Is Your Installation Safely Earthed

8/6/2019 Is Your Installation Safely Earthed

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Strategy & Solutions Limited 2.1

Is Your Power Installation Safely Earthed?

Trevor Charlton and Dr. Matthew Taylor, Strategy & Solutions Ltd, UK

Dr. Mark Davies, Earthing Measurements Ltd, UK

Author Biographical Notes

Trevor Charlton is the Managing Director of Strategy & Solutions and Earthing Measurements

Ltd. These companies supply earthing design, research, training and measurement services to

most of the UK electricity companies and their contractors. Via S&S, he has written the earthing

policy documentation for most of the UK electricity companies and is the named earthing

consultant for several large companies.

Via EA Services Ltd., he was the UK representative on earthing related issues on the IEC group

preparing IEC 61936-1 (concerned with electrical power installations).

He has published and presented numerous technical papers on power system earthing and

electric interference, one of which won the IEE Power Engineering Journal Premium Award.

He is an experienced lecturer on the subject and is involved in this role in earthing courses in the

UK, Europe, the Middle and Far East.

His previous engineering experience has been gained via WPD (SWALEC), National Power,

the Seychelles Electricity Corporation and Coopers Deloitte.

In addition to his engineering qualifications, he has an MBA (distinction) from Warwick

Business School and has provided business consultancy services (Strategic and Business

Planning) to a number of UK companies.

Mark Davies has carried out numerous measurements at substations throughout the UK using

standard equipment and a purpose built earth impedance measurement system. Before working

for Strategy & Solutions, he completed a three year industry-linked PhD at Cardiff University,

specialising in the high frequency performance of earthing systems. This was via the EPSRC

Total Technology Scheme sponsored by Strategy & Solutions Ltd. In addition to his studies, he

was involved with numerous earthing investigations and has assisted in presenting severalearthing courses. He is also currently a director of Earthing Measurements Ltd, who offer a

range of measurements services for high-voltage substation earthing systems. His special

responsibilities include site measurements, earthing system assessments, analysis of the

performance of electrode systems under lightning and impulses and interference studies.

Matthew Taylor graduated from a four-year Electrical and Electronic degree at Cardiff

University in 1995, which included a sandwich training year with SWALEC (now WPD).

Continuing at Cardiff University, he completed an industry-linked PhD (sponsored by EPSRC

and Strategy & Solutions Ltd), developing soil resistivity measurement and analysis techniques

and the condition monitoring of earthing systems at substations up to 132kV. Now at Strategy &

Solutions Limited, he has tutored on several earthing courses, contributed to industry policy

documentation, R&D and conducted numerous substation earthing assessments. His areas ofresponsibility include soil resistivity analysis and earthing design for transmission lines and

tower-based mobile phone/radio installations.


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Abstract

The earthing system of a power installation plays a pivotal role in providing a safe environment

for personnel and avoiding damage to equipment, particularly during fault conditions.

Interest in earthing has been rekindled in recent years due to injuries, equipment damage and

factors that have increased the external environmental effect of power faults. This has led to

changes in design, revised standards and greater control of installation practices. Integration of

power and telecommunication equipment at electricity company sites has highlighted the

difference between installation standards and led to the introduction of new codes of practice [1].

The paper summarises the procedure undertaken to ensure the safety of an installation where

power and telecommunication equipment are combined. Some examples of earthing related

defects and previous poor practices are included.

Importantly, because of the interest in earthing, supported by research and development, there is

a full portfolio of test equipment, test procedures and design tools to ensure that the earthing

system performs correctly and limits its external impact during faults.

1. Introduction

The electricity network is mature and has a large distributed array of assets through which

electricity is transmitted and distributed to end customers. Whilst extension of these assets (by

installing new, extending or re-organising existing assets) does occur, much of the recent

emphasis is on making the most effective use of existing assets. These form an attractive base

on which to add new technology and, in many cases, this has been telecommunication

equipment added to transmission line towers or substations with relative ease in terms of

planning and other general requirements. The technical requirements have however proven to be

much more demanding and have served to highlight the difference in earthing construction

standards between electricity (higher fault current) and telecommunication bodies. This has

helped serve as a catalyst into a deeper investigation of the earthing issues because the design

requirements were so demanding.

At the same time there has been a maturing in the application of earthing analysis skills, suchthat when diagnosing incidents, earthing is now more quickly established as a cause and efforts

have moved from the analysis area towards being able to take the measurements which either

prove the designs compliance with technical limits or as the cause of an incident. Measurement

of safety voltages is especially important as these are both the basis of the design and the

quantities to be measured in the case of an incident.

Earth faults on power networks cause an Earth Potential Rise (EPR). This appears on all the

connected metalwork at the point of fault, including the electrode system within the soil.

Voltages occur on the soil surface (Surface Potentials) and within the soil surrounding the

electrode system. If the EPR magnitude was high enough, these voltages may damage

equipment or cause an electrical shock to humans or livestock. The voltage differences around a

faulted installation are characterised using the terminology transfer, touch and steppotentials.

Electricity power installation (Substation) earthing systems within the UK are designed to

restrict touch and step potentials such that they are lower than the limits set out in EA TS 41-24

[2]. Best endeavours are used to reduce the EPR and design the earthing system such that the

voltage differences created are all within safe limits or special procedures are implemented. The

EPR, the electrode size, its geometric shape, the soil resistivity and its structure influence the

area affected by these voltages.

When a telecommunication installation is added, this may extend the affected area and the

design must ensure that no new zones of hazardous surface potentials are created.


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2. Defining the Environment/Characteristics

Typically, the environment is defined through measurements of soil resistivity and (particularly

when an existing asset is being used) the earth resistance.

The Wenner sounding method [3] is generally used to measure soil resistivity and the fall of

potential technique [3] for measuring the existing earth resistance.

Figure 1 shows a Wenner electrode array, which is characterised by four electrodes, with equal

separation a, driven into the soil in a straight line. Current is circulated between electrodes C1

and C2. The resulting surface potential is measured between electrodes P1 and P2.

Figure 2 shows an example data set from a Wenner sounding, with maximum spacing a of

200m. The corresponding three-layer soil model, derived using a computer software package, is

shown on the right of Figure 2. The soil structure and the resistivity of the various layers will

influence the earthing design used and its impact upon safety and surface potentials.

C1 C2P1 P2

Soil surface

3a2

Array centre

X

a

2

a a a

Figure 1: Wenner Sounding Array

1 10 100 1000 100 1000 10000

Wenner spacing (m) Layer resistivity (m)

Figure 2: Example Soil Resistivity Data and Corresponding Soil Model

An analysis of services and other equipment in the area of interest is also necessary, assessing

susceptibility to the potentials produced and impact on the local environment. For example, if

App

arentresistivity(m)

10

1

00

1000

1000

0 1

10

100

Depth(m)


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EarthingSystem

Under Test

C1 P1 P2 C2

Voltage Probe

Current Probe

Fuses

P2 C2

Four Terminal

Earth Tester

5 to 10 times dimensions of earth grid

equipment such as gas pipelines or telecommunication/signalling cables are involved an

investigation of the possible damage and ways of avoiding this must be carried out. If the land

has public access or is grazed by cattle or horses, then stricter design criteria are used.

3. Site Assessments and Examples of Typical Defects

Where existing assets are being used, an examination of their earthing system is necessary. The

first test (where practicable) would be to measure the earth resistance using the fall of

potential method, as illustrated in Figure 3. The procedure for carrying out this type of

measurement was explained in an earlier paper [4]. Where drawings are missing or suspect, the

location and depth of the installed earth electrode may also need to be located using surface

tracing techniques, as illustrated in Figure 4.

Figure 3: Fall-of-Potential Measurement Method

Figure 4: Surface Tracing Technique


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Examples of some earthing defects found on telecommunication equipment are shown in

Figures 5, 6 and 7. In Figure 5, the joint used for bonding the telecommunication equipment

would not be capable of carrying even a small proportion of the power system current without

failing. Examples have been found where such connections have been made without even

removing the tower paintwork. The insulated conductor and single, short earth rod of the

telecommunication installation shown in Figure 6 ensure that it is not safe when a fault occurs

as this type of arrangement increases prospective touch voltages. The earth connections at the

base of the telecommunication tower shown in Figure 7 will ensure problems in the event of a

lightning strike. To perform adequately, the earth connections must be as short and straight as

possible something to which great attention is paid to in new power system installations.

Of course defects are found in the power installations as well. These include old designs based

on plates or single electrodes with no consideration of safety voltages (the arrangement of

Figure 8 only has earth rods in the centre), incorrect maintenance or installation which has led to

corrosion of the electrode systems (see Figure 9), theft (see Figure 10, circled) and failure to

bond the earthwire to transmission line structures (this was not required in old standards). The

last defect is the most common reason for finding a much higher earth resistance during

resistance measurements than anticipated.

Once the useful parts of the existing earthing system and any important defects have been

identified, the design process will start.

Figure 5: Earthing Defect - Inadequate Bolted Connection


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Figure 6: Earthing Defect - Unsuitable Earthing Design

Figure 7: Earthing Defect - Poor Quality Telecoms Tower Connection


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Figure 8: Earthing Defect Only Single Rods Used

Figure 9: Earthing Defect Corrosion


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Figure 10: Earthing Defect - Theft

4. Typical Designs Proposed and Installed

An integrated design is generally the most desirable, but where there is significant physical

separation between electricity and telecommunication equipment, they may be treated as

separate entities and have their own earthing systems. This does of course require that there is

no significant impact from one to another, via transferred potentials through the soil for example.

We will assume, for this paper, that the design will integrate the electrode system of each into

one overall earthing system. Where the equipment is in close proximity to one another, this is

the only option available. This does mean that the telecommunication installation will see the

same voltage rise during faults as the power installation and its electrode system will need to

carry part of the power system fault current.The design must ensure that transfer voltages, touch, step and external surface potentials are

controlled [2]. Typical examples of completed designs include:


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Top View of Conductors

-8 -4 0 4 8 12-8

-4

0

4

8

12

Tower legs

Telecoms

equipment plinth

Figure 11: Typical Earthing Design for a Telecom Installation within an Electricity Substation

Figure 12: Typical Earthing Design for a Telecom Installation adjacent to a Transmission Tower

As part of the design process, touch, step and transferred potentials are calculated (normally

using computer software [5]) and the design optimised until such time that the calculated

voltages are lower that the limits set out in the applicable standards.

Figure 12 shows an earthing design developed for a mobile phone base station (MPBS) withantennae mounted on a 132kV transmission tower. Earthing was provided around the tower

footing and MPBS, including the provision of a safe area to be used during temporary generator

Telecoms

Equipment PlinthElectricity

Terminal Tower

Electricity

Substation

Compound

Telecom Tower


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connection. This design and others are covered in greater detail in an earlier paper [6]. Figure 13

shows an example of this type of earthing design being installed adjacent to a transmission

tower footing.

Figure 13: Earthing Design Being Installed at a Transmission Tower Site

5. Post Installation Tests

Once the earthing installation is complete, there is usually a need to confirm that the design

values have been achieved. The fall of potential resistance measurement carried out previously

to measure the existing earth resistance may be repeated and a lower overall resistance should

be one factor indicating a satisfactory outcome. However, as the designs are based on touch,

step and external surface potentials, in cases where there is any doubt about the design, these

quantities must be measured. In the past this has been difficult to achieve at live power

installations. This has mainly been due to the fact that small potentials must be measured against

significant levels of background electrical noise.

Following a number of developments in this area, equipment and procedures are now availableto carry this out and give a much more precise audit of the installed arrangement.

Figure 14 shows some potential measurements being taken on the gravel surface within a live

electricity substation. Figure 15 shows a high degree of correlation between the calculated and

measured values. Discrepancies that were found, such as that shown on the right of the graph,

identified the presence of buried metallic sheathed cables that had not be included in the

computer model that was used to generate the calculated curve.

Where the potential contours external to the site need to be investigated in order to establish the

degree of impact on third party equipment, again measurements are now possible to compare

against the calculated value. Figure 16 shows a contour plot as measured around a power

installation and this proved that further mitigation action was required in order to avoid damage

to a gas installation.

The advantage of the measured values is that they account for any local anomalies in the soil,

together with any buried metal structures which may not have been known about at the design


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stage. Once this information is made available to the designer, an assessment can be made of

which contour to use for the present and into the future.

Figure 14: Surface Potential Measurements within a Substation

0

10

20

30

40

50

60

70

80

90

100

-20 -10 0 10 20 30 40 50

Distance with reference to 33kV tower footing (m)

Surfacepotentialas%ageofEPR

CDEGScalculated

Measured

Figure 15: Calculated and Measured Results

Earth

electrode

Tower

footingVertical

earth rods

Difference due

to metallic

sheath cables


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Figure 16: Surface Potentials Measured Around a 132kV Substation

6. Conclusions

The renewed interest in earthing has enabled the development and re-evaluation of testing and

design methods to predict the performance of earthing systems during normal and fault

conditions. The process of integrating the design and installation practices of telecommunication

and power providers has now gone past the initial culture difference phase and with the

introduction of a new code of practice, the past problems should now be behind us and good

quality earthing installations are something which should now be expected.

An important gap has also now been closed, i.e. the previous inability to accurately measure the

design parameters (in particular safety voltages and external potential contours) in an

electrically noisy environment. So we now have the full portfolio of design tools and procedures

together with the test procedures to carry out a post installation audit. This enables the safetyvoltages to be checked and the impact on external plant and equipment measured.

7. References

1. E.A. Engineering Recommendation G78: Recommendations for low voltage connections

to mobile telephone base stations with antennae on high voltage structures, Electricity

Association Services Ltd, London, 2003.

2. E.A. Technical specification 41-24: Guidelines for the design, installation, testing and

maintenance of main earthing systems in substations, Electricity Association, London,

1992.

3. TAGG, G.F: Earth resistances, (George Newnes, London, 1964).

Substation

Gas installation

Residence

SW route (FOP)

N-NW route

NW route

SE route

390m

300m

225m

370m

Required

surface potential

contour

Substation

Gas installation

Residence

SW route (FOP)

N-NW route

NW route

SE route

390m

300m

225m

370m

SubstationSubstation

Gas installationGas installation

ResidenceResidence

SW route (FOP)SW route (FOP)

N-NW route

NW routeNW route

SE routeSE route

390m

300m

225m

370m

390m

300m

225m

370m

Required

surface potential

contour


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4. DAVIES. M, QUEENAN. J, CHARLTON. T and GRIFFITHS. H: Measurements For

Testing Earthing System Integrity, ERA Technology Earthing 2000 Conference

Proceedings, June 2000.

5. CHARLTON, T. and GAGLANI, M.: Designing the earthing system of a power

installation using computer software, ERA Technology Earthing 2000 Conference

Proceedings, June 2000.

6. CHARLTON. T, TAYLOR. M and DAVIES. M: Technical Issues, Design Approach and

Typical Solutions When Co-locating Telecommunication Equipment On Electrical Power

Installations or Towers, ERA Technology Conference Proceedings Earthing and Bonding

of Telecommunications Installations, 2002.

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