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Electricity ResearchAssociation

Earthing & Bonding ofTelecommunicationsInstallations andColocation Systems

Worldwide BuildingEarthing Arrangements:A Case Study and IssuesAffecting the Designer

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Contents

1 INTRODUCTION....................................................................................................................................1

2 LOW VOLTAGE SYSTEM EARTHING..................................................................................................2

2.1 Earthing Systems...... .................................................... ........................................................ 3

2.1.1 Technical Earthing Systems..................................................................................................3

2.1.2 Typical Earthing Arrangements........... ............................................... ................................... 3

2.2 Calculation for sizing earthing conductors.............................................................................5

3 SYSTEM CURRENTS DIVIDING BETWEEN THE NEUTRAL AND EARTHING SYSTEM ..................6

3.1 Three pole and four pole circuit breakers in dual supplies ................................. ................... 6

3.1.1 Three pole circuit breakers....................................................................................................6

3.1.2 Four pole circuit breakers......................................................................................................7

3.1.3 Three or four pole circuit breakers .............................................. .......................................... 8

3.2 Multiple earth-neutral connections in a distribution network (Case Study). ........................... 8

4 Earthing of Uninterruptible Power Supplies............................................................................................9

4.1 The UPS output neutral is directly connected to the UPS input neutral. ............................... 9

4.2 Isolation transformer ......................................... .............................................. ......................9

4.3 Earth fault protection on the static and manual bypass lines...............................................10

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Electricity Research Association

Earthing & Bonding of Telecommunications Installations and

Colocation Systems Worldwide

Building Earthing Arrangements: A Case Study and Issues Affecting the Designer

October 2002

Copyright Ove Arup & Partners Ltd 2002

Ove Arup & Partners Ltd

13 Fitzroy Street, London W1T 4BQ

Tel +44 (0)20 7636 1531 Fax +44 (0)20 7752312

www.arup.com

Job number 71570/10

Mike Hastings

BEng (Hons), CEng, MIEE, MPhilMike is a Senior Electrical Engineer within the Manufacturing Group,specialising in electrical distribution system design, technical advice and

assistance throughout the company, predominantly related to banks, data

centres and pharmaceutical facilities. His Experience has been gained

throughout China, Hong Kong and Europe. More recently he has completed

research work into supply quality in high voltage and low voltage distribution

networks. He co-ordinates and presents Arups internal training seminars on

electrical distribution design strategies to meet clients requirements relating

to the fundamental question what are we trying to achieve

Chris J Hewitt

BEng (Hons), CEng, MIEE

Chris is an Associate of Ove Arup & Partners. He is the Business Area

Leader of Arup Mission Critical Facilities Group. He has experience of the

design, site implementation and project management of large scale data

centre projects with multi-national blue-chip companies. He has experience

of the HV and LV design of supply critical facilities including large scale UPS

and generator systems. His knowledge of electrical design and construction

management has been gained throughout Europe and South East Asia.

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Electricity Research Association Earthing & Bonding of Telecommunications Installations andColocation Systems

Worldwide Building Earthing Arrangements: A Case Study and IssuesAffecting the Designer

Arup

C:\ARUP\ADMINISTRATION\MCF\PAPERS\AVAILABLE FOR ISSUE\EARTHING\ARUP - 2002 -EARTHING AND BONDING OF TELECOMMUNICATIONS INSTALLATIONS AND COLO SYSTEMSWORLDWIDE.DOC

Copyright Ove Arup & Partners LtdOctober 2002

Page 1

1 INTRODUCTION

This paper has been written so as to outline the fundamental considerations when designing building

earth systems. The paper particularly relates to the design of building earth systems specifically inmission critical facilities such as data centres.

The authors have experienced a number of building installations where fundamental design problems

have exist in the earthing arrangements. The paper is not intended to address the issues of earthing

within technical or server spaces where the object is to create a low impendance path for high

frequency leakage currents. Instead, the paper presents more fundamental arrangements upstream of

the technical space where earth connections are more important to the overall safe operation of the

building with minimal supply interruption.

The paper concentrates on the following:

Neutral current paths, presenting a case study where neutral current was allowed to divide

between the neutral and earth conductor systems. The following mitigating measures arediscussed:

Use of three or four pole circuit breakers.

Removal of multiple earths in a distribution network.

In addition, the paper presents two methods for earthing uninterruptible power supply systems, one for

a more simple single sources system, and one for a more complex multiple sources system.

Throughout the paper, it is emphasised that distribution systems must be designed as a single entities

and therefore.

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Page 2

2 LOW VOLTAGE SYSTEM EARTHING

Essentially, all plant that generates electricity or changes system voltage levels (i.e. transformers)

must to be earthed. There are fundamentally two types of earthing systems:

1. Protective earthing for protection of persons against electric shock.

2. Technical earthing (sometime referred to as functional or clean or IT/communications earthing)

for noise suppression or stable earth reference.

In general, metalwork that may become live during normal use or during a fault should be bonded to

the protective earthing system. The protective earthing system must therefore be bonded at some

point to the technical earthing system (if present). Technical earthing systems are not specifically

designed to clear earth fault currents (although by their nature they may be capable of performing this

function). Instead, they are designed to provide a high integrity, low impendance path to earth for high

frequency leakage currents and noise caused by the operation of switch mode power supplies.

As an example, for a server rack containing 8no. industry standard 2U servers, the leakage current

can typically be 10mA 20mA.. Clearly in the UK, compliance with the requirements of Section 607 of

BS 7671 is imperative for safety of personnel.

In general, the star points of the secondary winding of a distribution transformer, low voltage generator

or UPS inverters are normally solidly earthed. Solidly earthing the neutral point ensures that voltages

to earth will not exceed the phase voltage. If several sources are installed, regulations and practices

allow each transformer neutral connection to be taken to a common earthing bar; separate earth

electrodes are not required.

In broad terms, distribution systems are earthed for the following reasons:

To limit voltages due to impressed surges (including lightning) and faults giving a measure ofsafety to personnel.

To provide a known maximum voltage in the system.

To facilitate clearance of line to earth faults.

To reduce fire risk due to arcing earth faults.

To provide a low impendance route for high frequency leakage currents.

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Page 3

2.1 Earthing Systems

2.1.1 Technical Earthing Systems

Typically, technical earthing systems are subject to high frequency earth leakage currents of up to

30MHz. As the frequency increases, the distribution of current across the cross section of an earth

conductor becomes less uniform. Consequently, impedance of the earthing system increases. This

phenomenon is known as skin effects. It is possible to calculate the effect on the cable impendance of

skin effect by using equations indicated in IEC287. Typically, to mitigate the cause of skin effects it is

suggested that the technical earth system is designed to incorporate many parallel paths.

2.1.2 Typical Earthing Arrangements

2.1.2.1 TT System

TT systems have independent earthing system for the source earth and protective earth.

Figure 3.1 TT Earthing Arrangement

If a fault to earth occurs, current will flow from the phase conductors, into the protective earth system,

back through the earth (ground) to the transformer star point (thus completing the circuit). Since the

impedance between protective earth electrodes and source earth electrodes is relatively high, in TT

systems earth fault currents are relatively low when compared to other earthing systems such as TNS.

Therefore, it is normal practice to install earth fault protection through the installation.

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Page 4

2.1.2.2 Earthing systems - TNS system

TNS systems use a single earth system for both the source earth and protective earth. Earth fault

currents will be of a similar magnitude to a phase fault. The high fault currents will ensure minimum

circuit disconnection times. Therefore, it is not normally necessary to install earth fault protection.

Figure 3.2 TNS Earthing Arrangement

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Page 5

2.2 Calculation for sizing earthing conductors

Fault current paths from the position of the fault comprise protective conductors and other random

paths in parallel, such as structural metalwork. Calculations must be based on the actual known

conductors, including sheaths and circuit protective conductors, and cannot make allowance for

fortuitous parallel paths.

Earth faults occurring at main switchboards close to the transformer will develop high currents, most of

which will return to the transformer star point via main bonding conductors.

Earthing conductors are sized in accordance to adiabatic conditions and to ensure the earth loop

impedance is low enough to operate the protective device in the event of an earth fault. Overload

protection is omitted, since the earthing conductors are not intending to carry steady state current

during normal operation

The adiabatic formula can be used:

Equation 3.3

Where

S = Cross-sectional area of conductor (mm2). Standard size conductors should be chosen.

I = r.m.s. fault current (amps) (ignoring current limiting by the circuit protective conductor).

t = Disconnection time for operation of the circuit protective device (seconds). For disconnection

longer than five seconds, equation is pessimistic.

k = A factor related to initial and f inal temperature of the earthing conductors, conductor and insulation

materials. (For copper PVC cables, bunched k=115)

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Page 6

3 SYSTEM CURRENTS DIVIDING BETWEEN THE

NEUTRAL AND EARTHING SYSTEM

Under normal operating conditions it is important to ensure that any neutral currents caused by

unbalanced loads return along the correct neutral conductor and do not divide and flow along a

parallel earth path. Conversely, under earth fault conditions the fault current must low through the

correct earth path and not via a parallel neutral path.

There are a number of reasons why neutral currents could be flowing in the earthing system.

Three pole + solid neutral (TP+N) circuit breakers have been installed when four pole circuit

breaks should have been used.

Multiple earth neutral earth connections.

3.1 Three pole and four pole circuit breakers in dual supplies

Four pole switchgear is required for any three phase four wire systems with multiple infeeds, whether

these infeeds be transformer, generators or a combination of the two.

The correct installation for a multiple infeed switchboard would be to install 4 pole circuit breakers on

the incoming feeds (transformers and generators) and any bus-couples.

Additionally, if there are any auto-changeover devices down-stream of the main switchboard, these

should be 4 pole.

3.1.1 Three pole circuit breakers

The three pole circuit breaker situation is shown in the two figures 4.1.1.1 and 4.1.1.2, which shows,

the neutral current will divide between the neutral and earth conductors.

Figure 4.1.1.1 Electrical schematic with three pole circuit breakers

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Page 7

Figure 4.1.1.2 Neutral current dividing between the earthing system and neutral conductors

3.1.2 Four pole circuit breakers

This division of the neutral current could be prevented, if the bus section circuit breaker and the

transformer circuit breakers are four pole. The four pole circuit breaker situation is shown in the two

figures 4.1.2.1 and 4.1.2.2.

Figure 4.1.2.1 Electrical schematic with four pole circuit breakers

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Page 8

Figure 4.1.2.2 Neutral current does not dividing between the earthing system and neutral conductors

3.1.3 Three or four pole circuit breakers

It can be seen that the only circuit configurations that eliminates the divided neutral current problem is

utilising four pole circuit breakers with the neutral to earth connection up stream of the incoming of the

circuit breaker.

3.2 Multiple earth-neutral connections in a distribution network (Case

Study).

Where distribution systems are modified to increase the system capacity or during the installation of

UPS, it is common to inadvertently introduce additional earth-neutral connections. If there are

additional earth-neutral connections downstream of the main earth neutral connection, parallel neutral

paths are created via the earthing system.

During a recent site survey, substantial currents were found to be following in the earth system of a

telecommunications facility. It was noted that the earth current was varying as the building electrical

load changed. Visual inspections established that the star point of transformer was directly earthed

and four pole circuit breaker were used throughout the system. When reviewing the current flow within

the earthing system, the only feasible source of the earth current was due to a second earth neutral

link in the newly installed sub-switchboard.

Without shutting down the switchboard, it was not possible to physically verify the presence of the

earth neutral links within the sub-switchboard. Therefore, as built drawings were obtained from the

manufacturer. These confirmed that the earth neutral links were installed in the switchboard.

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Page 9

4 Earthing of Uninterruptible Power Supplies

Uninterruptible power supply systems are an integral part of the electrical distribution network and

their design must be tailored to that of the electrical distribution network, such that it operates as asingle entity. Therefore, there must be close coordination between the system designers and a

nominated UPS manufacture.

For safety reasons, the output neutral of the UPS must be electrically connected to earth. There are

number of solution to achieve this:

The UPS output neutral is directly connected to the UPS input neutral.

Isolation transformer.

4.1 The UPS output neutral is directly connected to the UPS input

neutral.

This solution is ideal for simple networks with only one source, as the neutral can be solid (i.e. not

switched) throughout the entire system. Consequently, four pole circuit breakers cannot be used

upstream of the UPS.

Figure 5.1 Schematic of a UPS system were the output neutral is directly connected to the UPS input

neutral

4.2 Isolation transformer

Where distribution systems incorporate two or more electrical sources, four pole circuits have to be

utilised for reasons previously discussed. In this case, in the event that the circuit breaker upstream of

the UPS is opened, the UPS will lose it earth reference. Therefore the UPS must have its own earth-

neutral connection. This can be achieved via a deltastar isolation transformer or delta /zigzag

isolation transformer with the star point connected to earth. The UPS output neutral and earthing

system is electrically isolated from the upstream earthing and neutral system.

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Page 10

4.3 Earth fault protection on the static and manual bypass lines

Earth fault protection must not be utilised on the static bypass and manual bypass lines. When the

UPS is switched from the static bypass to the manual bypass the two feeders are in parallel. The

current in each phase may not divide equally between the two parallel paths, and consequently, this

will be detected by RCD (residual current Devices) as leakage current or earth fault. Such an

installation with RCD protection would be prevalent in Asia, with Singapore as an example.

Figure 5.2 Schematic of a UPS system were the output neutral is directly connected to the UPS input

neutral

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