be at the main provide for inconvenience-free maintenance
LV switchboard for the unified circuits, and at the opportunities in the Category 3 or 4 risk areas.
point of use for the dual-unified circuits. A LV However, the same opportunities do not exist in
infrastructure of this kind is appropriate for clinical the Category 1 and Category 2 risk areas, where
risk categories 2, 3 or 4, where the dual-unified the infrastructure resilience is only achieved by the
circuits are used in Category 3 or 4 risk areas (see standby generator subject to the operating demand.
paragraphs 4.12–4.24). This infrastructure may There is no resilience with the distribution cables

Figure 15 Primary and secondary supply – unified and segregated infrastructure

Substation ISS

G1

Risk Risk
category 1 category 2

Risk category 3 or 4

Figure 16 Primary and secondary supply – unified and dual-unified infrastructure

From healthcare site’s network
or DNO connection

G1

Risk Risk Risk category 3 or 4

category 1 category 2

Substation on a healthcare site

35
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

and/or switchgear for the Category 1 and Category single-conversion UPS (see paragraphs 16.3–16.19)
2 risk areas. The unified circuit infrastructure lends may be connected to dedicated final circuits of the
itself to that part of the healthcare premises that do unified distribution. Enhancing the dual-unified
not operate 24/7, and the dual-unified circuit infrastructure resilience and adding additional
infrastructure (Category 4) lends itself to those standby generator units may improve the
which do operate 24/7. maintenance opportunities and business continuity
for the Category 3 or 4 risk areas.
6.45 Enhancing the infrastructure resilience may
be achieved by adding a tertiary power source. 6.50 An assessment should be made of the potential
A single-conversion UPS may be connected to for expansion and/or remodelling of the healthcare
dedicated final circuits of the unified distribution. premises and how the LV distribution of Figures 16
Enhancing the dual-unified infrastructure resilience and 17 could accommodate such adaptations. This
and adding additional standby generator units may arrangement may be appropriate for general acute
improve the maintenance opportunities and or large acute hospitals with additional support
business continuity for the Category 3 or 4 risk services, dependent on the assessed level of risk
areas. posed by failures.
6.46 An assessment of the potential for expansion and/or 6.51 Figure 17 shows a potential connection point for
remodelling the healthcare premises should be a CHP plant. The schematic only provides one of
made, to understand how the LV distribution could many potential electrical connections for the CHP
accommodate such adaptations. This arrangement plant. Designers and stakeholders should assess the
may be appropriate for a general acute or large ideal CHP connection based on the opportunity to
acute hospital with additional support services, “black start” the CHP sets, and to synchronise the
dependent on the assessed level of risk posed by a CHP with the PES supply and/or standby
failure. generator supply.
6.52 In reality, the CHP location may be driven by the
Dual primary and dual secondary supply – unified
thermal and environmental requirements rather
and dual-unified infrastructure
than the electrical connection. For example,
6.47 The LV infrastructure shown in Figure 17 has a locating the CHP plant close to the boiler plant
dual-primary supply, connected directly to either may provide a more beneficial connection for the
the DNO or the healthcare site’s transformer, and reclaimed heat energy into the boiler return
a dual-distributed secondary power supply also pipework. In addition, the CHP engine exhaust
connected at the intake LV switchboard. The can be ducted alongside the boiler flues.
transformer, standby generator, switchgear and
main cables should all be rated to take the AMD Dual primary and dual HV secondary supply
and an allowance for growth. – dual-unified infrastructure
6.48 The first single point of failure will be at the main 6.53 The LV infrastructure illustrated in Figure 18 has a
LV switchboard for the unified circuits, or at the dual primary supply, connected to the healthcare
point of use for the dual-unified circuits. A LV site’s transformer, and a dual distributed SPS
infrastructure of this kind is appropriate for clinical connected directly to the internal HV network.
risk categories 2, 3 or 4, where the dual-unified The transformer, standby generator, switchgear,
circuits are more appropriate for Category 4 or and main cables should all be rated to take the full
5 risk areas (see paragraphs 3.28–3.33). This assumed maximum demand and an allowance for
infrastructure may provide for inconvenience-free growth (see paragraphs 3.28–3.33).
maintenance opportunities in the Category 3 or 4
6.54 The first single point of failure will be at the point
risk areas. However, the same opportunities do not
of use for all circuits. A LV infrastructure of this
exist in the Category 2 risk areas, where there is no
kind is appropriate for clinical risk categories 4 or 5
resilience with the distribution cables and/or
where the dual-unified circuits are Category 5 (see
switchgear. Therefore, the unified circuit
paragraphs 4.12–4.24). The Category 5 risk areas
infrastructure lends itself to that part of the
have the added installed resilience of tertiary power
healthcare premises that does not operate 24/7.
supplies, double-conversion UPSs and IPSs. This
6.49 Enhancing the infrastructure resilience may be infrastructure may provide for inconvenience-free
achieved by adding a tertiary power source. A maintenance opportunities in all areas, particularly

36
 6 Distribution strategy

when the final subcircuits are interleaved (see directly to the internal HV network, via step-up
paragraphs 6.60–6.64). transformers. The transformer, standby generator,
switchgear and main cables should all be rated to
6.55 The potential for expansion and/or remodelling of
take the AMD and an allowance for growth (see
the healthcare premises should be assessed in terms
Chapter 3).
of how the LV distribution could accommodate
such adaptations. The arrangement may be 6.57 This form of LV infrastructure is the same as that
appropriate for a large acute hospital with in paragraphs 6.53–6.55 except that the standby
additional support services, dependent on the generators are low voltage with step-up
assessed level of risk posed by a failure. transformers.
6.58 The IPS connection arrangement in Figures 18
Dual primary and dual LV secondary supply – dual-
and 19 is only one such possible arrangement.
unified infrastructure
Designers may wish to consider not supporting the
6.56 This form of LV infrastructure is illustrated in IPS with a UPS as per Figure 41.
Figure 19 and has a dual primary supply connected
6.59 Many large healthcare premises will have a mixture
to the healthcare site’s transformer, and a dual
of clinical risk areas (see paragraphs 4.12–4.24) and
distributed secondary power supply connected
consequently may require an overall distribution

Figure 17 Dual-primary and dual-secondary supply – unified and dual-unified infrastructure

HV network

G1

G2

CHP

Substation on a
healthcare site

Risk Risk
category 2 Risk category 3 or 4 category 2

Areas served from substation

37
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 18 Dual primary and dual HV secondary supply – dual-unified infrastructure

HV generator substation
G1 G2

DNO DNO
Service 1 Service 2
The healthcare site network

Substation ISS

UPS UPS

May not IPS IPS

Risk always be Risk

category 1 via UPS category 2

Risk category 4 & 5

Risk category 3 & 4

Areas served from substation

The healthcare site

strategy based on a mixture of the above examples. Final circuits

Designs with a single distribution strategy, best
suited to the highest category of clinical risk across 6.60 This section describes how the design strategy of
the whole healthcare site, are less complex and final subcircuits can assist in the security of the
easier to control. The design strategy principles supply at the point of use. The nature and type of
promoted by this Health Technical Memorandum distribution to the final distribution boards are
achieve the best opportunities for flexibility and described in paragraphs 6.31–6.59 (for low
remodelling. voltage). For simplicity, it will be assumed that the
final distribution board only has one supply, but
adjacent distribution boards (where referred to) are
supplied from a different sub-distribution panel.

38
 6 Distribution strategy

Figure 19 Dual primary dual LV secondary – dual-unified infrastructure

LV generator substation
G1 G2

DNO DNO
Service 1 Service 2

Substation ISS
The healthcare site network

UPS UPS

May not IPS IPS

always be Risk
Risk via UPS
category 1 category 2
Risk category 4 & 5

Risk category 3 & 4

Areas served from substation

The healthcare site

Fixed equipment should be made. For example, if the interruption to

a socket subcircuit resulted in no available power in
6.61 Designers should carefully consider how to provide
a out-patient consulting room, the effect might not
protection of the final circuit to fixed equipment
be too dramatic – whereas in clinical risk Category
such as fluoroscopy machines. For example, two
4 and 5 areas, the patient environment should have
supplies with an auto-changeover switch could be
at least two IPS circuits at the bedhead, ideally
provided. Alternatively, a UPS could be provided
derived from different sides of a common
that would provide sufficient power for task
substation (see Figure 19). Electric bed motors,
completion in the event of circuit failure.
patient warming systems etc, which do not require
Power outlets the IPS facility, will be connected to a socket circuit
from the TN-S system. The outgoing circuits
6.62 Power outlets include sockets, spurs and connection should be interleaved between two adjoining
units regardless of how the circuit is wired (ring or theatres, or for 50% of sockets at the bedhead
radial). Assessments of the advantages of arranging where an IPS monitors the outlets. Designers
circuits in an interleaved manner per room or bed

39
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

should consider the advantages of providing a UPS does not provide 100% cover of the lighting
for each of the IPS systems in any clinical risk circuits, lighting circuits (in one area) derived
Category 5 area. from distribution boards can be interleaved with
secondary power supplies and those with primary
Lighting circuits power supplies only. Alternatively, interleaving
6.63 The distribution strategy for final circuits supplying
lighting circuits from the same distribution board
luminaires need not be significantly different from may be an acceptable derogation. It may be possible
the strategy employed for the low-power outlets in to justify only one lighting circuit in non-clinical
the same area. Where the secondary power plant areas.

40
7 Primary power – distribution centres

HV substations 7.6 HV substations should not be located under bulk

water (or any other fluid) storage areas.
7.1 Guidance on the number, type and location of any
HV substations incorporated within the site’s Construction
electrical distribution is given in Chapter 6. This
section provides guidance on the design of HV 7.7 External HV substations should be constructed
substations owned by the healthcare organisation on well-drained surfaces (with catchments for any
(or nominated agent). This Health Technical spilled oil, if appropriate). The electrical equipment
Memorandum does not govern substations that should not be within reach from outside the
are owned by the external DNO, usually limited perimeter fence of the substation.
to the main intake point. However, designers and 7.8 External HV substations can be constructed from
stakeholders should liaise with the external DNO brick, concrete or GRP, or be of steel fabrication to
to ensure that such substations have suitable access the same enclosure standard of an internal HV
and space provision. HV substations that include substation.
an integral space for the external DNO’s HV cables
and equipment may be appropriate, providing that 7.9 Where external substations have a metallic
adequate control and areas of responsibility can be enclosure construction or the substation is
clearly defined. For the purpose of this guidance, open and surrounded by a metallic fence, see
HV substations are deemed to be the total area of BS 7430:1998 for earthing arrangements.
the HV switchgear and transformer enclosure. 7.10 Construction of internal substations should
include adequate fire precautions to satisfy
Location the recommendations given in Firecode (Health
7.2 External and internal locations can be suitably Technical Memorandum 05). The construction
adapted for HV substations provided the design of internal HV substations should be sufficiently
adheres to the principle of the following guidance. robust to contain the effects of an electrical
External substations can be located at ground or explosion emanating from within, and should
roof level. Internal substations can be located at any provide suitable acoustic attenuation. HV
floor, including ground level. substations should be constructed to minimise
the effect of electrical interference.
7.3 HV substations located in close proximity to
the principal LV switchboard afford the best 7.11 Walls and fire-resisting partitions forming the HV
opportunity to regulate earth faults between the substation must comply with statutory Building
two items (see paragraphs 7.52–7.54 for the Regulations Part B or be of an equivalent fire-
location of LV switchrooms). resisting steel-fabricated modular construction.
Internal walls should have a suitable finish to
7.4 External substations should be located away from reduce dust formation and facilitate cleaning.
any live vegetation by a minimum distance of 3 m.
The clear zone includes above the construction and 7.12 HV substations should be constructed to prevent
subterranean areas. Low-maintenance grassed areas the ingress of water, including from flood. Specific
are an acceptable derogation from this requirement. precautions are required where cables enter the
substation from external areas (including
7.5 The location of internal substations should be in subterranean).
accordance with the recommendations given in
Firecode (Health Technical Memorandum 05) 7.13 Floors and ceilings should be constructed from
and the adjacencies described therein. reinforced concrete or equivalent fire-resisting

41
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

construction. Floors should have a non-slip, dust- connecting directly to a safe haven, on opposite
reducing finish. sides, to provide suitable escape routes. Additional
door openings will be required to ensure that the
7.14 HV switchroom doors should open outwards and
maximum travel distance to a safe haven is no
have a total clear opening to allow replacement
greater than 9 m.
of switchgear and transformers (see paragraphs
7.17–7.19). 7.19 The access to any HV substation, including the
HV side of a transformer, should be arranged so
7.15 Substations should be constructed without
as to prevent unauthorised access (see Health
windows or skylights to minimise the effect of solar
Technical Memorandum 06-03).
heat gain.
7.16 The construction of any HV substation, including Layout
the HV side of a transformer, should be designed so
7.20 The layout of HV substations will depend partially
as to prevent unauthorised access (see Health
on the distribution strategy employed (see
Technical Memorandum 06-02).
Chapter 6). Internal HV substations should
have level room height of 1 m greater than the
Access and egress
equipment height, and a clear maintenance space
7.17 External substations should have good access for of a minimum 0.8 m on the sides and rear of
road vehicles to facilitate plant replacement and equipment and 1 m plus the equipment depth in
maintenance. Where external substations are on front of the equipment. The requirement may be
the roof, a clear method statement describing derogated where the HV equipment is combined
the arrangements for plant replacement and onto a switchboard or close-coupled with the
maintenance should be provided, without the need transformer. However, it is essential to ensure that
to dismantle individual HV switches, circuit all cables and equipment can be serviced and
breakers or transformers. External substations replaced without any modification to the room.
should be so arranged and constructed as to Where the distribution strategy has dual supplies
prevent unauthorised access. See Health Technical and multiple transformers per substation, a physical
Memorandum 06-02 Parts A and B. Gates or other fire barrier between each section may improve the
purpose-made openings should provide adequate fire precautions and inherent system resilience.
clearance for plant replacement, inclusive of any Where the HV equipment includes a withdrawable
manual handling equipment and personnel. There section (see Chapter 9), the depth of the equipment
should be a minimum of two sets of gates, on should be added to the clear maintenance space.
opposite sides, to provide suitable escape routes. The maintenance space will include the headroom
For external substations, additional gates will be above all HV equipment and transformer. The
required to ensure that the maximum travel headroom should be a minimum of 1 m measured
distance to a safe haven is no greater than 9 m. between the soffit (and the underside of any drop
7.18 Internal substations should have good access for
beam) and the highest point of the equipment.
road vehicles to facilitate plant replacement and Designers should liaise with the structural engineer
maintenance. This will generally mean that they are for the coordination of services within the HV
located on the perimeter of the ground floor or in substation. Risk assessments should be undertaken
separated dedicated buildings. Where internal to determine the amount of space to be set aside for
substations are not at ground-floor level, a clear future expansion and flexibility (see Chapter 3).
method statement describing the arrangements 7.21 HV cables used for the supply and interconnection
for plant replacement and maintenance should be of HV equipment and transformers (including the
provided, without the need to dismantle individual LV secondary side cables) are best laid in a cable
HV switches, circuit breakers or transformers. trench or duct. Suitably-installed busbars would be
Internal substations should be so arranged and an acceptable derogation from this requirement.
constructed to prevent unauthorised access. See Cable trenches or ducts should be of adequate
Health Technical Memorandum 06-02 Parts A cross-section to facilitate the pulling-in and
and B. Door openings should provide adequate replacement of additional cables. Cables should be
clearance for plant replacement, inclusive of any positively fixed to the sidewalls of cable trenches
manual handling equipment and personnel. There and ducts, and so arranged as to prevent the need
should be a minimum of two sets of door openings for cable crossover.

42
 7 Primary power – distribution centres

7.22 HV equipment should sit partially over the cable four-hour fire-rated partition wall, the two sections
trench to facilitate final cable connection. Such being linked by a fully rated cable.
arrangements will require adequate sidewall
construction and edge-wall protection. Environmental requirements
7.23 Cable trenches and ducts should have a natural 7.31 External open-air HV substations do not require
drainage fall, and be sealed to prevent the ingress any environmental requirements. Lighting should
of water, where they pass through walls. Similarly, be provided for security and possible emergency
the cable trench/duct should provide the same fire working. Maintenance staff should be protected
integrity as the wall, where the trench/duct passes from bad weather during emergency working.
under the wall, that is, preventing ingress of gas or External HV substations of GRP or steel
foam fire-extinguishing fluids. fabrications should have the same environmental
conditions as an internal HV substation (given
7.24 HV substations should not be used for any purpose
below). Internal HV substations should be
other than HV equipment and cables. The room
illuminated by maintained lighting to an average
should not be used for the storage of other items
level of 150 lux at floor level. The illumination
at any time. The room should not be used as a
should not cast shadows on any instrumentation
conduit for other engineering services, including
and working surfaces of the equipment. Escape
drainage.
lighting should provide an average of 5 lux at floor
level for three hours, and be supported by grade A
Fire precautions
standby lighting. Internal HV substations should
7.25 HV substation construction must satisfy the have natural ventilation to prevent moisture and
requirements of the Building Regulations Part B. condensation. HV switchgear would not normally
Designers should comply with the “medical contribute to internal heat gains of the switchroom.
adjacencies” as defined in the Firecode series. A full Transformers typically radiate between 1.5% and
risk assessment (in conjunction with the healthcare 2% of their rating as heat, which should be
premises’ fire officer, the local authority’s fire officer removed by crossflow (low to high opposite sides)
and a specialist fire consultant) should be made, to natural ventilation. The supply and extract air for
address the form of suitable fire-fighting equipment an HV substation should connect directly to an
and precautions. external wall and should be arranged to prevent
7.26 Internal automatic fire-extinguishing equipment
short-circuiting. Thermostatically-controlled low-
of a gaseous type should be considered where the level background heating should be arranged to
HV equipment contains flammable material (for maintain a room temperature not less than 10°C.
example mineral oil). Specialist fire engineers Consideration may be given to the provision of LV
should undertake the design of such fire-fighting or SELV sockets, derived from a resilient routed
equipment. secondary source (standby generator) for the use of
competent persons (see Health Technical
7.27 The risk of a fire should also be determined by Memorandum 06-02).
the effect of an electrical fault causing explosions.
Such electrical faults will include those that can be Equipment and notices provided
assumed to happen and those that may arise from
7.32 HV substations should include the following
unauthorised interference.
equipment as a minimum set (see Health Technical
7.28 The fire-extinguishing equipment should include Memorandum 06-02):
an audible and visual alarm system within the
• safety posters as identified by Health Technical
room, immediately outside the room, and to a
Memorandum 06-02, including first aid/electric
suitable 24-hour staffed location.
shock treatment;
7.29 Fire-extinguishing equipment of the halon or CO2
• single line diagrams as identified by Health
type should be replaced and not considered for new
Technical Memorandum 06-02;
installations.
• rubber mats;
7.30 Where the HV substation is part of a ring network,
consideration may be given to having the two • a mimic board – intake sub only (including
network incomers in two rooms separated by a where appropriate key locks, keys and site
logbook);

43
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

• a sign positively identifying the LV earthing as a Access and egress

TN-S system;
7.39 External transformers should have good access for
• a battery charger (for the instruments and road vehicles to facilitate plant replacement and
power-driven switches, including trip circuits); maintenance. Where external transformers are
on the roof, a clear method statement describing
• storage space for maintenance tools, with tools;
the arrangements for plant replacement and
• fixed lifting equipment. maintenance should be provided, without the need
to dismantle the transformer. External transformers
Transformer enclosures should form an integral part of the HV enclosure
and do not require a dedicated access and egress
7.33 The adopted distribution strategy (see Chapter 6)
opening, provided the layout does not restrict the
should be used to determine the number of
plant replacement and maintenance routes.
transformers per substation.
7.40 Internal transformers should have good access for
Location road vehicles to facilitate plant replacement and
maintenance. This will generally mean that they are
7.34 The potential for harmonic interference, fault level,
located on the perimeter of the ground floor or in
and zone of protection should be addressed when
separated dedicated buildings. Where internal
locating transformers. The transformer should be
transformers are not at ground-floor level, a clear
located within 1 to 3 m from the respective HV
method statement describing the arrangements for
switchgear, and as close as possible to the respective
plant replacement and maintenance should be
LV switchgear. External transformers should be
provided, without the need to dismantle the
located away from any live vegetation by a
transformer. Internal transformers should be
minimum distance of 3 m. The clear zone includes
so arranged and constructed as to prevent
above the construction and subterranean areas.
unauthorised access (see Health Technical
Low-maintenance grassed areas are an acceptable
Memorandum 06-02 Parts A and B). Door
derogation from this requirement.
openings should provide adequate clearance
7.35 The location of internal transformer enclosures for plant replacement, inclusive of any manual
should be in accordance with the recommendations handling equipment and personnel. The footprint
given in Firecode and the adjacencies described of a typical single transformer enclosure (including
therein. maintenance areas) is 4 m by 4 m, and hence
consideration may be given to the provision of only
Construction one door opening. Where the distribution strategy
7.36 External transformer open compounds should requires two transformers per substation, each
be constructed on well-drained surfaces (with transformer should be located in its own enclosure.
catchments slightly greater than the volume of oil,
for any spilled oil, as appropriate). The electrical Fire precautions
equipment should be placed beyond the reach of 7.41 Transformer enclosures and/or rooms must satisfy
personnel stood external to the substation and/or the requirements of the Building Regulations Part
transformer. B. Transformer locations should satisfy the medical
7.37 External transformer enclosures of GRP or steel adjacencies as defined in the Firecode series.
fabrication should have the same environmental Designers and stakeholders should carry out a full
conditions as an internal HV substation. risk assessment (in conjunction with the healthcare
premises’ fire officer, the local authority’s fire officer
7.38 Construction of internal transformer rooms should and a specialist fire consultant) to address the form
include adequate fire precautions to satisfy the of suitable fire-fighting equipment and precautions.
recommendations given in Firecode. Where an oil-
filled transformer is installed, a bund area should be 7.42 Automatic fire-extinguishing equipment should
provided sufficient to hold more than the capacity be provided where internally-located transformers
of oil within the transformer. The transformer contain flammable material (for example mineral
enclosure should also provide suitable acoustic oil). Specialist fire engineers should undertake the
attenuation and should be designed to minimise design of such fire-fighting equipment.
the effect of electrical interference.

44
 7 Primary power – distribution centres

7.43 The risk of a fire should also be determined by H = height difference between the centre lines of
the effect of an electrical fault causing explosions. the two openings (m).
Such electrical faults will include those that can be
7.47 The above formula should be corrected for ambient
assumed to happen and those that may arise from
room temperatures above 20°C and/or altitudes
unauthorised interference. The fire-extinguishing
above 1000 m.
equipment should include an audible and visual
alarm system within the substation area, 7.48 Where natural ventilation cannot be secured to
immediately outside the substation, and within a maintain a room temperature of 20°C, designers
suitable 24-hour staffed location (telephonist). may wish to consider forced ventilation with an
airflow rate (q) calculated by:
7.44 Transformers suitably rated for external location
and located in the open air of a compound may q = 0.081P (for fluid transformers)
not require any specific fire precautions or q = 0.05P (for dry-type cast-resin transformers)
extinguishing equipment.
where
7.45 Fire-extinguishing equipment of the halon or CO2
type should be replaced and not considered for new P = total losses of the transformer (kW).
installations.
Figure 20 Air flow through a transformer room
Environmental requirements
7.46 External open-air transformers do not require any S1
environmental requirements. Artificial lighting
should be provided for security and possible
emergency working. Maintenance staff should be
protected from bad weather during emergency
working. Internal transformer enclosures should be H
illuminated by artificial lighting to an average level
of 150 lux at floor level. The illumination should
not cast shadows on any instrumentation and S
working surfaces of the equipment. Emergency
lighting should provide an average of 5 lux at
floor level for three hours. Internal transformer
enclosures should have natural ventilation to
prevent moisture and condensation, and 7.49 An alternative to the crossflow arrangement may
overheating of the space and equipment. The be to have the external wall fully louvred such that
radiated heat from a transformer is a function of there is a 40%–60% free-air area. Provisions for
the non-load losses (iron losses) and the full-load adequate ventilation rates of transformer rooms are
losses (copper I2R losses). Total losses for a fluid- a significant factor in determining the location of
cooled transformer will be between 1.5% and 2% internal transformer rooms. The transformer room
of their rating, and for dry-type (cast resin) should be thermostatically controlled with low-level
transformers the total losses are between 1% and background heating to maintain a room
1.5%. Natural ventilation may be achieved by a temperature above 10°C.
crossflow of air as illustrated in Figure 20. The total 7.50 Two 100%-rated transformers normally operating
area of an opening may be calculated from a typical at 50% and with a common load reduce the copper
formula, for example: losses to 40% of the full-load losses.
0.90S1 = S = (0.18P/(√H))
where
LV switchrooms
7.51 Guidance on the number, type and location of
S and S1 = lower and upper total opening areas,
any LV switchrooms incorporated within the
respectively (m2)
site’s electrical distribution is given in paragraphs
P = sum of the no-load and full-load losses of the 7.52–7.54. This section provides guidance on the
transformer (kW) design of LV switchrooms owned by the healthcare
organisation (or nominated agent). LV switchrooms

45
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

that are owned by the external DNO and usually 7.59 Floors and ceilings should be constructed from
limited to the main intake point, are not governed reinforced concrete or equivalent fire-resisting
by Health Technical Memoranda. However, a construction. Floors should have a non-slip, dust-
liaison with the external DNO will ensure that reducing finish.
such switchrooms have suitable access and space
7.60 Doors should open outwards and have a total clear
provision. LV switchrooms that include an integral
opening to allow replacement of switchgear (see
space for the external DNO’s LV cables and
paragraphs 7.62–7.63 below).
equipment may be appropriate providing that
adequate control and areas of responsibility can be 7.61 LV switchrooms do not require windows or
clearly defined. skylights, which may otherwise increase the effect
of solar heat gain.
Location
Access and egress
7.52 External and internal locations can be suitably
adapted for LV switchrooms, provided the design 7.62 External switchrooms should have good access for
adheres to the principle of the following guidance. road vehicles to facilitate plant replacement and
External switchrooms can be located at ground maintenance. Where external LV switchrooms are
level or on the roof level. Internal LV switchrooms on the roof, a clear method statement describing
can be located at each floor level. the arrangements for plant replacement and
maintenance should be provided, without the
7.53 The location of internal LV switchrooms should be
need to dismantle the LV equipment. External
in accordance with the recommendations given in
switchrooms should be so arranged and constructed
Firecode and the adjacencies described therein.
as to prevent unauthorised access. See Health
7.54 LV substations should not be located under bulk Technical Memorandum 06-02. Door openings
water (or any other fluid) storage areas. should provide adequate clearance for plant
replacement, inclusive of any manual handling
Construction equipment and personnel. There should be a
7.55 External LV switchrooms can be constructed
minimum of two sets of doors, on opposite sides,
from GRP or be of steel fabrication provided the to provide suitable escape routes. For internal
enclosure complies with the requirements of substations, additional doors may be required to
internal LV switchrooms. ensure that the maximum travel distance to a safe
haven is no greater than 9 m.
7.56 Construction of internal switchrooms should
include adequate fire precautions and satisfy 7.63 Internal switchrooms should have good access for
the recommendations given in Firecode. The lifting equipment to facilitate plant replacement
construction of internal LV switchrooms should and maintenance. This will generally mean that
be sufficiently robust to contain the effects of an they are located on the principal circulation
electrical explosion emanating from within. The corridors. Where internal switchrooms are not
construction of LV switchrooms should provide at ground-floor level, a clear method statement
suitable acoustic attenuation and should minimise describing the arrangements for plant replacement
the effect of electrical interference. and maintenance should be provided, without the
need to dismantle the LV equipment. Internal
7.57 Walls and fire-resisting partitions forming the LV switchrooms should be so arranged and constructed
switchrooms must comply with statutory Building as to prevent unauthorised access. See Health
Regulations Part B or be of an equivalent fire- Technical Memorandum 06-02. Door openings
resisting, steel-fabricated modular construction. should provide adequate clearance for plant
Internal walls should have a suitable finish to replacement, inclusive of any manual handling
reduce dust formation and facilitate cleaning. equipment and personnel. There should be a
7.58 LV switchrooms should be constructed to prevent minimum of two sets of door openings connecting
the ingress of water, including from flood. Specific directly to a safe haven, on opposite sides, to
precautions are required where cables enter the provide suitable escape routes. Additional door
substation from external areas (including openings will be required to ensure that the
subterranean). maximum travel distance to a safe haven is no
greater than 9 m.

46
 7 Primary power – distribution centres

Layout conditions as an internal LV switchroom (given

below). Internal LV switchrooms should be
7.64 The layout of LV switchrooms will depend
illuminated by artificial lighting to an average level
partially on the distribution strategy employed (see
of 150 lux at floor level. The illumination should
Chapter 6). LV switchrooms should have a clear
not cast shadows on any instrumentation and
maintenance space of a minimum 0.8 m on all
working surfaces of the switchgear. Emergency
sides of equipment contained therein. The room
lighting should provide an average of 5 lux at floor
height should be even, and at least 1 m greater than
level for three hours. Internal LV switchrooms
the equipment height. It is essential that all cables
should have adequate natural ventilation to prevent
and equipment can be serviced and replaced
build-up of moisture and condensation. LV
without modification to the room. Where the
switchrooms should be arranged so as not to give
distribution strategy has dual supplies and/or
concern for internal heat gain. The supply and
interleaved sub-main distribution, a physical fire
extract air for LV switchrooms should connect
barrier between each section may improve the fire
directly to an external wall and should be arranged
precautions and inherent system resilience. Where
to prevent the air flow short-circuiting.
the LV switchgear includes a withdrawable section
Thermostatically-controlled low-level background
(see Chapter 9), the depth of the switchgear should
heating should maintain a room temperature not
be added to the clear maintenance space. The
less than 10°C. Designers should consider the
maintenance space will include the headroom
provision of LV or SELV sockets, derived from
above all LV equipment. The headroom should be
a resilient routed secondary source (standby
a minimum of 1 m measured between the soffit
generator) for the use of competent persons (see
(and/or the underside of any drop beam) and the
Health Technical Memorandum 06-02).
highest point of the equipment. A risk assessment
to determine the amount of space set aside for
Equipment and notices required to be provided
future expansion (see Chapter 3) should be
undertaken. LV switchrooms should not be used 7.68 LV substations should include the following
for any purpose other than LV switchgear, controls equipment as a minimum set:
and cables. The room should not be used for the • safety posters as identified by Health Technical
storage of other items at any time. The room Memorandum 06-02, including first aid/electric
should not be used as a conduit for other shock treatment;
engineering services, including drainage.
• single line diagrams as identified by Health
Fire precautions Technical Memorandum 06-02;
7.65 LV switchroom construction must comply with the • rubber mats;
requirements of the Building Regulations Part B. • a mimic board (including, where appropriate,
The location should comply with the medical key locks, keys and site logbook);
adjacencies as defined in the Firecode series. A full
risk assessment should be made in conjunction • a sign positively identifying that the LV earthing
with the healthcare premises’ fire officer, the is a TN-S system;
local authority’s fire officer and a specialist fire • a battery charger (for the instruments and
consultant to address the form of suitable fire- power-driven switches including trip circuits);
fighting equipment and fire alarm and detection
systems. • storage space for maintenance tools, with tools;

7.66 The risk of a fire should also be determined by • fixed lifting equipment;
the effect of an electrical fault causing explosions. • other equipment such as safety keys as required
Such electrical faults will include those that can be by local rules.
assumed to happen and those that may arise from
unauthorised interference. 7.69 Note that a sign positively identifying the LV
earthing as a TN-S system should also be located at
Environmental requirements each final distribution board, motor control centre
and similar locations.
7.67 External LV switchrooms of GRP or steel
fabrication should have the same environmental

47
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

8 Secondary power centres and plant

8.1 This chapter deals with secondary power sources assessment are intended as a guide to best practice,
(SPSs) either directly connected to the PEI or which should not restrict any design innovation.
connected to a secondary electrical infrastructure Healthcare organisations may not have any control
(segregated essential/non-essential, A and B etc). over their DNO configuration, and therefore the
reliability of a duplicated DNO connection (to the
8.2 The use and operating configurations of CHP
PES) as an SPS should be the subject of a risk
plant are comparable with standby generators; but
assessment. Areas within a healthcare facility (or the
with CHP, the thermal energy produced can be
whole healthcare facility) where the clinical risk is
harnessed. It is for this reason that CHP plant may
equal to or greater than Category 3 will always
be considered as an SPS but not as standby plant.
require some form of standby supply. The design
The electrical connection and operating
process should assess the benefit provided by
arrangements for CHP plant are discussed in this
standby generators where the clinical risk areas are
Health Technical Memorandum only as a generator
Category 1 or Category 2.
operating in parallel with the PES (see paragraphs
8.7–8.11). Installation with photovoltaic and/or 8.6 Where permanent standby provision is installed,
wind turbines as an SPS is set out in the Energy the addition of strategically-located mobile
Network Association’s Engineering generator plug-in points may be an alternative
Recommendations G.83/1. solution for the maintenance provision of
embedded units. Such arrangements may be
8.3 Opportunities exist to allow the secondary electrical
particularly useful where the electrical
power sources, such as CHP plant, to become the
infrastructure is a unified system. Where mobile
prime power source and the DNO connection
generators are considered as part of the electrical
to become the SPS. Designers and stakeholders
resilience, consideration should be given to the
should consider the holistic risk strategy for such
hook-up location and the ease of installing
arrangements. Non-technical issues can influence
potentially very long, trirated cables. Designers and
the operating viability of alternative energies such
stakeholders should remain mindful of the fact that
as CHP, including reduced carbon emissions and
mobile generators are in fact “mobile” and should
the rejection of excess thermal energy. The design
be physically secured.
process should evaluate the resilience of generating
plant (such as CHP plant) with multiple sets
running at below full duty, or having spare capacity Secondary power general arrangements
on the DNO connection to the PES. 8.7 The use of alternative electrical energy sources has
8.4 The chapter concentrates on standby generators become more widespread in the UK since the late
provided with unified and/or segregated electrical 1990s. All forms of such energy have unique
infrastructures. The configurations are presented availability and viability considerations outside the
generally in order of resilience, from low to high. scope of this Health Technical Memorandum. A
The selection of a particular configuration will be useful source of such data can be found on either
dependent on the clinical risks and non-clinical the CIBSE or BSRIA websites. Designers and
risks. The selected configuration should clearly be stakeholders may wish to consider these energy
consistent with the distribution strategy (see sources as a way to reduce the carbon emissions
Chapter 6). locally and the possible benefits available from the
renewable obligations commitment (ROCS).
8.5 The configurations presented in this chapter should
not be taken as being definitive, prescriptive or
restrictive. The selected configurations and

48
 8 Secondary power centres and plant

Photovoltaic power secondary power source 8.14 The physical location of SPS plant and primary
plant (substation switchroom) should be considered
8.8 Photovoltaic cells may be a useful background
in similar ways. This should address the access for
supplementary energy source. Over the normal
maintenance requirements. With SPSs such as
range of weather conditions, these units can
photovoltaic cells and/or wind turbines, these may
provide an average of 15% to 20% of their total
be located on a flat roof where they may optimise
rated output. The use of photovoltaic cells is
their respective prime energy source (sun or wind).
therefore limited to smaller healthcare premises,
or dedicated small circuits of larger healthcare 8.15 Other secondary power plant locations should be
premises. considered by taking account of the medical
adjacencies and environmental conditions. The
8.9 CIBSE technical memorandum TM 25 provides
medical adjacencies are identified in the Firecode
a useful guide to the current applications of
series of documents.
photovoltaic cells (PV). Any PV system that is used
on the site’s electrical systems should run only in 8.16 Where the SPS is the standby power plant, the
parallel with the PES supply. There should be a environmental conditions include exhaust
form of positive isolation between the PV output terminations (the Clean Air Act) and noise
and the incoming PES to prevent island-mode emission (see paragraphs 8.91–8.93). Where the
operation and/or back-feeding into the PES via healthcare site includes a CHP plant, the CHP
the DNO. These requirements are set out in the plant should be located close to the boiler plant
Energy Networks Association’s Engineering to minimise the water distribution pipework (and
Recommendations G.83/1. hence distribution losses). Locating the CHP plant
close to the boiler plant will provide other benefits,
Wind turbine power source including a common location for boiler flue and
8.10 Any wind turbine system that is used on the site’s
exhaust locations to comply with the Clean Air Act,
electrical systems should run only in parallel with as well as a common location for the fuel.
the PES supply. There should be a form of positive
isolation between the wind turbine output and the Essential power capacity
incoming PES to prevent island-mode operation 8.17 An assessment of the essential power requirement
and/or back-feeding into the PES via the DNO. should be made from an understanding of the
These requirements are set out in the Energy clinical risk areas that require power to be restored
Networks Association’s Engineering within 15 seconds (see Chapter 4). Further
Recommendations G.75/1 and G.83/1, consideration should be given to the clinical risk
and the Technical Report ETR 113. areas that have certain items that should be
8.11 The use of wind turbines should include an reconnected within 0.5 seconds. Such items may
assessment of the available wind and potential initially remain connected to a supply, by either
output of any wind turbine that may be on site, internal batteries or a UPS system. Within all
including the space and access requirements. clinical risk areas above Category 2, there will
inevitably be some equipment of a non-clinical and
General – secondary power plant business continuity risk category. Such risks might
compromise the provision of healthcare treatment
location if they were not also connected to the standby
8.12 An approach to the DNO should be made to power source after an initial delay in excess of
establish an indicative reliability factor for the PES. 15 seconds, for example building services
This will place the designer team in an auditable environmental control, medical support services
position when determining the standby power etc.
plant location. 8.18 Assessments for essential power requirements for
8.13 Where the distribution strategy has placed the first new developments should be based on the ratings
single point of failure (see Chapter 6) nearer to the of the above equipment and the general power
point of use, the standby plant should be connected density of the healthcare premises with an
at the intake point. Where the first single point of acceptable allowance for growth. Actual detailed
failure is much nearer the intake point, distributed load profiles of existing sites may be a useful audit
secondary power centres should be provided. of the essential power capacity assessment, where

49
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

the profile covers at least one year. “Camera shot” the main switchboard to the essential services
measurements should be assessed with the risk switchboard, or in an emergency, from the ac
associated with oversized plant, which may be more standby power supply to only the essential services
flexible and accommodate the allowance of growth. switchboard. Thought should be given to the rating
of all associated cables with the respective loads and
8.19 When assessing the size and type of emergency
mode of operating the essential power source
power plant, designers and stakeholders should
(island or parallel).
be aware that electrical outages can be very short
(less than a few minutes) or for many hours.
Consequently, all emergency generating sets should Standby generators
be designed and rated to provide continuous full
load for prolonged periods. Where the essential Design criteria
power plant is not connected to the full electrical 8.23 A range of system designs is considered below,
load, thought should be given to the temporary for both LV and HV systems. In small healthcare
connections of plant such as the chilled water premises, the most economical and convenient
systems. The provision may require a manual or arrangement may be a single diesel standby
automatic control system with the ability to “load generator set to supply power to the essential
shed” a limited number of the secondary services services. However, for larger premises the better
such as non-essential lighting. The schedule should arrangement is to share the load between two or
be reviewed annually as part of the maintenance more machines. A system of two or more standby
regime. generators, with interlocked and interconnected
switching, may be necessary to ensure only a single
Essential and emergency power running supply to essential loads.
provision 8.24 The choice between LV and HV generation is
8.20 Standby power systems should always be available usually dependent on the nature of the total site
to provide electrical power to those areas that will supplies; 11 kV generators have a higher unit cost
enable the healthcare facility to carry out essential but can be cheaper or more convenient to
functions. The designation of these areas within the distribute electricity to the points of use.
healthcare facility should be decided at design stage 8.25 The design criteria for the standby generator system
with involvement of all the stakeholders, and should consider the advantages of managing the
particularly clinicians. The framework of such maximum demand profile (from the PES) by
decision-making should include the clinical risk operating the generators in parallel. This may be
categories identified in paragraphs 4.12–4.24. achieved by running any one of the multiple sets
Within this general objective, the aim should be to in parallel with the PES during high maximum
keep electrical installations as simple as practicable demand periods.
and avoid unnecessary segregation of essential and
non-essential circuits. Consequently, the design 8.26 LV standby generators connected to the HV
team should contribute towards the medical network may provide a practical solution. However,
planning process. consideration needs to be given to the space
required for the additional transformer(s) and
8.21 Developments in clear separate phases should earthing arrangements. Standby generator
design in the emergency power supply for the arrangements including step-up transformers
final steady state, as far as practicable, at the initial should comply with the Energy Networks
design stage. This will enable the total emergency Association’s Engineering Recommendations G.84.
power supply requirement to be assessed in the Designers and stakeholders should also consider the
planning stages and appropriate areas of capital and life-cycle costs of such transformers and
accommodation to be allocated. associated switchgear and equipment.
8.22 For ac standby power supplies, in island mode,
required to supply only segregated essential services, Component parts
a fully-rated four-pole main auto-changeover 8.27 In its basic form, the generating set configuration is
switch should be provided. It should be connected formed by an engine, alternator and control panel
to supply power to the healthcare facility from two with associated bed frame. Failure of any of these
sources, either from the DNO’s normal supply via items will cause the generating set to fail.

50
 8 Secondary power centres and plant

8.28 The generating set represents a single point of clinical risk is no greater than Category 2. Where
failure, and maintenance routines should be there is no requirement for a standby supply within
developed to reduce the risk of failure. Some of the 15 seconds, and essential health and safety supplies
commonest reasons for failure are given in Table 3. are provided by UPS or battery units, a plug-in
Standby power plant should have an N+1 point for a mobile generator may be adequate.
configuration. The benefit of having such a mobile plug-in point,
either for such a simple system, or as an addition
Table 3 Typical causes of generator set failure for any other supply configuration, is the facilities
it offers to effect planned maintenance of the fixed
Fault Typical cause
wiring system or downtime of permanent standby
Overload Inadequate testing onto actual site load generators. The design process should reconcile the
Cold engine Engine heater turned off or heater failed availability and security of mobile generators with
Flat batteries Battery charger turned off, charger failed, the benefits of embedded distribution and standby
or batteries too cold generator resilience. When evaluating the clinical
Cold room Room heater turned off or when the risks against viability of fixed generator provision,
generating set is at standby, room air the realistic response time to collect and connect
change rate set too high a mobile generator (for any scenario) should be
considered, particularly if the generator has to be
Generator configuration hired. Where it is considered advantageous to
provide a mobile plug-in point, managers should
8.29 Standby generators can be arranged in various ways
consider the purchase of a mobile unit to be
as described below. Each configuration provides kept at a central location, or make emergency
different opportunities for routine testing of the arrangements for mobile generators of suitable
generator. Full electrical system tests for a PES rating to be obtainable elsewhere at short notice. In
failure (blackout) are described in Health Technical planning an installation, it is desirable to reserve a
Memorandum 06-01 Part B. dedicated fenced-off location for mobile generators
where they may be easily connected to an allocated
Mobile plug-in generator island operation
switch or plug-in point.
8.30 A basic LV system comprising one PPS and a
8.31 The connecting of a mobile generator should
mobile secondary generator is illustrated in block
comply with the Energy Networks Association’s
format in Figure 21. This is a simple unified
Engineering Recommendations G.84, and may be
system, with standby supply provision likely
achieved by a plug-in facility (up to 175 kVA) rated
to be via a plug-in point for a mobile standby
to BS EN 60309-1:1999, IEC 60309-1:1999. An
generator. Such a simple system may provide a
external earth lead connection to the generator star
back-up supply to healthcare premises where the

Figure 21 Mobile generator connection

Healthcare
site
Changeover load
contactor/ BS EN 60309 socket
switch

Isolator
Isolator (generator)
(main)

DNO G
supply

51
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

point and, separately, to the generator metalwork 8.37 The electrical system resilience will be N+1 as
should be provided. An earth bar connection there is an embedded secondary power source (the
should be available for connection to the earth generator). Where the clinical risk Category 4 area
terminal of the generator base plate, regardless of its forms only a small part of the healthcare premises,
type and size. the generator control may be adjusted such that the
healthcare premises areas less than clinical risk
8.32 The electrical supply regulations do not permit
Category 4 are only connected to the standby
a mobile generator to feed back into the PES, as
generator when the actual demand is less than
such connections may feed a fault into the PES.
the generator rating (see Figure 22). Where a
Therefore, it is essential that positive isolation from
significant percentage of floor area is used as
the PES be made before and while any healthcare
clinical risk Category 4 or above, more than one
organisation’s mobile generator set is used to
generator should be provided – rated so that the
energise any part of the electrical network.
full essential circuits’ AMD can be supported while
one standby generator is not available (due to
Generator(s) in island operation
maintenance or faults). Under such circumstances,
8.33 Island operation represents the simplest generator the generator resilience would also be defined as
control arrangement requirements. Each ac N+1.
generator will supply electrical power to a discrete,
segregated part of the network. There will be no Generator(s) operating in parallel with PES
facility or opportunity to connect the generator
8.38 Parallel operation represents a more refined control
output to the normal DNO connection.
arrangement in the mode of standby generator
8.34 Where the SPS plant is connected in island mode running. Each ac generator will supply electrical
and the distribution strategy allows for the classic power to any part of the internal electrical
segregated non-essential and essential circuits (see infrastructure depending on voltage and the type
paragraphs 6.40–6.42), any single room (or space) of parallel operation. For parallel operation with
will have only non-essential or essential circuits and the PES, the generator control regime should be
not both circuit types. Where the clinical risk compliant with the Energy Networks Association’s
Category 3, 4 and 5 areas are dispersed or are a Engineering Requirements G.59/1 (short- or long-
small percentage of the overall electrical AMD, term).
the safety requirement tends to drive the standby
8.39 Short-term parallel operation requirements of
generator rating up. Designers and stakeholders
G59/1 allow the embedded generators to run
should consider the implications of not being able
(synchronised) in parallel with the PES for periods
to test the generators in parallel with the DNO
between 1 and 5 min, subject to approval of the
supply. This may mean that the non-essential
local DNO. Designers and stakeholders should
circuits have to be turned off when the physical
consider the advantage of this arrangement as a
essential load is used to test the generators. The
means of having a no-break return to normal PES
alternative to this (preferred) strategy will be to test
supply following an outage. The disadvantage of
the generators with a load bank. All generator
such an arrangement is that the whole building
testing and power restoring (after a DNO outage)
electrical load cannot be used to test the standby
will require a short interruption to the electrical
generator power capacity. This may mean that the
power. Standby generator operating in island mode
non-essential circuits have to be turned off, when
will not require compliance with the Energy
the physical essential load is used to test the
Networks Association’s Engineering Requirements
generators. However, as the sets meet the G59/1
G.59/1.
requirements for short-term parallel operation, the
8.35 Operating standby generating power plant in island sets can be synchronised with the PES supply and
mode may be considered for all clinical risk then the non-essential circuits re-energised before
category areas. the generators are isolated from the load. The
alternative to this (preferred) strategy will be to test
8.36 Figure 22 shows a classic LV system comprising a
the generators with a load bank.
single PPS with an SPS (the standby generator).
The generator(s) is configured to operate in island 8.40 Where the standby power plant is connected in
mode only. short-term parallel operation and the distribution
strategy allows for the classic segregated non-

52
 8 Secondary power centres and plant

Figure 22 Generator(s) in island operation

DNO or healthcare site network connection

Generator and PES not synchronised

Generator running in island mode
G1
Energised but
with no load On line and
connected connected to
the load
Open Closed
Open

Closed Closed Closed Closed

Healthcare site substation

Closed Closed Closed Closed

No power No power No power Power and

energised
Risk Risk
category 1 category 2 Risk category 3 or 4

The healthcare site

essential and essential circuits (see paragraphs 6.40– 8.42 Where the standby power plant is connected in
6.42), any single room (or space) will have only long-term parallel operation and the distribution
non-essential or essential circuits and not both strategy allows for the classic segregated non-
circuit types. Where the clinical risk Category 3, 4 essential and essential circuits (see paragraphs
and 5 areas are dispersed or are a small percentage 6.40–6.42), any single room (or space) may have a
of the overall electrical AMD, the safety mixture of non-essential and essential circuit types.
requirement tends to drive the standby generator 8.43 Best-practice standby-generator connecting
rating up. Designers and stakeholders should arrangements will allow for long-term parallel
consider the implications of not being able to test operation with the PES (that is, fully compliant
the generators in parallel with the DNO supply. with G59/1). Assessments of the advantages for
8.41 The long-term parallel operation requirements business continuity with the additional control and
of G59/1 allow the embedded standby generators switchgear regulation should be made.
to run (synchronised) in parallel with the PES for 8.44 Operating standby generating power plant in short-
any unspecified period of time. Advantages are or long-term parallel operation may be considered
available with arrangements that provide a facility for all clinical risk categories provided that any
to test the generators with the essential circuits of areas of Category 5 have an intermediate tertiary
the building load. Where the standby generator power source UPS.
capacity is less than 100% of the AMD, the
generators will run up and synchronise with the 8.45 Figure 23 shows a classic LV system comprising a
PES (for the test regime), and consequently there single PPS with an SPS (in this case a standby
will not be a need to isolate any part of the generator). The generator(s) is configured to
electrical system while testing the generators. operate in island mode only.

53
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

8.46 The electrical system resilience will be N+1, as generator connection). Where the site has
there is an embedded SPS (the standby generator). generators above 20 kV and/or 5 MW, its design
Where a significant percentage of floor area is used should ensure that the generators are fully
as clinical risk Category 4 or above, more than one compliant with the Energy Networks Association’s
standby generator should be provided – rated so Engineering Requirements G.75/1.
that the full essential circuits’ AMD can be
supported while one generator is not available LV generators feeding HV ring main
(due to maintenance or faults). Under such
circumstances, the generator resilience would also 8.48 The design process might consider using LV
be defined as N+1. (0.4 kV) standby generators connected to the HV
network (11 kV), via step-up transformers, where
8.47 This chapter has discussed the various operating the distribution strategy includes for an HV
configurations for standby generators based on the network. Generators in this configuration can
generators having a terminal voltage up to 11 kV operate either in parallel or in island mode as
and/or each generator having an output of less than described in paragraphs 8.29–8.47. When the
5 MW (see Figures 22 and 23; also see Figure 18, standby generators have a design potential to allow
which can be used as the configuration for an HV

Figure 23 Generator(s) operating in parallel with PES

DNO or healthcare site network connection

Generator and PES are synchronised

Generator running in parallel mode
G1
Energised and On line and
with the load connected to
connected the load

Closed Closed

Closed

Closed Closed Closed Closed

Healthcare site substation

Closed Closed Closed Closed

Power and Power and Power and Power and

energised energised energised energised

Risk Risk Risk category 3 or 4

category 1 category 2

The healthcare site

54
 8 Secondary power centres and plant

for parallel operation, the electrical system should discriminate against a fall in normal voltage due
include neutral-switching contactors. to a voltage transient or auto-reclose switching
operation (that is, a time delay to establish that
Generator control the under-voltage is an outage rather than a
disturbance). When the chosen time delay confirms
Generating set management the loss of normal supply voltage, the engine start
is initiated. A time delay of up to 15 seconds
8.49 The generating sets are defined as a standby system.
(following the initial confirmation time) is allowed
The modes of operation should be well-defined and
between loss of normal supply and connection of
clearly stated.
the standby generator to the essential circuits. The
8.50 Where the healthcare facility has SPSs including essential circuits are defined as those which cannot
power sources from alternative energy plant (CHP, accept an interruption of electrical energy greater
wind turbines, photovoltaic), designers should liaise that 15 seconds plus the detection time (that is,
with the local DNO to ensure compliance with the clinical risk Category 4 and above). The ac standby
requirements of the Energy Networks Association’s generator circuit breaker should close when the
Engineering Requirements G.83/1. generated voltage and frequency are at 95% of
nominal values and before the auto-changeover
8.51 Designers are also required to prevent these power
load switch operates. The initial step load applied
sources feeding back into the PES at the instance of
to the generator should be less than the maximum
an under-/over-voltage or an under-/over-frequency
acceptance factor to prevent the generator’s
of the SPSs in either the power source or PES.
protection shutting the set down again.
However, such power sources may be arranged
to continue to supply the internal essential 8.57 The time-delayed start of the motor and high
distribution, and allow the standby generators to inductive loads may be achieved by the individual
synchronise to the CHP supply. (Note – subject to motor controls or by a centralised network control
the rating and step load response of the CHP, such (see Chapter 9).
arrangements may require the non-essential loads
8.58 Regardless of the actual duration of the outage
to be isolated, while the standby generators are
(PES or internal distribution), provision should
initialised and connected.)
be made to ensure a minimum run-time of
8.52 Detection of any under-voltage on any phase 20 minutes to allow the generator engine
should be made at the input terminals of the point lubrications to reach operating temperatures and
of common coupling of the primary supply and fully circulate. The minimum run-time will also
secondary supply (generator). facilitate the recharging of the batteries. The
minimum run-time should be exclusive of the time
8.53 Where the generator(s) cover the full essential load,
required to establish a returned and stable PES
the phase failure detection will be at the intake
supply. Where the mains supply has been restored
point.
prior to load transfer, this will mean the generator
8.54 Where the generator(s) provide cover to only part may be operated off-load.. Where the mains has
of the electrical load, this will be at the respective been re-established within the minimum run-time,
switchboard (point of segregation of local non- the load will be transferred back to the DNO
essential and essential circuits). supply and the generator will continue to run off-
8.55 Detection monitoring is required on all phases
load until completing the minimum run time.
of the normal supply, such that any single-phase 8.59 The source of run/stop signals should be clear, to
voltage failure in the normal supply initiates a start ensure that the set automatically runs when mains
signal to the essential generator controls. failures occur at points in the systems that are to be
8.56 The engine-driven generator set for the supply
supplied by the set. The position of any simulate-
of essential circuit power and lighting should be mains-failure switches and mains-return push-
designed for automatic starting in the event of buttons should be decided with respect to the
either a total failure of supply or a prolonged system as a whole and in particular the needs of the
variation in supply voltage from its specified limits. operator.
A short delay of between 0.5 and 6.0 seconds is 8.60 The opportunities to maintain and/or test will
normally chosen at the voltage detector device to affect the management control requirements. For

55
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

example, the maintenance requirements may follow. This should be a minimum of five minutes,
require a level of redundancy in sets, which will and determined by local experience.
then require sequencing controls (see paragraphs
8.66 Designers should give further consideration to how
8.61–8.64). Test regimes should facilitate the
the load is actually transferred, either manually or
generator(s) to run in parallel with the PES for
under an automated control system. Where the
business continuity and clinical risk control.
load is transferred manually, a key switch will be
Additional details are available in Health Technical
required for this function, and the transfer can
Memorandum 06-01 Part B, “Operational
be coordinated to the benefits of the healthcare
management”.
premises. This mode of operation may only require
a short-term (less than five minutes) parallel
Multi-set operation
operation under the G59/1 requirements. The load
8.61 For standby generators connected to different transfer under an automated control system should
points within the network, arrangements with be gradual to minimise transient voltages, which
parallel operated sets (which have collective may otherwise cause an outage of either the DNO
redundancy) should be adopted. Where the clinical supply or the running standby generators. Where
risk areas are Category 4 or above, arrangements the load transfer from the generator(s) to the DNO
with multiple generator sets that have inbuilt supply is gradual, a long-term (greater than five
redundancy should be adopted. minuets) parallel operation agreement under the
8.62 Where the SPS is via multiple standby generator
G59/1 Regulations will be required.
sets, the total run-up time (detection time of 8.67 Long-term parallel operation of generators with
between 0.5 and 6.0 seconds plus run-up of less the PES as described by the G59/1 regulations has
than 15 seconds) should include the time it will clear advantages for testing generators with the
take to synchronise sufficient sets so that their minimum inconvenience to end-users. See the
combined acceptance load is greater than assessed routine online testing of primary and secondary
essential load (clinical risk Category 3 and above). power sources in Health Technical Memorandum
8.63 In applications where the system is operating with
06-01 Part B.
an N+1 capacity, it should be decided whether 8.68 Additional information can be found in Chapter 9
the resilient set should operate continuously for the automated management and control of a
throughout a mains failure or only run on the stage transfer system.
failure of a running set. All sets should be started at
8.69 When all electrical loads have been transferred
the instance of a mains failure. After the generator
back to the DNO supply, the standby generator(s)
operation has stabilised (normally less than five
should be allowed to run on for a period to
minutes), the number of online sets is adjusted to
facilitate natural cooling of the engine. This period
suit the actual demand.
can be a pre-set time of say ten minutes or at pre-
8.64 If the number of sets that operate is to be varied to set return temperatures of the lubricant and/or
match the required load, care should be exercised in water cooling systems.
defining the power levels that disconnect a set from
load. This figure, which may vary over months or Computerised load management of generators
years, should be reviewed on a regular basis. The
8.70 The electrical infrastructure and distribution
risk of stopping a running set due to light load
strategy may minimise the effect of an electrical
running, compared with the reduced risk of
fault to the clinical risks areas. However, the most
operating with excess capacity, should be
resilient system cannot eliminate the risk. The
considered.
highest risk of generator failure is during the first
five minutes of the set starting online. While the
Mains return
standby generators are providing the only electrical
8.65 To return the electrical power source back to the power, variation in demand may result in the
DNO service, a minimum delay should be agreed generators running on a light load. This is
to allow the DNO service to be established and particularly the case with multiple sets. Similarly,
stabilised, otherwise the risk of repeated outages the demand variations may cause the generator to
(caused by the number of auto-reclose devices) may become overloaded.

56
 8 Secondary power centres and plant

8.71 Where the standby generators do not provide prescribed maintenance having been carried out.
support for the total electrical system of the site, This is known as a Class A rating.
problems may rise during prolonged outages (of
8.77 Diesel or gas engines should generally be
the PES). For example, where the chiller plant
manufactured in accordance with BS 5514,
is not supported by default, building internal
ISO 3046. Four categories of load acceptance are
temperatures may arise above acceptable levels.
available for various types of engine operation on
8.72 The design process may consider supervisory the basis of percentage load acceptance for the
control and data acquisition (SCADA) computer Class A rating:
systems to automatically control the generators and
• Category 1 – 100% load acceptance;
switchgear status and connected load. This function
may also be provided using a PLC system. • Category 2 – 80% load acceptance;
• Category 3 – 60% load acceptance;
Standards and references
• Category 4 – 25% load acceptance.
8.73 Generating sets should be specified that are
compliant with the relevant parts of the following 8.78 The advantages of higher acceptance factors should
specifications. Particular attention should be given be reconciled with the increased cost of larger
to the governing system of the engine and the generator sets, and the time taken to reach
voltage regulation system of the alternator: acceptance point with synchronised sets. Naturally-
aspirated generators have a higher acceptance factor
• generating sets are specified in BS 7698, for a given output rating, but are also physically
ISO 8528; larger. Generators that can satisfy the Category 2 or
• engines are specified in BS 5514, ISO 3046; 3 of a Class A specification to BS 5514, ISO 3046
may be more economic and appropriate for most
• alternators are specified in BS 4999. healthcare premises.

Generator engines Batteries and battery charging

8.74 The choice of generator engine type is determined 8.79 For most generating sets, the means of starting is
by the required output and speed. For generators by an electric starter motor. Air start is available,
up to 50 kVA, the prime mover may be either a but for economic reasons is generally restricted to
petrol or a diesel engine with four or six cylinders. generators greater than 2 MVA for sets at 0.4 kV or
Generators between 50 kVA and up to 500 kVA greater than 3 MVA for sets at 11 kV.
are best driven by diesel engines with six or eight
cylinders in V formation. Generators in the range 8.80 The reliability and maintenance of batteries is
of 500 kVA to 1500 kVA are best driven by diesel extremely important. For a generating set to
engines having 12 or 16 cylinders in V formation. start consistently, the batteries should be in good
From the early 2000s some engine manufacturers condition and maintained fully charged while
have been making 20-cylinder engines used to drive the set is both running and stationary. The
1500 kVA to 2 MVA generators. The advantage maintenance procedures should include the
here is that with more cylinder displacement and requirements given by the particular battery
equal engine speed, a greater load acceptance factor manufacturer.
can be applied to the generator. 8.81 Usually, two battery-charging systems are supplied
8.75 The larger engines should all have turbocharge with a generating set:
units fitted, while the smaller sets (less than • a charger for operation while the set is
100 kVA) may be more economical with natural stationary, usually in the control panel;
aspiration.
• a belt-driven charge alternator that maintains
8.76 Engines should be continuously rated, as defined the battery when the set is running.
in BS 5514, ISO 3046. They should be capable
of operating at the rated load for a period of 8.82 For both charging systems the battery should be
12 consecutive hours inclusive of an overload of charged at the correct “float voltage”, and for
10% for a period not exceeding one hour, the engine starting the battery should be adequately
sized for the “breakaway” (initial starting) voltage

57
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

to be acceptable to the engine manufacturer. days’ continuous peak thermal boiler-plant

The use of a BMS should be considered for the demand. Designers should give consideration to
monitoring of the battery condition. how to minimise the effect of stratifying fuel oil
where the stored generator fuel is not shared with
8.83 Table 4 gives a range of battery types in ascending
the boiler plant, which may mitigate such effects.
cost order. The type of battery to be selected should
There are ranges of systems for fuel storage and
be assessed with regard to the risk, cost and planned
supply that can be considered, and a brief
maintenance. The length of battery life should be
description of some follows.
checked with the battery manufacturer.
8.86 Figure 24 shows a basic system comprising a day
Table 4 Battery types tank and isolating valve with bulk fill point and
hand pump. The tank could be filled using the
Type of battery Typical life
bulk fill point or via the hand pump from fuel
Lead acid 3 to 5 years brought to the side of the day tank. However, the
Sealed lead acid 3 to 7 years generator has no automatic means of maintaining
Planté 5 to 10 years the fuel tank full.
Ni-Cad (Nickel cadmium) >10 years 8.87 The addition of bulk tank storage as well as a day
tank, as in Figure 25, allows extra capacity to be
Fuel and fuel storage kept on site. The day tank can now be filled either
from the bulk tank, or via the hand pump from
8.84 The design process should evaluate the fire and
fuel brought to the side of the day tank. The bulk
pollution implications of storing diesel fuel (the
tank can fill the day tank either by gravity feed
generators’ prime energy source). Further advice is
or as a pumped supply. Where the day tank is
available from a guidance note for the Control of
automatically maintained full by the transfer
Pollution (Oil Storage) (England) Regulations
pumps, at least one pump, powered by an extra LV
2001 as published by Defra (Department for
source or diesel, should be provided. This may
Environment, Food & Rural Affairs). Designers
assist when the day tank is empty, the generator has
should see the Firecode series and medical
stopped and there is no mains electrical power.
adjacencies when determining the location of any
bulk fuel storage. The volume of diesel fuel oil 8.88 Where fuel can be dumped from day tank to bulk
stored within the day tank and arranged for gravity tank, it is important to reduce, by design, the
feed of fuel oil to the engine should be no more chance of accidental system operation. The entire
than the greater of 750 L or the equivalent of generating set installation is at risk if the fuel dump
10 hours’ full-load (maximum capacity) running is accidentally released, since the day tank would
of the generating set. In addition, a fuel oil main be empty, and during a mains failure there is no
reserve for 200 hours’ full-load running for each supply available to operate the transfer pumps.
standby generator set should be available on site. Where a 24 V dc supply is required to maintain the
Where the standby generators are decentralised, dump valve closed, the source of that supply should
fuel should be pumped from the centralised storage be carefully considered, as a reduction or failure of
area. this voltage would also cause all fuel to be dumped
from the day tank. Operation of the dump valve
8.85 Where the fuel is not pumped to decentralised
should also be monitored and alarmed. The bulk
standby generator(s), a hand-operated semi-rotary
tank capacity should maintain sufficient empty
oil pump should be available for transferring fuel
space to receive the full contents of the day tank.
oil from oil drums or other vessels to the standby
generator(s) day tank. The hand pump should have 8.89 The addition of the fire safety-valve feature needs
a filter fitted with screw caps to prevent ingress of to be carefully considered and risk assessed against
dirt when in storage. Where the oil-fired boiler the potential disruption of a premature generator
burner plant (or CHP plant) can use the same low- set failure. If the generator is sited away from
sulphur fuel as the generators, designers may wish other buildings and provided with automatic fire
to consider sharing the bulk fuel storage. Under detection, the ability to maintain electrical supply
such strategies, the stored fuel volume should be while making a managerial decision regarding the
assessed on the worse-case demand of 200 hours’ fire condition may be preferential in terms of risk.
continuous full-load generator(s) demand or 10 In addition to the guidance given by Defra,

58
 8 Secondary power centres and plant

Figure 24 Fuel day tank with hand fill

Day tank
Isolating
valve

Bulk fill
point
Engine Alternator

Isolating Isolating
valve valve

Hand pump
Feed
Return

Figure 25 Fuel day and bulk tanks with dual pumps and fire dump

Thermal link

Day tank

Fire Fire
Engine Alternator valve dump
valve
Isolating
valve

Isolating
valve
Hand pump
Feed
Return

Isolating
valve
Bulk fill
tank
Duty/standby
electric pumps
Bulk tank

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

installations must comply with local regulations Environmental considerations

from the fire department, district surveyor and
local council. For large generating sets, the quantity 8.91 A generating set should be configured to operate on
of fuel to be stored could become significant in low-sulphur fuels. Noise from standby generators
both the day and bulk tanks. Both the size and the can cause significant disturbance if not attenuated.
location of bulk fuel tanks should be carefully Designers should address the air intake noise by
considered. Fuel in the bulk tank can remain suitable attenuation so that the sound pressure level
unused for significant periods and may deteriorate. in the generator enclosure is less than 85 dbA,
It should be subject to routine testing. The risk or operatives should wear personal protective
associated with fuel leakage should be assessed. equipment (PPE). The generator enclosure and
Containment may be required for day tank, bulk air removal system should also be attenuated so
tank and associated fuel transfer pipework, both that the sound pressure level satisfies the local
from the bulk tank to the day tank and from the environmental conditions. This will require an
day tank to the engine. The use of tank bunds and understanding of the nighttime background noise
double-skinned transfer pipes should be considered. levels near the generator house. The need for PPE
To reduce the possibility of fuel spillage, controls should be assessed, and a risk assessment should be
should be included to ensure that should a day tank undertaken.
become overfilled, any fuel transfer systems are 8.92 The following is a list of typical conditions in
automatically switched off; similarly if a bulk tank a generating set that will require operational
is overfilled, visual indication is given at the fill procedures to provide a safe environment:
point.
• hot surfaces:
Exhaust systems – a running engine operates at
approximately 90°C and the exhaust at
8.90 The exhaust system associated with a generating
550°C, which requires guarding;
set should include a silencing system that reduces
the noise level to acceptable limits at the point of • rotating parts:
discharge. Due consideration should also be given – all moving parts should be protected by
to the position of the discharged gases with regard guards;
to smell and gas condensation temperatures. The
engine exhaust will be the hottest part of the • batteries filled with acid:
generating set and will operate in the region of – leakage, venting, filling together with
450°C at full load. The system should be lagged electrical connection and disconnection
where it is considered to present a safety hazard should be controlled;
or heating problem. Design of exhaust systems
should take due consideration of the possibility of – procedure for cleaning a spill, together
condensation from the exhaust gases at the final with controlled disposal of waste materials;
exit to avoid the possibility of corrosion. Where • antifreeze that can spill:
the discharge point of the gases is remote from the
generating set, it may be necessary to increase the – procedure for cleaning a spill, together
diameter of the pipework to overcome any back- with controlled disposal of waste materials;
pressure. Exhaust gases are a fire and pollution • noise levels that typically exceed 105 dbA in
hazard, and are increasingly regulated, with the close proximity to set:
need to fit catalytic converters and particulate
filters. Installations must comply with local – the use of ear protection is essential;
regulations for environmental health, fire safety • electricity generation at voltages of 0.4 kV or
officers, district surveyors and local councils, and 11 kV:
must comply with the Clean Air Act. The final
– all protective cover plates should be in
termination point of the exhaust should be kept
position.
away from any fenestration and/or air intakes.

60
 8 Secondary power centres and plant

8.93 The maintenance of a generating set will

periodically result in batteries, lube oil and
antifreeze requiring to be replaced. Each of these
items has environmental consequences, and a safe
disposal policy should be enforced that includes an
audit trail documenting the controlled disposal.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

9 Protection and switchgear

9.1 This chapter considers the various types of connect between the common busbar and
switchgear at 11 kV and 0.4 kV that may be cableway. Withdrawable units tend to have the
appropriate for healthcare premises. It is important largest physical size of all switchgear for a given
to review the distribution strategy (see Chapter 6) rating. The units have a very safe interlock and
before selecting switchgear and protection types. shutter mechanism that prevents access to live parts
This is of particular importance when making when the truck is removed. The truck can be
modifications to existing electrical network(s). An located in the “cable busbar” position, “cable earth”
understanding of the implications for maintenance position or “busbar earth” position, just by the
and the spare-part requirements should be ensured relative position of the truck in the housing. To
before selecting different generic types of replace a withdrawable unit, the unit is lowered
switchgear. Detail of spatial planning for switchgear from its normal in-service position and wheeled
is provided in Chapters 7 and 8. away from the chamber. Withdrawable units can
be replaced (with a spare unit) within half an hour.
High-voltage switchgear With withdrawable units it is possible to prove a
circuit dead, while the truck has been removed,
9.2 The type of HV switchgear selected should be with the aid of a purpose-built “voltage indicating
comparable with the type of HV substation. stick”. There is no equivalent method of proving a
While this statement may seem obvious, many circuit dead on the other types of HV switchgear.
manufacturers are making compact ingress Note that the oilswitch oil circuit breaker is only
protection-rated enclosures for internal switchgear available as a withdrawable unit. Where the
to be used semi-externally. Similarly, some installers withdrawable unit includes electrical interlocks, the
are housing typical external switchgear (ring main electrical interlock integrity to other withdrawable
units) inside buildings due to the competitive units should be maintained when the device has
pressures for available land space and reduced been withdrawn.
component cost.
9.3 The main forms of HV switchgear are oil, SF6 Semi-withdrawable units
and vacuum, all of which can be used for a switch 9.5 Semi-withdrawable units are generally frame-
disconnector or circuit breaker. Switchgear mounted components which sit in a specific
assemblies (functional units) can be in the form of chamber and connect between the common busbar
a single component or multiple units, linked via a and cableway. Semi-withdrawable units tend to
busbar, to form a composite HV switchpanel. have the medium physical size of all switchgear for
Functional units can be withdrawable, semi- a given rating. The units have a very safe interlock
withdrawable or fixed-pattern. The difference and shutter mechanism that prevents access to live
offered by each system is the compactness and parts when the unit is withdrawn from its frame.
opportunities for servicing or replacing faulty The unit only has one service position and
functional units. (Many devices include two provision for cable earthing. To replace a semi-
functions: for example, disconnector and circuit withdrawable unit, the unit is released from its
breaker in one device, except the switch is often a normal in-service position; fixing bolts have to be
single-function device.) removed before the unit can be fully removed.
Semi-withdrawable units can be replaced (with a
Withdrawable units spare unit) within one to two hours. In order to
9.4 Withdrawable units are generally truck-mounted prove a circuit dead, specially-connected indicating
components which sit in a specific chamber and lamps are connected between cable and busbar.

62
 9 Protection and switchgear

However, there is not a 100% foolproof method of (normally held at 1 bar g). The SF6 circuit breaker
proving the lamp circuit. can still operate satisfactorily with a reduced gas
pressure. The disadvantage of SF6 is that the gas
Fixed-pattern units can dissociate and produce an odour when exposed
9.6 Fixed-pattern units are frame-mounted to a high-energy spark of a fault condition. The
components which sit in a specific chamber dissociated gas can produce particulate dust and
and connect between the common busbar and other by-products that are a skin irritant. The
cableway. Fixed-pattern units tend to have the health and safety aspects of SF6 have tended to
smallest physical size of all switchgear for a given drop this form of circuit breaker from favour.
rating. The units have a safe interlock and shutter Where SF6 switchgear is used, appropriate hazard
mechanism that prevent access to live parts. signs should be fixed to the switchroom doors.
However, the units cannot be withdrawn or used 9.10 Vacuum switchgear uses a vacuum chamber to
for cable/busbar earthing, as the unit only has one interrupt the arc generated by the circuit breaker
service position. To replace a fixed-pattern unit, tripping or automatically opens under fault
the full switchpanel has to be isolated and stripped conditions. The disadvantage of vacuum switchgear
down. Fixed-pattern units will therefore take the is the instability of the arc under fault conditions.
longest time of all switch assembly types to replace. As the vacuum bellows opens, the spark can
In order to prove dead, specially-connected collapse and remake three or four times before the
indicating lamps are connected between cable and energy is sufficiently lowered to effect isolation.
busbar. However, there is not a 100% foolproof The repetitive arcing may cause HV transients in
method of proving the lamp circuit. Consideration the load circuit.
should be given to the method of cable earthing
9.11 The selection of any particular switchgear type
where fixed-pattern units are used. Cable earths
should include a review of the life-cycle costs of the
may be difficult when fixed-pattern units are used
respective types. However, best-practice designs are
at both ends of the cable. It is also recommended
focused on the implications of a fire arising from
that every switchpanel have a local means of
any explosion within the HV equipment. Equally
earthing the busbars.
important will be the protection type that can be
9.7 Where the switchgear device includes a circuit- used and the method of reconfiguring the HV
breaker function, thought should be given to network following a fault. The selection of the
the type of arc-interrupting material (oil, SF6 or particular switchgear type should also consider the
vacuum) in terms of the environment, health and means of earthing the cable (at both ends).
safety, and maintenance requirements.
9.8 Oil switchgear has a good legacy and reliability; High-voltage busbar sections
however, any controlled or fault operation of the 9.12 As mentioned in the previous section, certain
switchgear creates a controlled spark interrupted switchgear types allow for earthing cables and
by the oil, which degrades the oil. Hence, the oil busbars. In addition, they provide a means to
circuit breaker device (OCB) should be serviced positively prove dead. Where the selected type of
after any three fault operations of the breaker. switchgear does not allow this function, the HV
Mineral oils are not environmentally friendly, and switchpanel should be split into two sections
special disposal requirements should be observed. separated by a cable length. If the cable terminates
Silicon oil is a suitable replacement for mineral oil, onto each section of the switchpanel via a
but the switchgear maintenance requirement disconnector circuit breaker combination, it may
remains unchanged. be possible to replicate the advantages of the
9.9 SF6 switchgear uses the properties of sulphur withdrawable device (except, that is, the
hexafluoride (SF6) for arc interruption and hence is opportunity for fast replacement times).
smaller than the OCB for the same rating. Under 9.13 One clear advantage of the cable link on the HV
normal use, SF6 is a colourless, odourless, non- bus section is the opportunity to build a fire barrier
toxic and non-flammable gas, giving advantages for between the two sections of the panel. Similar
internally located switchgear. The switchgear facilities can be achieved with two ring main units
maintenance requirements are related to a fault used on a common substation.
condition or a lowering of the gas pressure

63
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

High-voltage protection devices Inverse definite minimum time (IDMT) relays

9.14 On HV networks, the protective devices are fuse 9.17 The two previous devices are current-operated only.
links or relays which automatically operate local The IDMT relay in its basic electro-mechanical
or remote circuit breakers. At high voltage the form is adjustable both in the current setting
opportunities to grade fuse links and provide a level known as a plug setting (PS) and a time setting
of discrimination are significantly less than those know as a time multiplier setting (TMS). The relay
for the LV fuse link, particularly as the HV current is fed via CTs by varying the CT ratio. Plug and
approaches 45 A (800 kVA). Relays can have very time settings enable the relay to be used in any part
fast operating time compared with fuses, which of the distribution network and in series with each
explains why relays are the preferred protective other. The modern relays are electronic; these have
device for voltages above low voltage, typically the added advantage of more settings and curves,
50 ms, 2.5 cycles (digital/numerical relays) and for enabling them to mimic HRC fuses, time fuse links
electromechanical relays 150 ms (7 cycles). Time and LV ACBs. They can also be configured to
fuse links are used for some HV applications. display HV current, removing the need for
ammeters. The latest IDMT relay can also be
High-rupture-capacity (HRC) fuse links to connect to BMSs or connected to the Internet for
BS 2692, IEC 60298 remote interrogation or operation.
9.15 These fuse links are suitable for fitting into HV Bias differential relays
ring main units. They are equipped with a striker
pin (actuated by a small pyrotechnic device) which 9.18 Bias differential relays are used in unit protection
is used to operate a trip mechanism disconnecting schemes (the trade names for these relays being
all phases. The speed of the device at large currents Translay and Solkor). They are configured in pairs
is such that they are able to limit the current to the at either end of a feeder cable or at a transformer.
fault. The size of the fuse is determined by the They will only operate if the fault is within their
transformer size and its inrush current (normally zone of protection; all other faults will cause no
12 times transformer full load for 0.1 s). The range action. Unit protection schemes are used on closed
of fuses available are from 5 A to 125 A, covering ring networks and on interconnectors, that is,
transformer sizes from 50 kVA to 1600 kVA. cables connecting two sources of supply (primary
To assist with network discrimination it is and secondary).
recommended that HRC fuses are limited to
protection of transformers up to and including Earth fault passage indicators
800 kVA. 9.19 Earth fault passage indicators are devices which are
connected to cable entry points on HV switchgear.
Time fuse links to ESIS 12.6 They are used to indicate when an earth fault has
9.16 Time fuse links, also known as time-lag fuses passed through the cable. Early versions dropped
(TLF), are designed to Electricity Supply Industry a coloured disc on the unit, and the Authorised
Standard (ESIS) 12.6. The links are used in Person (HV) would then walk the system and
conjunction with a current transformer (CT) to disconnect the faulty section. The latest devices can
operate an HV circuit breaker via an ac trip coil. be individually connected to a central location and
The range of fuses available is 3 A, 5 A, 7.5 A, used as part of an automatic restoration system.
10 A, 12.5 A and 15 A. When used with the
appropriate CT ratio they can be used to protect Grading of protection systems
transformers having a range from 200 kVA to 9.20 Grading of protection systems is carried out to
2000 kVA. TLFs are normally designed to protect ensure, so far as is possible, that only the faulty
against overcurrent and earth faults. The yellow equipment is disconnected when a fault occurs.
phase is used for earth fault protection and the fuse
on this phase is either reduced or omitted; this 9.21 Discrimination by time separation is achieved by
makes grading between TLFs in series with each making the protective devices, which all detect and
other very difficult. respond to the fault current, progressively slower to
operate the further they are from the point of fault.
This is the normal method of ensuring grading on
open ring or radial distribution systems using HRC

64
 9 Protection and switchgear

fuses, time fuse links and IDMT relays. In order to Network reconfiguration after a fault or outage
ensure that grading is achieved there are typical
9.23 The HV protection system should be designed
minimum acceptable time separations between the
to disconnect the faulty part of the system with
various devices, and these are as shown in Table 5
minimum disruption. The type of HV distribution
and Figure 26. However, fixed grading margins are
strategy (see paragraphs 6.26–6.30) and the type of
only appropriate at high fault levels that lead to
functional unit used will determine the time taken
short relay operating times. At lower fault current
to restore supplies to the health section of the
levels with longer operating times, typically when
network.
the HCP is supported using standby generators,
relays may fail to grade corrrectly. 9.24 A fault on a radial circuit will cause all users on
the circuit to be affected until the system can be
9.22 Discrimination with stability is achieved by making
reconfigured, and any users downstream of the
the protective devices detect, and respond to, only
fault will be disconnected until the fault has been
faults which require their operation, thus ensuring
repaired. On ring circuits utilising ring main units
that only the faulty equipment is isolated. This is
(RMUs), users on the affected parts of the ring
the normal method of ensuring grading on closed
(depending on the position of any open point) will
ring distribution systems using unit protection
be disconnected until the fault is located. Normally
relays. As the unit protection relays detect, and
all users will be restored after reconfiguration of the
respond to, only faults calling for their operation,
system. Protection control units are now available
there is no necessity to build in time delays for time
that will reconfigure the network automatically
separation purposes.
using fault detection systems and powered switches
on RMUs and circuit breakers, restoring power in

Table 5 Time separation of protective systems

Smaller protective device Larger protective device Minimum time separation (seconds)
(nearer to fault) (further from fault)
HRC fuse HRC fuse Limited options for grading using I2t
(ie let-through energy) values
HRC fuse Time fuse links 0.2
HRC fuse IDMT relay 0.4
Time fuse links Time fuse links Will not grade satisfactorily
Time fuse links IDMT relay 0.4
IDMT relay IDMT relay 0.25–0.3

Figure 26 Progressive time separation

1.0 s 0.6 s 0.2 s 0.0 s

0.4 s 0.4 s 0.2 s

65
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

minutes rather than the normal manual restoration transformer used in healthcare premises is a Dyn11.
which can take up to an hour – even longer when This means the primary windings are delta-
staff are not in attendance. connected, the secondary windings are star-
connected, and the secondary windings lag the
9.25 Networks with a closed ring topology using circuit
primary windings by 30°. “Zigzag” transformers
breakers and unit protection will disconnect the
have a Z as the winding notation.
section of the ring under fault, normally leaving all
users unaffected. 9.30 Transformers used in healthcare premises fall into
one of two types, defined by the method of cooling
9.26 The design of the HV protection should be
the windings. The types are fluid cooled (either
consistent with the distribution strategy. The
mineral or synthetic oils), cast resin, and air-cooled
protection systems relate to the network type, and
or exposed winding type.
should not be the principal selection process. The
advantage of using IDMT relays with adjustable 9.31 Transformers used for IPS units are discussed in
settings to allow for the remodelling of the HV paragraphs 16.27–16.48.
network is that they may also provide greater
flexibility for future developments of the healthcare Fluid-type transformers
site. 9.32 The windings are ideally insulated to Class F (that
9.27 Where the standby plant consists of HV generators, is, allow a 100°C differential temperature between
it will be essential to incorporate an automatic the winding and the adjacent area). The windings
control system to reconfigure the network(s) after are cooled by circulating oil, usually synthetic
any HV fault conditions. HV protection systems silicon oils. The oil transfers the winding heat to
that include facilities to reconfigure the network the external face of the transformer where it is
automatically should be associated with the clinical radiated by air. This type of cooling is referred to as
risk and business continuity assessment. “oil natural circulation, air natural flow” (ONAN).
Silicon oils are a dielectric fluid (K3), which are
Distribution transformer types preferred because of their high flash point (greater
than 300°C).
9.28 The number and rating of transformers should
be determined by the distribution strategy (see 9.33 Fluid-cooled transformers less than 1600 kVA are
Chapter 6). The transformer rating should also generally hermetically sealed. This requires the
be selected according to the AMD, fault level, and transformer oil tank to take up any expansion in
the prospective short-circuit current (PSCC). The oil volume due to the heating. Larger oil-cooled
transformer rating should be limited to 2000 kVA. transformers have an oil conservator located on
The parallel operation of power transformers is the top of the transformer. Expansion of the oil is
not recommended by this Health Technical controlled by the volume of dried and filtered air
Memorandum. Transformers may be operated in allowed into the conservator.
parallel only with due care and consideration for 9.34 Designers may wish to consider a further advantage
the increased fault level. Where more than one of the oil-cooled transformer with conservator.
transformer is used in a common substation A gas detector can be used in the conservator to
(either independent or in parallel), they should operate a relay that disconnects the primary side
all be of the same vector group, same voltage supply if the oil contains gases caused by faults or
transformation, and have a percentage impedance air impurities. This type of relay is known as a
within 10% of each other, for example 6% and “Buchholz relay”. However, the conservator
5.4% or 6.6%. and Buchholz relay tend not to be viable on
9.29 Transformers of all types are denoted by their transformers less than 10 MVA. For transformer
winding configuration, phase displacement ratings less than 10 MVA, alternatives such as the
between primary and secondary windings, and distribution strategy given in Figure 17 may be
percentage impedance. The notations start with appropriate.
the HV winding configuration, followed by the 9.35 Where the transformer oil tank contains more
LV winding configuration, and then phase than 50 L of silicon oil, the transformer should
displacement expressed in clock-hour positions. be enclosed in a two-hour fire-compartmented
Capital letters are used to denote the higher enclosure. Where the fluid is a dielectric such as
voltage. The most common type of distribution

66
 9 Protection and switchgear

mineral oil (O1), high hydrocarbons (K1) or esters 9.43 See Chapter 7 for details of the package substation’s
(K2), fire compartmentation is required with fluid location and construction.
capacities above 25 L.
9.36 See paragraphs 7.33–7.50 for details of the Transformer protection
transformer room location and construction. 9.44 For a general overview of transformer protection
systems, see paragraphs 9.14–9.27. HRC fuse links
Dry-type transformers are suitable for transformers rated up to 800 kVA,
9.37 The windings are ideally insulated to Class F (that while TLF fuse links, which can also provide earth
is, allow a 100°C differential temperature between fault protection, are a more appropriate protection
the winding and the adjacent area). The resin form for transformers in the range of 200 kVA to
transfers the winding heat to the transformer outer 2 MVA. Unit protection is not economic or
casing where the heat is radiated in much the same effective on transformers less than 10 MVA.
way as with the fluid-cooled transformer. This type Protection of fluid-filled transformers can be
of cooling is referred to as “no inner circulation and achieved with a Buchholz relay. Dry-type cast-resin
air natural flow secondary” (AN). transformers have a thermistor integral with the
windings which will isolate the transformer on high
9.38 Cast-resin transformers may be totally enclosed in winding temperatures caused by a fault current or
their own housing, or of the open-winding type. other reasons. The most common faults with
Clearly, the open-winding type cannot be used transformers used in healthcare premises are more
externally. Further safety precautions regarding to do with the cables connecting the primary and
access to the transformer room are required for secondary windings to the network. An IDMT
open-winding dry-type transformers. Cast-resin relay with the sensing CTs configured to give “earth
transformers tend to vibrate and hum more than fault and protection” can monitor either the HV
fluid-cooled transformers and consequently collect connecting cables or the LV connecting cables.
dust, which requires regular cleaning, say annually. (Note that overcurrent/overload protection will be
(See Health Technical Memorandum 06-01 provided by the LV circuit breaker.) By connecting
Part B.) pilot wires between the HV and LV circuit breaker,
9.39 See paragraphs 7.33–7.50 for details of the a system known as “intertripping” can be used to
transformer room location and construction. ensure that no power can be supplied into the fault,
regardless of the IDMT relay being on the HV or
Package substation LV side.
9.40 Package substations are composite units with the
HV switchgear close-coupled to the transformer Generator protection
side. In some cases the LV switchgear is also close- 9.45 Generators are essentially provided to maintain
coupled to the transformer. Package substations a supply when part of the internal distribution
always use dry-type cast-resin transformers. has failed, or the PES has failed. Therefore, the
9.41 Designers and stakeholders should consider
protection design intent should be different, and
package substations, which offer a cost-effective more tolerant of fault conditions, before operating
solution in terms of the requirements and access any generating isolating devices.
for maintenance. Due to the close-coupled 9.46 Designers and stakeholders should consult with
arrangement, maintenance may take longer and manufacturers regarding the generator damage
will affect larger parts of the systems. Care should curve and short circuit decrement curve before
be taken to ensure that only HV Authorised calibrating the protective devices.
Persons (AP(HV)) have access to the HV
9.47 The main fault condition to be considered in
equipment.
relation to a generator is the earth fault. Generators
9.42 Package substations may provide an effective should be able to generate adequate fault currents
solution for dedicated single loads such as large to clear a system earth fault without shutting down.
chiller stations. Package substations may also However, an earth fault on the local cable between
provide effective solutions where the distribution the generator and network should be cleared
strategy has a high resilience, such as the dual- instantly. An IDMT relay with two separate relays,
unified network (see Chapter 6). one configured for restricted earth fault (REF)

67
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

and the other affording overcurrent and overload Form 2

protection and network earth fault protection, can
9.51 Form 2 assemblies are enclosures that provide
be considered. However, care needs to be taken
protection against contact with any live parts and
in their grading, as the generator short-circuit
provide internal separation between the busbar and
decrement current may influence the operation of
functional units, but there is no separation between
any other IDMT relays on the network.
individual functional units. Compliant variations
include insulated or non-insulated busbars, cable
Low-voltage switchboards terminations separated or not separated from the
9.48 The type of LV switchgear selected should be busbar but not from the functional units. Each
comparable with the type of LV substation. functional unit should have a facility that enables it
While this statement may seem obvious, many to be locked in the off (de-energised) position (see
manufacturers are making compact IP-rated Health Technical Memorandum 06-02 – ‘Electrical
enclosures for internal switchboards to be used safety guidance for low voltage systems’). Final
semi-externally. distribution boards with Form 2 separation are an
acceptable standard.
9.49 The main feature of LV switchboard switchpanels
is the form of construction (form of separation). Form 3
The forms are defined in BS EN 60439-1 and
BS EN 60439-3 for final distribution boards 9.52 Form 3 separation units are available; however, by
(see Figure 27, which illustrates the main forms). considering the merits of distribution boards and
Designers and stakeholders should assess the switchpanels with Form 2 and Form 4 separation
opportunities for maintenance and remodelling respectively, such units have little advantage within
of the distribution when selecting the form of a healthcare environment.
separation. The selection of a switchboard
switchpanel form can be related to clinical risks Form 4
and business continuity risks. Note that any work 9.53 Form 4 assemblies are enclosures that provide
on a switchboard of any type should be managed protection against contact with any live parts and
under the electrical safety regulations. See Health provide internal separation between the busbar and
Technical Memorandum 06-02 – ‘Electrical safety functional units and between functional units.
guidance for low voltage systems’. Cableways are also separated from each other.
9.50 Designers should select the form of separation for Compliant variations include insulated or non-
switchboards, switchpanels or final distribution insulated busbars, cable terminations separated or
boards based on the area covered and type of load not separated from their respective functional unit.
connected to the outgoing circuits. Switchboards 9.54 The main compliant variations of Form 4
and switchpanels may serve more than one clinical switchboards, switchpanels are how the external
function, and therefore the opportunity to isolate cables are terminated (glanded-off ). Cables can
the switchboard switchpanel (for any form of be glanded-off at the switchpanel frame, with
maintenance) is reduced. A switchboard or conductors terminated in a common cableway,
switchpanel with a minimum form of separation or separated from the cableway. Alternatively, the
(Form 4) should be used. Form 2 separation may cables can be brought into the switchpanel and
be suitable for final distribution boards. Where glanded-off at individual gland boxes associated
space is available, all LV switchboard switchpanels with one functional unit. Each functional unit
should be located in dedicated electrical should have a facility that enables it to be locked
switchrooms, electrical risers, or plantrooms with in the off (de-energised) position (see Health
controlled access. Where this is not achievable, Technical Memorandum 06-02 – ‘Electrical safety
electrical switchboard switchpanels should have guidance for low voltage systems’).
lockable devices to prevent unauthorised access or
interference. See Health Technical Memorandum Motor control centre (MCC)
06-02 – ‘Electrical safety guidance for low voltage
9.55 Motor control centres tend to include an
systems’.
LV protection section and a control section.
Control technology continues to advance, from

68
 Figure 27 Forms of separation

BS EN 60439-1 Switchgear forms of separation

Form 2A
Terminals NOT separated
from Busbars

Form 2B
Terminals ARE separated
from Busbars

Form 4 Type 6. Busbars separated from functional units. Functional units separated and separated from cable terminals which are in
separate boxes. Cables glanded externally

69
9 Protection and switchgear
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

electromechanical devices to pneumatic controls to Low-voltage protection devices

electronic numeric controls and so on.
9.61 The type of LV switchgear selected should be
9.56 Designers should therefore consider the main safety comparable with the type of LV substation.
requirements and opportunities to isolate small While this statement may seem obvious, many
sections of the MCC. Best-practice solutions manufacturers are making compact IP-rated
separate any LV protection devices from the control enclosures for internal switchgear to be used semi-
section, making the two sections a minimum of externally.
Form 3 separation. Within the LV section, the
form of separation should be at least Form 4. 9.62 The main forms of LV switchgear are air circuit
Where the protection and control sections are in breaker (ACB), high rupturing capacity (HRC)
one composite panel, it should be possible to open fuse, high breaking capacity (HBC) fuse, moulded-
the control section without providing direct access case circuit breaker (MCCB), miniature circuit
to the protection section. breaker (MCB) cartridge fuse, and rewireable fuse.
These forms of protective device can be assembled
9.57 An MCC should be located within a plantroom in the moving part or fixed part of the respective
(which itself has controlled access). The supply switchgear. Assemblies can be in the form of a
to the MCC should be limited to 250 A, in single component or multiple units, linked via
accordance with BS EN 60439-3. a busbar, to form a composite LV switchpanel.
Switchpanel components can be withdrawable,
Final distribution boards and consumer units semi-withdrawable or fixed-pattern. The difference
9.58 Consumer units and distribution units (DBUs) offered by each system is the compactness and
do not fall into the same forms of separation opportunities for servicing or replacing faulty
construction requirements of BS EN 60439-1. components. In spite of the switchgear-type name,
Their requirements are identified in BS EN many devices include two functions, for example
60439-3. a disconnector and circuit breaker in one device,
except that the “switch” is often a single-function
9.59 DBUs should be located within dedicated electrical device.
switchrooms, risers or plantroom areas. Where this
is not practical, the DBUs should have a local Switch
device to prevent unauthorised access, or be
surrounded by a lockable cupboard. 9.63 A switch is a mechanical device that can carry and
break a current under normal circuit conditions.
9.60 The benefits of remodelling and design flexibility The switch may be modified to allow for
cannot be understated when selecting switchboard automated operation by other protective devices.
switchpanel forms. Standardising on one form, A switch may not provide adequate separation
particularly when the building is large, has distance between disconnected parts of a circuit
significant merits. Form 4 type 1 switchboards conductor.
which have separate compartments for insulated
busbars and each functional unit, with cables Disconnector
glanded-off externally to the panel frame and
conductors terminated within the respective 9.64 A disconnector is a mechanical device that carries
functional unit, may provide best-practice solutions the design current for its intended purpose. A
for healthcare premises with clinical risk categories disconnector cannot break a normal current nor
1, 2 or 3. Form 4 type 6 switchboards which have make or break a fault current. The disconnector
separate compartments for busbars and each will provide adequate separation distances between
functional unit, with cables glanded-off externally disconnected parts of the circuit conductor.
to the panel frame and conductors terminated in
termination boxes external to the respective Fuse
functional unit but in a common cableway, may 9.65 A fuse can provide the fundamental function to
provide best-practice solutions for healthcare rupture a fault current that may flow in a correctly
premises with clinical risk categories 4 or 5. designed electrical circuit. The fuse can provide
overcurrent protection and fault current (both
short-circuit and earth fault) protection. The fuse
has different characteristics, making it suitable for a

70
 9 Protection and switchgear

range of electrical loads, for example the general 9.72 Designers may wish to consider the use of RCBOs/
range and motor range. RCDs for areas where the natural environment may
be damp and hence the body contact resistance
Circuit breaker may be lowered. Such areas will include
9.66 The circuit breaker is a more advanced form of
laboratories, kitchens and workshops.
protective device than the standard fuse. The fusing 9.73 Designers may wish to consider the use of RCBOs/
element can have a tolerance range to delay the RCDs for dedicated cleaners’ sockets.
rupturing action. The circuit breaker is available in
9.74 Designers should be mindful that RCBOs may
five basic formats for LV circuit protection:
be subject to nuisance tripping caused by the
• air circuit breaker – ACB; occasional high earth leakage currents that may be
• moulded-case circuit breaker – MCCB; generated when equipment is switched on.

• miniature circuit breaker – MCB; Low-voltage busbar sections

• residual current device – RCD; 9.75 Where the LV distribution strategy includes for
• residual current breaker with overcurrent – dual-unified circuits supported by two 100%-rated
RCBO. transformers, it may be useful to link the two
sections of the main LV switchpanel via a “bus
9.67 The fusing elements of all types of circuit breaker coupler-bus-tie”. However, maintenance access to
have two parts: a current transformer providing the bus coupler may require the full isolation of the
adjustable electromagnetic setting, and a bimetal switchpanel. Although such devices have very low
strip providing adjustable thermal setting. Note maintenance requirements, it may also be useful
that some smaller MCBs do not have an adjustable to consider splitting the LV switchpanel into two
range, but still operate with a tolerance range. sections, linked by a cableway and two ACBs.
9.68 The RCD provides protection against earth An advantage of such an arrangement may be to
leakage, with typical ranges at 10 mA, 30 mA, install a fire barrier wall between the two sections.
100 mA, 150 mA and 300 mA. Designers and stakeholders should see this as an
expensive option and carefully assess the merits
9.69 The RCBO is a combination protective device of
accordingly.
the MCB and RCD functions; however, the earth
fault sensing element of 150 mA and 300 mA are
not normally available in this combination. Discrimination of protective devices
9.76 The IEE Regulations BS 7671:2001 require that
9.70 MCBs and RCBOs have a range of characteristic
curves, Type A, B, C or D (earlier devices were 1, 2 the characteristics and setting of a protective device
and 3). The separate RCD devices only sense an for overcurrent should provide any intended
earth leakage current. RCBOs and RCDs used in discrimination within its operation. While the
medical locations should be type A or B and should regulations do not call for discrimination for fault
have a tripping current of 30 mA. currents, by default a protective device should so
discriminate.
9.71 The design should consider the selected protective
device that will clear overload currents and short- Discrimination with HBC/HBC fuses
circuit faults within the prescribed disconnection
9.77 HBC fuses will conform to the requirements of
times of BS 7671:2001. Clearly, the protective
BS 88.
device rating and disconnection times are related
to the earth fault loop impedance. Designers 9.78 Discrimination will ensure that the total let-
should consider the use of RCBOs or RCDs where through energy (I2t) of the minor (downstream)
the earth loop impedance cannot generate sufficient device is not greater than the pre-arcing energy
earth fault currents to operate the protective device (I2t (pa)) of the major (upstream) device. Where the
within the appropriate disconnection times protective device has an operating range (MCB,
(5 seconds for stationary equipment and MCCB), it is important to check that the normal
0.4 seconds for portable equipment). Designers operating current (In) of the major device is greater
should be mindful of the earth leakage current that than x times the prospective short-circuit current at
may flow in the protective conductor under normal the downstream protective device in question
conditions.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

(where x equates to the multiplying factor applied Discrimination with RCDs

to the downstream device; see BS EN 60898-2).
9.82 RCDs will provide earth fault protection. In order
to discriminate between two RCDs as the upstream
Discrimination with MCB/MCCB
and downstream protective devices, designers need
9.79 The MCB (or MCCB) device can provide to use the time-delay setting on the upstream
discrimination for overload and fault current device.
between the upstream and downstream device. In
9.83 Designers may wish to consider that full
order to provide overload protection, the nominal
discrimination may not be required on all circuits.
current setting (In) of the upstream device should
Opportunities exist to take advantage of a grading
be greater than the instantaneous rating of the
between fuse element curves. Where the
downstream device (which will be x times In
discrimination is a little uncertain and the risk of
according to the MCB type). In order to provide
such relative high fault currents are low, the circuit
discrimination of fault currents (both short-circuit
is said to have “limited” discrimination and may
and earth fault), knowledge of the prospective
be acceptable. The advantage here would be the
short-circuit current (PSCC) and earth fault
reduced size of protective devices, especially with
current is required. Discrimination will occur if
the more upstream devices.
these values, for the downstream device, are less
that the nominal current setting of the upstream
device. Manufacturers’ advice should be obtained Automatic load management of
for the actual let-through energy (I2t) which will switchgear (HV, LV)
also determine the discrimination of series MCBs. 9.84 The electrical infrastructure and distribution
9.80 The use of MCBs on some final circuits may cause strategy may minimise the effect of an electrical
nuisance tripping; for example using MCBs (or fault to the clinical risk areas, but the most resilient
RCBOs) on fluorescent lighting circuits, where system will not totally eliminate the risk. The
the non-linear transient current of the inductive fundamental reasons electrical systems have
control circuit may cause early tripping of the protective devices is to limit the effect of a fault.
protective device through its internal CT. Effective discrimination (see above) and correct
Consideration should therefore be given to using selection of protective devices will isolate the
an RCBO/RCD with earth leakage characteristics smallest appropriate section of the infrastructure.
of type A or B. Best-practice distribution strategies may provide an
alternative supply route which could be initiated
Discrimination with MCB/fuse more quickly or safely than replacement of a
protective device.
9.81 Discrimination between a fuse (as the upstream
protective device) and an MCB for the downstream 9.85 The distribution system may initiate the standby
device may occur if the lower instantaneous generator plant until the fault is rectified
operating current (range) of the MCB crosses the or isolated. The reconfiguration of an electrical
tripping curve of the fuse above the prospective network (whether the system is a dual-unified
fault current level and earth fault current level of supply or segregated supply) may be made
the downstream device. Discrimination checks manually or automatically.
should be made based on the manufacturers’
9.86 Where the network is an HV ring circuit (open or
declared let-through energy (I2t). See Table 6.
closed), the operation of a protective device may
result in the standby generators supplying some
areas.

Table 6
MCB type Nominal Current (In) Overload Characteristics Instantaneous Tripping range Instantaneous
Tripping
B All 1.45 In 3 In to 5 In 5 In
C All 1.45 In 5 In to 10 In 10 In
D All 1.45 In 10 In to 20 In 20 In

72
 9 Protection and switchgear

9.87 Healthcare premises with significant sections of SCADA systems are modular in design and may be
clinical risk Category 3 areas and above may benefit added to retrospectively. The SCADA system can
from quick reconfigurations of the electrical be applied to any part of the HV and/or LV
distribution following a fault on the network. networks including all power sources.
Therefore the use of a supervisory control and
9.88 SCADA systems should be hard-wired with
data acquisition (SCADA) computer system to
monitored circuits wherever they are used.
automatically control the switchgear status and
reconfigure the network(s) would be useful.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

10 Tertiary power supplies

10.1 Tertiary power supplies (TPSs) should not be standard five-year-life batteries, they offer
considered as a long-term energy source in the same significant long-term benefits in terms of security
way that primary power or secondary power units of function and reduced long-term costs.
are. TPSs are generally used as a back-up supply for
a given period of time (autonomy) or to start SPSs. Battery life
The batteries considered are those used for 10.5 Battery life is a function not only of load cycling,
uninterruptible power supplies, battery inverter but of charging methods and the environment.
units and batteries used to start standby generator VRLA batteries will function for a short time
engines. period over a wide range of temperature typically
10.2 Batteries used in control systems, such as motor from –15°C to +50°C. However, for normal
drives on switchgear, have been excluded from this continuous use their ambient operating
Health Technical Memorandum. Likewise, batteries temperature should be ≈20°C, otherwise their life
used for electric vehicles are also excluded from the expectancy will be reduced considerably, typically
content of this Health Technical Memorandum. to 50% at 30°C and to 25% at 40°C. Continued
The foregoing exclusions are justified, as their operation at high temperatures also may bring fire
respective systems do not form part of the fixed danger due to case splitting and resultant acid
wiring systems of healthcare premises. spillage, which in turn may result in uncontrolled
battery dc earth faults. It is therefore very
Batteries for uninterruptible power important that the battery location has a suitable
environment with adequate ventilation/cooling to
supplies maximise battery life. Note that in practical terms,
Battery type even with the recommended regular maintenance,
VRLA batteries are normally changed at 80% of
10.3 There are a number of battery cell types in use their designed life. Battery life should be in
today for UPS applications. The most appropriate accordance with the range given in Table 4.
are the valve regulated lead acid (VRLA) battery
types, which are more commonly known as sealed 10.6 Correct charging of VRLA batteries is very
lead acid cells. The VRLA battery is a near-zero- important, and should be with low or minimum ac
gassing battery cell, and hence presents a lower ripple and typical charge values of 2.27 V per cell.
environmental hazard to the UPS or surrounding At elevated temperatures, it is necessary to reduce
area. With no toxic gasses emitted from the battery, the battery charge voltage to below 2.27 V per cell
there are no special venting requirements for the to prevent over-charging.
battery unit. The VRLA battery is almost
Battery arrangements
universally used for modern UPS systems, due to
their low maintenance and because of the reduced 10.7 Designers should consider the opportunities for
requirements for vented gas extraction, this being a maintenance of the UPS battery assembly. Batteries
serious consideration when using wet cells. can be arranged as single or split banks. The use of
split battery banks allows the UPS to remain online
10.4 VRLA batteries complying with BS 6290-4:1997 –
(at reduced battery autonomy) while half of the
with threaded insert connection posts, flame-
battery system is being serviced.
retardant case materials and a ten-year-designed
life – are the minimum acceptable standard. While
BS 6290-4, 10-year-designed-life batteries have the
initial penalty of higher investment costs than

74
 10 Tertiary power supplies

Battery autonomy Battery arrangements

10.8 Single-conversion UPS units are generally used 10.13 Three main types of battery inverter unit are used
for small personal computers or computerised in healthcare premises. Batteries within the self-
processors dedicated to medical/laboratory contained emergency escape lighting and signage
equipment, such as blood gas analysers. Battery are generally in small packs with cells connected in
autonomy is typically in minutes up to say 15 series or parallel series groups. Their physical size
minutes, depending on the particular application. allows these battery packs to be replaced in a single
Designers should consult with staff for the actual step, taking only minutes. Batteries for either the
requirement. Single-conversion UPS units are central emergency escape lighting signage or
most commonly used to safely shut down systems operating theatre operating lamps are housed in
following an outage of the PPS, or depending on cabinets and connected in parallel-series cell
the particular need, between the period of mains groups. Battery maintenance is achieved by
failure and SPS standby generators becoming disconnection of any one parallel group. Designers
available. The battery autonomy for single- should consult with manufacturers to ensure that
conversion UPS units should be less than the optimum number of parallel cell groups are
30 minutes. provided to minimise the reduction of battery
autonomy during replacement.
10.9 Double-conversion UPS units are most commonly
used for TPS to dedicated final circuit outlets, used 10.14 Designers should consider the opportunities for
for example in clinical risk Category 4 or 5 areas. maintenance of the inverter units’ battery pack.
The most usual application of a double-conversion Batteries can be arranged as a single or split bank.
UPS is to provide tertiary power to IPS systems, The use of split battery banks allows the inverter
particular those for the IEC 60364-7-710 Group units to remain online (at reduced battery
2. The batteries maintain an electrical supply autonomy) while half of the battery system is
following an outage of the PPS and prior to being serviced.
the SPS standby generators becoming available.
Where the UPS battery provides TPS to non- Battery autonomy
operating-theatre low-power applications, the 10.15 Four main types of inverter unit are used in
battery autonomy should provide clinical staff healthcare premises. Battery inverter units used
with enough time to start “hand bagging” or for self-contained emergency escape lighting and
connecting supplementary equipment battery signage have a three-hour battery autonomy as
packs. Consequently, battery autonomy of required by BS EN 1838, BS 5266-7. Central
15–30 minutes may be appropriate. Where the battery units for emergency escape lighting should
UPS battery provides tertiary power to operating also have a three-hour battery autonomy.
theatre low-power applications, the battery
autonomy should provide operating theatre staff 10.16 Battery inverter units for theatre lamps should
enough time to facilitate “patient closure” for all have a minimum of three hours’ battery autonomy.
theatre cases. Consequently, battery autonomy of 10.17 Battery inverters used for fire alarm and detection
60 minutes may be appropriate. systems, or other alarm systems, should have
10.10 Clearly, designers should consult with stakeholders sufficient autonomy to drive the systems (in
and clinical staff to determine the most quiescent mode) for 24 hours, followed by a
appropriate battery autonomy. 30-minute period where all sounders, indicators
and communications are operated with the normal
Batteries for inverter units sound pressure level outputs. For a healthcare
facility that may be closed over a weekend and
Battery type bank-holiday period, an autonomy of 100 hours
may be more appropriate. This requirement is
10.11 See paragraphs 10.3–10.4. independent of any secondary power supply (SPS)
that may be available.
Battery life
10.12 See paragraphs 10.5–10.6.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Generator batteries 0°C. Generator battery systems should be capable

of turning the generator engine continuously for
Battery type 60 seconds at an ambient temperature of 0°C.
10.18 See paragraphs 10.3–10.4. 10.21 Usually two battery-charging systems are supplied
with a generating set: the constant charger is a
Battery life charger for operation while the set is stationary,
usually in the control panel; and a belt-driven
10.19 See paragraphs 10.5–10.6.
charge alternator maintains the battery when the
set is running.
Battery autonomy
10.22 For both charging systems the battery should be
10.20 Generator batteries are normally specified for
charged at the correct float voltage, and for engine
x Ampere Hours (AHr), where the battery capacity
starting the battery should be adequately sized for
x should be able to provide sufficient power when
the breakaway (initial starting) voltage to be
discharged by 25% to attempt three successive
acceptable to the engine manufacturer.
starts each of ten-second duration with a three-
second interval, while the ambient temperature is

76
11 Electromagnetic compatibility

Standards Procurement requirements

11.1 The Electromagnetic Compatibility (EMC) 11.4 Problems from electromagnetic interference (EMI)
Regulations enact the requirements of the EMC will be minimised by procuring equipment that
Directive for the UK. From the UK Regulations, complies with relevant standards, is supplied with
regulations 28 and 30 require that those who a relevant EMC declaration of conformity (DOC),
supply relevant equipment should show that: and is installed and maintained using good EMC
practices.
• it conforms to the protection requirements;
11.5 It is essential that those designing and specifying
• it meets the conformity assessment
equipment for use in the NHS environment must
requirements;
give their relevant purchase departments an EMC
• the CE marking is properly applied; specification that is sufficiently detailed that
• it has an IEC declaration of conformity suppliers are made aware of their contractual
certificate. obligations with respect to EMC.
11.6 Procurers, system integrators and designers should
11.2 Regulation 29 requires that no person should take
into service relevant equipment unless it conforms be knowledgeable about the EMC performance
to the protection requirements. For example, levels that equipment is expected to meet when
equipment covered by the EMC directive is taken correctly installed and operated. To be able to
into service when the end-user that operates the distinguish between the requirements and
equipment, for example a building management declarations of compliance statements for the
system, first uses it. “Taking into service” does various directives, a procurement document should
not include the area of energising, testing and be written that covers the various directives.
commissioning of the equipment by the 11.7 As a first step, EMC requirements should consider
manufacturer before handover to the end-user. appropriate standards; a selection is provided in
The equipment manufacturer will be in a position Tables 7 and 8. The standards are a selection for
of overall control in ensuring that the essential equipment that may be expected to be present in
protection requirements are satisfied, and assumes healthcare premises. This is not an exclusive list,
legal responsibility for compliance. Data should and as standards evolve over time, the relevant
also be provided for the end-user to ensure that websites should be consulted for changes. A
these requirements are satisfied throughout the complete list of all regulations quoted in this
operational life of the equipment. Health Technical Memorandum can be found at
11.3 From the point of view of the legislation, it is not
the end of the document.
sufficient to integrate CE-marked equipment and 11.8 As well as helping to set the environment, standards
claim that the large “system” hence complies are used to show compliance with the EMC
because compliant equipment has been used. Directive and with UK regulations. For equipment
Compliance of the large “system” should be to be legally sold within Europe, the equipment
demonstrated either by testing and/or by must comply with harmonised standards, reference
presentation of a rationale as to why the system to which has been published in the Official Journal
complies. of the European Union (OJEU). Again, reference
to the European Union’s website will help identify
those standards that have been referenced in the
OJEU.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Table 7 EMC standards

BS EN 12015:2004 Product family standard for lifts, escalators and passenger conveyors – emissions
BS EN 12016:2004 Product family standard for lifts, escalators and passenger conveyors – immunity
BS EN 45502-2-1:2003 Active implantable medical devices. Particular requirements for active implantable medical
devices intended to treat bradyarrhythmia (cardiac pacemakers)
BS EN 62040-2:2006 Uninterruptible power systems (UPS). Electromagnetic compatibility (EMC) requirements
BS EN 50098:1999 Customer premises cabling for information technology; Part 1: 1999: ISDN basic access; Part 2:
1996: 2048 kbps ISDN primary access and leased line network interface
BS EN 50130-4:1996 Product family standard – Immunity requirements for components of fire, intruder and social
alarm systems
BS EN 50173-1:2002 Information technology. Generic cabling systems. General requirements and office areas
BS EN 50174:2001 Information technology. Cabling installation; Part 1: 2001: Specification and quality assurance;
Part 2: 2001: Installation planning and practices inside buildings; Part 3 (draft for comment):
2002: Installation planning and practices outside buildings
BS EN 50310 2000 Application of equipotential bonding and earthing in buildings with information technology
equipment
BS EN 55015:2001 Limits and methods of measurement of radio disturbance characteristics of electrical lighting and
similar equipment
BS EN 55022: 1998 Information technology equipment – Radio disturbance characteristics – Limits and methods of
measurements
BS EN 55024: 1998 Information technology equipment – Immunity characteristics – Limits and methods of
measurements
BS EN 60947 (1996–2003) BS EN 60947: Specification for LV switchgear and control gear (8 parts)
BS EN 61000-3-2: 2006 BS EN 61000-3-2:2006. Electromagnetic compatibility (EMC). Limits. Limits for harmonic
current emissions (equipment input current ≤ 16 A per phase)
BS EN 61000-3-3:1995, Electromagnetic compatibility. Limits. Limitation of voltage fluctuations and flicker in LV supply
IEC 61000-3-3:1994 systems for equipment with rated current ≤ 16 A
BS IEC 61000-3-4: 1998 Electromagnetic compatibility. Limits. Limitation of emission of harmonic currents in LV power
supply systems for equipment with rated current greater than 16 A
BS EN 61000-3-11:2001, Electromagnetic compatibility. Limits. Limitation of voltage changes, voltage fluctuations and
IEC 61000-3-11:2000 flicker in public LV supply systems – Equipment with rated current ≤ 75 A and subject to
conditional connection
BS EN 61000-6-1: 2001 Generic Standards – Immunity for Residential, Commercial and Light Industrial Environments
BS EN 61000-6-2:2005 Electromagnetic compatibility (EMC). Generic standards. Immunity for industrial environments
BS EN 61000-6-3: 2001 Generic Standards – Emission for Residential, Commercial and Light Industrial Environments
BS EN 61000-6-4: 2001 Generic Standards – Emission for Industrial Environments
BS EN 61547:1996, IEC Equipment for general lighting purposes. EMC immunity requirements
61547:1995
BS EN 61800-3: 2004 Adjustable speed electrical power drive systems – EMC product standard including specific test
methods

78
 11 Electromagnetic compatibility

Table 8 EMC standard by equipment type

Type of equipment Applicable standard(s)
Access control units Generic for emissions BS EN 50130-4:1996
Air handling units Generic
Audio amplifiers Generic
Battery charger Generic
Boilers Generic
CCTV control panels Generic but not BS EN 55022/BS EN 55024
Chillers Generic
DRUPS BS EN 62040-2
EDS BS EN 61000-6-2
BS EN 61000-6-4
Extract fans Generic
Fire detection and voice alarm system BS EN 50130-4:1996
BS EN 50270
HV switchgear Generic
HVAC control system BS EN 60730-2
ISM equipment BS EN 55011
BS EN 55014-1
BS EN 55014-2
IT equipment used in BMS/EMS, CCTV, access control, BS EN 55022
intruder alarm, and fire detection systems BS EN 55024
Lifts BS EN 12015:2004 (emission)
BS EN 12016:2004 (immunity)
Lighting equipment BS EN 55015:2001
BS EN 61547:1996, IEC 61547:1995
LV switchgear BS EN 60947
Power distribution units Generic
Water pumps (for either potable or fire water) Generic

EMC phenomena the first reference is to EMC standards. Those

standards that are specific to the health
11.9 EMC phenomena are divided into radiated environment will have included in them the
and conducted aspects, and a special case for relevant phenomena that the equipment can be
electrostatic discharge (ESD), that is, radiated expected to operate to.
emissions, conducted emissions, radiated
immunity and conducted immunity.
11.11 Given the information in Tables 9–11, the system
Standards and levels designer who is advising on the type of equipment
11.10 In any analysis of the healthcare environment for procurement and/or installation should advise
or for setting procurement requirements for the suppliers of the relevant standards for any
equipment to be installed in those environments, compliant equipment.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations
Table 9 Radiated emissions: levels for some typical equipment standards
Specification Frequency (MHz) Limit (dB(μV/m)) Comments
BS EN 61000-6-3 30–230/ 30/37
230–1000
BS EN 61000-6-4 30–230/ 40/47
230–1000
BS EN 12015 30–230/ 40/47
230–1000
BS EN 50130-4 30–230/ 30/37
230–1000
BS EN 55015(1) 0.009–0.07 88(1) Measured in a 2 m diameter loop
0.07–0.15 88–58(1) (2)
0.15–2.2 58–26(1) (2)
2.2–3 58(1)
3–30 22(1)
BS EN 55022 30–230/230–1000 30/37 Class B
40/47 Class A
BS EN 62040-2:2006 30–230/230–1000 30/37 Class B
40/47 Class A
BS EN 61800-3 30–230/ 30/37 First environment
230–1000 40/47 Unrestricted <25 A
40/47 Restricted <25 A
40/47 Unrestricted >25 A
Restricted >25 A
Notes:
(1) conducted emissions on AC port
(2) conductor disturbances

Table 10 Conducted immunity levels for some typical equipment standards

Specification Frequency Modulation Applied test level (V) Comments
(MHz) (% AM) Power port Signal port Functional
earth
BS EN 61000-6-1 0.15–80 80% 1 kHz 3 3 3
BS EN 61000-6-2 0.15–80 80% 1 kHz 10 10 10 ITU bands: 3 V
BS EN 12016 N/A N/A N/A N/A N/A
BS EN 50130-4 0.15–100 80% 1 kHz 10 10 N/A CCTV: 3 V
BS EN 55024 0.15–80 80% 1 kHz 3 3 N/A
BS EN 62040-2:2006 N/A N/A N/A N/A N/A
BS EN 61800-3 N/A N/A N/A N/A N/A
BS EN 61547:1996, 0.15–80 80% 1 kHz 3 N/A N/A
IEC 61547:1995

Table 11 Electrostatic discharge (ESD) test levels for some typical equipment standards
Specification Discharge (kV)
Air Contact
BS EN 61000-6-1 8 4
BS EN 61000-6-2 8 4
BS EN 12016 8 4
BS EN 50130-4 8 6
BS EN 55024 8 4
BS EN 62040-2:2006 8 6
BS EN 61800-3 8 6
BS EN 61547:1996, IEC 61547:1995 8 4
80
 11 Electromagnetic compatibility

Electromagnetic environment within the building has the potential to affect the
performance of other installed systems.
11.12 The environment within a building is made up of
sources that are located within the building (that 11.16 The electromagnetic environment should be
is, equipment that is the source of electromagnetic divided into zones where equipment will be
radiation, for example transformers, MRI suites) compatible for both emissions and immunity. At
and sources that are generated externally to the boundaries, a risk assessment will be required to
building. The external sources will usually be determine whether mitigation measures need to be
intentional transmitters, together with strong implemented to reduce the potential cross-
radiating unintentional transmitters such as boundary interference.
railways (see Table 12).
EMC control for power systems
Table 12 Electromagnetic sources 11.17 Uninterruptible power supplies (UPS) and battery
Frequency (MHz) Description rectifiers are a source of mains-injected harmonic
interference. For this reason, they should be
70–85 Fire and rescue radio
located in zones away from equipment which may
122.15 Air band communications be affected by their emissions, for example IT
153.675 Pagers systems.
170.65 PMR mobiles
11.18 Power transformers are a concentrated source of
197.325 PMR mobiles low-frequency magnetic interference. For each
427.7 PMR base station type, their location and cubicle screening should
461.65 PMR mobiles be considered in relation to sensitive equipment
380–420 TETRA (that is, those likely to be affected by radiated
magnetic fields). This particularly applies to
450 Police radio
theatres where cathode ray tube systems are used,
486–606 TV broadcasting band as on-screen distortion effects will occur.
903–951 GSM
11.19 The influence of the transformer and the route of
1812.75 DCS base station
unscreened or single-core main LV cables should
2144.25 3G UMTS base station not be ignored. There may be magnetic coupling
11.13 Emergency services’ mobile units (PMR and with the steel and reinforcement bars of the
TETRA) will also be present, as these operate building structure, thus inducing a network
at much higher transmission levels than GSM of currents flowing in the steel to earth with
mobiles and can be expected to be present in the associated localised secondary magnetic fields.
non-specialist areas of healthcare premises, that is,
clinical risk categories 1–3 inclusive. Building and EMC control for cables and cable-containment
system control panels located in corridors will be systems
subject to these higher levels. 11.20 Single-phase power cables, including power feeds
11.14 In hospitals particularly, cable lengths in excess of and lighting circuits, carrying up to 250 V should
30 m, running either horizontally or vertically, will not be grouped with sensitive cables (that is, data
be encountered. These lengths are ideal for picking cables).
up and conducting frequencies up to around 11.21 No data, telecommunications or any other
400 MHz. System designers should always sensitive cabling should be placed near three-phase
consider the use of screened cables, metal trunking cables, as these are normally used for heavy
and cable ladders to minimise interference into electrical inductive loads, for example air-
plant or building systems equipment. conditioning, welding equipment and motors.
11.22 All cabling should avoid any close proximity to
Designing systems for EMI control radio or television transmitters, beacons and
11.15 Electromagnetic interference (EMI) does not overhead transmission lines.
stop at interfaces, either conducted on cables
11.23 Cables carrying high-level impulse energy produce
or radiated. The positioning of M&E systems
a large frequency distribution of disturbances due
to their fast rise times. Special precautions need to

81
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

be taken with these types of cabling: efficient 11.32 If a large aperture is unavoidable, the principle of
screening, clean earthing at both ends, and an using a “waveguide beyond cut-off ” can be used
increase in the separation with adjacent cables (see Figure 28). Using this type of aperture will
would need to be implemented. enable cables etc to pass through, although
attenuation performance is reduced. Multiple
11.24 All cables should be terminated whenever possible
waveguides should be used (see Figure 29) where
in accordance with their intended terminating
many services need to pass through a screening
impedance.
shield.
11.25 The characteristic impedance of cables should be
selected to match closely the impedance of the Figure 28 Single waveguide
terminating equipment. This reduces the
amplitude of standing waves created by reflections
due to mismatches in impedance transition.
L
11.26 All power-cable screens or armour should be
bonded at both ends of the run to an earth plate
using 360° peripheral glands.

EMC control for general systems W

11.27 Personal transmitters/receivers, main transmitters
and local radar devices should be evaluated to For f << fc
ensure that they do not cause random operations AdB = 27 L/W
or failure of electronically controlled equipment.
Personal transmitter/receivers are particularly likely Figure 29 Waveguide array
to cause this problem.
11.28 Checks should be made with these devices on all
new plant installed, at a convenient time, to ensure
there is no susceptibility.

Intentional apertures
11.29 Apertures are always required in rooms to allow
services to enter and leave. Rooms that are
required to have a screen to prevent
electromagnetic interference (EMI) in the
healthcare environment, rarely require the same
performance as a screened room used for EMC 11.33 Where screened cables need to penetrate through
measurements. However, the same techniques for a screen, conductive cable penetration blocks (see
screening apertures can be used to allow services to Figure 30) should be considered to maintain the
enter. continuity of the cable screens and screened room.

11.30 Where holes in the shield are essential for such Figure 30 EMC compliance measures
items as ducting, pipework or cables, the hole
should be filled by placing a mesh screen over the
hole and ensuring that the duct, pipework or
cables are electrically connected to the mesh
screen.
11.31 A mesh screen has an attenuation determined by
the size of the largest hole in the mesh. It is better
to use a number of similar size apertures to run
multiple items through the mesh, than one large
aperture that takes all the items required to go
through the screen.

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 11 Electromagnetic compatibility

Cable segregation and separation 11.38 Using trays or racks of sufficient wall thickness
to separate cables can provide both PEC and
11.34 To reduce the possibility of power-cable to signal- reduction in crosstalk. They can often be laid next
cable coupling and the associated EMC risks, the to each other. Another solution is to keep some
best approach is to use separation between power distance between shallow conduits for the different
and sensitive service cables. It is advised to run types of cable, for example by stacking them (see
fire alarm cables in a separate conduit from other Figure 31).
service types. Signal and telecommunications
cables should not be run in the same tray as power Figure 31 Stacking cable trays to avoid crosstalk
cables. Table 13 indicates the separation distances
between power and signal cables, where the cables
are not screened or screened at their respective Control
voltage level.
11.35 Note the screening should be bonded to an earth Measurement

return at both ends of the cable.

Auxiliary
Cable screening, trunking and trays
11.36 Various types of cable tray or conduit may be used
Power low voltage
and run in parallel over an appreciable distance.
The crosstalk between the cables they contain Power high voltage
may be important. The recommended separation
distance between the cables in the trays depends
on two parameters:
• the quality of the cable tray as protective earth 11.39 Cable-tray stacking achieves a combination of
conductor (PEC); separation of segregated cable types with the
• low transfer impedance (high shielding additional benefit of screening introduced by the
effectiveness). trays themselves. Solid trays with no gaps are the
ideal tray type for this application. Trays often
Crosstalk characteristics have slots for easy attachment of cables; the most
beneficial of these are those with a short slot
11.37 Cables with a low crosstalk may require shielding parallel to the axis of the tray. Those with slots
against the (magnetic) fields causing the crosstalk perpendicular to the tray axis should not be used.
currents.
11.40 Caged trays constructed with large gaps in the
screen should certainly not be used where

Table 13 Recommended minimum separation distance between power and signal cables
Not enclosed environment Minimum separation distances for various power cables (mm)
for example on tray/basket
Signal cable No metallic sheath or screen for example twin Steel wire armoured MICC
and earth or singles
Plain 150 125 Touching
UTP 75 below 100 MHz 50 Touching
125 above 100 MHz
Screened Touching Touching Touching
Enclosed environment for example trunking Metal separator Plastic separator
Unscreened power lines, or electrical equipment and unscreened data IT lines 150 300
Unscreened power lines, or electrical equipment and screened data IT lines 30 70
Screened power lines, or electrical equipment and unscreened data IT lines 2
Screened power lines, or electrical equipment and screened data IT lines 1 2

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

electromagnetic screening is an issue, as they offer 11.44 For right-angle and corner interfaces, the same
no screening benefits and are generally insufficient principles should be applied with L-shaped joints
as parallel earthing conductors. attaching interconnecting sections.

Trunking and tray interconnection and termination Using conductive structural supports as runs for
cables
11.41 When a metallic cable tray or trunking system is
implemented, inevitably sections will need to be 11.45 Metallic structural support elements in buildings
interconnected for extended runs. Particular care can also serve EMC objectives where room for
will be necessary in order to maintain electrical cable trays or trunking is limited. Steel beams of
continuity between the various sections. Ideally, L-, H-, U- or T-section can form a continuous
the parts should be welded over their full earthed structure that offers relatively large cross-
perimeter, although it is recognised that this is not sections and therefore low impedance and large
always achievable in practice. Riveted, bolted or surfaces with many potential intermediate
screwed joints are adequate if the contact surfaces connections to earth. Cables can be laid against
are good conductors (there should be no insulating such beams as shown in Figure 33.
coating or paint). Ensure that they are safeguarded
against corrosion and that good electrical contact Identification of critical systems
between the separate sections can be maintained.
11.46 Mechanical and electrical equipment being
11.42 It is important that the shape of the metallic procured currently will have been designed to
section should be maintained over the full comply with either the light or heavy industrial
length of the run. Bonding via a short earth wire generic or product-specific standards. Such
connection between two sections of the tray or equipment will be generally immune when located
trunking system may have a low dc resistance, but in its intended environment. Designers should
will have high impedance to high-frequency (a few identify environments where levels higher than
MHz upwards) currents. those specified in standards will be encountered,
11.43 This means that for extended runs the centremost and apply mitigating measures, for example
sections are effectively floating at high frequencies, prevention of the use of mobile phones close to
thus reducing performance. This has both control systems while screening enclosure doors
personnel safety and EMC implications. are open. Many M&E systems not normally
Figure 32 shows the recommended practice for considered critical are critical when their
interconnecting cable trays and trunking systems. misoperation causes reduced operational efficiency,

Figure 32 Recommended interconnection of cable trays and trunking

Unacceptable: earth wire

connection between sections

Not recommended: plate

connection between sections

Best practice: plate connection

on three sides (for trunking 360°)
termination recommended

84
 11 Electromagnetic compatibility

Figure 33 Location of cables inside metallic structural supports

Recommended Acceptable Not recommended

for example heating and ventilation systems, fire or where a high continuity of supply is required by
alarm systems. the application (for example in hospitals) or by
national regulations.
Earthing and bonding 11.51 Non-linear loads (fluorescent lamps, switched-
11.47 Earthing and bonding (or equipotential bonding) mode power supplies etc) on distribution networks
is often confused. The terms are defined in the can generate harmonic currents which may
following way. Earthing is the connection of the overload the neutral conductor. Correction
exposed conductive parts of an installation to methods for such systems are provided in
the main earthing terminal of that installation. Chapter 5. The controlled earth return current
Bonding is an electrical connection maintaining of a TN-S system is shown in Figure 34.
various exposed conductive parts and extraneous 11.52 A clean earth will utilise dedicated earth
conductive parts at substantially the same conductors, which are fed back to the main earth
potential. Earthing arrangements are described in terminal (MET).
detail in Chapter 13.
11.53 The intent for any earthing system should be to
11.48 Earthing is also used to contribute to the maintain a low impedance at most harmonic
mitigation of disturbances for installations with frequencies. This means maintaining an
sensitive and interconnected electronic and equipotential between all the cabinets in the data
electrical systems. These requirements, that is, centre. The likelihood that this can be achieved is
shunting of unwanted power-frequency and high- increased by a localised connection to the mesh
frequency currents and lowering the voltage bonding network of the room.
difference between two points of the system,
are the same for lightning, personnel safety, 11.54 The mesh bonding network is intended to be
installation protection and EMC. Each one of created by the use of interbonding sections of the
these considerations places constraints on the 1.2 m2 earth mesh (25 mm2 copper straps) in the
design, since lightning and personnel safety dictate raised flooring using bonding straps between 50%
the design of the earth electrode; safety and of the pedestal supports for each floor panel. At
installation protection dictate the size for the least a 16 mm2 earthing conductor should be
earthing conductors; and EMC behaviour connected to the nearest point on the mesh
requirements determine the layout of the earthing bonding network from the local distribution
network. board’s earth bar. All underfloor cable trays and
risers which pass through the raised floor should
11.49 Given the above, the following EMC be bonded to the earth mesh using an earth strap.
implementation rules are recommended. All surrounding metallic cable trays, conduits,
11.50 Wherever possible, the TN-S system should be pipework, risers and ducts in the roof void should
used. Exceptions exist with IT configured systems, be interbonded using earth straps or preferably

85
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

solid metal strips which are galvanically compatible

with the contact metal. Galvanic potentials should
not exceed 300 mV otherwise there is a potential
risk of long-term degradation of the bonds.

Figure 34 Controlled returned current flow in a TN-S installation

L
N

PE

TN-S earthing network

Controlled flow of
return currents

Cabinet frame
ground

Main distribution board

earth bar

Main building
earth mat

Common bonding network

including trays, pipes and ducts.
Connection via PE

86
12 Wiring systems

12.1 All wiring systems will be of a form defined in Protected extra low voltage systems
the IEE Regulations BS 7671:2001. Where
appropriate, the primary internal distribution will 12.6 In general, PELV systems as described by the IEE
be a three-phase HV network. The next tier of Regulations BS 7671:2001 are not covered by this
distribution will be a mixture of three-phase and Health Technical Memorandum. PELV systems
single-phase LV systems. may be used within medical locations Group 1
or 2. PELV systems may also be considered
appropriate for wet areas such as kitchens, trolley
High voltage wash-down areas or mortuaries.
12.2 HV wiring systems will, in general, be used only
12.7 The nominal limit for PELV is 50 V ac or 120 V
for distributing high power around the site. This
ripple-free dc. However, as prescribed by BS EN
Health Technical Memorandum does not promote
60601-1, IEC 60601-1, this limit is reduced to
the use of HV equipment. However, consideration
25 V ac and 60 V ripple-free dc when these systems
may be given to the use of HV chiller plant where
are used in medical locations of Group 1 and
the chillers have a high mechanical duty and hence
Group 2.
electrical load requirement. The manufacturer’s
advice should be used.
Separated extra low voltage systems
12.3 Certain radiographic and diagnostic equipment
generates a high voltage as part of the equipment 12.8 The nominal limit for SELV is 50 V ac and 120 V
process. Such applications are not part of the fixed ripple-free dc. However, as prescribed by BS EN
wiring systems and are therefore not covered by this 60601-1, IEC 60601-1, this limit is reduced to
Health Technical Memorandum. 25 V ac and 60 V ripple-free dc when these systems
are used in medical locations of Group 1 and
Group 2.
Low voltage
12.9 Normally, protection by insulation of live parts
12.4 All LV systems will be installed as TN-S systems
and by barriers or enclosures applies only to SELV
as defined by the IEE Regulations BS 7671:2001,
systems where the nominal voltage exceeds 25 V ac
unless the wiring is of a type defined below.
or 60 V ripple-free dc. Where the SELV system is
within a medical location Group 1 or 2, protection
Medical IT by insulation of live parts and by barriers or
12.5 The system may also be known as an isolated power enclosures should always be provided. Placing live
supply (IPS; see Definitions). The system will parts out of reach, only, is not acceptable within
include a monitoring device with an alarm for medical locations Group 1 or Group 2.
disconnection, insulation failure, overload and high
temperature. Medical IT wiring systems will in
general be limited to a medical location of Group 1
or Group 2 areas (see Definitions) and post-
mortem facilities.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

13 Earthing

13.1 The earthing arrangements for the full electrical at ground or at subterrain level, the substation
system should comply with the requirements of exposed metalwork will be earthed via a copper
BS 7430:1998 and BS 7671:2001. In general drain wire of the HV network cable.
terms, the earthing arrangements will take the
form of a TN-S system. The exception to this High-voltage network cables
fundamental requirement will be that certain areas, 13.4 All HV cables forming part of the HV distribution
defined in this chapter, will meet the earthing network should have a copper wire as part of the
requirements of an IT-earthed system. armouring of the cable. HV cable glands should be
rated above the prospective fault current of the
High-voltage earthing methods system to which they are assembled. The glands
13.2 Where the PES is rated at high voltage (11 kV) and should have integral earth lugs from which
the termination point is at low voltage (0.4 kV), equipotential bonding copper strip connects to
the responsibility of the HV earthing will lie with the main copper earth bar. Consideration may be
the DNO. Where the healthcare organisation given, if required, to the cable armour secured at
meters and purchases electricity at a high voltage, the cable gland being isolated or separated from the
but has no internal HV network, the DNO will equipment by an island-type insulating gland. In
remain responsible for the earthing provision of order to prevent dangerous high earth fault currents
the HV earthing. Designers will need to liaise circulating within the structure of the healthcare
with the DNO whenever any new development or facility, the HV cable earths should not come into
significant internal remodelling of the healthcare direct contact with any exposed conductive part of
facility’s electrical services is undertaken. Managers the facility.
of healthcare premises will be required to provide
the DNO with full access rights to any part of the High-voltage generator earths
facility that they may require to access in order to 13.5 All HV generators will be earthed. Designers
maintain the HV earthing systems. Where the should evaluate the earthing by a neutral earthing
electrical distribution strategy includes an HV reactor or an earthing transformer. Thought should
network, the designer of the electrical system be given to the potential for circulating neutral
should ensure that the electrical systems are currents and/or harmonic currents in the delta-
adequately earthed. Where the healthcare facility wound generator stator, and how these may
includes more than one HV substation, each be negated with the addition of an earthing
substation should be linked by an HV earth transformer. The generator earthing arrangements
conductor. This will be particularly important should ensure that an adequate fault current can be
where a single building is served from more than developed to operate any protective device within
one HV substation. the electrical network. Figures 35 and 36 show
13.3 A suitably-sized copper conductor will collectively typical high-voltage generator earthing
bond all exposed metalwork associated with HV configurations.
equipment at an HV substation. The cross-bonding
conductor should have a green-yellow sheath and
be buried at a depth of 600 mm within the
substation area. Where the substation is not

88
 13 Earthing

Figure 35 HV generator earths – island mode

Interlock

11 kV generator

PES Site
11 kV 11 kV
supply network

Earth
resistor
if required

Low-voltage main earthing methods primary and secondary electrical distribution (see
Chapters 7 and 8) should have a dedicated MET
13.6 Where the PES is rated at low voltage (0.4 kV), directly connected to the earth electrode with the
designers will liaise with the DNO to determine earthing conductor. The earthing conductor will
the responsibility of earthing the PES supply cable, be sized to carry, without risk of danger, the
which will usually be with the healthcare greatest earth fault current and earth leakage
organisation. currents likely to occur, having due regard for the
13.7 A suitable supplementary equipotential bonding thermal and electromechanical stresses. An
copper conductor will collectively bond all exposed earthing conductor and demountable link will
metalwork and conductive parts associated with interconnect each MET where the substation has
the LV switchpanels in the switchroom to the local multiple distribution transformers.
ERB. The cross-bonding conductor will be a bare 13.8 Where appropriate, any fixed equipment clean
hard-drawn copper tape of a minimum cross- earths will be bonded to the MET of the respective
sectional area of 50 mm by 6 mm and in LV substation either directly or via an ERB in the
accordance with BS 7671 IEE Regulation 543. LV switchroom.
A suitable copper earth cable or tape will bond
each ERB to the respective LV substation main 13.9 Where appropriate, any information management
earthing terminal (MET). A suitable and technology clean earths will be bonded to the
supplementary equipotential bonding copper MET of the respective LV substation.
conductor will collectively bond all exposed 13.10 Where the LV and HV system earths are separated,
metalwork and conductive parts associated with the resistance of the LV earth electrode should be
the LV substation to the local MET. All extraneous less than 20 Ω.
metalwork will be cross-bonded and either directly
or indirectly connected to the MET. The MET 13.11 Where the electrical system includes both HV and
will be directly connected to the star point of the LV networks, designers may wish to interconnect
respective distribution transformer secondary the earths from the two earthing systems. Subject
winding. All transformers associated with the to approval of the DNO, this may be achievable if

89
90
Figure 36 HV generator earths – parallel operation (TN-S)

When the bus tie is closed, only one neutral earthing transformer will be connected to the LV system

IL

Earthing contactor interlock so that

the contactor cannot be closed
during parallel operation with
the incoming mains supply
Site PES PES Site
11 kV 11 kV 11 kV 11 kV
network supply supply network
11 kV 11 kV
generator generator
Neutral
earthing Neutral
transformer earthing
transformer
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations
 13 Earthing

the combined earth resistance is less than 1 Ω, 13.20 Where the radiographic room has high
and any earth fault current does not give a rise of electromagnetic field radiation, such as in a
a 430 V potential in the parallel earth circuits. magnetic resonance imaging (MRI) room, the
Where the combined earth resistance cannot be room will be provided with a Faraday cage to
reduced to 1 Ω, the LV earth electrode should be isolate any such magnetic fields from the building
at least 3 m from the HV earth electrode and/or structure and surrounding rooms. The ERB in
any HV extraneous conductive metalwork. these rooms will be made of a non-magnetic
material and housed in a non-magnetic enclosure
Low-voltage generator earths (usually clean ABS). The ERB will be directly
13.12 It should be ensured that an adequate fault current
bonded to the room side of the Faraday cage.
can be developed to operate any protective device An earthing conductor will be directly connected
within the electrical network. The earthing between the Faraday cage and the MET of the
arrangement may require an earthing reactor or respective substation.
earthing transformer. 13.21 The Faraday cage will have suitable apertures for
13.13 The resistance of the generator star-point-
the provision of any EMC filter equipment for
connected earth electrode should be less than conductors of any electrical or communication
20 Ω. system. The radiographic equipment
manufacturer/supplier should specify the detail of
13.14 Figure 37 shows a typical earthing configuration. the filter equipment.
13.22 All fixed electrical equipment connection points
Switchroom earths will be positively bonded to the ERB with a
13.15 All LV distribution switchrooms should have a resistance no greater than 0.1 Ω.
visible earth reference terminal made from hard-
13.23 The electrical installation for radiographic
drawn bare copper.
diagnostic and imaging facilities should comply
13.16 A suitably-sized copper conductor will collectively with the Medicines and Healthcare products
bond all extraneous and exposed conductive parts Regulatory Agency (MHRA) document ‘Medical
associated with the switchroom LV switchgear to Electrical Installation Guidance Notes’
the local ERB. The circuit protective conductor (MEIGaN).
(CPC) from each final distribution board should
be bonded to the local ERB, and covered by a Medical IT or isolated power supply
green-yellow sheath. All extraneous metalwork will earths
be cross-bonded and either directly or indirectly
connected to the ERB. 13.24 Isolated power supply (IPS) circuits should have an
IT earthing system as defined by BS 7430:1998
13.17 Where appropriate, any fixed equipment clean and BS 7671:2001.
earths will be bonded to the ERB of the respective
LV switchroom. 13.25 In all areas defined as a clinical risk Category 4 or
5, an ERB will be provided adjacent to the local
Earths for radiographic rooms final distribution board of the IPS.

13.18 In general terms the designer of the electrical 13.26 The IPS circuits will be bonded to a protective
hard-wired system will have a responsibility for earth terminal (PET), which should be easily
the earthing system to the ERB within each accessible from the IPS distribution board and
radiographic room (see also paragraph 13.23). IPS isolation transformer housing. An earthing
conductor will be directly bonded between the
13.19 In all radiographic rooms an ERB will be provided. PET and a local ERB. Both the PET and ERB will
Designers should liaise with the radiographic be visible and accessible by authorised people only.
equipment manufacturer to establish the size of See Health Technical Memorandum 06-02 –
the earthing conductor and associated ERB. An ‘Electrical safety guidance for low voltage systems’.
individual earthing conductor will be directly
connected to the ERB (of each radiographic room) 13.27 Where the IPS serves a diagnostic room, the PET/
and the MET at the respective distribution board. ERB should be located within the room (see the
MEIGaN).

91
92
Figure 37 LV generator earths (TN-S)

When the bus tie is closed, only one neutral earthing transformer will be connected to the LV system

IL

Site PES Site

0.4 kV 0.4 kV 0.4 kV
network supply network
0.4 kV 0.4 kV
generator generator
Neutral Neutral
earthing earthing
transformer transformer
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations
 13 Earthing

Microshock S = the nominal cross-section area of the conductor

in mm2;
13.28 Microshock is the passage of a low level of
electricity through the body which causes no I = fault current;
perceptible sensation. The threshold of sensation is
t = the operating time of the disconnecting device
at about the 1 mA level. The subject cannot detect
in seconds corresponding to the fault current;
currents below this level. These low-level events
are of no consequence unless the current passes k = a factor taking account of the resistivity,
through the cardiac muscle, in which case temperature coefficient and heat capacity of the
ventricular tachycardia or ventricular fibrillation conductor material, and the appropriate initial
may be triggered. Currents of the order of 10 μA and final temperatures.
can be enough to initiate ventricular fibrillation. 13.33 Where circuit cables or conductors have an
13.29 A patient undergoing any procedure which integral metallic sheath, the sheath will not be used
involves the placing of an electrical conductor in as the sole earth return path. Designers should
the central circulatory system is particularly at risk. consider the use of multicore cables with an earth
In this context, an electrical conductor includes conductor, or where this is not possible, installing
insulated wires such as cardiac pacing electrodes or a separate CPC.
intracardiac ECG electrodes, or an insulated tube
(catheter) filled with conducting fluid inserted into Functional earth
the central circulatory system.
13.34 Functional earthing systems are a method used to
13.30 In order to limit any potential rise due to the provide a zero reference point or a signalling path
effects of leakage current, the voltage between the for communications equipment. A functional
hard-wired system and the ERB should not be earth does not strictly provide any protection
greater than 20 mV. A further voltage of 30 mV against electric shock or danger. Functional
is allowed between the exposed conductive parts earths should comply with the requirements of
of the medical equipment and its supply cord Chapter 47 of the IEE Regulations BS 7671:2001.
(BS EN 60601-1, IEC 60601-1). This means
13.35 The functional earth conductor may be connected
that the maximum obtainable voltage between
directly or indirectly to the main earthing terminal
the exposed conductive parts of the medical
(MET) in an installation where earth currents flow
equipment and the ERB should not exceed 50 mV.
due to the normal function of load equipment.
To achieve this low voltage the maximum
resistance between the socket-outlet terminals, 13.36 Functional earths for telecommunication systems
fixed equipment terminal or extraneous metalwork should be installed with a cream-coloured sheath.
should be 0.2 Ω (0.1 Ω from any point to the The telecommunications engineer should
ERB). determine the functional earth conductor size and
install it in accordance with BS 6701.
13.31 Figure 38 shows a typical earthing arrangement.

Circuit protective conductors Monitored earthing systems

13.37 Where it is assessed that a high degree of earth
13.32 All parts of the LV distribution including final
integrity is required, an earth monitoring system
circuits will have a separate circuit protective
provides a means of maintaining a high degree of
conductor (CPC). The size of the conductor will
confidence in the impedance level of the protective
be assessed from the prospective short-circuit
conductor from the monitoring unit to the remote
current (PSCC) and the current-carrying capacity
protected equipment. The monitoring unit may be
of the conductor. The assessment will take the
connected between the source of energy (if
form of the calculation:
accessible) and the equipment to be protected. The
2 source of energy may be, for example, a generator
S=√I t or a transformer. It is therefore essential that any
k
plug, socket and flexible cable provide not only the
(given in BS 7671:2001) main protective path but also a return path, which
is usually known as the pilot conductor.
where

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94
Figure 38 IPS theatre earthing arrangement

Insulation
monitor

TEMP
SENSE
PTC LOAD
CT

Isolation
transformer

To Local
Distribution
Board or UPS

Medical IT (IPS)
Earth Bar

Alarm panel
Medical gases & boom

**
Earth
Reference
Antistatic grids
Bar (ERB)
Taps & pipes
*
*
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

** Note if no IPS installed the ERB should then * If installed * If required

be connected to the local distribution board PET
 13 Earthing

13.38 The system proves or monitors the protective Lightning protection system components
conductor and pilot conductor loop in a flexible
13.42 The LPS consist of three principal components as
trailing cable supplying transportable or mobile
listed below.
equipment, the proving or monitoring unit
being arranged to disconnect the supply to the Air terminals – finials
equipment at the point of connection of the
trailing cable to the wiring installation. A wall- 13.43 The air terminals consist of a copper or aluminium
mounted protective-conductor proving or tape installed around the roof, and cross-bonded to
monitoring unit is directly connected to a section all exposed metalwork and plant. The air terminals
of the fixed electrical installation and is arranged to can be insulated with a PVC sheath, or bare. Metal
feed the flexible trailing cable, which may be roofs can be used as the air terminals providing
connected to the unit either by means of a plug that the conductivity and thickness of metal does
and socket or by a permanent connection. When not impede the discharge of the lightning strike.
connected in this manner, both the trailing cable Air terminals can be laid under a slate roof subject
and the equipment will be disconnected when the to building regulations and approval of the
unit operates in the event of failure of the building control officer.
monitored loop.
Down conductors
Lightning protection 13.44 The down conductors consist of aluminium or
13.39 The energy of a lightning flash can be very high, copper tapes clipped down the exterior façade of
with typical strikes currents in excess of 20 kA the healthcare premises at a maximum spacing of
within the UK. The damage from lightning strikes 20 m. Each down conductor should be cross-
can be very significant and blow electrical bonded to any exposed metalwork within 1 m of
components off the wall in the worst case. The the conductor and installed at least 1 m from any
damage may not be limited to items that have entrance way. Where the down conductor cannot
direct contact to the conducting path of the be installed on the external façade, a segregated
lightning protection system (LPS). Air (or other internal duct (conforming to the requirements of
conducting materials) are locally ionised around BS 6651:1999) may be utilised. Designers should
a lightning flash or conducting path, which can consider the location of data communications and
induce damaging currents in electrical equipment electromagnetic storage systems before using the
not directly connected to the LPS. In poorly steel structure of any healthcare building.
designed or installed LPSs, flashover of high
Earth electrodes
currents can occur between the LPS conducting
path and items not bonded to the LPS. Flashover 13.45 The earth electrode consists of a high-conductive
and/or ionised fields can cause damage to metal rod connecting the down conductor to the
communications systems and electromagnetically mass of earth. The resistance to earth of an LPS
stored data. network should not be greater than 10 Ω, with the
resistance of each individual electrode less than ten
13.40 BS 6651:1999 provides maps with the statistical
times the number of earth electrodes in the LPS.
frequency of lightning strikes and their energy
Where the soil resistivity is high, the earth
values throughout the UK.
electrode can consist of a high-conductive metal
13.41 Designers should carry out a risk assessment plate or mesh. In very poor soil resistivity areas,
based on the approach given in BS 6651:1999 to the local resistance can be improved by the use of
determine the need (or otherwise) of an LPS. The high-conductive concrete, such as bentonite, to
risk assessment should consider the location of the provide a bond between the electrode and mass of
healthcare premises (urban/rural, high/low), and earth. Each earth electrode should have a test point
the value of the equipment and data stored, as well close to ground level to aid the routine testing of
as any protection afforded by the proximity of the LPS. See Health Technical Memorandum
taller buildings. 06-01, Part B “Operational management”.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Ionised fields • Type B on each main distribution switchboard;

13.46 The effect of a lightning flash can ionise the air • Type C on the equipment itself (or the
surrounding the flash up to several metres. The equipment’s supply lead).
ionised air can give rise to high transient currents 13.48 Surge arrestors should be capable of attenuating
in cables and equipment. Designers should the induced ionised current such that the transient
consider the use of “surge arrestors” to mitigate the current is no greater than twice the steady-state
effects of such transient currents. normal supply current.
13.47 Surge arrestors can be located in one of three 13.49 Further details on the design and installation of an
locations: LPS can be found in BS 6651:1999 and BS EN
• Type A on all cables as they enter/leave a 50164-2:2002.
building;

96
14 Containment

14.1 Due diligence should be given to the protection of 14.4 The routing of any containment system should
all cable routes throughout the healthcare premises. preserve the recommended segregation distances
The various types of cable and busbar system are from other services, including other electrical
described in Chapter 15. This section addresses services. General containment routes should not be
the method of installation. Where the primary installed in lift shafts including dumb waiters (see
distribution cables etc are installed external to any BS EN 81 for more information). Containments
building, the cables should be direct-buried. Where should not be routed in laundry shafts.
the cable route passes under roadways etc, the
14.5 Where cable containments pass through a fire
cables should be installed in ducts of not less
compartment wall, a fire-stopping material will
than 100 mm diameter. Appropriate inspection
be used to make good the opening. A fire barrier
chambers should also be provided. Main cables,
should be installed within the containment
where direct buried in open ground, should be
(close to the fire compartment wall), where the
initially laid in, covered by sifted soil or sand, and
containment has an internal air space (for example
over-covered with reinforced interlocking fibre
trunking systems).
boards or concrete tiles to BS 2484:1995. Boards or
tiles afford protection against hand tools but not
against mechanical excavators. Red warning tapes Trenches, service tunnels and ducts
for HV cable routes and yellow warning tapes for 14.6 Where cables of any type and/or voltage band are
LV cable routes should be provided, and placed installed in a trench, service tunnel or duct, they
300 mm above the tile or cable. Accurately located should be installed on other containment types
concrete surface markers should be provided such as ladder rack or tray work. The arrangement
at intervals of approximately 6.5 m (where of the secondary containment should keep the
practicable) in open ground, road crossings etc cables out of any accumulated water and not
along the cable route, and at any change of impede access along the trench/tunnel/duct.
direction or entry to buildings. For information on Trenches, service tunnels and ducts should be
preventing damage to buried cables, see Appendix self-draining.
5 in BS 7671:2001.
14.7 Where the containment system is used for other
14.2 Electrical services of any type and/or voltage band services, the space should have natural ventilation.
installed on or in any containment type should The effect that other services, such as heating
have a current-carrying capacity for the grouping of pipes, in the same trench or duct may have on the
cables and local environment of the containment local environment should be taken into account.
system. Advice for de-rating a cable’s current-
14.8 Chapter 11 gives details on how cables should be
carrying capacity from the nominal values is given
arranged in voltage-band groups and the respective
in BS 7671 IEE Wiring Regulations. Additional
separation distances to achieve EMC.
information can be obtained from the cable
manufacturer. 14.9 HV cables should not be routed in enclosed areas
close to flammable gases such as piped medical
14.3 Where single-core cables are used for heavy-current
oxygen.
three-phase circuits, the cables of the three phases
should be laid in close proximity in trefoil or flat 14.10 When sizing a trench/tunnel/duct, consideration
formation, mechanically braced, and tied along for maintenance access should be assessed. The
the route. Eddy currents should be reduced, for recommended minimum clearances are given in
example by the use of non-ferrous clamps, fittings, Defence Works Functional Standard DMG 08:
spacers, non-ferrous gland plates and cable ‘Space requirements for plant access operation and
terminations. maintenance’. Manholes or access holes should be

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

provided for entry into cable tunnels and ducts. 14.19 Metallic ladder-rack, tray or basketry should
SELV lighting and a power supply at entrances to be electrically continuous and may be used as a
trenches and service tunnels should be provided. supplementary earth return path. Each length of
The provision for portable forced ventilation the containment should be mechanically joined
systems for use of maintenance staff may be with overlapping fillets on all three sides, and it is
required under the Health and Safety at Work etc recommended that these are supplemented with
Regulations. copper links to ensure earth continuity. Where
the installation topology prohibits the mechanical
14.11 On main cable routes where additional cables
jointing of the containment system, an earth cable
may subsequently be required, spare cable ducts,
(of 6 mm2 minimum size) should be used to
trenches or service tunnel space should be
provide the earth continuity.
provided.
14.20 In order to limit the effect of electromagnetic
14.12 Where HV cables are installed, they should be
radiation and reduce high fault currents, the
identified with “DANGER 11,000 Volts” notices
containment system should not form the only
provided at points where access to HV cables can
earth return path of any circuit on the
be obtained.
containment.
14.13 Open trenches (ha-has) are not a recommended
containment system for electrical services. Where Trunking – conduits
such trenches are used, the cables should have
additional mechanical protection. Additional 14.21 Steel trunking for cables represents the most
safety precautions for the public should also be satisfactory type of installation where a number
reviewed. of circuits can conveniently follow the same path.
Cable trunking is suitable for use in voids, above
suspended ceilings, in surface applications and
Ladder rack – tray – basketry in service risers. Trunking layouts should be
14.14 Steel cable trays and aluminium or steel ladder- predetermined and be dimensionally coordinated
rack can simplify installation where several cables with other building components to enable
are to be installed in close proximity. In damp standard prefabricated lengths to be used whenever
areas and in order to reduce the risk of corrosion practicable.
by electrolytic or water action, the containment
14.22 In installations with segregated essential and non-
should have a galvanised finish.
essential circuits, complete segregation of non-
14.15 Such containments should only carry cables of essential and essential subcircuit wiring is
one voltage band. Basketry can be considered for a desirable, but may not be possible in all instances.
mixture of cables at low voltage and voltage bands Where either the essential or the non-essential
below, provided all such cables are insulated to LV wiring is less than, say, 30% of the total wiring,
grades. separate containment systems may not be practical
14.16 Where these types of containment are installed in a
or justified. See Chapter 6 for more information.
common service route, each containment system 14.23 Circuits for emergency and escape lighting from a
should preserve the segregation of the various central battery system should always be segregated
voltage bands. The highest voltage band should from both essential and non-essential circuits
be installed on the lowest containment rail. The (guidance is given in BS 5266), and those circuits
containments should not be used to support any should be wired in an appropriate fire-resistant
other services. cable (see Chapter 15).
14.17 Manufacturers’ data should be used to assess 14.24 Extra-LV circuits can be installed with LV circuits
the maximum mechanical loading and fixing operating at the mains potential providing that the
arrangements of each containment system. insulation is equally rated to the maximum circuit
14.18 Such fixings should not be connected to any
voltage present. Wires of mixed service should be
demountable building element (for example suitably screened to reduce inter-circuit
ceiling tiles, wall partitions) or other engineering electromagnetic interference.
services. 14.25 Small TP & N cables installed in trunking should
be tied or clipped together in small convenient

98
 14 Containment

bunches. Groups of four single-core larger cables, 14.31 Where large quantities of data and computer
comprising a three-phase supply and neutral, equipment are installed, such as hub room and
should be laid in trefoil, interleaved at suitable floor-distribution patch cupboards, raised floors
intervals and labelled to assist identification of with removable square sections to permit sub-floor
circuits. The number and size of any cable bunch access for any later cable works are recommended.
in any trunking should not exceed that allowed in
14.32 Cables bunched in steel conduit of 20 mm,
the IEE Wiring Regulations Guidance Note 1
25 mm or 32 mm diameter are economical.
Selection, Appendix A.
Conduits less than 20 mm in diameter are not
14.26 Metal trunking should have a suitable anti-rust recommended.
finish (for example zinc-coated steel). For damp
14.33 The number and sizes of cables pulled into any
environments, galvanised trunking will provide
trunking and/or conduit should not exceed the
suitable protection.
circuit-loading guidance in the IEE Wiring
14.27 All equipotential contact surfaces should be free Regulations Guidance 1 Selection Appendix A and
of rust or corrosion or have an anodised finish to BS 7671 Chapter 52. The conduit system for each
ensure electrical continuity to earth and between distribution board should be kept separate, and
trunking sections. Tinned copper bonding links cables from different distribution boards should
should be used across all trunking section joints not be enclosed in the same conduit.
to complete the equipotential bond and earth
14.34 Conduit should be heavy-gauge quality to BS 31.
connection. The metallic trunking or metal
Enamel finish is satisfactory for indoor dry
conduits should not be used as the sole earth
locations. A passivated, galvanised, Class 4 finish
return path of the circuits within the containment.
should be specified where damp conditions are
14.28 All conduits and trunking systems should be likely. The use of only passivated, galvanised,
solidly fixed. Such fixings should exclude the use Class 4 finishes may be more cost-effective, as it
of demountable building elements (for example will negate the need of any retrospective touch-up
ceiling tiles, wall partitions) or other engineering painting of installed metallic conduits and
services. All fixing systems should be suitable for trunking.
the mass of the containment and wiring systems.
14.35 The effect of electromagnetic interference from
14.29 Approved non-flammable fire barriers and non-metallic trunking and conduits should be
penetration seals should be inserted in cable evaluated before they are used. Electromagnetic
trunking where it penetrates floors and partitions energy can be radiated from or absorbed by wiring
which are intended to form fire barriers (that is, systems unless they are adequately screened and
fire compartment walls). The outside of the earthed (see Chapter 11). Electrical containments
trunking should also be locally fire-insulated on should be resilient to effects from thermal and/or
both sides for 500 mm to prevent heat transfer by mechanical impact. The risks may be acceptable in
conduction along the metal trunking and the clinical risk category 2 and 3 areas, but is unlikely
passage of smoke. Unenclosed cables entering/ to be acceptable in clinical risk category 4 and 5
leaving barriers or seals should also be fire- areas (see paragraphs 4.12–4.34 for more
protected with ready-mixed inert material or information). It is best practice to use metallic
fire-resistant paint. trunking and/or conduits.
14.30 Fire barriers and penetration seals should be
provided for all cable installations entering/leaving Preformed wiring containment
switchrooms and plant cubicles where gland plate 14.36 Preformed wiring consists of wiring systems that
sealing is not provided. Underfloor trunkings or are manufactured off-site, and the individual
flush lay-in trunkings are a useful containment circuit conductors are installed in a form of
system for services to “island” (mid-floor area) containment. The containment in this case is
equipment such as radiography units and theatre generally a spiral metallic sheath or interweaved
tables, computer hub rooms and laboratory metal-and-paper spiral wrap. The system is
benches. In such locations, it is essential that the delivered with pre-made terminations and in
manufacturer, structural engineer and architect all standard lengths (primary runs are 40–50 m while
be consulted. final circuits are 3 m, 4 m and 5 m). Most systems
have a range of distribution boxes and fuse boxes

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

and therefore the installed system becomes a capacity. Consideration can be given to providing
spider’s web of cables. The systems allow for facilities for re-modelling by allowing other cabling
lighting and low power. Lighting circuits can have systems to be installed (retrospectively) from a
additional control wires for switching and lighting common distribution board used for preformed
control systems. Low-power circuits can be wired wiring systems.
as radial or ring circuits. Preformed wiring systems
tend to be sized at 50 mm diameter, while the final Circuit segregation
runs are typical 20 mm diameter. The number of 14.40 Designers should consider the holistic, coordinated
multi-circuits in any one length of preformed installation with all other electrical and non-
system may be dependent on the installation. electrical services within the installation area.
However, all conductors of any one circuit should Designers should obtain the manufacturer’s data
be installed in the same wiring lengths. The system on the system’s compliance with electromagnetic
sheath should not be relied on for any part of the radiation and absorption, which will need to be
earth loop impedance. specific for the particular environment (see
Chapter 11 for additional information).
Layout considerations
14.41 All primary preformed wiring systems that may be
14.37 Designers should consider how to provide for any
used should be secured on secondary containments
flexibility and/or spare capacity within the system.
such as tray work. Similarly, all final runs of
As the systems are preformed, it is not possible to
preformed wiring system should be solidly
cut into an existing length, and the installed routes
fixed. Such fixings should exclude the use of
follow the building room layouts. Designers
demountable building elements (for example
should therefore consider providing the spare
ceiling tiles, wall partitions) or other engineering
capacity at local distribution points or at the fuse
services. Clearly, all fixing systems should be
box. A spare capacity of 25% should be made
suitable for the mass of the preformed wiring
available, partly at the distribution boards and
system, and not leave any catenary effect.
partly at the ends of the primary routes.
Alternatively, consideration can be given to all 14.42 Wiring systems installed within a clinical risk
spare capacity being available at one location only. Category 5 area should be exclusive to the use
of equipment and fittings in that location.
Fire precautions
Access for maintenance
14.38 Where preformed wiring systems penetrate floors
and partitions which are themselves intended to 14.43 Designers and stakeholders should consider
form fire barriers (that is, fire compartment walls), the risks associated with the installed routes for
the outside of the trunking should also be locally preformed wiring and the need to provide suitable
fire-insulated on both sides for 500 mm to prevent access for maintenance. (See Health Technical
heat transfer by conduction along the metal Memorandum 00 and Defence Works Functional
trunking, and the passage of smoke. Containments Standard DMG 08 ‘Space Requirements for Plant
should be treated in such a way as to prevent Access’ for additional information).
any smoke that may travel on the inside of
containments from linking separate fire Suitable locations
compartments. 14.44 Designers and stakeholders should consider the
risk associated with installing the systems in
Remodelling and extensions certain locations. Clinical risk Category 1 areas
14.39 Preformed wiring systems do not provide an should not be adversely affected by preformed
easy way for additional circuits to be pulled into wiring systems. The risks may be acceptable in
existing wiring systems. Hence, any circuits to clinical risk category 2 and 3 areas, but may
be added retrospectively will require additional present a higher risk in clinical risk category 4
preformed lengths, which in turn erodes the spare and 5 areas.

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15 Cable and busbar types

15.1 All current-carrying conductors (cables, busbars 15.4 For example, the code AA4 signifies:
etc) should be suitably sized to carry their design
A= Environment
load after the application of any de-rating factors
generated by their installation environment and AA = Environment – Ambient temperature
in accordance with manufacturers’ data. All cables AA4 = Environment – Ambient temperature –
should be of an approved type tested by an external range –5°C to +40°C.
body such as the British Approvals Services for
Electrical Cables (BASEC) or CBS ENELEC. The 15.5 Further advice should be obtained from cable
conductor size should limit the volt drop between manufacturers’ data sheets to validate the
the network origin and point of use to the values appropriateness of the cable for the intended
given in BS 7671 IEE Wiring Regulations. application.
Designers may optimise the conductor power 15.6 Cross-linked polyethylene (XLPE) is well
dissipation (I2R losses) by designing the final established at higher voltages and is the preferred
circuits to carry the majority of the permissible volt type of cable construction. XLPE cables have an
drop. improved operating temperature (90°C) over PVC,
15.2 The environmental protection grades can be found which means that XLPE cables do not require de-
in BS 7671 Chapter 52 and Appendix 5. The rating (for temperature) as much as an equivalent
electrical properties can be found in BS 7671 PVC cable. This can be a particular advantage
Appendix 4. in plantroom and energy-centre locations.
Significantly higher symmetrical short-circuit
15.3 Each condition of external influence is designated ratings are also possible, corresponding to a
by a code comprising a group of two capital letters conductor temperature of 250°C during fault
and a number, as follows. The first letter relates to conditions. This is compared to 150°C for PVC
the general category of external influence: cables. XLPE will ignite and burn readily, but has
A Environment low smoke and fume-emission characteristics.
B Utilisation 15.7 Elastomeric (or thermoset) materials return to their
original shape and dimensions after deformation.
C Construction of buildings
They tend to have a wider operational temperature
The second letter relates to the nature of the range and superior mechanical properties compared
external influence: with general-purpose thermoplastic materials. This
makes them particularly suited to cable sheathing
...A
applications, especially in harsh environments.
...B Elastomeric materials are suitable for all cable
...C applications. Ethylene vinyl acetate forms the basis
of most modern low-smoke zero-halogen cable
The number relates to the class within each external sheaths.
influence:
15.8 Designers should evaluate whether the cable will be
......1 suitable for all normal and fault conditions. The
......2 fault calculations should include both overload and
short-circuit condition (between live conductors
......3 and/or live conductor phase to earth). The fault
conditions should be modelled for all circuit
conditions, which will vary according to the

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

number of motors etc running. Cables should be orientated to face a nearby reinforced concrete
suitable for power supplies from the DNO as well vertical surface or 200 mm brick wall. A splash-
as any secondary power supplies (SPS). protected breather hole with an external
replaceable silica gel dryer with screwed insert
15.9 Where the primary power is supported by parallel-
should be provided to prevent the accumulation of
running CHP plant, the fault calculations should
condensed water vapour within the cable box.
reflect various power supply ratios of no CHP,
25% CHP and say 50% CHP. 15.19 All cables should be marked and terminated in an
approved manner to indicate phases. The far and
15.10 Designers should consider the use of computer
near phase cable ends should be checked by a
software applications to simulate all scenarios for
continuity meter to confirm identical phase
fault calculation and cable selection. Any software
markings.
used for such purposes should have an auditable
quality control system such as ISO 9001.
Low-voltage distribution
15.11 Where there is large radiographic equipment
which derives radiation from short-impulse high 15.20 Multi-core LV distribution cables should have a
voltages, the distribution cables may not be black outer sheath to denote their voltage rating.
required to be rated at the full load. Designers 15.21 The core colours should be defined by
should liaise with the radiographic equipment BS 7671:2001.
suppliers to determine any opportunity to use
under-sized cables. 15.22 LV distribution conductors are made from copper
or aluminium. Aluminium cables as rated are
15.12 This chapter addresses the various cable types larger, require greater space, are difficult to lay,
available for each system within the electrical and require larger glands and cable lugs for
network of healthcare premises. terminations. Copper conductors have a better
thermal and mechanical impact resistance and are
High-voltage distribution more durable.
15.13 HV cables have a higher power density than the
Cable identification
equivalent-sized LV cable; therefore, where an
electrical network includes an HV system, the HV 15.23 The colour of the conductor sheath of multi-core
system should be made to cover as large an area as LV three-phase distribution cables should be as
is practical (see Chapter 6). illustrated in Figure 39.
15.14 The grades of cable insulation normally used are 15.24 Where single-core LV distribution cables are
XLPE cables complying with BS 6346:1997. installed, the phase colour should be brown with a
blue neutral conductor as in Figure 40.
15.15 HV cables may be direct-buried, laid in a trench
or, where practical, installed on heavy-duty cable 15.25 Note: where single-core cables are installed for
trays. LV distribution, all conductors of a common
circuit should be enclosed in the same metallic
15.16 HV cable boxes should be made of fabricated steel,
containment such as trunking. In accordance with
and terminations should be air insulated up to
IEC 60364-7-710 any wiring system within
11 kV. Spacing between the terminals must
Group 2 locations should be exclusive to the use
conform to BS 4999-145 or IEC standards
of equipment and fittings in that location (see
requirements for the rated voltage.
BS 7671:2001). The terminations of single-core
15.17 All HV terminations and terminating cable tails LV conductors should be identified by the
should also be encapsulated in heat-shrinkable, appropriate colour or notation, which may include
voltage-graded plastic insulation, approved and IPS circuit identification.
guaranteed by a reputable manufacturer for the
15.26 Existing installations may continue to use the
rated voltage.
pre-April 2004 BS 7671 conductor sheath phase
15.18 Steel cable boxes for the HV terminations of colours (red, yellow and blue), black neutral and
rotating machines should be provided with an yellow-green protective conductors. However,
aluminium foil explosion diaphragm and, as a these should be replaced when modifications are
safety precaution, the boxes should preferably be undertaken to the electrical system.

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 15 Cable and busbar types

Figure 39 LV three-phase multi-core cable identification

L1 Brown L1 Brown

L2 Black L2 Brown
Or
L3 Grey L3 Brown

N Blue N Blue

Figure 40 LV single-core cable identification

Busbar distribution
L1 Brown
15.30 LV busbar distribution systems are becoming a
cost-effective solution for high-current circuits.
N Blue LV busbar systems with current ratings from 63 A
to 2.5 kA (depending on type) are available, with
the insulation being air or cast-resin encapsulation.
15.27 Note: where the conductors form an IPS circuit,
Some systems provide insulated bars only. The
both conductors should be coloured brown and main advantages of LV busbar distribution are the
identified as L1 and L2. In composite cables, reduced space, and the standard tap-off facility to
conductors can be sleeved brown. add additional outgoing circuits later (via fused
15.28 The LV distribution strategy should focus on the switches).
cable size with a view to installing the cable and
giving access for maintenance. Designers should Control alarm and communication
allow adequate space for the bending radius of cables
cables (including the respective containment
system – see Chapter 14). 15.31 There are many types of alarm and
communications system in a healthcare facility.
15.29 Distribution and sub-main cables above 240 mm2 This section identifies some of the more common
are difficult to install, which means either having wiring systems used for such circuits. Since the
smaller distribution circuits (which in turn means mid 1980s, many communication and alarm
more switchgear) or installing single-core cables. systems have moved to digital networks and data
Where the distribution uses single-core cables, highways. As such systems have expanded and
each core should be laid in a trefoil arrangement. their relative speed and bandwidths increased,
In order to limit electromagnetic radiation (see many data highways are being used to carry a
Chapter 11), the group should be 0.75 diameters multitude of systems ranging from information
from a wall or any other distribution cable (cable technology systems, BEMS, nurse call, blood bank
group), and generally means more space. alarms, security and fire alarm signals. Since the
mid-1990s, some of the communication and alarm
systems have moved to wireless systems.

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

15.32 This Health Technical Memorandum is only the containment routing for such systems. IT
concerned with fixed wiring. However, designers containments should be in separate vertical risers
and stakeholders should consider the effects of to any other building services containment route.
wireless systems and electromagnetic compatibility Horizontal containment used for IT should be
(see Chapter 11). The use of any wireless systems at least 300 mm to 600 mm from other building
in clinical risk Category 3 and above areas should services containment, subject to the voltage band
be the subject of a risk assessment. Although the of any distributed power cabling system. The IT
wireless signals may not have any common distribution strategy and separation distance are
frequency or side-frequency with electro- exclusive of any maintenance access requirements
biomedical equipment etc, the clinical risk may that should also be considered.
be high.
Fire alarm cables
Control communication and non-fire-alarm cables
15.36 Cables used for any part of a fire alarm system
15.33 Designers should liaise with system suppliers should be an enhanced grade cable as defined by
before selecting the type of cabling used for general BS 5839-1:2002.
communication and alarm systems.
15.37 All fire alarm cables should also satisfy the CWZ
15.34 The distribution and installation of alarm and rating of BS 6387:1994; that is, the cable should
communication systems should follow (as far as be able to withstand water and impact and be
practical) the general route of containment used subjected to a temperature of 950°C for three
for power systems, provided a suitable segregation hours.
distance (100 mm to 300 mm depending on
15.38 Cable systems may be derogated from their
voltage screening bands) is maintained.
respective mechanical cable impact requirements
Information technology cables of BS 6387:1994 by installing enhanced-grade fire
alarm cable in a continuous containment, which
15.35 The construction and type of cable used for then satisfies the impact requirement of
IT systems fall outside the scope of this Health BS 6387:1994.
Technical Memorandum. Designers should liaise
with the IT staff at an early stage to coordinate

104
16 Final circuits

16.1 This section deals with final circuits and point- Uninterruptible power supplies
of-use connections of the PEI that present best-
practice configurations for final circuits, UPS and Standards
IPS for the emergency protection of final outlets,
16.3 UPS systems should be to, but not be limited to,
circuits and equipment. The configurations are
presented generally in order of resilience from low the following design and manufacturing standards:
to high. The selection of a particular configuration • BS EN 62040-1-1:2003. ‘Uninterruptible
will be dependent on the specific factors of each power systems (UPS). General and safety
individual design. The selected configuration requirements for UPS used in operator access
should be based on a risk analysis to determine the areas’;
appropriate level of resilience.
• BS EN 62040-2:2006. ‘Uninterruptible power
16.2 The configurations presented in this section should systems (UPS). Electromagnetic compatibility
not be taken as being definitive, prescriptive, or (EMC) requirements’;
restrictive of innovation. They are intended as a
• BS EN 60146, IEC 60146. ‘Semiconductor
guide to best practice (see Figure 41).
convertors. General requirements and line
commutated convertors’;
• BS EN 60439-2, IEC 60439-2. ‘LV switchgear
and control gear assemblies’;
• VDE 0510-2 paragraph 6.5, Ripple current for
battery charging systems.

Figure 41 Final circuit connectivity

Low Voltage Low Voltage Low Voltage

Sub Main (Unified) Distribution Sub Main (Segregated) Sub Main (Dual Unified)
Board Distribution Board Distribution Board

TERTIARY POWER
SUPPLY
UNINTERRUPTIBLE
POWER SUPPLY

ISOLATED
POWER SUPPLY

FINAL CIRCUITS

CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK CLINICAL RISK
CLINICAL RISK
CATEGORY 1 CATEGORY 2 CATEGORY 3 CATEGORY 4 CATEGORY 5
CATEGORY 1

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Rating 16.9 Single-conversion UPS units are configured so that

the supply is normally via the static switch and, on
16.4 UPS system ratings range from 250 VA up to
loss of supply or when the supply quality falls, the
several hundred kVA; the small units may be single-
static switch instantaneously connects the battery
phase units used to support a single circuit, and the
output to the load. The battery is held fully
larger UPS systems may be single- or three-phase
charged by a trickle charger supplied from the
units for supporting a complete department.
normal supply. The battery autonomy (see
16.5 Central UPS systems may be considered where the Chapter 10) of single-conversion UPSs working
need covers several small distributed areas. Where offline is typically up to 15 minutes.
centralised UPS systems are considered, a diesel
16.10 Double-conversion UPS units are configured so
rotary UPS (DRUPS) may provide an economic
that the supply is normally via the rectifier and
solution. The location of DRUPSs should be based
inverter line and, on loss of supply (or poor supply
on the same environmental criteria used for the
quality), the battery output supplies the load via
standby generators and/or CHP plant.
the inverter. The static switch will also bypass the
16.6 Other types of rotary UPS, which use the stored rectifier inverter if the UPS circuit develops a fault.
energy of a flywheel under full torque connected to The battery is held fully charged by a trickle
a motor, may also be used. The autonomy of these charger supplied from the normal supply. The
UPS devices are essentially time-based and largely battery autonomy (see Chapter 10) of double-
independent from the actual load. These so-called conversion UPSs working online is typically up to
“silent rotary UPSs” currently have high kVA 1 hour. The battery autonomy should be assessed
ratings and are more suited to a centralised system. to ensure that adequate power can be provided to
allow the medical therapies to be concluded safely
UPS environment (within the area of concern). Usually 1 hour will
16.7 Designers should consider the local space of the facilitate the closure of any patient in an operating
UPS, in terms of its access for maintenance and theatre. However, on the most complex of
heat generated. Depending on the UPS type, single surgeries or medical therapies, periods of up to
or double conversion, a UPS will radiate about 3% three hours may be required. Designers and
to 8% of its input power, which will need to be stakeholders should liaise with surgical staff to
vented. Ideally, the ventilation should be natural. understand the most appropriate cost-effective
The environmental conditions should control the strategy.
room space to the limits recommended by the 16.11 The rectifier and bypass may have a common
battery manufacture. supply connection. The ideal connection should
provide separate connections for the rectifier and
UPS description and configurations bypass line (see Figure 42a/b).
16.8 A UPS consists of three principal parts: a rectifier,
a battery unit and an inverter. The rectifier converts UPS fault condition design
the ac power supply (single- or three-phase) to a 16.12 UPS protective devices should be capable of
dc supply. The rectifier output maintains the clearing downstream circuit faults in similar
battery in a fully-charged condition. The inverter fashion to other distribution boards. UPS output-
reconverts the rectifier output (or battery output) circuit protective devices should discriminate from
to a synthetic sinusoidal waveform output (again upstream devices. Designers should consider the
either single- or three-phase according to the effect of overload and short-circuit fault condition.
input). The UPS should include a static bypass, a Short-circuits in the UPS load are isolated either
manual internal bypass and an external bypass; all by a downstream protective device, or by the
three bypass switches should be installed. The static insulated-gate bipolar transistor (IGBT) control
bypass will electronically divert the normal supply circuit of the inverter. UPS units may tolerate
from the rectifier inverter line through the static overloads of 125% for 10 minutes, 150% for
switch whenever a fault in the UPS conversion 1 minute, or 200% for 100 milliseconds
occurs. The static bypass operates at such a high (depending on the manufacturer’s selected
speed that it is considered as a no-break supply internal protective device). The actual overload
switch. characteristics vary from manufacturer to
manufacturer. Designers should verify the

106
 16 Final circuits

Figure 42a UPS resilient arrangements

Secondary Power Source SPS Primary Power Source PPS

Rectifier Rectifier By-Pass
Input 1 Input 2 Input

A B

RCB SBCB
MBCB

BCB

UPS Unit 2
Normally “Off-Line”

OCB

Electrical Interlock
D C

E F

RCB SBCB
MBCB

BCB
UPS Unit 1
Normally “On-Line”

OCB

Electrical Interlock
H G

UPS – Basic Cascade Arrangement

107
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 42b UPS resilient arrangements

Secondary Power Source SPS Primary Power Source PPS

A B
LV Panel

NOTES
Isolator Switches A & B should
be separate in case it
becomes necessary to
Input Switch Panel totally isolate input SWBD
........A ........A ........A ........A to work on switches or
MCCB MCCB MCCB MCCB busbars etc

Input & output SWBDs

should be separate as
input SWBD only has
mains whereas output
has UPS power
SBCB RCB SBCB RCB
Switch X is essential
to fully isolate total
system, electrical
Remote BCB Remote BCB interlocking with
Status Status switch Y should
Panel Panel be fitted as
standard

Static Switch Static Switch

OCB OCB

........A ........A
ROCB ROCB
Switch Switch

........A ........A
External System SOCB X Y SOCB
MCCB Electrical Aux for Safety Interlock MCCB
Output/Bypass
Cabinet

To Load Distribution
UPS – Basic Parallel Arrangement

coordination of fault conditions when selecting the 16.14 With three-phase UPS units, designers may wish to
UPS type. consider the use of a zig-zag transformer on the
UPS bypass lines. Such transformers provide a
16.13 Designers need to consider carefully the protection
local earth point, may help to ensure that an
systems used by the UPS. The UPS subcircuit
adequate fault current is developed, and assist in
protective devices should provide adequate
harmonic control.
discrimination with the inverter/static switch
protection. If the inverter/static switch protection 16.15 Where the UPS is supported by the secondary
operates before the subcircuit protection, the UPS power supply, a transformer with a double-wound
may shut down. Clearly, this would then isolate all secondary may assist with the limiting of the initial
the subcircuits and not just the faulty circuit.

108
 16 Final circuits

acceptance load as the harmonic currents have the fixed wiring. However, designers should be
been reduced. mindful of such units when assessing the overall
electromagnetic characteristics of the wiring system
UPS power quality (see Chapter 11).
16.16 UPS systems are a significant non-linear supply.
Central battery units
The rectification stage provides a pulsed ripple dc
circuit, and the inversion stage provides a synthetic 16.21 The wiring used in central battery units should be
sinusoidal ac circuit. Thought should be given to of an enhanced grade as defined by BS 5839-1:
UPS units which use IGBTs and to the duration 2002.
and value of any inrush currents.
16.23 Central battery inverter units should be directly
16.17 The output of the rectifier will be a ripple dc connected to the secondary power supply and
voltage. The ripple effect is normally smoothed be so arranged that the output can energise all
by the use of IGBT devices. However, the IGBT connected emergency escape lighting and signage
circuitry will reflect a harmonic current into the within five seconds as required by BS 5266.
supply line. The level of harmonic currents should
16.24 Central battery inverter units should be
be controlled such that the net harmonic current
constructed with maintenance bypass switches.
reflected to the DNO connection is in accordance
The switch should isolate the battery charging unit
with the Energy Networks Association’s
and the batteries from the output, but maintain a
Engineering Recommendations G.5/4. See also
normal supply to the output. Note: if there were
paragraphs 5.11–5.17.
to be an outage of the primary supply during the
maintenance of a central battery inverter unit,
UPS resilience
there would be no output supply until the
16.18 UPS units can be grouped as multiple units secondary power supply was available. This
connected either in cascade (redundant) or in period of circa 15 seconds is beyond the
parallel (see Figure 42a/b). Either arrangement 5-second requirement of BS 5266. See paragraphs
provides N+1 resilience, as described in paragraphs 10.14–10.16 for details of the battery capacity
6.8–6.30. UPSs connected in cascade provide a relating to central battery inverter units.
“redundant” arrangement. With redundant UPS
arrangements, each UPS should be able to fully Rectifier units for theatre operating lamps
support the full load, that is, be 100% rated. The
16.25 Each separate operating theatre should have its
output of the first cascade-connected UPS should
own rectifier battery unit, external to the theatre,
supply the bypass of the second UPS. UPS units
exclusively for the operating lamp(s).
connected in parallel are normally all online, but a
standby unit (in parallel) may be provided. UPS 16.26 See paragraphs 10.14–10.16 for details of the
units connected in parallel may be rated at a battery capacity relating to inverter units for
percentage of the full load, provided that when theatre operating lamps.
one unit is not available, the remaining units can
provide the full load. The common point of Isolated power supplies (IPS)
coupling for parallel UPS units should be
16.27 Medical IT systems are IT electrical systems having
downstream of the external bypass of each unit.
specified requirements for medical applications.
16.19 The selected arrangement for UPS resilience Medical IT systems are commonly termed
should ensure that the internal and external bypass “isolated power systems”, and have monitored
switches provide a safe maintenance strategy. circuits. A medical IT system should comply with
the following standards:
Inverter units • IEC 60364-7-710;
16.20 The inverter units considered in this Health
• BS 7671 Special Guidance Note 7, Chapter 10;
Technical Memorandum relate to central battery
units and stand-alone units used for theatre • BS EN 61558-2;
operating lamps. Inverters used as a self-contained • BS EN 61557-8.
power pack for emergency escape lighting and
signage are excluded, as they do not connect to

109
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

16.28 It is a basic requirement that an IPS system should insulation monitors installed in IPS systems should
be able to sustain power on its subcircuits during be capable of correct function (that is, no nuisance
and following a first earth fault on the system. This alarms) when dc levels (of either polarity) are
requirement differentiates an IPS from a UPS: the present on the monitored medical IT systems.
IPS maintains power when an earth fault occurs Furthermore, insulation monitors should be able
on the transformer output circuits, while a UPS to function correctly in systems with capacitive
maintains power output when its source of supply filters such as MRI installations.
is interrupted. Therefore, these systems may be
16.32 The construction and design of the IPS units
used to full advantage to complement each other
should provide adequate access for maintenance.
in critical medical locations to improve patient
This is especially important in “multiple channel”
safety.
IPS panels, where it should be possible to safely
16.29 For the purpose of this Health Technical isolate and maintain each individual IPS channel,
Memorandum it may be assumed that any electro- without detriment to the operation of the other
medical equipment used in either a Group 1 or IPS channels. Where the multi-channel IPS
Group 2 location (as defined in the above panels also include EDS systems, each channel
standards) should be compliant with the should be suitable for full EDS function either
requirements of BS EN 60601-1, IEC 60601-1 simultaneously with other IPS channels, or
(as required by the Medicines and Healthcare independently, when other IPS channels are
products Regulatory Agency, MHRA). isolated for maintenance.
16.30 In medical locations, the distribution strategy 16.33 The installation of IPS systems, together with
should be designed to facilitate the automatic additional equipotential bonding and other
changeover from the primary distribution network measures described in the standards/guidance
to the SPS (standby generator feeding essential referred to above, are necessary to ensure the safety
circuits) when and if the primary supply voltage of patients and medical staff in medical locations.
drops by more than 10%. The LV distribution However, the increased use of electrical equipment
circuitry, up to the sub-distribution board used to for the purpose of life support and/or complex
connect the IPS and UPS systems, should be surgery used in special medical locations requires
deemed an essential circuit. enhanced reliability and safety of the electrical
installation in hospitals to ensure the security of
16.31 The medical IT system provides a monitored,
supplies and to minimise incidents of microshock.
isolated, floating power supply, which will sustain
the first single earth fault. Consequently, IPS 16.34 The earth leakage current from the secondary
systems do not require any overload protection on winding of the isolation transformer, when
isolating transformer input or output circuits. An measured in no load condition and in single earth
advance warning of potential faults on the medical fault condition, should not exceed 0.5 mA. It is
IT system, which includes the final subcircuitry, is essential that this requirement for leakage currents
raised by the monitoring of insulation, transformer is specified as an additional requirement to
overcurrent and temperature. The medical staff IEC 61558-2-15. As it stands, IEC 61558-2-15
can unplug medical equipment from the affected specifies these leakage currents to a limit of
circuit and replace or reconnect to a healthy 3.5 mA; however, there are moves in hand to
circuit. To enable the transfer of equipment in this modify IEC 61558-2-15 to reduce the leakage
manner, sockets at patient locations should be currents specified to 0.5 mA. This requirement
from two interleaved IPS systems. Automatic earth enhances the safety applications of the transformer
fault location systems (EDS) may be used to and brings it in line with BS EN 60601-1,
advantage in interleaved IPS systems areas and IEC 60601-1.
especially in wards, to provide rapid, detailed earth
fault location information to clinical staff at the IPS environment
staff base; generally this should be in the form of a 16.35 Designers should consider the local space of the
simple text message, which for example would IPS in terms of its access for maintenance and heat
state “IPS 1 Earth (Insulation) Fault, ITU bed 4, generated. The IPS unit should be located on
left side”. In addition to the requirements of the same floor and just outside the medical
IEC 60364-7-710 for these locations and department clinical risk category area it serves.
BS EN 61557-8 (insulation monitor standards), Where this is not practical, derogation may be

110
 16 Final circuits

given to locating the equipment on the floor any future remodelling of the clinical risk category
immediately above or below, or within 30 m on 4 and 5 areas.
the same floor as the clinical risk category area.
16.39 Figure 43 shows an IPS arrangement suitable for
The IPS unit includes an isolating transformer,
Group 1 or Group 2 Locations. Figure 38 shows
typically 3.5 kVA–10 kVA radiating about 2–5%
the earthing arrangement for a typical theatre.
of its output power as heat, which should be
(Other IPS arrangements that satisfy the
ventilated. Ideally the ventilation should be
requirements of IEC 60364-7-710 and BS 7671
natural, unless the forced ventilation power is
may also be possible.)
derived from a standby generator.
The patient environment
IPS communication
16.40 Figure 44 relates to the patient treatment location,
16.36 Each IPS system will have audible and visual alarm
where all sockets associated with medical
indication of any first fault in accordance with the
equipment should be connected to the IPS and
requirements of BS EN 61557-8 and IEC 60364-
any other socket (or fixed equipment) connected
7-710 insulation monitoring devices. Remote
to the TN-S supply with an RCD/RCBO
indication, where required, will be at the nurse/
protective device. Although the figure relates
management station for the medical area covered
essentially to a theatre location, it should be
by the IPS system. Connecting the remote alarm
reasonably clear how the zone would be modified
indication to a networked BEMS communication
(at the patient head) when used to illustrate an
system and terminals within the estates office will
area such as a high-dependency unit (HDU).
have added advantages.
16.41 Theatre operating lamps do not require an isolated
Resilience power supply and should not be connected to IPS
circuits.
16.37 IEC 60364-7-710 and BS 7671 require Group 2
areas to have at least two separate socket-outlet 16.42 In Figure 44 the dark grey area represents the
subcircuits at each patient treatment location (for theatre table/bed, while the light grey shows the
example bedhead or theatre pendant). This applies patient treatment area (exclusion zone). Any
to Group 1 areas also. This can be achieved from exposed or extraneous conductive parts within the
a single IPS unit with an integral single-phase exclusion zone, or that could be reached from
distribution board. The resilience would be further within the exclusion zone, should be connected
enhanced if the IPS had dual 100%-rated isolation to the ERB with an impedance less than 0.1 Ω.
transformers serving different integral distribution The theatre table (or bed) could be moved as
boards. Such arrangements would provide an N+1 illustrated, in which case the exclusion zone would
resilient IPS isolation transformer as defined in also move. Therefore, any exposed or extraneous
paragraphs 6.8–6.14. conductive parts within the outer boundary
(above) should be bonded to the ERB with
16.38 IEC 60364-7-710 and BS 7671 require luminaires
impedance less than 0.1 Ω. All exposed or
and life support equipment used in Group 2 (or
extraneous conductive parts within the theatre
occasionally Group 1) locations which need power
(or ward) should be bonded as described above.
supply within 0.5 seconds or less, to be restored
Theatre operating lamps, pendants, beams,
within 0.5 seconds of a supply failure and other
equipment gantries etc should be considered as
equipment to be restored within 15 seconds. To
extraneous metalwork and therefore should be
achieve the safety requirements, IPS units serving
bonded to the ERB (see Figure 38).
life-support equipment should be connected to a
UPS supply which has a derived power supply,
IPS low-power circuits
supported by the standby generator. However,
other equipment may not require the same level of 16.43 This section refers to sockets in Group 1 or 2
UPS/generator support. In order to ensure that the areas as defined by IEC 60364-7-710 (the IEE
appropriate support is always available to cover a Guidance Note No 7) and mortuary post-mortem
range of treatment options, any IPS used should rooms.
be supported by a UPS and standby generators. 16.44 Activity Database (ADB) and the Department of
This arrangement will provide greater flexibility in Health’s ‘Health Building Notes’ (HBNs) provide

111
 Figure 43 IPS/UPS high-security supply arrangements

112
Secondary power Primary power Secondary power
source (SPS) source (PPS) source
single phase
AC 230V PE
UPS

Typical clinical risk area Typical clinical risk

Medical isolation category 5 medical IT area category 5
transformer system TN-S system
IEC61558.2.15, to IEC60364-7-710 to IEC60364-7-710
IEC60364-7-710
typically 8 kVA

ICU remote alarm

IEC60364-7-710

Insulation/load/ Protective Supplementary

temperature monitor Link Protective
En61557-8, IEC60364-7-710
earth equipotential earth bar earth
B20 double pole
MCBs Extraneous
Medical IT system metalwork
radial 13 Amp Taps & Standard TN-S
socket-outlet pipes 13A socket-
sub-circuit
outlet
distribution
sub-circuits
max 24 sockets per
Pendant 6
sub-circuit Pendant 5
Bed 1 Bed 2 Bed 3 Bed 4 Bed 5 Bed 1 Bed 2 Bed 3 Bed 4 Bed 5 Medical IT system Bed 1 Bed 2 Bed 3 TN-S system
Pendant 4
13A 13A 13A 13A 13A 13A 13A 13A 13A 13A socket-outlet Pendant 3 13A 13A 13A socket-outlet circuit
sockets sockets sockets sockets sockets sockets sockets sockets sockets sockets circuit PE Pendant 2 sockets sockets sockets PE connections
(50%) (50%) (50%) (50%) (50%) (50%) (50%) (50%) (50%) (50%) connections Pendant 1
Socket-outlets for X-ray, electric
Socket-outlet distribution circuits for the supply of medical electrical equipment, medical systems intended bed and all other non-critical
for life support, complex surgical applications and patient monitoring and other electrical equipment in the electrical equipment
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

patient environment, excluding that listed under TN-S socket-outlet circuits (non-life support)
 16 Final circuits

Figure 44 The patient environment

2.5 m

1.5 m 1.5 m

1.5 m

Exclusion zone

The patient environment must take into account all possible exclusion
zones when the patient position is not fixed

advice on the number of sockets. The number (IPS circuits) may be supplied as unswitched items
of sockets at the patient location of clinical risk to prevent accidental switch-off. Where such
Category 4 and 5 areas will be significantly large. sockets are supplied as switched items, the switch
IEC 60364-7-710 recommends that each patient will be double-pole. A means of identifying
location has two IPS socket circuits and one TN-S individual medical IT circuits should be provided
circuit. Final circuits of an IPS system may have up at each socket-outlet. Medical IT socket-outlets
to 24 sockets per circuit. should be blue in colour to distinguish them from
any TN-S earthed socket within the same vicinity.
16.45 Socket-outlets in clinical risk category 4 and 5
areas and connected to the medical IT system (IPS 16.47 Initial concepts and remodelling of clinical
circuits) will be connected in a radial format. The departments will always require an understanding
protective device for such circuits should be a 20 A of the intended use of each medical location prior
MCB with a Type B characteristic. Socket-outlets to designing the IPS-UPS configuration.
within clinical risk category 4 and 5 areas not
16.48 Some of the sockets in these areas will be earthed
connected to the medical IT supply should be
as part of the TN-S system, while others are part
protected by an RCD/RCBO protective device.
of a medical IT system. Sockets earthed by the
The RCD/RCBO may be incorporated within the
medical IT method will be connected via the IPS
distribution board. The RCD/RCBO will be
distribution board.
30 mA and Type A or B characteristic.
16.46 Sockets-outlets in clinical risk category 4 and 5
areas, and connected to the medical IT system

113
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

General low-power circuits supplied with final circuits protected by RCD/

RCBO protective devices. Consideration may be
16.49 In general, socket-outlets (as defined by BS 7671) given to the provision of a PELV system within
will conform to BS 1363 or IEC harmonised post-mortem rooms.
standards and be connected to a ring or radial
circuit. The protective device for a single-phase 16.55 Other wet areas, such as hydrotherapy pools, will
socket circuit should be rated no higher than 32 A require all final circuits to be equally protected by
for a ring circuit and 20 A for a radial circuit. RCD/RCBO protective devices. See the IEE
Regulations BS 7671 for more details.
16.50 All sockets should be suitable for the local
environment. While this may appear obvious, 16.56 Healthcare premises include engineering
it ensures that suitable precautions (IP ratings) workshops for mechanical, electrical and
are made for sockets in kitchens, laboratories, biomedical repairs. The maintenance of electrical
plantrooms and general circulation spaces. Metal- equipment and biomedical equipment may require
finished sockets should be installed within a testing with the supply connected, that is, working
clinical risk Category 3 area and above in order to live. Great care for such working arrangements
limit the effect of electromagnetic interference and must be observed for compliance with the
the increased mechanical protection. Electricity at Work Regulations (regulation 14)
and HSE guidelines etc.
16.51 Socket-outlets and switches, regardless of the
location, should be installed at a distance of at least 16.57 Designers should consider providing a special test
0.2 m horizontally (centre to centre) from any room or test bay within the engineering workshops
medical gas outlets. This requirement is specified and biomedical workshops. The low-power circuits
in BS EN ISO 11197:2004. within the test room and/or test bay should be
from an isolating transformer, providing an earth-
Socket-outlets/connection units free environment. These circuits should be very
clearly identified, and labels should be provided to
16.52 Designers should assess the maximum number
alert the occupier to the earth-free environment.
of socket-outlets on a final circuit by calculation The circuits within this area should be protected
of the minimum disconnection times given in by a 20 A MCB Type A or B, and require a
BS 7671, and the likely simultaneous connected monitoring system.
power on the circuit. Designers should consider
the distribution strategy and risk ownership when Sockets for operating theatre suites
determining whether the socket-outlet circuit
should be supported by the SPS. However, for 16.58 The patient environment of an operating theatre is
some areas in clinical risk categories 1, 2 or 3, a clinical risk Category 5, and hence the sockets
stakeholders may wish to have a different may be served from a UPS or IPS circuit.
approach. In such areas, consideration can be given Consideration may be given to connecting the full
to socket-outlets connected to the SPS where the theatre suite from a UPS supported by the SPS
area has standby lighting of Grade B or above. In standby generator, or just the SPS standby
such cases, designers and stakeholders should be generator. Other socket-outlets within the
mindful of the implications for the capacity of the operating theatre should be connected to the TN-S
essential SPS. wiring system and have an RCBO or RCD with a
30 mA trip.
16.53 Socket-outlets (in any location) connected to the
essential SPS should be positively identifiable from Socket for mobile X-ray units
any non-essential sockets in the same area.
16.59 Mobile X-ray units supplied since the mid-1980s
Sockets for special locations do not present any real disturbance to the electrical
distribution. However, designers should enquire
16.54 There may be areas that require special electrical whether any provision should be made for mobile
installations, providing safety measures for specific X-ray units which derive their high ionisation
purposes. In general, post-mortem rooms will have voltage by inductive means, and provide dedicated
medical IT circuits having an earth fault trip for sockets circuits accordingly.
enhanced safety in a wet environment. However,
other rooms within the mortuary should be

114
 16 Final circuits

Spark-proof sockets equipment manufacturer/provider and MEIGaN

before designing the electrical services to these
16.60 The use of anaesthetic gases with a very low flash
areas. Dedicated sub-main circuits should be
point has virtually been eliminated from UK NHS
used for these areas. However, the use of dedicated
hospitals (see College of Anaesthetic Consultants
earth cables between the radiography sub-main
and the Medicines and Healthcare products
switchboard and the LV switchpanel ERB or the
Regulatory Agency). However, an assessment by
MET at the transformer is strongly encouraged.
enquiry should evaluate the likelihood of such
anaesthetic gases being used, and provide mercury- 16.66 Designers may wish to consider using a dedicated
operated switched sockets (and light switches) transformer for the sub-main supplies to large
accordingly. fixed equipment. There are strong positive
advantages of such an infrastructure strategy,
Number of outlets per final circuit particularly where the fixed equipment has a very
16.61 Designers need to consider the potential earth
high inductive load, such as vapour compression
leakage current that may flow on the protective chillers or radiography departments.
conductor under normal conditions, which should
be minimal. For clinical risk categories 4 and 5, Supplies to external buildings
the earth leakage current is regulated by cable 16.67 Some healthcare premises have small annexes used
leakage capacitance, the design of the IPS isolating as stores and/or plantrooms not intended to be
transformer and associated medical equipment. occupied for long periods. The standard of
In other clinical risk areas, the potential earth electrical installation for these buildings should
leakage current will be determined by the medical be the same as for the main healthcare building.
equipment (assumed to be compliant with BS EN Electrical installation standards should reflect the
60601-1, IEC 60601-1), other equipment loads, nature of the stores, which may contain medical
and cable leakage capacitance. gases or flammable material. In such cases the
16.62 The steady-state earth leakage current expected on electrical equipment, including containments,
a TN-S final-circuit protective conductor should cabling, luminaires and accessories, may require to
not exceed 50% of the sensing element of any be intrinsically safe.
RCD or RCBO used as the protective device on
the circuit. Designers may therefore wish to Temporary supplies
consider this statement when designing final 16.68 Designs that comply with the guidance given in
circuits as ring mains or radial circuits. this Health Technical Memorandum should avoid
the need of temporary supplies. Where they are
Fixed equipment needed, the electrical standards should be as high
16.63 Large fixed equipment such as lifts, compressors, as for the permanent supply. Derogation may be
air-handling units, laundries, engineering given on the containment requirement, when a
workshops and radiographic imaging equipment is clear understanding of the intended temporary
addressed here. Such items of plant include heavy period has been given.
inductive loads, which may cause disturbances to 16.69 Designers’ attention is drawn to the application
the distribution network. to connect (see paragraphs 3.60–3.62). The site
16.64 Where the electrical supplies are for high-inductive engineer will reserve the right not to connect a
motors, “soft-start” or “inverter speed drives” temporary installation where the installation does
should be used. All such inductive loads should not comply with the guidance given in the Health
have local power factor correction and harmonic Technical Memorandums and local electrical safety
filtering. Dedicated earth cables should be rules.
provided between the MCC and LV switchpanel
ERB, or the main earthing terminal (MET) at the Connections for mobile trailer units
transformer. 16.70 Where mobile Treatment Centre (TC) units
16.65 Where the electrical supplies are for radiographic (for example MRI scanners), or similar units,
imaging diagnostic and treatment facilities, are connected to the electrical distribution of the
designers and stakeholders should liaise with the healthcare facility, it is important to maintain a

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Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

high degree of electrical safety. This will include Examination lamps lighting
suitable protection to any cables and switchgear
that might be more readily accessible to 16.79 Final circuits used for any fixed examination lamps
unauthorised persons. Suitable solid earthing located within a clinical risk Category 4 or 5 area
between the healthcare premises building earth should be protected by RCD/RCBO protective
and the mobile unit should be provided. devices. The connection of the examination lamp
to the output of any IPS circuit is not encouraged
16.71 Stakeholders should ensure that the internal by this Health Technical Memorandum. However,
electrical systems of any mobile TC unit used on any exposed conductive part of the examination
healthcare premises could not compromise the lamp should be bonded to the ERB.
safety of patients and/or the electrical system of the
healthcare premises. Emergency escape lighting
16.72 The mobile unit should be earthed as a TN-S 16.80 The emergency escape lighting circuits should be
system, and where the clinical risks are of Category designed in accordance with BS 5266 and BS EN
4 or 5, a suitable IPS system (with medical IT 1838. This Health Technical Memorandum
earthing) should be used (see MEIGaN and IEE considers emergency escape lighting to consist only
Guidance Note 7). of escape-route emergency lighting. Emergency
16.73 Designers and stakeholders should ensure that the lighting circuits should be so arranged as to
final supply/connection cable to any mobile unit provide escape-route lighting throughout the
includes a monitored earth as described in healthcare facility. Where the facility has muster
BS 4444. points for progressive horizontal evacuation
(as defined in the Firecode series), at least two
General lighting circuits should be provided. Emergency lighting
emergency power should be derived from integral
16.74 The design of the lighting systems and lighting battery packs (tertiary power). Consideration can
levels are outside the scope of this Health Technical be given to central emergency battery units, but
Memorandum. the additional fire-rated cabling cost may make
16.75 Lighting circuits used should be wired as a radial this uneconomic.
circuit with a maximum protective device rating of
10 A. Standby lighting
16.76 In any room of clinical risk Category 3 and above, 16.81 This Health Technical Memorandum considers
at least two lighting circuits should be provided. standby lighting as a secondary form of emergency
lighting (defined by BS 5266 and BS EN 1838).
16.77 Lighting circuits within the patient environment
All areas that require standby lighting should also
should be supported by the standby generator to
have emergency lighting.
ensure that Grade A standby lighting is achieved.
Consideration may be given to connecting such 16.82 Standby lighting will derive its power from the
lighting circuits to a UPS. Connecting lighting SPS. There are two grades of standby lighting:
circuits to any IPS circuit is not encouraged by this Grade A and Grade B. Grade B standby lighting
Health Technical Memorandum. provides lighting at a reduced level compared to
the normal lighting level. Standby lighting to
Theatre operating lamps Grade B is best provided by an increased number
of emergency light fittings with integral tertiary
16.78 All fixed theatre operating lamps, including power battery packs. Grade A standby lighting
the main unit and any satellite units, should be provides lighting at the same level as normal
connected to a battery inverter unit providing lighting. Standby lighting to Grade A is best
3-hour autonomy. The connection of the theatre provided by the SPS standby plant.
operating lamp (including its battery inverter) to
the output of any IPS circuit is not encouraged by 16.83 Clinical risk Category 3 areas should be provided
this Health Technical Memorandum. However, with Grade B standby lighting, and clinical risk
any exposed conductive part of the operating lamp Category 4 and 5 areas with Grade A standby
should be bonded to the ERB. lighting.

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16.84 Designers and stakeholders may wish to consider 16.89 Cables used for security and other alarm systems
the implication of any additional circuitry required should be installed as per the manufacturers’
to provide a mixed standby-lighting facility. requirements.
Consideration may be given to all lighting circuits
16.90 Designers should provide an independent tertiary
being connected to the SPS.
power source (battery inverter unit) for the central
16.85 Designers should be mindful that operating head-end of a security system. The system
theatres, which by definition are clinical risk suppliers should specify the battery autonomy.
Category 5, should have an independent tertiary Designers and stakeholders should liaise with all
power source (battery inverter unit) for the theatre staff, especially security staff, when determining
operating lamp(s) and satellite lamps. The battery which, if any, security detection and alarm
autonomy should be at least three hours. In component parts are supported by the SPS. As
addition to the inverter unit, the electrical a minimum, if there is a pharmacy on site with
distribution supply to the theatre operating controlled and/or dangerous drugs, the security
lamp(s) should be derived from the secondary system should be connected to the standby
emergency power source (SPS). generators.
16.91 Designers should provide an independent tertiary
Fire alarm, security circuits and critical power source (battery inverter unit) to any blood-
alarms bank alarm system. The system suppliers should
16.86 Designers should provide an independent tertiary
specify the battery autonomy.
power source (battery inverter unit) for the fire
alarm system. The battery autonomy should be BEMS communication and control
compliant with the requirements of BS 5839-1: wiring systems
2002. The fire alarm systems should be connected
16.92 Designers should provide an independent tertiary
to the SPS, where appropriate to the distribution
power source (battery inverter unit) for the central
strategy. Consideration may be given to connecting
head-end of any system used for these facilities.
any fire-door detents to the SPS.
BEMS outstations should have an integral battery
16.87 See paragraphs 10.11–10.16 for details of battery unit to maintain internal software parameters. The
inverter capacity. BEMS equipment etc should at least operate in the
fail-safe position. More critical plant and service
16.88 All cables associated with the fire alarm system
(controlled through the BEMS) should be
should be of an enhanced grade as defined by
connected to the SPS standby emergency
BS 5839-1:2002, and should be installed as a
generator.
Category 3 cable as defined by BS 7671.

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17 Validation and commissioning

17.1 This chapter describes the recommended level of • BS EN 60034-2;

validation and commissioning required for all new
• BS 5000-50:1982, IEC 60681-1:1980; and
and modified fixed wiring systems. The chapter
does not provide a fully comprehensive scope of • BS 7698.
works, but gives a general overview. Designers
and stakeholders may wish to consider acceptance Factory testing
of standard equipment factory or type test 17.5 The manufacturer should conduct a full set of tests
certificate items, rather than repeat the test after as described for site and dynamic tests below. For
installation, which in certain circumstances may be verification of dynamic load tests, a reactive and
difficult to perform. resistive load bank should be used. The project
17.2 Procurement of projects, which includes electrical engineer should witness all factory testing. The
installations (and others), should include adequate generator should be located in an environment
time and organisation to perform the required similar to that of the main site during any factory
validation and commissioning programme for any test.
works associated with the fixed wiring of the site.
Clearly, for the range of fixed wiring schemes Site testing
within healthcare premises, it is not possible to 17.6 Before any dynamic tests are carried out on a new
provide a general rule of thumb. Design teams engine, the following procedures and static tests
should consult with the contractor and planners should be carried out: all generator lubrication and
when allocating resources to the validation and cooling circulation systems should be fully filtered;
commissioning process. Inappropriate validation after descaling the circulation, systems should be
and commissioning may lead to failure of the fixed sealed; the oil circulation systems should be filtered;
wiring system. and the filters should be replaced after all tests have
17.3 The CIBSE Commissioning Manual (CCM), been completed and prior to handover. Checks on
which contains very useful data and commissioning the engine crankshaft deflection (at the bearings)
techniques for building services in the construction should be made and recorded for operational
industry, provides valuable guidance in general maintenance records. Verification of the stator
commissioning strategies. The CCM also describes insulation resistance with the manufacturer’s
the design considerations for the construction type test records should be made. The ratio of
industry in a similar manner to Chapter 3. the one-minute reading and ten-minute reading
(Polarisation Index PI) should be at least 2.
Validation of specific plant The installation resistance of all control circuits
should be measured. Verification of the contract
Generators and CHP plant documents with installed plant should be made,
with all appropriate indications, including fault and
17.4 Generating plant, including wind turbines, PV cells control indication lamps and alarms.
and CHP, should be tested as a complete system,
including the actual equipment control panel and Dynamic tests
functionality of the controls. Plant should be tested
in accordance with the relevant British Standards; 17.7 The dynamic tests on site should include the
see: following witnessed observations:

• BS 5514; a. lubricating oil pressure and pressure trip;

• BS 4999;

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 17 Validation and commissioning

b. lubricating oil and jacket water bypass terminal voltage should not vary by more than
automatic valves opening during engine warm- 15% following a step load increase from no load to
up including a series of test starts and checks, as 60% load, and then return to within 3% of the
follows: rated voltage within 0.5 seconds.
– the ability to start up within the specified
Multiple generators
time;
17.9 It should be verified that multiple generators,
– overspeed trip;
running in parallel (whether with the PES supply
– speed variation within specified limits; or not, G59/1 regulation), share the connected
load in equal proportions. The connected load
– voltage regulation and open-circuit
should be varied and a measure of each generator
characteristic;
terminal voltage made. The generator engine
– electrical trips of generator by overcurrent, speeds should also be equal. Excessive differences
reverse power protection relays at in generator field currents may lead to the
minimum plug settings with generator generators drifting out of synchronisation.
below 25% FL (or primary injection);
Parallel operation with the PES
– a full-load run of not less than four hours,
followed by a one-hour, 10% overload test 17.10 Where generators are intended to operate in
and full-load protection trip – the test full parallel with the PES, tests to verify that the
load should be obtained by either a ballast generator speed varies in conjunction with any
load bank, or synchronised to the normal change in the PES frequency should be made.
supply; When the supply frequency varies, the generator’s
fuel governor should modulate similarly and adjust
– fuel-oil inlet pressure;
the fuel input accordingly. The governor speed
– fuel-oil injector settings; characteristic over a speed range of 100%–105%
– temperature rise of jacket cooling water; should be at synchronous speed, given a load
change from full load to no load respectively. From
– temperature rise of lubricating oil; no load to 110%, the governor should be stable
– temperature rise of charge air across and sensitive, and should respond to prevent
turbocharger, if fitted; overspeed excursions reaching 110%. If a speed
of 110% is reached, the governor overspeed
– temperature of exhaust gases at each protection should close the engine fuel rack,
cylinder head; cutting off the fuel supply to the engine.
– 240 V stator winding heater disconnects
when circuit breaker closes; Power factor correction

– ambient conditions; 17.11 Any installed power factor correction (PFC) units
connected to any part of the generator-supplied
– noise acoustic levels, engine/background; network should be able to be isolated when the
– verification of the generator voltage rise. generator is supplying the load. Verification of this
control should be demonstrated at commissioning.
Voltage regulation Where the PFC units continue to be connected
across the generator output, a reduced field
17.8 The generator terminal voltage should be verified excitation current may result, making the
to be within ±2.5% from no load to 110% load generator output become unstable. PFC units may
conditions. The voltage regulation should be be fitted with enhanced modulation control such
checked with the applied load varied up and down that the PFC does not produce a leading power
in the range from no load to 110% load several factor while the generators are connected to any
times, hence simulating actual conditions. The part of the network.
generator terminal voltage on starting should not
overshoot the nominal terminal voltage by more Operational tests
than 15%, and return to within 3% of the rated
voltage within 0.15 seconds. The generator 17.12 After the generator has been fully tested as
identified above, an assessment of the actual fuel

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consumption should be made, and checks to verify This part of commissioning is essential to protect
that there is adequate fuel storage (on site) for the operational life of the batteries.
200 hours of continuous full-load operation. The
manufacturer should hand over all test records and Indications and alarms
insurance certificates, which should be held in the 17.16 All local and remote indications and associated
building logbook and operational maintenance alarm combinations for normal use or failure in
manuals. The generator should be run against the operation should be demonstrated and recorded.
building load, and verification of all phase failure
and control devices established. Where the Isolated power supplies
generator is arranged to synchronise with other
generators, this should be demonstrated within the 17.17 IPSs should be commissioned and validated in
required time, voltage and frequency tolerances. accordance with the requirements of IEC 60364-
Where the generator is designed to operate in 7-710 and manufacturers’ recommendations.
parallel with the DNO connection, verification 17.18 Designs should ensure that the IPS integral
of the G59/1 relay should be established. The distribution board has the correct protective
commissioning and operational testing of devices and is correctly labelled.
generators will require the DNO’s engineer to
witness and authorise. 17.19 Verification of alarm indicators, local and remote,
should be demonstrated.
Uninterruptible power supplies 17.20 Measurements of all individual circuit insulation
17.13 The uninterruptible power supply (UPS) should resistances should be made as part of the general
provide a no-break supply rated to the load testing and commissioning stage carried out by the
equipment for the required endurance period. The electrical installations contractor and as required
equipment should continue to function normally by the IEE Regulations. The results should be
when the normal supply is disconnected. The compared with the reading indicated on the
battery endurance capacity in ampère-hours should insulation monitoring device (IMD). The
be verified under load conditions. IMD should be tested by decreasing the circuit
insulation resistance to prove the alarm system.
17.14 Typical commissioning tasks should include:
17.21 The leakage current of the isolation transformer
a. the supply (a UPS) should include a test to should be tested when the transformer is energised
verify that the supply changeover occurs within and with the secondary open circuit. The value
0.5 seconds; should be <0.5 mA.
b. verifications to ensure that the UPS synthetic 17.22 Where the IPS is connected to a primary supply
sinusoidal output is within specification and secondary supply (generator), a test should
tolerance of the normal mains sinusoidal ac verify that the supply changeover (at the point
waveform; of common coupling) occurs within 0.5 seconds
c. verification of the total harmonic distortion or 15 seconds (depending on the actual circuit
(THD) should be within the tolerance given in intention). This test will require the input of the
the design specification; main electrical contractor and IPS contractor.
d. the UPS should be operated at a load greater
than 50% on battery duty to establish the true
Fixed wiring distribution, switchgear
battery autonomy. and protection
17.23 The fixed wiring system should be verified and
Environment commissioned in accordance with BS 7671.
17.15 The commissioning of the environment systems 17.24 All testing, verification and commissioning
of the UPS room should be coordinated with all will only be undertaken by suitably competent
parties to establish that design conditions have personnel (that is, having obtained a qualification
been satisfied. Deviations from the design compatible with the City and Guilds Certificate
conditions may be best achieved by changes to the C&G 2391; see Health Technical Memorandum
ventilation system, rather than replacing a UPS. 06-02).

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17.25 The recommended initial test and verification As-installed drawings

for the fixed wiring systems is given in the IEE
Guidance Note No 3 ‘Inspection and Testing’ 17.32 The following list provides a minimum acceptable
Sections 1 to 3 inclusive. level for the as-installed drawings. Project contract
documentation should be written and agreed with
17.26 Verification and commissioning of the fixed the healthcare organisation, and should clearly
wiring system should demonstrate that the indicate which drawings are relevant to the
earthing systems employed comply with the TN-S particular project and any additional drawings that
system as defined in BS 7671. The only exception may be required:
to such earthing methods will be any IT earthing
systems associated with the ISS employed in the a. HV network – layout and single-line schematic
patient environment Groups 1 and 2. to cover the whole site:

17.27 Verification and commissioning should (i) the drawings should include substation
demonstrate that a consistent voltage rise (phase and equipment references;
rotation) is employed throughout the electrical b. HV switching and transformer schedule to
infrastructure, including the connection to the cover the whole site on one drawing:
PES and any secondary or tertiary power sources.
(ii) comprehensive equipment details with CT
and VT relay ratings settings etc;
Records to be kept
c. principal earthing drawing – layout and single-
17.28 All tests and inspections should be recorded.
line schematic to cover the whole site on one
A collection of sample record sheets covering the
drawing:
more common elements of the fixed wiring is
provided in Appendix 2. Designers may wish to (i) the layout drawings should use the site
adopt other forms put forward by manufacturers general arrangement as a background and
or from software design programs. These will be show all main earthing points, regardless
accepted if they cover the minimum information of being an HV earth, LV earth, generator
provided on the sample forms. earth or form the lightning protection
systems;
17.29 The records should include all test certificates
relating to electrical test and pressure test as (ii) the schematic drawing should clearly
appropriate. Records for all (off-site) manufactured show the interconnectivity of all earthing
items demonstrating conformity to the European systems, and the measured resistances of
Community legislation (CE marking) should be each earth electrode;
provided. d. LV main distribution – layout and single-line
17.30 As appropriate, a comprehensive operational schematic – one drawing per substation:
maintenance manual for all plant and accessories, (i) the layout drawings should use the
including protection and switchgear items, should building general arrangement as
be provided at project handover or during the background. The layout drawing should
validation and commissioning period. The show all containment sizes;
operational maintenance manual should describe
how the design satisfies the design strategy and (ii) the schematic drawing should indicate all
should indicate the intended mode of operation. cable sizes, protective device rating and
The operational and maintenance manual should setting, switchgear and fault levels at
include a section to describe any action required to switchboards;
change the distribution for power supplied from e. LV sub-main distribution – layout and single
the PES, and/or generator-supplied power. line schematic per switchroom:
17.31 The operational maintenance manual should (i) the layout drawings should use the
include a full single-line diagram to show all points building general arrangement as
of isolation (with room name/number references). background. The layout drawing should
show all containment sizes;

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(ii) the schematic drawing should indicate all Building logbook

cable sizes, protective device ratings and
settings, switchgear and fault levels at 17.33 The building logbook is now a standard
switchboards; requirement for all new buildings throughout the
construction industry, and is referenced in the
f. LV final circuit distribution – layout and single Building Regulations. The items identified
line schematic per distribution board: throughout Chapter 17 fulfil the requirements of
(i) the layout drawings should use the the building logbook. Where the capital project
building general arrangement as relates to only part of the site or adaptations of
background. The layout drawing should existing electrical circuits, the existing building
show all containment sizes; logbook should be updated.

(ii) the schematic drawing should indicate all 17.34 The purpose of the building logbook is to provide
cable sizes, protective device ratings and a single collection of all relevant information
settings, switchgear and fault levels at relating to the architecture and building services at
distribution boards; the site. The information should facilitate a source
of all data to enable modifications to any part of
g. general arrangement drawings of 1:20: the building services, and to operate the plant and
(i) all substation HV rooms; services in an energy-efficient way homogeneous to
the design intent.
(ii) all substation transformer rooms;
17.35 The CIBSE technical memorandum TM31
(iii) all substation LV rooms; ‘Building Logbook’ provides a validated guide
(iv) all generator house/enclosures; template for small businesses. The CIBSE Building
Logbook, CD-ROM, Logbook Template Standard
(v) all rooms with CHP or other alternative
(LBTS) or Logbook Template Customisable
power sources;
(LBTC) may prove more useful when the project
(vi) all LV main distribution switchrooms or relates to a new build. The CD-ROMs contain
rooms with LV distribution equipment; electronic templates. LBTSs are the standard
templates, which may or may not dovetail into
(vii) all LV sub-distribution switchrooms;
the project, while LBTC contains customisable
(viii) all electrical risers; templates that may be user-adjusted to suit the
(ix) typical cross-section ceiling voids showing specific job.
principal routes and areas of high service 17.36 The building logbook will fulfil some of the
density; designer’s duties for compliance with the CDM
h. system and control wiring: Regulations.

(i) where the project includes any associated

electrical services (for example fire alarms,
nurse-call systems), layout drawings (using
the building general arrangement drawing
as a background) to show the location of
any associated devices and a single-line
schematic of the system should be
provided, including any associated panel
wiring diagrams.

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 Appendix 1 – Maximum interruption times to
the primary supply

Figure 45 Maximum interruption times – primary supply

Clinical IEC 60364- Maximum Electrical Supply Interruptions Times (Seconds)
Risk Service 7-710
Category Group < 0 to 0.5 > < 0.5 seconds to 15 seconds > < 15 to 10800 >

Medical Equipment with IPS 2

General Medical Equipment 0–1
General Electrical Circuits 0 C
5
Fixed Medical Lighting and Escape Lighting 0
General Lighting 0 A
Mechanical Services 0
Medical Equipment with IPS 2
General Medical Equipment 0–1
General Electrical Circuits 0 C
4
Fixed Medical Lighting and Escape Lighting 0
General Lighting 0 A
Mechanical Services 0
Medical Equipment with IPS 0–1
General Medical Equipment 0
General Electrical Circuits 0 C
3
Fixed Medical Lighting and Escape Lighting 0
General Lighting 0 B
Mechanical Services 0
Medical Equipment with IPS 0
General Medical Equipment 0
General Electrical Circuits 0 C
2
Fixed Medical Lighting and Escape Lighting 0
General Lighting 0 B
Mechanical Services 0
Medical Equipment with IPS 0
General Medical Equipment 0
General Electrical Circuits 0 C
1
Fixed Medical Lighting and Escape Lighting 0
General Lighting 0 B
Mechanical Services 0
NOTES A Standby Lighting Grade A (Lighting provided to the same, or nearly the same, lighting levels, achieved
at normal electrical supply)
B Standby Lighting Grade B (Lighting provided at a reduced lighting level, 33%, of that achieved at
normal electrical supply)
C Battery Inverter Unit provided for items such as fire alarms, security, computer network servers, and
local computer systems as appropriate.
When the alternative power source has been connected, it should remain connected until the primary
power source has been restored and stabilised.
Tertiary power sources (UPS) will be required for periods less than 0.5 seconds (refer to Chapter 14)
Secondary power sources (generators) will be required for periods greater than 0.5 seconds (refer to
Chapter 8)
Indicates that an electrical supply must be available within the specified timeband
Indicates that an electrical supply must be available where equipment requires

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Appendix 2 – Sample test record sheets

Figure 46 Test sheet – fixed panels

Plant Item Fixed Panels and Switchboards Inspection Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
1 Check switchboard for damage or incomplete work
2 Check all labels warning symbols, switchboard circuit identification labels
are correct
3 Check switch is fixed and mounted correctly
4 Check switchboard protective earth conductor are connected to the main
earth terminal (MET)
5 Check termination lugs and bolts for tightness
6 Check VT & CT compartment assembled correctly
7 Check shutter linkage and the locking facilities
8 Rack all devices into service position
Note: all shutters should have a smooth movement
9 Check all busbar joints with torque spanner and inspection contact spaces
Bolt size
Specified torque setting
10 Isolate VT, remove fuselinks of Voltmeter and CTs
Measure (i) IR py/Sy (iii) IR CT Sy
(iii) IR busbar and circuit bar phases
11 Measure total conductance of HV busbar phases along the switchboard
by ohmmeter measurement
a) between adjoin cubicle busbar phase spouts (BS)
b) between circuit spouts (CS) and cable box (BX)
Note: Estimate Resistance from 1.0 m of conductor
Between Ph1 Ph2 Ph3 Res
Spouts
BS1 and 2 μΩs
BS2 and 3 μΩs
BS3 and 4 μΩs
1CS and 1 BX μΩs
2CS and 2 BX μΩs
3CS and 3 BX μΩs
4CS and 4 BX μΩs

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 Appendix 2 – Sample test record sheets

Figure 47 Test sheet – HV switchgear pressure test

Plant Item HV Pressure Test Switchboards Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
1 Before HV test ensure all covers and fittings are replaced and secure
2 Check components correctly assembled and fitted
3 Check free operation of all switch movement etc
4 Check all earthing facilities and switch positions
5 Check
(i) all instrument fuselinks removed
(ii) VT isolated and CT fuselinks removed
(iii) IR test busbar before and after pressure test
MegΩ Values
Ph1-Ph2/Ph2-Ph3/Ph3-Ph1
Ph1-N/Ph2-N/Ph3-N
Ph1-E/Ph2-E/Ph3-N
6 Adhere to the Electrical Safety Rules Health Technical Memorandum
06-02
7 Pressure test busbars as
0.4 kV system @ 2 kV for one minute
11 kV system @2 kV for one minute
Voltage ……….. kV
Humidity …...… %
Temperature ….. °C
Phase Ph1-Ph2/Ph2-PH3/Ph3-Ph1
Leakage Current
Phase Ph1-N/Ph2-N/Ph3-N
Leakage Current
Phase Ph1-E/Ph2-E/Ph3-E
Leakage Current
8 Check IR of close, open and control circuits
Note: HV Equipment should be energised as soon as practical after test,
to ensure faults are checked
9 Verify switch labels with circuits and record drawings

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Figure 48 Test sheet – Switchboard devices electrical test

Plant Item Switchboard Devices Electrical Test Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
1 Ensure cubicle busbar/circuit shutter door mechanisms are locked shut, if
board energised
2 Carry out IR test between devices open contacts and when open, closed
between phases and frame earth
Values Ph1/Ph2/Ph3
3 Pressure test busbars as
0.4 kV system @ 2 kV for one minute
11 kV system @ 2 kV for one minute
Voltage ……….. kV
Humidity …...… %
Temperature ...... °C
Phase Ph1-Ph2/Ph2 -PH3/Ph3-Ph1
Leakage Current
Phase Ph1-N/Ph2-N/Ph3-N
Leakage Current
Phase Ph1-E/Ph2-E/Ph3-E
Leakage Current
4 Rack devices into cubicle isolated position for the close open operational
test
5 Check local control, close and trip of device at the rated battery voltage,
minimum of ten operations.
Check the operation of the close and trip at 80% of the rated applied
close battery voltage
6 Check the trip mechanism at 50% of the rated applied trip battery
voltage
7 Check time of closing mechanism operating spring to recharge, at 80% of
rated applied voltage
8 Check operation of “auto-change” devices used for Emergency Generators
and normal DNO supply as appropriate for the distribution strategy

126
 Appendix 2 – Sample test record sheets

Figure 49 Test sheet – Transformer mechanical test

Plant Item Transformer Mechanical Test Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
1 Check drawing, general inspection for damage and completeness
2 Check all components fitted to general arrangement
3 Prove tightness of all fastenings
4 Check all labelling to transformer schedule
5 Check transformer correctly positioned in bay for cable box entries/
bushing connections
6 Check colour of desiccant crystals (as supplied)
7 State type of coolant in tank
8 Check if transformers filled with oil/fluid to operating level yes/no
9 Check for any coolant leaks
10 Check cable box details agree with cable details and requirements
11 Check location of loose CTs, if provided, and method of connection in
cable box or to star point neutral
12 Check position of transformer earth lug and connection to main earth
system

127
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 50 Test sheet – Transformer electrical test part A

Plant Item Transformer Electrical Test Part A Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of 2
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
1 Check IR of transformer cooling fan motors, and cable terminations
(where appropriate)
2 Check transformer cooling fan motor electrical function in local and
remote modes
3 Check transformer cooling fan motor overload/time by three-phase and
single-phase injection
4 Analyse the tank and Buchholz relay oil for clarity and resistance
5 Take IR readings of HV and LV windings
6 Check operation of all protection trips and alarms at initiating and
control sections
7 Check fan controls are operational
8 Check cable box and bushing connections tight, oil tank free and secure
9 Transformer enclosure locked and secure
10 Check marshalling box wiring connections at termination blocks for
tightness and correct labelling
11 Check IR of control wiring using megger
(i) Marshalling box control wiring
(ii) Buchholz relay (if fitted)
(iii) Temperature indicators Coolant Core
12 Fill transformer with coolant to operational level with new oil complying
with BS 148
13 Check IR of Core insulation to earth before link is covered with coolant,
during the fill operation
14 Check IR when transformer filled with coolant
HV LV
PPh1–PPh2 /
PPh2–PPh3 /
PPh3–PPh1 /
Sph1–Sph2 /
Sph2–Sph3 /
Sph3–Sph1 /
Ph1–Ph1 /
Ph2–Ph2 /
Ph3–Ph3 /
N–E /
All Primary Phases to Earth
All Secondary Phases to Earth

128
 Appendix 2 – Sample test record sheets

Figure 51 Test sheet – Transformer electrical test Part B

Plant Item Transformer Electrical Test Part B Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 2 of 2
Project Engineer
No Activity Witness Date
Healthcare Project Engineer
Premises Engineer
15 Winding ratios at each off-load tap position and the transformer vector
group
(i) apply 0.4 kV 3-phase ac to HV winding terminals and interconnected
Ph1 HV to Ph1 LV
(ii) Winding ratio
HV Ph1–Ph2, Ph2–Ph3, Ph3–Ph1
LV Ph1–Ph2, Ph2–Ph3, Ph3–Ph1
Tap
–10%
–5%
–2.5%
0%
2.5%
5%
10%
(iii) Vector Group
Ph1–Ph2
Ph2–Ph3
Ph3–Ph1
Ph1–Ph2
Ph2–Ph3
Ph3–Ph1
PPh2–Sph3
PPh3–Sph2
16 Check trip/alarm supplies voltages
(i) At circuit breaker
(ii) At transformer
(a) Buchholz
(b) coolant temperature
(c) Tank pressure
(d) cooling fans running
17 Check IR of Tap changer control pane (if fitted)

129
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 52 Test sheet – Secondary injection IDMT relay

Plant Item Secondary Injection Test (IDMT Relay) Completed
Identifications/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet
Healthcare Premises Engineer 1 of
Project Engineer
Manufacturer’s Description Setting for Test
Test R Y or N R
1 General Inspection
2 Check Contacts close at zero Tm time and follow through
3 Check Flag operation
4 Measure time to reset from contacts
Close at 1.0 Tm
5 Check trip isolation contacts
6 Set 100% Pm, check no creep at 1.0 Psm, and creep
commences at/or before 1.25 Psm current values
7 Check Plug bridge continuity, max Pm setting and with
plug out
8 Check relay, T shorts removed
CT ratio ……/…… type Relay Controls Relay Operating Times
Time/current characteristic at 100% Pm Tm Psm Amps R Y or N R

Pm and at applied setting 1.0 1.3

2
100% 0.5 2
1 4
Applied setting 2
Fag Setting Final setting applied

Remarks

Note: Settings for electronic IDMT relays are generally software set. Therefore the maintenance test of electronic IDMT relays
may be reduced to a check that the commissioning settings have not been changed, or the network (protected by the
IDMT relay) has not changed, which would require a re-commissioning of the IDMT relay. The manufacturer’s data sheet
should be used in all circumstances

130
 Appendix 2 – Sample test record sheets

Figure 53 Test sheet – secondary injection instantaneous relay

Plant Item Secondary Injection Test Instantaneous Relay Completed
Identification/Location Incomplete
PC Address File
Contractor
Manufacturer
Serial Number
Witness Print Name and Sign Date Sheet 1 of
Healthcare Premises Engineer
Project Engineer
Witness Date
Healthcare Project Engineer
Premises Engineer
Test R Y or N B

General Inspection
Check Trip isolation contacts
Check Flag operation
Check CT shorts
Plug bridge continuity (Inst o/c relays)
R Y or N B
Plug setting Op Amps Plug setting Op Amps Plug setting Op Amps

Plug out Plug out Plug out

R Y or N B
Stab resistor value
With stabilising resistor series and CT in
Applied setting
shunt
Operating volts
Operating current
Flag reset Final setting applied

131
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Figure 54 Test sheet – LV final distribution board test results

Description of Works
Circuit Over- current Wiring Test Results
Description device conductor

Polarity
Continuity Insulation resistance Earth loop Functional
Short circuit Impedance Testing
capacity
kA
Type Rating Live cpc R1 + R2 Ring Live/ Live/Earth Zs RCD Other
in A R2 Live Time
mm2 mm2 Ω MΩ Ω
Ω MΩ mS
1 2 3 4 5 6 7 8 9 10 11 12 13 14

Deviations from the Wiring Regulations and Special Notes

Note the test sheet shown here is a much reduced format of the form provided by the IEE Regulations

132
 Appendix 2 – Sample test record sheets

Figure 55 Lighting commissioning certificate

LIGHTING COMMISSIONING DETAILS
Location
Building
Areas Covered
Relevant Distribution Board
Relevant Controls
Test Engineer
Approved Engineer
Test Date
Test Commissioning Test Result Follow Up
Complete
Groups of luminaires are assigned to the correct positions in grid switch or grid
single circuit dimmer
Emergency lighting complies with recommendations of BS 5266/BS 12464-1
Luminaires and remote control gear are of the correct make and type
Fixed luminaires have been installed at the correct orientation
Fluorescent lamps have the correct phosphor
Lamps are of the correct colour temperature (Rendering Index Ra **)
All lamps are the correct wattage and voltage ratings
Exterior floodlights have been aimed to drawing and according to terms of planning
permission
Horizontal illuminance on horizontal tasks(s) is at specified level
Vertical illuminance on vertical tasks(s) is at specified level
PIR detector systems are programmed and operate correctly
Lighting levels associated with control signals have been chosen

When commissioning lighting installations, grouping rooms with similar functions and lighting designs, for example toilet areas
may reduce the number of repeated tests.
A more comprehensive lighting commissioning schedule is available from CIBSE

133
Electrical services – HTM 06-01 Electrical services supply and distribution: Part A – Design considerations

Appendix 3 – Drawing symbols

Figure 56 Drawing symbols used in this Health Technical Memorandum

The symbols below are all generic versions of the British Standard symbols. In some case where the device type is not
specific to the figure in the Health Technical Memorandum text, a symbol representing more than one device type is
indicated.

High voltage isolator or disconnector

Bus coupler with dual supply
and changeover arrangement
and control wires
Transformer

G High voltage generator

G Low voltage generator

CHP Combined heat and power plant

Circuit breaker no control implied

Fused switch or switch fuse

Low voltage isolator or disconnector

Non-specific load

Typical substation High voltage ring main unit

with two ring switches and
tee circuit breaker
(no implied control or protection)

134
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