MAKING ELECTRICAL
TECHNOLOGY
ITEC Guide to Datacomms
WORK
PART 12
Telecomms Earthing
01 August 04
In this leaflet, we will start by defining the common terms used in the process of earthing
(or ‘grounding’ in the USA).
Telecomms Earthing
Foreword
Datacomms and electronics earthing is often seen as a ‘black art’. It must be said that
often a particular approach is taken because it was tried in the past and was found to work
rather than being based on a sound engineering derivation.
In this leaflet, we try to guide you through terminology and the main world standards that
exist for datacomms earthing, and give practitioners some tips for achieving a successful
outcome.
An Introduction
Earthing and Bonding Definitions
It is essential to understand the difference between earthing and bonding.
Earthing is defined in BS 7671 as: “The act of connecting exposed-conductive-parts of an
installation to the main earthing terminal of an installation” (MET).
Equipotential bonding is defined as: “Electrical connection maintaining various exposed-
conductive-parts and extraneous-conductive-parts at substantially the same potential”
(N.B. referred to as bonding throughout this document).
Both of these functions are carried out using protective conductors (as defined in the BS
7671 Regulations). Protective conductors for Earthing are known as Circuit Protective
Conductors (CPCs), protective conductors for Bonding are known as Bonding Conductors
and may be either Main or Supplementary.
The Earthing Conductor is defined as “a protective conductor connecting the Main
Earthing Terminal of an installation to an earth electrode or to other means of earthing”.
For a TN systems the means of earthing is either a Protective Multiple Earthing (PME)
terminal or a separate conductor which may be the cable sheath. In a TT system the
means of earthing is the earth electrode which must comply with the requirements of sub-
section 542-02.
The definition of an extraneous-conductive-part is “a conductive part liable to introduce a
potential (generally earth potential) and not forming part of the electrical or telecomms
installation”. Isolated metal door handles, shelf brackets, metal windows, etc. are unlikely
to be extraneous-conductive-parts. However, racks, trays, patch panels, etc. do require to
be bonded.
An acceptable test to establish whether or not an item is an extraneous-conductive-part is
as follows:
1
Using a 500 volt DC insulation tester, measure the insulation resistance between
the item and the main earthing terminal. If the resistance value is 22,000 ohms or
greater, and inspection confirms that the resistance is unlikely to deteriorate, then
the item can reasonably be considered not to be an extraneous-conductive-part.
The two main sets of standards for telecomms earthing, excluding exchanges, etc. which
are covered by ETS 300 253 Equipment Engineering (EE) and standards which support
domestic I.T. solutions, are for the USA and for Europe.
In addition, there also exists international standards ISO/IEC 11801:2002 Second Edition
(Amendment 2) – Generic Cabling for Customer Premises and IEC:61140:2002
Protection Against Electric Shock – Common Aspects for Installation and Equipment
(same as EN 61140 & BS EN 61140, etc).
IEC 61140:2002 is an electrical standard and does not cover telecomms earthing other
than referring to generic class equipment.
However, these standards are not as widely quoted as the required contract standards from
the regions previously mentioned.
Until recently, the USA stood alone as the sole producer of telecomms standards, but in
the last few years Europe have produced their own standards and are now arguably in
front of the USA in some areas of Telecomms. At some stage, I am sure that standards
for the Telecomms and the electrical industries will be the same for the whole world, and
as I speak, the standards are moving increasingly closer together.
Starting with the Telecomms earthing standards in the USA, as they were the first to be
published.
There are two main standards:
1) ANSI/J-STD-607-A-2002 Commercial Building Grounding (Earthing)
and Bonding Requirements for
Telecommunications
2) National Electrical Code 2002 Note: this is a generic electrical standard
which also includes requirements for
grounding (earthing) and bonding
2
FIGURE 1
ANSI/J-STD-607-A-2002
Commercial Building Grounding (Earthing) and Bonding for Telecommunications
This standard was developed jointly between the American National Standards Institute
(ANSI) and the USA Telecommunications Industry (TIA).
The standard covers planning, design and installation of telecommunications grounding
and bonding systems within a building with or without prior knowledge of the
telecommunication systems that will subsequently be installed. The standard also
provides recommendations for grounding and bonding customer owned towers and
antenna. The telecommunication infrastructure specified in this standard is for a multi-
vendor/multi-product environment as well as for various system installation practices.
You will notice on the drawing that the telecomms earthing is tied into the main building
earth bar/connection which includes the main electrical earthing system, lightning
protection, BMS, etc.
3
Whilst ANSI/J-STD-607-A-2002 gives guidance on the sizes of the grounding (earthing)
cables, one should always seek advice from the telecomms equipment vendor regarding
their requirements for these cable sizes, as failure to comply with their recommendations
may result in invalidating any warranty which they offer for the system. Also, included in
the standard is a picture of the preferred type of telecomms earth bar which recommends
the use of two screw cable lugs as shown below for connecting the grounding (earth)
cables. The ANSI document calls for earth bars and clamps to be type tested and listed by
a nationally recognised testing laboratory.
FIGURE 2
National Electrical Code 2002
This standard was produced by the National Fire Prevention Association (NFPA) in the
USA and covers all electrical work and includes information regarding bonding and
grounding of all systems as well as those for use on telecommunication systems.
In Europe, our main standards are:-
BS EN 50310:2000 Application of Equipotential Bonding and Earthing in Buildings
with I.T. Equipment
HD 384.5.54 Earthing Arrangements and Protective Conductors
Although we are looking at Earthing standards, it would be very incomplete not to include
BS EN 50174 Part 2 Section 6.7 (I.T. Cabling Installation Planning and Practices Inside
Buildings) which gives further information on earthing and bonding telecomms systems.
The advice given helps deal with surge magnetic fields, reducing common mode current,
shielding and noise suppression.
4
In the UK we use:
BS EN 50310:2000 Application of Equipotential Bonding and Earthing in Buildings
with IT Equipment
BS 7671:2001 IEE Wiring Regulations
BS 7430:1998 Code of Practice for Earthing
BS EN 50174 Part 2 I.T. Cabling – Installation Planning and Practices inside
Buildings
BS EN 50310:2000
This standard was developed by CENELEC and the National Committees from the
countries within the European Union. It covers earthing and bonding of information
technology equipment in buildings with relation to safety, functional and electromagnetic
performance. It acknowledges that level of earthing and bonding required by a building
depends upon the degree of complexity and size of the information technology
installation.
Figure 3 shows a typical telecomms earthing arrangement to BS EN 50310. Once again,
you will notice in the drawing that the telecomms earthing is tied into the main building
earth bar/connection which includes the main electrical earthing system, lightning
protection, BMS, etc. The underlying concepts of the standard are in harmony with ETS
300 253.
It should be noted that BS EN 50310 does not give any guidance on the sizes of bonding
conductors, and whilst the recommendations of BS 7671 or HD 384 may prove to be
satisfactory, it is somewhat unlikely. The ECA recommends that a minimum earth
conductor of 16mm² is used and the sizes of cables listed in Table 1 (which is in harmony
with the ANSI/J+STD-607-A-2000) are used. However, once again, it is recommended
that equipment vendors’ requirements are sought, as failure to comply with their
requirements may result in invalidating any warranty which the vendor may offer on their
system. Unlike ANSI/J-STD-607-A-2002, BS EN 50310 does not give any pictures or
recommendations with regard to the style or specification of the earth bar. However, the
ECA recommend using a 50mm x 6mm earth bar as a minimum with a disconnecting link
between the main CPC connection and the rest of the earth bonding connections in order
to facilitate future periodic inspection/testing and fault finding, similar to that shown in
Picture A.
PICTURE A
5
It should be noted that the ANSI standard requires a 100mm main earth bar at the origin
of the installation (MET). However, the ECA consider this to be unnecessary in small to
medium installations. The ANSI standard also calls for steps to be taken to prevent
corrosion of busbars including electroplating or an anti-oxidant (eg. Petroleum jelly)
coating.
TABLE 1
Supplementary Bond Cable Sizes
ECA Suggested Supplementary Bonding Conductors (PC, CPC or TBB)
Bond Cable Length (m) Bond Cable Size (mm²)
Less than 4 16
4–6 25
6 – 12 35
13 – 20 50
Greater than 20 95
FIGURE 3
Note: The Earth Bars may be part of the Distribution Board
6
BS 7671:2001
This standard is produced jointly between the IEE (Institute of Electrical Engineers) and
BSI (British Standards Institute) and covers all electrical work excluding vehicles, ships,
lifts, lightning protection and general RF suppression and should be read in conjunction
with BS 7430:1998, the Code of Practice for Earthing. Although BS 7671:2001 does not
give guidance on telecomms earthing, it gives extensive generic practice which should be
complied with in order to comply with the ‘Electricity at Work Regulations 1989’.
BS 7430 – The Code of Practice for Earthing
This standard gives guidance on the methods that may be adopted to earth an electrical
system on land based systems and should be read in conjunction with BS 7671:2001.
Whatever grounding (earthing) systems are installed, always remember that in most
instances they are tied together usually close to the point of entry into the building and
there should only be one building main earth connection.
The main building grounding system protects against the effects of electrical hazards,
lightning hazards, and electrostatic hazards in addition to telecommunication hazards.
The primary purpose of earthing, grounding and bonding is to ensure safety from
electrical shock or fire. In the UK and Europe, earthing is designed to ensure as low a
voltage as possible on contactable surfaces with a maximum of 50V (special dispensation
is granted to the UK for 55V for portable tools).
FIGURE 4
7
FIGURE 5
By tradition the earth lead was a zero voltage lead and had no current flowing in it with
the exception of short duration faults which are cleared when the fuse or circuit breaker
trips.
Unfortunately, this is rarely the case as a number of things have changed which prevent
this from happening.
FIGURE 6
The increased use of TN-C-S (which is still more commonly referred to as PME,
Protective Multiple Earthing) by RECs (Regional Electricity Companies) means that the
earth termination point in a small premises always has a voltage present. This is not true
8
for larger buildings where a Three Phase T.N.S. supply is provided by the REC and the
main earth terminal (MET) of the installation is held at very near zero volts by a circuit
protective conductor (CPC).
FIGURE 7
FIGURE 8
Another problem arises mainly on electronic and I.T. equipment where the manufacturer
has installed two capacitors, one between the live and earth and the other between the
neutral and earth which continuously bleed off a small current (about 0.5mA) to earth in
order to help comply with the EMC Directives. These capacitors are usually fitted to all
switchable power supplies and are thus present in most active I.T/telecomms equipment.
For a standard P.C. station there are normally 3 fitted, one for the main computer case,
one for the monitor and one for the printer.
In the UK, the IEE cover this topic in Section 607 of the Wiring Regulations (BS 7671)
and also in their Guidance Note 7 “Special Locations”.
9
It does not take long for anyone to realise that although the currents involved are small,
they soon add up to over 30mA, which is the start of current range generally perceived to
cause damage to humans. It also causes concern that when the capacitor breaks down it
can cause either a dead short from live to earth (for the time it takes for the protective
device to trip) or can cause a link between the neutral conductors and earth placing a
higher than normal voltage on the earth system due to the neutral current being shared
between the neutral and earth conductors. In this event the protection will not normally
trip unless protected by an RCD or from another form of earth leakage device.
For further advice on the effects of current on human beings and livestock, see the Pd
6519/IEC 60479 (Parts 1 to 3) series guides.
Please note that all the standards mentioned in this leaflet are available from the ECA
Membership Department (ECA members receive a discount of up to 30%).
10
ABBREVIATIONS LIST
A/Amp Ampere
ANSI American National Standards Institute
BCS Building Control System
BSI British Standards Institute
CENELEC Central Electrical
CPC Circuit Protective Conductor
CGLI City and Guilds of London Institute
D/B Distribution Board
E Earth
ECA Electrical Contractors’ Association
EE Equipment Engineering
EMC Electro Magnetic Compatibility
EMI Electro Magnetic Interference
EN European Norm
ERA Electrical Research Association
ETS ETSI Specification
ETSI European Telecomms Standards Institute
FAA Federal Aviation Administration
FCC Federal Communications Commission
GE Grounding Equaliser
I Current
I.T. Information Technology
IE Earth Current
IEC International Electrotechnical Commission
IEE Institute of Electrical Engineers
IN Neutral Current
ITEC Information Technology Electronics Controls Committee of the
ECA
J Joint
JTL Joint Technical Training
L Line or Live
m Milli
MEB Main Electrical Earth Bar
MET Main Earth Terminal
N Neutral
NEC National Electrical Code
NET National Electrotechnical Training
NFPA National Fire Protection Association
NVQ National Vocational Qualification
P.C. Personal Computer
P.E. Protective Earth
Pd Published Document
PME Protective Multiple Earth
QCA Quality Curricular Authority
R Resistor
RCD Residual Current Device
REC Regional Electrical Company
RF Radio Frequency
SQA Scottish Qualifications Authority
11
STD Standard
TBB Telecommunications Bonding Backbone
TGB Telecommunications Ground Bar
TIA Telecommunication Industry Association
TMGB Telecommunications Main Ground Bar
TN – CE Earth Neutral – Combined
TN – CS Earth Neutral – Combined Separate (PME)
TN – S Earth Neutral – Separate
UK United Kingdom
USA United States of America
V Voltage or Volts
12
