Company Directive

ENGINEERING SPECIFICATION
EE SPEC : 89/2

Relating to Fixed Earthing Systems for Major Substations

Summary

This document specifies requirements for design, installation and commissioning of fixed
earthing systems at major substations. It is intended for use as part of Invitations to Tender or
for schemes involving Competition in Connections or for connections where the customer
site will be designed to accommodate WPD >33kV intake equipment. It does not replace
ST:TP21B which is for internal WPD use and contains guidance on the application of the
above requirements.

Author: Simon Scarbro

Implementation Date: November 2012

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 Document Revision & Review Table
Date Comments Author
22/11/12 Minor amendments to pages 3, 4, 11, S. Scarbro
12, 17 & 18, all of which are self-
explanatory.
20/12/12 Commissioning tests are now defined S. Scarbro
in section 9.0.

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1.0 INTRODUCTION

1.1 This document specifies requirements for design, installation and commissioning
of fixed earthing systems at major substations.

1.2 Guidance relating to the application of this Specification to designs involving

33kV Competition in Connections is provided in ‘Outline Design Guide for
Customers Seeking WPD Adoption of Indoor 33kV Switchgear and Associated
Equipment Housings’, available on request.

2.0 SCOPE

2.1 This document is intended for use as part of Invitations to Tender or for schemes
involving Competition in Connections or for connections where the customer site
will be designed to accommodate WPD >33kV intake equipment. It does not
replace ST:TP21B which is for internal WPD use and, whilst meeting the
requirements set out in this document, provides additional guidance on the
application of these requirements.

3.0 COMPLIANCE WITH STANDARDS

3.1 This document makes reference to or implies reference to the following

documents and it is important that users of all standards and Technical
Specifications ensure that they are in possession of the latest issues together with
any amendments. Design and installation shall comply with the listed
standards unless otherwise specified to the contrary. EA TS 41-24 shall take
precedence over BS 7354 and BS 7430.

3.2 Standards

3.3 Whilst the IEC base document is listed for information, the prime document
which shall take priority is the British Standard enacting the European Standard
(EN) or European Harmonisation Document (HD).

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 British EN/HD/ISO IEC/ISO
Title
Standard Reference Base

BS 7354 Design of High Voltage Open Terminal

Substations
BS 7430 Code of Practice for Earthing
BS 1432 Specification for copper for electrical purposes:
high conductivity copper rectangular conductors
with drawn or rolled edges
BS 6360 Specification for conductors in insulated cables
and cords
BS 4109 Specification for copper for electrical purposes:
Wire for general electrical purposes and for
insulated cables and flexible cords
BS 2898 Specification for wrought aluminium and
aluminium alloys for electrical purposes. Bars,
extruded round tube and sections
BS 1377-3 Methods of test for soils for civil engineering
purposes. Chemical and electro-chemical tests
BS EN13043 Aggregates for use in bituminous mixtures and
surface treatments for roads, airfields and other
trafficked areas
BS EN60298 EN 60298 A.C. metal-enclosed switchgear and controlgear IEC 298
for rated voltages above 1kV and up to and
including 52kV.
Guide to the short-circuit temperature limits of IEC 60724
electric cables with a rated voltage not exceeding
0.1/1.0kV
Effects of current on human beings and IEC 60479-1
livestock. Part 1: General aspects

3.4 ENA Technical Specifications and Engineering Recommendations

Technical Specification Title

EA TS 41-24 Guidelines for the Design, Installation, Testing and Maintenance
of Main Earthing Systems in Substations
EA TS 43-94 Earth Rods and the Connectors
EA ER S34 A Guide for Assessing the Rise of Earth Potential at Substation
Sites
EA ER S36 Identification and Recording of ‘Hot Sites’ – Joint Electricity
Industry/British Telecom Procedure

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4.0 DEFINITIONS

CCITT International Telegraph and Telephone Consultative

Committee.

CDEGS A software package permitting the investigation of earthing

problems.

EARTH ELECTRODE A conductor or group of conductors in intimate contact with,

and providing an electrical connection to, earth.

EARTH IMPEDANCE The impedance between the earthing system and remote
reference earth.

EARTH POTENTIAL The difference in potential which may exist between a point
on the ground and remote reference earth.

EARTH RESISTANCE The resistance of the earth between the earth electrode and
remote reference earth.

EARTHING CONDUCTOR Conductor which connects plant and equipment to the earth
electrode.

EARTHING SYSTEM The complete interconnected assembly of earthing conductors

and earth electrodes (including cables with uninsulated
sheaths).

HOT SITE A substation where the rise of earth potential exceeds the
appropriate CCITT level. The CCITT level is 430V RMS
unless the power circuits contributing to the earth fault
currents are high reliability type, having an operating voltage
of 33kV or greater and controlled by switchgear with main
protection that will clear both a line or busbar earth fault
current within 500ms and generally within 200ms. In this
latter case the CCITT level is 650V RMS.

HOT ZONE The area over which the rise of earth potential may exceed
the appropriate CCITT level.

LIVE Electrically charged.

MALT/MALZ Modules of CDEGS which permit the analysis of earth

electrode arrangements in earth structure models. MALZ is
suited to large electrode systems.

RESAP A module of CDEGS which permits the construction of

equivalent earth structure models given soil resistivity data.

STEP VOLTAGE The part of the earth potential rise due to an earth fault which
can be picked up by a person with a step-width of 1m,
assuming that the current is flowing via the human body from
foot to foot.

TOUCH VOLTAGE Voltage appearing during an insulation fault, between

simultaneously accessible parts; hand-to-foot or hand-to-
hand.

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5.0 GLOSSARY OF SYMBOLS

IE Total current passing to ground

IF Total earth fault current
IES Component of IE passing to ground through mesh electrode
ISR Current returning through earthwire or cable sheath
RES Grid electrode earth resistance
ZE Substation earth impedance
UE Rise of earth potential of substation relative to remote reference earth
Utouch Touch voltage
Ustep Step voltage

6.0 FUNDAMENTAL REQUIREMENTS

6.1 Design Requirements

6.1.1 Earthing systems shall be designed to:

a) Comply with the Electricity Quality and Continuity Regulations

b) Comply with EA TS 41-24, as modified by Company Policy
c) Control the risk of human electric shock
d) Permit the flow of sufficient earth fault current for satisfactory operation of
System Protection
e) Permit the flow of earth fault current without exceeding electrical, thermal
and mechanical limits
f) Permit correct operation of lightning protection
g) Minimise earth potential rise to avoid 'hot site' classification where
reasonably practicable
h) Minimise the extent of the 'hot zone' so far as reasonably practicable
i) Avoid voltage rise above equipment withstand levels (i.e. 3kV)
j) Minimise lifetime ownership costs
k) Perform the above functions through its expected lifetime.

6.1.2 The risk of electric shock shall be controlled by use of voltage limits derived from
the C1 current-time curve of IEC 60479-1.

6.1.3 The short-time current rating of earthing system components that may carry fault
current shall be consistent with the specified rating of the plant (e.g. as specified in
the Invitation to Tender) and current division. Back-up protection operation shall
be assumed with an operating time of 3s.

6.1.4 The thermal rating of earth electrode shall be in excess of the current-time curve
defined by protective devices.

6.1.5 Earthing system components that are not required to carry earth fault current
(intentional or otherwise) shall be sized according to mechanical and corrosion
performance considerations.

6.1.6 Lifetime ownership cost considerations shall include purchase cost, corrosion
performance and theft deterrence.

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6.2 Installation Requirements

6.2.1 Earthing systems shall be installed such that the design requirements are met. The
method of installation shall:

a) Employ safe systems of work, controlling the risk of electric shock and
applying WPD’s Distribution Safety Rules
b) Avoid damage to/from other works (e.g. drains, cables etc)
c) Maximise the lifetime by suitable measures where hostile soil conditions
exist.

6.3 Commissioning Requirements

6.3.1 Post-installation commissioning tests shall be performed to verify compliance

with the design requirements. Safe systems of work shall be employed.

7.0 DESIGN

7.1 Soil Model

7.1.1 Perform soil resistivity measurements at two representative locations near the
substation using the Wenner test method and appropriate probe spacings (i.e. 1m,
1.5m, 2m, 3m, 4.5m, 6m, 9m, 13.5m, 27m, 36m and 54m).

7.1.2 Analyse the soil resistivity data, taking account of available geotechnical data,
using CDEGS RESAP software (full, unrestricted version) and determine a
representative multi-layer soil model.

7.2 Substation Earthing System Layout

7.2.1 The substation earthing system layout shall be based upon the following but
modified as necessary to meet the fundamental requirements:

a) Comprise a perimeter ring of electrode with interconnections forming a

mesh/grid.
b) Enclose the area containing all exposed conductive parts that may become
LIVE during an earth fault.
c) Have a maximum spacing between parallel mesh electrode of 10m at the
outer edges of the mesh and 12m in the inner area. Actual mesh size and
electrode position shall be determined by electric shock considerations.
d) Provide duplicate ‘independent’ connections to the mesh where connections
are critical (e.g. transformer neutrals, phase-earth fault throwers, overhead
line terminal towers, indoor switchgear earth bars).
e) Where reasonably practicable, have vertical earth rods at the corners of the
mesh, minimum 3m length.
f) Achieve EITHER 2m separation from the compound fence (substation earth
electrode within fence and fence separately earthed) allowing for the swing
arc of metal gates and doors OR encompass the fence with multiple
connections to it. The separately earthed fence is the preferred method.
g) Be at a depth of 0.6m.
h) Connect to piling.

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 i) Where necessary for touch voltage control, connect to building floor steel
reinforcing (e.g. control room base) with a parallel loop of earth electrode
under the plinth.
j) Have the fence earthed at corners and additional points where crossed by
overhead conductors and not less than every 50m.
k) Have gateposts bonded together and gates bonded to gateposts.

7.2.2 Special arrangements are required for substations containing G.I.S switchgear due
primarily to high-frequency switching transients. Special arrangements are also
required for substations containing air-cored reactors due to the possibility of
induced currents from strong localised magnetic fields.

7.3 Substation Mesh Earth Electrode Resistance, RES

7.3.1 The resistance, RES, of the substation mesh earth electrode arrangement in the soil
model shall be determined. This shall be performed using CDEGS MALT/MALZ
software, whichever is more appropriate. NB MALZ is necessary for large
electrode systems (e.g. wind farms).

7.4 Substation Earth Impedance, ZE

7.4.1 The chain impedance of overhead towers and impedance of external horizontal
electrode shall be determined using ER S34 or equivalent. The substation earth
impedance, ZE, shall be determined from the sum of the parallel impedances.

7.5 System Protection Clearance Time

7.5.1 The total time to clear each relevant earth fault on the WPD system for use in
touch and step voltage assessment shall be provided by WPD.

7.6 Earth Potential Rise

7.6.1 The current flowing into ZE, IE, shall be determined from the total fault current, IF,
by allowing for current returning via other (non-ZE) paths:

a) Current returning by local transformer neutrals, IL, shall be determined by

WPD
b) Current returning via earth wire or cable sheath, ISR, shall be determined
from ER S34 or equivalent.

7.6.2 No allowance for fault arc impedance shall be made.

7.6.3 The earth potential rise (EPR), UE, = IE x ZE = IES x RES, shall be determined for
the internal and external fault conditions.

7.7 Touch and Step Voltage Limits

7.7.1 EATS 41-24 defines touch and step voltage limits by fault clearance time. The
appropriate limits shall be determined for the internal and external fault conditions
assuming correct protection operation.

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7.8 Assessment of Touch and Step Voltages

7.8.1 The earth electrode layout shall be analysed in the soil model using CDEGS
MALT/MALZ software, whichever is more appropriate. The worst-case touch
and step voltages across the substation, internal and external to the compound
fence, including nearby buildings, shall be determined.

7.8.2 Compliance with the touch and step limits shall be checked, taking account of any
effective high resistivity ground surface covering (i.e. aggregate, tarmacadam etc).
For a ground surface covering to be considered effective it shall meet the
requirements of Table F1 in Appendix F. The electrode layout shall be adjusted
as necessary to achieve touch and step voltage compliance taking account of all
the fundamental design requirements.

7.8.3 Reducing the depth of the earth electrode reduces touch potential and increases
step potential. A minimum depth of 0.5m applies to conductors included in RES to
minimise variation should surface soil freeze.

7.9 Connectivity and Detailed Earthing Layout

7.9.1 The detailed design shall include connection of the following to the substation
earth electrode mesh via two independent routes:

a) Devices as necessary to provide System Earth(s) including neutral terminals

of power transformers, neutral earthing devices (i.e. liquid earthing resistors
or neutral earthing reactors) or earthing transformers.
b) Earthing conductor from earth terminals of phase-to-earth fault throwers
c) Overhead line terminal towers, where continuity is not provided by cable
sheath/armour
d) Indoor switchgear earth bar.

7.9.2 The design shall include connection of the following to the substation earth
electrode:

a) Earth switch terminals

b) Cable sheath/armour, subject to special requirements covered below
c) HV steelwork
d) Earth mats
e) Ancillary metalwork (accessible)
f) Structural earth electrode (i.e. control room floor steel reinforcing) where
necessary for touch voltage control.

7.9.3 Note, where frame-leakage busbar protection is present, the bonding shall not
circumvent the associated current transformers.

7.9.4 Above ground exposed conductive parts that can attain different potentials and
would present a hand-to-hand touch hazard shall be either:

a) Effectively bonded together, or

b) Effectively shrouded/insulated, or
c) Physically separated to control the hazard. A 2m separation shall be taken to
give effective separation.

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7.9.5 Note that it is important not to compromise the design (e.g. avoid connection of
earth mesh to metalwork outside of the interconnected mesh or connection to the
fence if this is intentionally separate etc).

7.9.6 Stance earth mats shall be provided to control touch potential for metalwork
which is not fully integral with the main equipment:

a) Air-break switch disconnector handles

b) Earth switch handles
c) Fault thrower handles.

7.9.7 The earth mat design shall ensure control of touch potential for all operator
positions.

7.9.8 The earth mat shall be connected direct to the handle with a spur connection to the
substation earth mesh electrode below ground level.

7.9.9 See below for cable sheath earthing and for fence and gate requirements.

7.10 Choice of Materials

7.10.1 Only Approved Materials shall be used to construct the formal substation earthing
system. These are given in Appendix A by standard and role. Other materials
may be offered for consideration for approval.

7.10.2 Aluminium shall not be used below ground level or within 150mm of it.

7.11 Selection of Earthing System Component Size

7.11.1 Appropriate size for each part of the earthing system depends on function:

a) Earthing conductor that may carry fault current to the earth electrode
b) Bonding conductor that is not reasonably likely to carry fault current
c) Earth electrode.

7.12 Earthing Conductor

7.12.1 Earthing conductor that may carry fault current intentionally (e.g. connection to
transformer neutral or phase-earth fault thrower) or following insulation
breakdown or other failure (e.g. fallen conductor) shall have a short-time current
rating consistent with Clause 6.1.3. For switchgear the requirements for earthing
of the enclosure set out in BS EN 60298, clause 5.3.1 shall also be met, subject to
the minimum conductor size requirements in 7.12.8 below.

7.12.2 Where, through switching the System Earth, connection can be made solid rather
than through a neutral earthing device, earthing conductor ratings shall take
account of this possible operational arrangement.

7.12.3 For 'critical' connections that are duplicated (e.g. connections to transformer
neutrals etc) each connection shall be fully rated.

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7.12.4 The short-time ratings of earthing conductor shall be assessed using the formula in
IEC 60724. Appendix B gives the formula and assumptions that shall apply
regarding material constants and permissible temperature rise. Appendix C details
calculated short-time ratings for standard conductor sizes.

7.12.5 Consideration shall be given to faults fed from one system voltage into that of
another system voltage (e.g. 132kV conductor dropping onto 33kV equipment if
physically possible).

7.12.6 For simplicity, selected standard sizes are given by substation type in Table 1
below. Note that these assume all joints are welded or brazed and shall be
checked against the requirements above.

7.12.7 Table C4 in Appendix C gives copper equivalent sizes in other materials.

Nominal 3s Fault Rating Required Standard size

Voltage cross-sectional in copper
(kV) area in copper
132/66 25kA (3-phase or single-phase, whichever 200 mm2 50mm x 4mm
higher) strip
2
132/66 31.5kA (3-phase or single-phase, whichever 260 mm 50mm x 6mm
higher) strip
2
132/66 40kA (3-phase or single-phase, whichever 330 mm 75mm x 6mm
higher) strip
2
33 16kA (3-phase or single-phase, whichever 130 mm 50mm x 3mm
higher) strip
2
33 20kA (3-phase or single-phase, whichever 160 mm 50mm x 4mm
higher) strip
2
33 25kA (3-phase or single-phase, whichever 200 mm 50mm x 4mm
higher) strip
2
11 12.5kA (3-phase or single-phase, whichever 100mm 25mm x 4mm
higher) strip or 100mm2
stranded
2
11 16kA (3-phase or single-phase, whichever 130 mm 50mm x 3mm
higher) strip or 150mm2
stranded
2
11 20kA (3-phase or single-phase, whichever 160 mm 50mm x 4mm
higher) strip

Table 1 - Standard earthing conductor sizes by substation nominal voltage

and fault rating

7.12.8 Where the required short-time current rating implies a cross-sectional area less
than 70mm2 copper-equivalent (ce), a minimum of 70mm2 ce shall apply. The
minimum permissible thickness of copper and aluminium strip shall be 3mm and
4mm, respectively. For stranded copper conductor the minimum strand diameter
shall be 3mm.

7.12.9 Fence earthing conductor, connecting the fence to its earth electrode, shall be a
minimum of 70mm2 ce.

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7.12.10 Earthing conductor used to connect to earth mats shall have a rating equivalent to
the associated HV steelwork earthing conductor.

7.13 Use of Steel Support Structure as Earthing Conductor

7.13.1 To minimise conductor theft, it is preferred that, where suitable, the steel support
structure is utilized as a conductor, replacing copper/aluminium earthing
conductor for part of the earthing conductor path. See Standard Substation
Construction drawing numbers XP1062 and XP1063. The steel support structure
shall not be used to replace copper/aluminium earthing conductor for earth
connections to earth switches or fault throwers and nor for connections to surge
arresters or capacitor voltage transformers.

7.14 Bonding Conductor

7.14.1 Bonding conductor that is not reasonably likely to carry fault current shall be a
minimum of 50mm2 ce (e.g. inter-gatepost bond, hand-rail bond, metal lamp-post
or security post bond. However, for bonds which may be subject to movement
(i.e. gate-gatepost bonds and metal trench cover bonds) a minimum of 35mm2 ce
shall apply. The minimum permissible thickness of copper and aluminium strip
shall be 3mm and 4mm, respectively. For stranded copper conductor the minimum
strand diameter shall be 3mm.

7.15 Earth Electrode

7.15.1 Each section of the interconnected earth mesh shall have a short-time current
rating:

a) No less than 60% of the earthing conductor rating given above

b) Equal to the earthing conductor rating given above where it connects single-
phase switchgear together.

7.15.2 Spur connections shall be fully rated where they connect to above ground earthing
conductor. In other cases, the spur connections shall be rated according to the
distribution of fault current.

7.15.3 In addition to the above, earth electrode shall be a minimum of 70mm2 ce and,
where applicable, the minimum permissible thickness of copper strip shall be
3mm. For stranded copper conductor the minimum strand diameter shall be 3mm.

7.15.4 Table 2 defines minimum sizes of driven earth rod.

Type of Earth Rod Minimum Nominal Diameter

Solid copper rod 15.0mm
Copper-clad steel 12.5mm
Table 2 - Minimum driven earth rod size

7.15.5 To avoid excessive temperature rise at the surface of the earth electrode it is
necessary to provide sufficient surface area in contact with the soil. The total
surface area of the buried bare earth electrode system shall be determined. The
adequacy of this value shall be checked using the formula in EA TS 41-24
assuming a three second fault. Appendix D gives the formula.

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7.16 Additional Issues

7.16.1 Power Cable Sheath Earthing

7.16.1.1 Three-core power cables shall have their sheaths and armouring earthed at each
end. Depending on the earth fault current that can flow in the cable sheaths it
may be necessary to interconnect the ends with separate earth electrode.

7.16.1.2 Single-core cables require special consideration as load-related circulating current

may cause significant de-rating. Each installation shall be individually assessed.
Precautions shall be taken to control induced touch voltage to not exceed 50V.

7.16.1.3 Care is required with the routing of sheath earthing with respect to current
transformers to avoid protection maloperation by through fault current or
capacitive current.

7.16.2 Multicore Sheath, Pilot Cable Sheath and Gas/Oil Pressure Alarm Earthing

7.16.2.1 Multicore cable sheaths entirely within the substation shall generally be earthed at
each end by connection to the substation earthing system. If it is necessary to
single-point earth multicore cable sheaths, precautions shall be taken to control
induced touch voltage to not exceed 50V.

7.16.2.2 Pilot cable sheaths and gas/oil pressure alarms from remote external cables require
special consideration and each installation shall be individually assessed.

7.16.3 Fences and Gates

7.16.3.1 Substation compound fences and gates shall be effectively earthed.

7.16.3.2 To control hand-to-hand touch hazards between exposed conductive parts that can
attain different potentials, two mutually exclusive arrangements for the substation
compound fence are permitted:

a) Independently earthed fence with minimum 2m separation from substation

earth electrode and effective separation from all plant connected to it,
including power cables with conducting sheaths.
b) Fence earthed via substation earth electrode.

7.16.3.3 Consideration shall be given to the touch voltage on the fence. Where necessary,
a perimeter potential grading electrode shall be provided.

7.16.3.4 Gates in substation compound fences shall be connected together via their
gateposts.

7.16.3.5 Metal fences which abut the substation compound fence shall be effectively
separated either by:

a) 2m physical separation, or
b) Effective shrouding/insulation.

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7.16.4 Stays

7.16.4.1 Care is required with the positioning of overhead line stay wires relative to the
substation compound fence and substation earthing system. The following
arrangements are permitted:

a) Stay within the substation earth mesh and connected to it.

b) Stay effectively separated by 2m above and below ground from equipment
that can attain a different potential. With this arrangement insulation of the
stay from the pole steelwork earth is required to reduce possible touch
voltage if the steelwork is earthed via the substation earthing system. The
Electricity Safety, Quality and Continuity Regulations define requirements
for stay insulators (minimum height of 3m). Furthermore, it may be
necessary to add a potential grading electrode around the stay with
connection to the stay to control touch voltage. In this case, the separation
requirements shall still apply. The method of installation must take account
of mechanical forces acting on the stay and associated ground.

7.16.4.2 Note that failure to insulate the stay from the pole steelwork may cause the steel
stay to corrode at an accelerated rate.

7.16.4.3 Overhead line stays shall not be installed such that they compromise the
independent earthing of a fence from a substation earthing system.

7.16.5 Surge Arrestors and Capacitor Voltage Transformers

7.16.5.1 The earthing associated with surge arrestors and capacitor voltage transformers
shall provide effective high-frequency and power-frequency earthing.

7.16.5.2 Where the two functions (i.e. high-frequency earthing and power-frequency
earthing) are nominally performed by separate conductors care shall be taken to
ensure that the power-frequency current flowing in the ‘high-frequency’ path does
not exceed its power-frequency rating.

7.16.5.3 Surge Arrestors

7.16.5.3.1 The power-frequency earthing shall be so arranged and supplemented to also

perform as an effective high-frequency earthing system.

7.16.5.3.2 The following general requirements shall apply.

7.16.5.4 Arrestor Position

7.16.5.4.1 To maximise the benefit from the surge arrestors:

a) Position surge arrestors as close as practicable to the protected equipment

(e.g. transformer bushing, ideally mounted on the transformer turrets).
b) Minimise connection lead length to minimise the ‘inductive’ loop length.

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7.16.5.5 Above-ground Earthing Conductor

7.16.5.5.1 Minimise impedance and, in particular, inductance of the above ground earthing
conductor:

a) Ensure there are no ‘sharp bends’. Where practicable use a minimum bend
radius of 20cm.

b) Avoid ‘loops’ where possible. If loops cannot be avoided (e.g. bund walls
for transformer surge arrestors), avoid loop length more than eight times the
width of the loop open end (as flashover is possible). Minimise loop length
relative to the width of the open end.

c) Avoid changes in conductor direction.

d) Avoid unnecessary length. Route conductor to earth electrode directly.

Ensure buried earth electrode established at closest practicable point (e.g.
just outside bund for transformer surge arrestors).

e) Use multiple paths where practicable (e.g. for transformer turret-mounted

surge arrestors ideally four but no less than two across the transformer plinth
to the buried earth electrode system).

f) For turret-mounted arrestors, a dedicated connection from the base of the

surge arrestor to the transformer tank is required. Ideally, this would be to a
dedicated earth tab, but where this is not possible a suitable bolt or drilled
hole shall be used taking care not to impair the transformer tank integrity.
The connection shall be below the level of transformer lid gasket to ensure
the most direct current path.

g) For turret-mounted arrestors, although the transformer tank provides a large

cross-sectional area for the path of surge current, it is considered that it is not
immediately obvious to personnel what the effect of disconnecting the lower
transformer tank connection(s) would be upon surge arrestor performance.
Given this, a dedicated connection shall be provided that is electrically in
parallel with the transformer tank as part of the surge current path.

7.16.5.6 Below-ground Earth Electrode

7.16.5.6.1 Minimise the high-frequency impedance of buried earth electrode at the point(s)
where surge current would enter:

a) Create an electrode arrangement that ‘fills’ the volume of soil, a hemisphere,

around the surge current injection point(s) by installing vertical rod (ideally
6m) and horizontal electrode to approach a ‘radial star’ arrangement. As a
minimum, install the ‘radial star’ arrangement at the surge current injection
point nearest to the surge arrestor(s).

b) Increase mesh ‘density’ in the horizontal plane to further fill the effective
volume, up to a radius of 20-50m. Spacing between parallel mesh electrodes
of the mesh shall be on an ‘exponential’ basis with highest mesh density
closest to the rod (e.g. spacings between parallel electrodes of 0.6m, 1.6m,
4.4m within a 12m mesh square).

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 i. Note: electrode further than about 20-50m, measured from the surge injection point along
the electrode, has little effect on reducing the high-frequency impedance. Horizontal
electrode is generally cheaper to install than deep-driven vertical rods.

c) Integrate the above into the substation power frequency earthing system.

7.16.6 Capacitor Voltage Transformers

7.16.6.1 As a capacitor voltage transformer acts as a short-circuit to earth at high

frequencies, it shall have an effective high-frequency earth as well as power
frequency earth. The approach detailed for surge arrestors shall be used with the
following amendments:

a) Rod length shall be a minimum of 2.4m.

b) No increased earth electrode mesh density is necessary.

7.16.6.2 Note that if protection signalling through the capacitor voltage transformer is
employed then care is required as carrier equipment may be relatively remote.
Specialist advice shall be sought.

7.17 Design Deliverables

7.17.1 The following shall be provided to WPD prior to installation:

a) Drawing showing earthing design and including electrode layout, depth and
materials, including sizes.
b) Design report
c) CDEGS input files in electronic format on CDROM

7.17.2 The design report shall include:

a) Soil model and field test results

b) Substation mesh earth electrode resistance, RES
c) Substation earth impedance, ZE, and constituent parts (e.g. tower chain
impedance etc)
d) Earth potential rise, UE
e) Hot or cold classification
f) Hot zone contour (ie 430V/650V) plot and 1150V/1700V contour plot,
where applicable, superimposed on OS geographical plan
g) Plots of touch and step voltages (plan view) superimposed on grid electrode
h) Confirmation that touch and step limits are not exceeded both internally and
externally.

7.17.3 Post-installation earth test(s) shall be performed to verify the design has been
achieved. Interpretation shall be via CDEGS, MALT/MALZ software, whichever
is more appropriate. Where the measurements indicate the design objectives have
not been met then remedial action shall be taken.

EE SPEC : 89/2 15 November 2012 - 16 of 29 -

8.0 INSTALLATION PROCESS

8.1 Safety

8.1.1 To minimise the risk of electric shock it is necessary to control the hazards
presented by:

a) 'Touch', 'step' and 'transfer' potential.

b) Failure of insulation
c) Current flowing in earthing systems
d) Induced voltage
e) Electrostatically-induced current.

8.1.2 Safe working methods shall be used. In addition to the normal requirements for
safe working, appropriate control measures may include:

a) Using suitable insulated gloves and safety footwear

b) Using temporary earth mat over a rubber mat at work position
c) Verifying insulator integrity by visual inspection prior to contact with
earthing system
d) Ensuring compound fencing remains earthed
e) Temporary adequately rated cross-bonding when making/breaking
connections in earthing systems
f) Insulation to prevent simultaneous contact across earth zones (e.g. insulated
isolating links).

8.2 Separation From Power Cables And Multicores

8.2.1 Damage to power cables and multicore cables due to high voltage transients or
thermal effects of earth fault current on/in earth electrode shall be prevented by
adequate separation. In the absence of other information, it shall be assumed that
150mm separation meets this requirement.

8.2.2 Where separation is not practical, suitable protection shall be applied. To achieve
this a suitable covering shall be applied. The length of covering shall be
minimised and shall be such that it does not compromise the earthing design.

8.3 Joints

8.3.1 Where reasonably practicable brazed or exothermically welded joints shall be

used in major substations for joints in the formal earthing system which may carry
fault current (e.g. connections to neutrals, fault throwers, HV steelwork and earth
electrode); in cases where the metal support structure forms part of the earthing
conductor path (see 7.13.1) the above ground joints to the metal support structure
shall be double-bolted. Joints to reinforcing bar shall only be exothermically
welded. In all other situations, brazed, exothermically welded, crimped and, when
above ground, bolted joints are permitted; in the case of bolted joints, where
reasonably practicable, double-bolted joints shall be used. Riveted joints shall not
be used. The drilled hole for bolted connections shall not exceed one third of the
tape width.

8.3.2 Bolted connections shall not be used below ground.

8.3.3 Precautions shall be taken to prevent corrosion (e.g. bimetallic (dissimilar metals)
and chemical).

EE SPEC : 89/2 15 November 2012 - 17 of 29 -

8.3.4 Inadvertent joints between copper earthing conductor and galvanised steel (e.g.
where copper passes earth switch handle support metalwork) shall be avoided or,
where this is not possible, the copper at the contact point shall be tinned or,
alternatively, greased with a high-temperature copper-based grease (eg ‘Copaslip’
or equivalent).

8.4 Soil Around Earth Electrode

8.4.1 Earth electrode shall not be installed direct into highly corrosive soil. Where
necessary, non-corrosive soil or other suitable products shall be imported to
achieve this requirement - refer to Tables E1 and E2 in Appendix E. In
particularly hostile soil, other measures may be required. Soils which shall be
considered potentially hostile are given in Table E3 of Appendix E.

8.4.2 Soil around earth electrode shall be free of stones for a radius of 150mm and
firmly compacted.

8.5 Anti-theft Measures

8.5.1 To prevent theft of the earth electrode by pulling it out of the ground sections of
the electrode may be encased in Marconite concrete or low sulphur concrete.

8.5.2 Earthing conductor theft can be discouraged by suitable fixing to structures and
use of security-coded paint where required. For security fixing to concrete,
stainless steel anchors (i.e. Alcomet FNA II or approved equivalent) shall be used,
generally spaced at 300mm centres. For security fixing to steel support structures,
M12 x 40 stainless steel countersunk screw, M12 stainless steel round flat washer
and M12 stainless steel shear nut shall be used, generally spaced at 300mm
centres; holes shall be 14mm diameter, countersunk on rear face; measures shall
be taken to prevent corrosion where tape is fixed along its length to structural
steelwork – see Standard Substation Construction drawing numbers XP1062 and
XP1063.

8.6 Ground Surface Covering

8.6.1 To reduce possible body current, high resistivity material shall be applied to the
surface in major substations where touch hazards may arise (e.g. open terminal
compound, around control building with earthed external metalwork etc).
Suitable materials and required thickness are given in Appendix F.

8.7 Loops for Portable Earths

8.7.1 Loops shall be provided attached to open-terminal HV steelwork earthing

conductor suitable for connection of portable earthing equipment. The cross-
sectional area shall not be less than that of the earthing conductor (i.e. they shall
be fully rated). The loop length parallel with the support structure shall be
180mm long and 75mm clear of the earthing conductor. For steel support
structures see Standard Construction Drawings XP1062 and XP1063; each
connection of the loop to the steel support structure shall be a double-bolted
connection – suitable corrosion precautions shall be taken (e.g. tinned copper).

EE SPEC : 89/2 15 November 2012 - 18 of 29 -

8.8 Addition of Metal Plant and Equipment

8.8.1 The addition of metalwork (e.g. lamp-posts, noise enclosures etc) may cause the
design to be compromised such that unsafe touch voltages could arise. Examples
include:

a) Locating metalwork in an area where there is no potential grading (e.g.

outside the ‘mesh’)
b) Addition of metalwork (e.g. lamp-posts or security posts) outside the earth
'mesh' connecting the fence to the substation earthing system directly or
unintentionally (i.e. cable or earth electrode under fence, changing the
compound fence position or addition of internal fences)
c) Addition of metal doors or gates creating hand-to-hand touch hazards
between independently earthed fence and metalwork connected to the
substation earthing system
d) Addition of external fences adjacent to earth electrode/compound fence.

8.8.2 Options for controlling the touch voltages include locating the metalwork such
that it is a minimum of 1m within the earth 'mesh'. This will not normally cause
an unsafe touch hazard unless the design did not control the touch voltage in the
particular area concerned.

8.8.3 Where the equipment is to be sited beyond the earth 'mesh', options include:

a) Ensuring there are no metallic parts exposed (e.g. plastic lighting column).
If it has a power supply ensure this is derived from the substation LV AC
supply using insulated sheath cable.
b) Extending the earth mesh to enclose the metalwork with 1m separation
around the perimeter. If this compromises an intentional 2m separation with
the compound fence move the fence or use insulated panels.

8.9 Multicore Cables

8.9.1 To prevent double-end earthed multicore cable sheath damage due to passage of
earth fault current, earth electrode shall be connected between each end. If
possible, the route of the multicore cable shall be made longer than that of the
earth electrode connection.

8.10 Cables and Earth Electrode Passing Under Independently Earthed Fencing

8.10.1 Cables with uninsulated sheaths which pass under independently earthed
compound fencing shall be insulated 2m either side to limit the fence potential
rise.

8.11 Protection of Communication Circuits at Hot Sites

8.11.1 Measures shall be applied at hot sites to telecommunication circuits and private
pilots to protect equipment, personnel and telecommunication customers. These
communication circuits shall be electrically isolated from the substation earthing
system. They shall be terminated on appropriate isolating links and isolation
equipment. For telecommunication circuits (e.g. BT) the telecommunication
operator's requirements shall apply.

EE SPEC : 89/2 15 November 2012 - 19 of 29 -

8.11.2 Isolation equipment shall be capable of withstanding the maximum substation
earth potential rise. In the absence of specific requirements, the withstand
capability shall be a minimum of 10kV ac RMS at 50Hz for 1 minute. This
equipment shall allow the connected apparatus to perform its function adequately.

8.11.3 To maintain isolation, the communication cable on the substation side of the
isolation equipment shall not run across or with the incoming communication
cable.

8.11.4 Hot site status and associated hot zone shall be determined together with
mitigation requirements.

8.12 Water Services at Hot Sites

8.12.1 New water service pipes into hot substations shall be non-metallic.

8.12.2 Metallic water service pipes shall be fitted with insulating insert(s) to prevent
transfer of earth potential rise externally to other customers. The insert(s) shall
provide a minimum 2m effective separation from the substation earthing system.

8.13 Records

8.13.1 A diagram showing 'as-installed' earthing layout shall be prepared. All

subsequent modifications to earthing shall be recorded. The record shall comprise
a plan view and shall include:

a) Earth electrode layout with depths, material, sizes, connectivity and rod
position/depth, joint types
b) Earthing and bonding conductor layout, material, sizes, connectivity and
joint types
c) Method of fence/gate earthing and connectivity with substation earthing
system
d) Special measures (e.g. imported soil, anti-theft measures, insulation under
fences, insulating inserts in water services and isolation equipment)
e) Details of plant crossing under fences
f) Connectivity with towers and earthed poles
g) Connectivity with power cable sheaths
h) Stay positions
i) PVC/PVC insulation
j) External earth electrode.

8.13.2 A record of the post-installation earth test (required by clause 7.17.3) and its
interpretation shall be provided.

9.0 COMMISSIONING

Commissioning tests shall be performed at appropriate times in the process. These

shall include:

a) Soil resistivity tests using approved earth tester. See 7.1.1 & 7.1.2.
b) Joint resistance test using approved micro-ohmmeter of each joint in the earthing
system.

EE SPEC : 89/2 15 November 2012 - 20 of 29 -

 c) Bonding test to verify all appropriate metalwork is bonded as per the design. Note
bonding tests to terminal towers shall be performed correctly to ST:TP21O, as
amended, to control the elevated risk that may arise due to induction.
d) Earth resistance/impedance test using approved earth tester. See 7.17.3. Note that
interpretation by the Slope Method and 61.8% rule are not appropriate for major
substations.

ST:TP21O, as amended, defines WPD test method policy.

EE SPEC : 89/2 15 November 2012 - 21 of 29 -

 APPENDIX A

Material Standard & Designation BS EN 13601 Material

Designation
Copper – strip BS 1432 - C101 CW004A
Copper - insulated strip BS 1432 - C101 & BS 5252 CW004A
Copper - stranded BS 7884 - C101(C100/C102) CW004A (CW003A/CW005A)
Copper - insulated stranded BS 6360
Copper - rod EA TS 43-94
Copper - flexible BS 6360
Copper - flexible braid BS 4109 - C101 CW004A
Copper-clad steel - rod EA TS 43-94
Aluminium - strip BS 2898 - 1350
Structural Steel NB Not for high- BS EN 485, BS EN 573, BS EN
frequency current (e.g. surge 574, BS EN 755
arrestor or capacitor voltage
transformer).
Structural Aluminium NB Not for BS EN 10025
high-frequency current (e.g. surge
arrestor or capacitor voltage
transformer).

Table A1 - Approved materials by standard and type/grade

Role Position Approved Materials

Earth electrode Below ground Copper - strip
Copper - stranded conductor
Copper-clad steel - rod
Copper - rod (where soil classified as 'hostile')
Steel piles
Steel reinforcing bar
Earthing Above ground Copper - strip
conductor Copper - stranded conductor
Copper-clad steel - stranded conductor
Aluminium - strip (not within 150mm of ground level)
Aluminium - support structure (subject to adequate
cross-sectional area)
Insulated Above or below Copper - insulated strip
earthing ground Copper - insulated stranded conductor
conductor Copper-clad steel - insulated stranded conductor
Gate-gatepost Above ground Copper - flexible or flexible braid
& door bond

Table A2 - Approved materials - formal earthing system

EE SPEC : 89/2 15 November 2012 - 22 of 29 -

 APPENDIX B

The short-time current rating formula applicable to earthing conductors given in IEC 60724
is:

Ic = A.k√[(ln[(θf +β )/(θi +β )]).1/t] where

Ic = conductor current rating (Amperes)

A = conductor cross-sectional area (mm2)
k = material k-factor (A/mm2)
θf = final temperature (oC)
θi = initial temperature (oC)
β = material β-factor (oC)
t = time (s).

Material Initial Final k β

Temperature Temperature
Copper 30oC 405oC 226 234.5
o
Copper-clad steel (30% conductivity) 30 C 405oC 131.7 245
Copper-clad steel (40% conductivity) 30oC 405oC 152.1 245
Aluminium 30oC 325oC 148 228
o
Galvanised steel 30 C 400oC 78 202

Table B1 - Assumptions for short-time rating assessment based on EA TS 41-24

EE SPEC : 89/2 15 November 2012 - 23 of 29 -

 APPENDIX C

Material Size Cross-sectional Area Rating 3s (1s)

Copper strip 25mm x 3mm 75mm2 9.2kA (15.9kA)
Copper strip 31mm x 3mm 93mm2 11.4kA (19.7kA)
Copper strip 25mm x 4mm 100mm2 12.3kA (21.2kA)
Copper strip 38mm x 3mm 114mm2 14.0kA (24.2kA)
Copper strip 31.5mm x 4mm 126mm2 15.4kA (26.8kA)
Copper strip 50mm x 3mm 150mm2 18.4kA (31.9kA)
Copper strip 40mm x 4mm 160mm2 19.6kA (34.0kA)
Copper strip 31mm x 6mm 186mm2 22.8kA (39.5kA)
Copper strip 38mm x 5mm 190mm2 23.3kA (40.3kA)
Copper strip 50mm x 4mm 200mm2 24.5kA (42.5kA)
Copper strip 38mm x 6mm 228mm2 28.0kA (48.4kA)
Copper strip 50mm x 6mm 300mm2 36.8kA (63.7kA)
Copper strip 50mm x 6.3mm 315mm2 38.6kA (66.9kA)
Copper strip 75mm x 6mm 450mm2 55.2kA (95.7kA)
Copper stranded 16mm2 16mm2 2.0kA (3.4kA)
Copper stranded 32mm2 32mm2 3.9kA (6.8kA)
Copper stranded 35mm2 35mm2 4.3kA (7.4kA)
Copper stranded 70mm2 70mm2 8.6kA (14.9kA)
Copper stranded 100mm2 100mm2 12.3kA (21.2kA)
Copper stranded 120mm2 120mm2 14.7kA (25.5kA)
Copper stranded 150mm2 150mm2 18.4kA (31.9kA)

Table C1 - Short-time ratings for standard copper earthing conductor sizes

NOTE: For tape with of 75mm it may be necessary to specially specify portable earthing end-clamps
with this capability – ENA TS41-21 (portable earthing) caters for a maximum of 50mm width.

NB The ratings above assume brazed or welded joints. For bolted or crimped joints the de-
rating multiplication factor shall apply as given in Table C3.

Material Size Cross-sectional Area Rating 3s (1s)

Aluminium strip 25mm x 3mm 75mm2 5.6kA (9.7kA)
Aluminium strip 25mm x 4mm 100mm2 7.5kA (12.9kA)
Aluminium strip 25mm x 6mm 150mm2 11.2kA (19.4kA)
Aluminium strip 40mm x 4mm 160mm2 11.9kA (20.7kA)
Aluminium strip 50mm x 4mm 200mm2 14.9kA (25.8kA)
Aluminium strip 40mm x 6mm 240mm2 17.9kA (31.0kA)
Aluminium strip 50mm x 6mm 300mm2 22.4kA (38.8kA)
Aluminium strip 60mm x 6mm 360mm2 26.9kA (46.5kA)
Aluminium strip 80mm x 6mm 480mm2 35.8kA (62.0kA)

Table C2 - Short-time ratings for standard aluminium earthing conductor sizes

NOTE: For tape of 80mm it may be necessary to specially specify portable earthing end-clamps with this
capability – ENA TS41-21 (portable earthing) caters for a maximum of 50mm width.

NB The ratings above assume brazed or welded joints. For bolted or crimped joints the
de-rating multiplication factor shall apply as given in Table C3.

EE SPEC : 89/2 15 November 2012 - 24 of 29 -

Material De-rating Factor -
Bolted/crimped Joints
Copper 0.83
Copper-clad steel (30% conductivity) 0.83
Copper-clad steel (40% conductivity) 0.83
Aluminium 0.90
Galvanised steel 0.84

Table C3 – De-rating factor - bolted/crimped joint - final temperature limited to 250oC.

The method of jointing may limit the rating. The permissible final temperature is reduced for
bolted or crimped joints to 250oC. Table C3 gives the de-rating multiplication factor.
NB: No de-rating should be applied to bolted joints onto substantial metalwork (e.g. HV
steelwork etc).

Material 100mm2 Copper-equivalent

Based on 3s Rating (mm2)
Copper 100
Copper-clad steel (30% conductivity) 174
Copper-clad steel (40% conductivity) 151
Aluminium 165
Galvanised steel 280

Table C4 - Material equivalent cross-sectional area table

EE SPEC : 89/2 15 November 2012 - 25 of 29 -

 APPENDIX D

The minimum surface area formula given in EA TS 41-24 is:

Ig/St =< 0.001√(57.7/(ρ.t)) where

Ig = current into earth electrode system (A)

St = total surface area of buried bare earth electrode (mm2)
ρ = uniform equivalent soil resistivity (Ωm)
t = fault duration (s).

EE SPEC : 89/2 15 November 2012 - 26 of 29 -

 APPENDIX E

Soil Type Description

Silty soils A fine soil will predominant particle size of
0.002-0.06mm
Sandy soils A coarse soil with predominant particle size
of 0.06-2mm
Gravelly soils A coarse soil with predominant particle size
of 2-60mm

Table E1 - Non-corrosive soils suitable for import around copper earth electrode

Note that gravelly soils may need to be sieved to remove large particles and ensure good
contact with earth electrode.

Material
Bentonite
Marconite concrete
Low sulphur concrete

Table E2 –
Non-corrosive proprietary material suitable for import around copper earth electrode

Table E3 lists materials which may cause accelerated corrosion of earth electrode.
Geotechnical tests can be performed which may help quantify the problem. Difficulty may,
however, be encountered in interpreting the results.

Soil Characteristics Example Soil Types

High sulphide and carbon content Cinders
Pulverised fuel ash (PFA)
Clinker
High organic acid content Organic soils (peat,
Soils with high content of decaying vegetation organic clay, silt or sand)
Soils with high humus content
Strongly alkaline Some clays
High dissolved salt content - sulphate and chloride Poorly drained soils
High humus soils

The acidity or alkalinity of the soil can be determined using the standard test method given in
BS 1377-3. This determines the ‘pH’ of samples. Values less than 6 and greater than 10 are
considered hostile to copper.

BS 1377 also defines how the ‘redox potential’ of a soil sample can be determined. This
provides a means of assessing whether a soil is conducive to the activity of sulphate-reducing
bacteria, which cause corrosion of metals.

Unusually low soil resistivity (e.g. <10 ohm metres) may also be indicative of corrosive soil.
This is best measured using the Wenner four-probe test.

EE SPEC : 89/2 15 November 2012 - 27 of 29 -

 APPENDIX F

Material Grading Target Wet Surface Dressing or Other

Category for Resistivity Construction Thickness
20mm Single
Size Aggregate
Limestone aggregate to 14/20 Gc 85/20 1500 ohm m 75mm with underlying
BS EN 13043. See Note 1 geotextile membrane or
150mm without
Granite aggregate to 14/20 Gc 85/20 1500 ohm m 75mm with underlying
BS EN 13043. See Note 1 geotextile membrane or
150mm without
Tarmacadum N/A 1500 ohm m 75mm

Table F1 –
Approved materials and required specification for substation surface covering

Note 1 Particles shall be hard, durable, inert and free from deleterious material (e.g. metallic
compositions).

EE SPEC : 89/2 15 November 2012 - 28 of 29 -

 APPENDIX G

SUPERSEDED DOCUMENTATION

This document supersedes EE SPEC : 89/1 dated November 2007 which should now be
withdrawn.

APPENDIX H

ASSOCIATED DOCUMENTATION

See section 3

APPENDIX I

IMPEMENTATION

Immediate

APPENDIX J

IMPACT

This specification is relevant to WPD personnel involved in primary system design,

preparation of invitations to tender, specification of requirements for competition in
connections and associated projects. It is also relevant to external recipients associated with
the above.

APPENDIX K

KEY WORDS

Competition in Connections, earth electrode, earthing, earthing system, hot site, hot zone,
tender, invitation to tender

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