PRODUCED BY THE OPERATIONS DIRECTORATE OF THE ENERGY NETWORKS ASSOCIATION

Technical Specification
41-24
Issue 1 1992 - Addendum, Section 15,
Incorporated November 2009
Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing
Systems in Substations

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 Technical Specification 41-24
Page 1 Issue 1
1992
Addendum, Section 15, incorporated November 2009

1. FOREWORD ..................................................................................................................... 3
2. SCOPE.............................................................................................................................. 3
3. DEFINITIONS ................................................................................................................... 3
4. FUNCTION OF AN EARTHING SYSTEM ........................................................................ 4
5. FEATURES OF AN EARTHING SYSTEM ....................................................................... 5
6. SAFETY CRITERIA .......................................................................................................... 5
6.1 The Effects of Substation Potential Rise on Persons ............................................ 6
6.1.1 Transferred Potential ......................................................................................... 6
6.1.2 Step Potential .................................................................................................... 6
6.1.3 Total Potential .................................................................................................... 6
7. DESIGN ARRANGEMENT ............................................................................................... 8
7.1 Layout .................................................................................................................... 8
7.2 Design Guidelines .................................................................................................. 8
7.2.2 Indoor Substation Installation ............................................................................ 9
7.2.3 Revision of Test Facilities for Monitoring Earth System Efficiency.................... 9
8. DESIGN DATA .................................................................................................................. 9
8.1 Soil Resistivity ........................................................................................................ 9
8.2 Fault Currents ...................................................................................................... 10
8.3 Earthing Conductor and Earth Electrode Current Ratings .................................. 10
8.3.1 Earthing Conductor Current Rating ................................................................. 10
8.3.2 Earth Electrode Current Rating........................................................................ 11
9. DESIGN ASSESSMENT ................................................................................................. 12
9.1 Approximate Assessment Procedure .................................................................. 13
9.2 Refined Assessment Procedure .......................................................................... 13
9.3 Assessment of 'Touch' Potential and Electrode Conductor Length ..................... 14
9.3.1 Substations with Separately Earthed Fence (See Figure 3) ........................... 15
9.3.2 Substation with Integrally Earthed Fence (See Figure 3) ................................ 16
10. INSTALLATION ........................................................................................................... 16
10.1 Laying Conductors Forming a Horizontal Earth Mat ........................................... 16
10.2 Fixing Conductors ................................................................................................ 18
10.3 Jointing Conductors ............................................................................................. 18
10.4 Loops for Portable Earths .................................................................................... 19
10.5 Operating Mechanisms of HV Equipment ........................................................... 19
10.6 Provision for Testing Earth Electrodes ................................................................ 20
10.7 Earth Electrodes .................................................................................................. 20
10.7.1 Structural Earths .......................................................................................... 20
10.7.2 Driven Rod Electrodes ................................................................................. 20
10.7.3 Plate Electrodes ........................................................................................... 21
11. ITEMS REQUIRING SPECIAL CONSIDERATION ..................................................... 22
11.1 Overhead Line Terminals .................................................................................... 22
11.2 Fences ................................................................................................................. 22
11.2.1 Independent Earthing .................................................................................. 22
11.2.2 Connected to the Substation Earthing System ............................................ 23
11.2.3 Plastics Covered Chain Link Fencing .......................................................... 23
11.3 Arc Horn Anchorage Arrangement ...................................................................... 23
11.4 Water Pipes ......................................................................................................... 23
11.5 Cable Metallic Sheath/Armour Earthing .............................................................. 24
11.5.1 Non-Insulated Sheath/Armour Cables ......................................................... 24
11.5.2 Insulated Sheath Cables .............................................................................. 24
11.5.3 Cables Within Substations ........................................................................... 24
11.5.4 Cables Entering Substations ....................................................................... 25
11.5.5 Outdoor Cable Sealing-Ends ....................................................................... 25
11.5.6 Polymeric Cables ......................................................................................... 25
11.6 Metalclad Substations .......................................................................................... 26
11.7 Handles of Manually Operated HV Switches and PoIe-mounted Reclosers with
Ground Level Control Boxes................................................................................ 26
Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

11.8 Fault-throwing Switches ....................................................................................... 26

11.9 Equipment which Passes High-frequency Current .............................................. 27
11.10 Light-current Equipment Associated with External Cabling ................................. 27
11.11 Anti-climbing Precautions along the Tops of Walls ............................................. 27
11.12 Earthing of Ancillary Metalwork ........................................................................... 28
11.13 Earthing Requirements at Substations Providing Railway Electrification Supplies
............................................................................................................................. 28
11.14 Earthing of LV System Neutral and Associated Metalwork ................................. 28
12. TESTING ..................................................................................................................... 29
12.1 Measurement of Substation Earthing Resistance and Impedance ..................... 29
13. MAINTENANCE ........................................................................................................... 30
13.1 Earth Connections ............................................................................................... 30
13.2 Earthing Electrodes and Conductors ................................................................... 30
13.3 Earthing and Bonding .......................................................................................... 30
13.4 Neutral Connections ............................................................................................ 30
13.5 Effectiveness of Earthing ..................................................................................... 31
14. EXAMPLES ................................................................................................................. 31
14.1 Small Area Outdoor or Indoor Substations (Figure 7 Ex 1) ................................. 31
14.1.1 Calculations ................................................................................................. 31
14.1.2 Application of General Criteria ..................................................................... 34
14.2 Medium Area Outdoor Substation (Figure 8 Ex 2) .............................................. 36
14.3 Large Area Outdoor Substation (Figure 9 Ex 3) .................................................. 39
14.4 The Use of Design Assessment to Achieve Acceptable 'Touch' Potentials
(Figure10 Ex 4A and Ex 4B) ................................................................................ 42
15. EARTHING ASSOCIATED WITH HV DISTRIBUTION OVERHEAD LINE NETWORKS
(EXCLUDING TOWER LINES & POLE TRANSFORMERS) ...................................... 46
15.1 General Comments & Assumptions .................................................................... 46
15.2 HV Earth Electrode Value .................................................................................... 46
15.3 Electrode Arrangement Selection Method ........................................................... 46
15.4 Earthed Operating Mechanisms Accessible From Ground Level ........................ 48
15.5 Air Break Switch Disconnector (ABSD) with an isolated operating mechanism . 52
15.6 Surge Arresters .................................................................................................... 54
15.7 Cable Terminations .............................................................................................. 54
15.8 Operations at Earthed Equipment Locations ....................................................... 55
15.9 Installation ............................................................................................................ 55
15.10 Inspection & Maintenance of Earth Installations.................................................. 56
15.10.1 Items to Inspect ........................................................................................... 56
15.10.2 Items to Examine ......................................................................................... 56
15.10.3 Items to Test ................................................................................................ 57

Appendices ...................................................................................................................... from 58

 Technical Specification 41-24
Page 3 Issue 1
1992
Addendum, Section 15, incorporated November 2009

GUIDELINES FOR THE DESIGN, INSTALLATION, TESTING AND MAINTENANCE OF

MAIN EARTHING SYSTEMS IN SUBSTATIONS

1. FOREWORD

This Specification relating to main earthing systems in substations is a companion document

to BS Code of Practice 1013 (1988) and supersedes Engineering Recommendation S5/1
(1966). It is to be used in conjunction with Engineering Recommendation S34 (May 1986). In
this document account has been taken of:

(i) the requirements for Protective Multiple Earthing systems as outlined in

Engineering Recommendation G12/2. (The relevant items concerning substation
earthing in Engineering Recommendation G12/2 have now been transferred to
this document);

(ii) metrication;

(iii) the increasing use of plastic sheathed cables;

(iv) the differing requirements of earthing systems at various voltages and for differing
types of substation installation.

An Addendum, Section 15, was incorporated in November 2009. This covers earthing
associated with HV Distribution overhead line networks (Excluding tower lines and
pole transformers.)

Section 15 supersedes any requirements in Sections 1 to 14 which will be updated in

due course.

2. SCOPE

This Specification applies to fixed earthing systems for all supply systems and equipment
earthing within EHV, HV/LV and MV/LV substations.

It also applies to:

(i) terminal towers adjacent to substations and cable sealing and compounds;

(ii) pole mounted transformer or air-break switch disconnector installations. Earthing

systems for power stations and railway supply substations are excluded;

(iii) pole mounted reclosers with ground level control.

It does not apply to earthing systems for power stations, quarries and railway supply
substations.

3. DEFINITIONS

APPROVED EQUIPMENT Equipment Approved in operational policy document

for use in the appropriate circumstances.
BONDING CONDUCTOR A protective conductor providing equipotential
bonding.
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Addendum, Section 15, incorporated November 2009

EARTH The conductive mass of earth whose electric

potential at any point is conventionally taken as zero.
EARTH ELECTRODE A conductor or group of conductors in intimate
contact with, and providing an electrical connection
to, earth.
EARTH ELECTRODE POTENTIAL The difference in potential between the 'EARTH
ELECTRODE' and a remote 'EARTH'.

EARTH ELECTRODE RESISTANCE The resistance of an 'EARTH ELECTRODE' with

respect to 'EARTH'.

EARTH ELECTRODE RESISTANCE That area of ground over which the resistance of an
AREA 'EARTH ELECTRODE' effectively exists. It is the
same area of ground over which the 'EARTH
ELECTRODE POTENTIAL' exists.
EARTH POTENTIAL OR GROUND The difference in potential which may exist between a
POTENTIAL point on the ground and a remote 'EARTH'.

EARTHING CONDUCTOR OR A protective conductor connecting a main earth

EARTHING CONNECTION terminal of an installation to an 'EARTH
ELECTRODE' or to other means of earthing.
EARTHING SYSTEM The complete interconnected assembly of
'EARTHING CONDUCTORS' and 'EARTH
ELECTRODES' (including cables with uninsulated
sheaths).
GROUND VOLTAGE PROFILE The radial ground surface potential around an
'EARTH ELECTRODE' referenced with respect to
remote 'EARTH'.
RESISTIVITY The reciprocal of conductivity. It is numerically equal
to the resistance measured across two parallel faces
of a unit cube of material with uniform current
distribution. Dimensionally, it is resistance x length
and for a 1 metre cube the units are ohm metres.
STEP POTENTIAL See Section 6 for definition.

TOUCH POTENTIAL See Section 6 for definition.

TRANSFER POTENTIAL See Section 6 for definition.

4. FUNCTION OF AN EARTHING SYSTEM

Every substation shall be provided with an earthing installation designed so that in both
normal and abnormal conditions there is no danger to persons arising from earth potential in
any place to which they have legitimate access. The installation shall be able to pass the
maximum current from any fault point back to the system neutral without establishing
dangerous potential gradients in the ground or dangerous potential drops between parts of
the substation with which a person may be in simultaneous contact.

In a non-effectively earthed system, as with a tuned reactor (arc suppression coil) connected
between the transformer neutral and earth, the magnitude of the earth-return current is very
small due to the high impedance of the reactor. Thus, if the supply to all circuits entering the
substation employs this type of neutral earthing, then a heavy current earth electrode system
 Technical Specification 41-24
Page 5 Issue 1
1992
Addendum, Section 15, incorporated November 2009

is superfluous. However, if the tuned reactor can be shorted out, e.g. for maintenance or
repair or automatically for protection purposes and the transformer is still providing supplies,
then a heavy current earth electrode system shall be installed.

The design shall be such that the passage of fault current does not result in any thermal or
mechanical damage or damage to insulation of connected apparatus, and also such that
protective gear, including surge protection, is able to operate correctly.

Any exposed normally un-energised metalwork within a substation which may be made live
by consequence of a system insulation failure and can present a safety hazard to personnel.
It is a function of the station earthing system to eliminate such hazard by solidly bonding
together all such metalwork and to bond this to the substation earth electrode system in
contact with the general mass of earth. Dangerous potential differences between points
legitimately accessible to personnel shall be eliminated by appropriate design.

5. FEATURES OF AN EARTHING SYSTEM

The earthing installation requirements are met principally by providing in each substation an
arrangement of electrodes and earthing conductors which act as an earthing busbar. This is
called the 'main earth grid' and the following are connected to it:

(i) all possible high voltage fault points within the substation such as transformer and
circuit breaker tanks, arcing rings and horns and metal bases of insulators;

(ii) neutral connection of windings of transformers required for high voltage system
earthing. For high voltage systems up to and including 66kV the connections may
be via earthing resistors or other current limiting devices. (The neutral earthing of
low-voltage systems is separately considered in Section 11);

(iii) earth electrodes, additional to the main earth grid which may itself function as an
earth electrode;

(iv) earth connections from overhead line terminal supports;

(v) earthing mats, provided as a safety measure, to reduce the potential difference
between points on the area of ground adjacent to manually operated plant and
the metalwork and handles of that plant (but see also 11.7);

(vi) all other exposed and normally un-energised metalwork wholly inside the
substation perimeter fence, e.g. panels, kiosks, lighting masts, oil tanks, etc.
Items such as fences and cables, water pipes which are not wholly inside the
substation are separately considered in Section 11.

Substation surface materials, for example stone chippings which have a high value of
resistivity, are chosen to provide a measure of insulation against potential differences
occurring in the ground and between ground and adjacent plant. Although effective bonding
significantly reduces this problem the surface insulation provides added security under
system fault conditions.

6. SAFETY CRITERIA

The basic criteria adopted in this Specification for the safety of personnel are those laid down
in the CCITT Directives which, for 50Hz induced or impressed voltages derived from HV
supply networks, presently prescribe safe limits of 430 volts rms or, in the case of high
Technical Specification 41-24
Page 6 Issue 1
1992
Addendum, Section 15, incorporated November 2009

security lines, 650 volt rms. High security lines are those with fast acting protection which, in
the majority of cases, limits the fault duration to less than 200 milliseconds.

These values are applicable to 'transferred' potentials where direct contact is assumed
between remote earth potential and the full rise of potential of the substation earthing system.
Where they are exceeded appropriate precautions must be taken to safeguard personnel
from the conductors concerned. Touch and step potentials involve additional insulation of
footwear and surface coverings; hence higher values than those above can be justified.
Figure 2 shows curves giving acceptable touch and step potentials as a function of fault
current duration. These curves are derived from IEC 479 (1984).

6.1 The Effects of Substation Potential Rise on Persons

During the passage of earth-fault current a substation earth electrode is subjected to a

potential rise and potential gradients develop in the surrounding ground area. These gradients
are highest adjacent to the substation earth electrode. The actual ground potential reduces to
zero or true earth potential at some distance from the substation earth electrode. This
distance forms a physical separation which, if it is not bridged by a metallic connection,
renders any person in the high potential area immune from the possibility of simultaneous
contact with zero potential. However, local potential gradients, if great enough, can present a
hazard to persons and thus effective measures to limit them must be incorporated in the
design.

6.1.1 Transferred Potential

A metallic object having length - a fence, a pipe, a cable sheath or a cable core, for example,
may be located so as to bridge the physical separation referred to in 6.1 above. By such
means zero earth potential or some low value of earth potential can be 'transferred' into an
area of high potential rise or vice-versa.

6.1.2 Step Potential

As noted in 6.1, potential gradient in the ground is greatest immediately adjacent to the
substation earth electrode area. Accordingly the maximum 'step potential' at a time of
substation potential rise will be experienced by a person who has one foot on the ground of
maximum potential rise and the other foot one step towards true earth. For purposes of
assessment the step distance is taken as one metre.

6.1.3 Total Potential

The 'step potential' referred to in 6.1.2 relates to the ground surface potential which, relatively,
is somewhat lower in value than that present on the buried earth electrode itself. It is thus
evident that a metal structure bonded to the earth electrode will assume the same potential. If
the structure is accessible, a person standing on the ground 1 metre away and touching the
structure will be subject to the 'touch potential'. For a given substation the maximum value of
'touch potential' can be up to two or three times greater than the maximum value of 'step
potential'. As a consequence, if a substation is safe against 'touch potentials', it will normally
be safe against 'step potentials'.

The above three conditions of 'transfer', 'step' and 'touch' potential are illustrated in the
following sketch. The various figures are exposed to the undernoted potential differences
when earth-fault currents pass through the earth electrode:
 Technical Specification 41-24
Page 7 Issue 1
1992
Addendum, Section 15, incorporated November 2009

The pilot cable is shown installed in a particular manner to illustrate the two most significant
conditions of 'transfer' potential, although normal practice is to earth the cable sheath at both
ends. If the potential rise of the earth electrode exceeds the appropriate CCITT level, then the
arrangement as shown is unacceptable and measures shall be taken to counter the risk or
danger. These measures may, for example, consist of physical isolation or insulation, sheath
discontinuities, non-metallic inserts, isolating transformers or other precautions appropriate to
the circumstances.

It will be appreciated that the particular locations in a substation where a person can be
subjected to the maximum 'step' or 'touch' potential depends on the configuration of the earth
electrode arrangement. If this comprises one long length of buried conductor the maximum
danger to a person is within a 'step' on either side of this conductor at any position along its
length. If the conductor length were short (the earth electrode current remaining the same) the
danger is greater because the more compact electrode results in an increased current density
and steeper potential gradient.

The presence of a surface layer of very high specific resistivity material provides insulation
from these ground potentials and greatly reduces the associated risks. Thus substations
surfaced with hard core and stone chippings are inherently safer than those with grass
surfacing.

Where there is danger of excessive potential gradient around above-ground metalwork such
as structures, tanks, etc, this shall be countered by providing an earth mat at or just below the
surface of the ground and bonded to the metalwork, so arranged that the metalwork can only
be touched while standing above the mat.

Pole-mounted equipment presents a particularly difficult ground potential gradient problem

and the special precautions noted in Section 11 be shall observed. It may be necessary to
apply these precautions in some ground-mounted substations.
Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

7. DESIGN ARRANGEMENT

In order to determine fully the requirements for and adequacy of an earthing system it is
necessary to produce a preliminary design arrangement of that earthing system. The design
should take into account those features that are unique to the site location, for example site
area available, soil conditions, system currents etc. To this end the following procedure is
recommended which includes guidelines for general design arrangements.

7.1 Layout

From a site layout drawing showing the location of the plant to be earthed, a preliminary
design arrangement of the earthing system for the substation should be prepared,
incorporating the relevant 'functions' of Section 4 and the relevant 'features' of Section 5. The
particular layout arrangement will be unique to each substation but all will have some
dependence on, inter alia, a combination of the following factors:

(i) an indoor or outdoor substation;

(ii) the value of soil resistivity;

(iii) the value of fault current to be carried by the earthing conductors;

(iv) the design fault clearance time;

(v) the value of ground current to be dissipated by the earth electrode.

7.2 Design Guidelines

This Section gives an outline of those features of earthing system arrangements which have
proved to be most satisfactory in practice.

7.2.1 Outdoor Substation Installations

Except for single plant type installations, e.g. pole or ground-mounted single transformer or
single switch installations, which typically employ a single point earth electrode, it is
recommended that the earthing arrangement be based on a peripheral buried main bare
earthing conductor generally encompassing the plant items to be earthed, with spur
connections, which may be fully or partly buried, from the main conductor to the items of
plant. In addition, discrete earth electrodes, e.g. rods or plates, may be connected to this main
earthing conductor. These electrodes may variously be employed to reduce the surface
current and/or the electrode resistance of the overall earth electrode system.

For small area substations a single bar main earthing conductor may be adequate for bonding
purposes but additional electrodes may be required to obtain a satisfactory earth electrode
resistance.

For large area substations the main earthing conductor may be augmented with inter-
connected buried bare cross-connections to form a grid. Such cross-connections increase the
quantity of earth electrode conductor as well as providing local main conductors to keep the
spur connections short.

Fault current flowing through an earth electrode system to ground uses the outer extremities
of the electrode system to a greater extent than the inner parts of the system. Thus, adding
more earth electrode conductor, whether as vertical rods or as horizontal tape, to the inner
area of a small loop or well integrated grid electrode system, will have little impact in reducing
 Technical Specification 41-24
Page 9 Issue 1
1992
Addendum, Section 15, incorporated November 2009

the current density in the outer electrode conductors of the system. Such reductions as may
be desirable are best achieved by extending the electrode system to cover a greater area of
ground, or by driving rods around the periphery of the system or by a combination of both.

The vertical rod electrode is most effective for use in small area substations or when low soil
resistivity strata, into which the rod can penetrate, lies beneath a layer of high soil resistivity.
For large area substations employing a grid electrode system, the addition of vertical rods,
even when optimally installed around the periphery of the system, may make only a marginal
improvement.

7.2.2 Indoor Substation Installation

Apart from some few installations, the plant of indoor substations will normally be erected on
a concrete raft. The provision of a buried main earthing conductor within the confines of a
building is impractical and thus a surface laid main earthing conductor loop is normally
installed with surface run spur connections to the various items of plant. The earth electrode
system employed with this arrangement may differ depending on the magnitude of earth fault
current that the electrode system is required to carry.

For low values of earth fault current, discrete electrodes, for example rods or plates,
distributed in the ground outside the building and connected to the main earthing conductor
loop may be sufficient. Some care, however, may have to be exercised in the placement of
these electrodes close to entrances to the building where the resulting step and touch
potential gradients may be unacceptable.

For substations subject to high earth fault current it is recommended that an outer main bare
earthing conductor be buried in the ground at approximately 0.5 metre depth and at 1.0 metre
distant from the building and frequent bonds made, say every 15 - 20 metres, between the
outer and inner main conductors and to the building if this is metalclad. Rod electrodes may
be connected to this outer main conductor, if appropriate and where ground areca is
restricted, otherwise a further external buried loop, separated from the outer main conductor
by about 10 metres and similarly regularly bonded to it may be employed.

7.2.3 Provision of Test Facilities for Monitoring Earth System Efficiency

For medium area and large area substations, consideration should be given to provision of
test facilities to enable monitoring of the earth system efficiency.

8. DESIGN DATA

The final design of the earthing system can only be undertaken when sufficient knowledge is
available of the proposed physical and electrical arrangements of the substation. Any special
features about the site such as subsoil of a corrosive nature and the suitability of the site for
driven earth rods or other forms of electrode, requires to be ascertained. Other relevant
features, such as existing earth electrodes, nearby earthed structures, buried pipes or piled
foundations are also required to be noted and taken into consideration. In urban areas in
particular the substation may be served by an underground cable network which, if
incorporating non-insulated sheaths/armours, might obviate the need for an extensive earth
electrode system.

8.1 Soil Resistivity

The value of the specific resistivity of the soil may be ascertained by reference to published
data or by direct measurement. Table 1 sets out typical values relating to types of soil but
these should be used for very preliminary assessments only. Direct measurement will
Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

normally be necessary and the recommended method is described in Engineering

Recommendation S34.

8.2 Fault Currents

Methods for calculating the appropriate values of fault current are not included in this
Specification.

For earthing conductors which are required to carry the full fault current it is recommended
that the value to be taken should be that corresponding to the ultimate symmetrical three-
phase short circuit rating of the substation. This value may be slightly exceeded in certain
cases for non-symmetrical faults but, having regard for the conservatism of other factors
which are recommended in this document, it is not considered necessary to work to a value
greater than that given by the symmetrical fault.

8.2.1 Earth Electrode Current

Earth electrode currents are associated with earth fault currents only. In many instances this
latter value of current is very much less than the above ultimate three-phase fault value. For
design economy, therefore, it is the practice to assess the value of earth electrode current
based on the value of earth fault current corresponding to the foreseeable future (typically a
five-year period) development of the system.

A detailed explanation and guidance on the assessment of the value of earth electrode
current is described in Engineering Recommendation S34.

8.3 Earthing Conductor and Earth Electrode Current Ratings

Earthing conductors which are not required to function as earth electrodes can have a higher
fault current density than those conductors designated to function as earth electrodes only.
The soil surrounding earth electrodes is of a much higher sensitivity than the electrode
conductor material and thus the passage of current through the soil will develop, relatively, a
much higher temperature rise. The effect of high temperature in the soil causes drying of the
surrounding soil, thus further increasing its resistivity, or even the production of steam which
can force a separation between the electrode conductor and its interfacing soil. For this
reason the current rating of an earth electrode, inter alia, is specified in terms of its surface
current density. As a consequence the cross-section current rating of discrete electrode
conductors in practical installations is very much less than that permitted in above-ground
earthing conductors. Where a multi-mesh buried main earth grid is employed in the dual role
of an earthing conductor and an earth electrode, the density of fault current in the earthing
conductors should rapidly reduce as the distance from the point of fault increases. Provided,
therefore, that a sufficient quantity of grid conductor is buried and is well distributed, the
surface current density will generally be satisfactory and high surface temperature restricted
to a small area close to the fault point and thus have negligible effect on the value of total
earth electrode resistance or on the efficacy of the earthing system as a whole.

8.3.1 Earthing Conductor Current Rating

Earthing conductors should normally be selected from standard copper or aluminium

sections. For conductors which have to carry the full fault current from the point of fault to the
main earthing system, it is recommended that the value of current rating to be taken is that
corresponding to the ultimate system maximum symmetrical three-phase short circuit current.
Subsequent redistribution of these currents into two or more paths allow a reduction in the
size of the branch conductors,
 Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

The conductor temperature should not exceed 405°C for copper and 325°C for aluminium
based on an initial temperature of 30°C. The fault duration times to be used in these
assessments are 1 second for 275kV and 400kV substations and 3 seconds for all others.
Tables 2A and 2B give declared current ratings for a range of standard conductor sizes for
both 1 second and 3 second fault duration times. The short time rating of other conductors
can be calculated from the following formula taken from IEC Publication 724 (1984):

1
2
1 θ β
ic Ak log e f amperes
t θi β

where ic = conductor current (amps)

2
A = cross-sectional area (mm )

θf = final temperature (ºC)

θi = initial temperature (ºC)

t = time (seconds)

k = see Table below

β = see Table below

Material k β
Copper 226 234.5

Aluminium 148 228

Lead 41 230
Steel 78 202

8.3.2 Earth Electrode Current Rating

The discrete earth electrode shall at all times retain its functional properties, i.e. both its
current carrying capability and its value of resistance to earth. For these reasons the
temperature rise of the electrode conductor and the density of current dissipation from
electrode to soil, during the passage of fault current through it, shall be limited. The limit of
surface current density is given by the following formula:

1
57.7 2
3
is 10 amperes/mm2
ρt

where is = conductor surface current density

ρ = soil resistivity (ohm. metres)

t = fault duration (seconds)

Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

Limiting values of surface current rating calculated from the above formula for some typical
electrodes are given in Table 3 and Figure 1 for a range of soil resistivities for 1 second and 3
second fault duration times.

In most practical installations the actual values of surface current density will be considerably
less than the above limiting values, due to the quantity of electrode conductor employed in the
installation to provide effective bonding and in some installations where extra electrodes have
been added, to comply with the 'touch' potential limit.

9. DESIGN ASSESSMENT

Using the preliminary design arrangement of the earthing system developed in Section 7 and
the design data of Section 8 the following methods and associated formulae describe the
processes of assessment.

While there is no design requirement which directly limits the overall rise of earth potential of
a substation to any particular value, the design shall comply with the safety criteria and with
the earthing conductor and earth electrode conductor current ratings. These latter
requirements will, as a consequence, tend to restrict the overall rise of earth potential.

Two conditions of earth fault may have to be considered to determine the maximum value of
earth electrode current. In one, the earth fault is external to the substation; here the current of
concern is that returning to the neutral(s) of the transformer(s) at the substation under
consideration. The other is for an earth fault in the substation; here the current of concern is
now that value returning to the neutral(s) of the transformer(s) external to the substation
under consideration. These currents are components of the system earth fault currents. If
these return currents have available to them other conducting paths directly connected to the
earthing system of the substation, for example overhead line earth-wires and cable sheaths,
then the currents in these paths shall be deducted from the appropriate return current to
derive the value of current passing through the earth electrode system of the substation.
Evaluation of this current is described in Engineering Recommendation S34.

Two procedures are given for assessing tile design, the first outlined in 9.1 being an
approximation which, if furnishing satisfactory results, avoids the need for a more detailed
assessment. If the results of this approximate assessment indicate that the safety criteria
could be exceeded or the rise of earth potential is considered to be excessive, then the more
refined assessment in 9.2 should be employed.

When an entirely theoretical approach is used for assessing the design of an earthing system,
doubts on the reliability of the result may arise due to uncertainties as to the correct value of
soil resistivity to be used or of the effects that other buried structures may have. In these
circumstances recourse may have to be had to direct measurement to obtain a more reliable
result.

Recommended methods of measurement are given in Section 12. On the basis that the earth
electrode system will not yet be installed, measurement may be made on representative test
electrodes and the results extrapolated to the intended final design. Measurement may be
delayed until a sufficiently representative part of the intended system is installed to obtain a
better prediction of any improvements necessary. In any event a final check measurement of
the completed installation is recommended.
 Technical Specification 41-24
Page 13 Issue 1
1992
Addendum, Section 15, incorporated November 2009

9.1 Approximate Assessment Procedure

This assessment considers both the internal and external earth fault conditions as explained
above but disregards any contribution that external electrodes, e.g. overhead line earth-wires
or cable sheaths, may have.

(i) Determine the soil resistivity. See Section 8.1.

(ii) Estimate from the preliminary design arrangement layout using data given in
Engineering Recommendation S34, the value of the site earth electrode system
resistance, disregarding the contribution of any external electrode, e.g. overhead
line earth-wire or cable sheath.

(iii) Obtain the appropriate total values of system earth fault current for both an
internal and external earth fault (see explanation above) and deduce the greater
value of the two following quantities:

(a) for an internal fault on a non-auto transformer, the total fault current less that
returning to any local transformer neutrals; for auto-transformers, more
specific calculations are required to indicate the direction of flow of current in
neutral;

(b) for an external fault, that proportion returning to the local transformer
neutrals.

(iv) Estimate the rise of earth potential based on the product of items (ii) and (iii)(a) or
(iii)(b) above, whichever is the greater.

If the value derived in (iv) above does not exceed the appropriate 'transferred' potential limit of
430/650 volts, then the 'touch' and 'step' potentials cannot exceed the safety limit and no
further assessment needs to be done. The finalised design of the earthing system may be
prepared taking into account the earthing and electrode conductor ratings.

If the value derived under (iv) above exceeds the appropriate safe value of 'transferred'
potential, the more refined assessment shall be made.

9.2 Refined Assessment Procedure

(i) Determine the soil resistivity by measurement using the detailed procedure given
in Engineering Recommendation S34.

(ii) Estimate the value of the substation earth electrode system resistance, including
the contributions made by any overhead earthwires and/or earthed cable sheaths
radiating from the site using the preliminary design assessment layout and the
data provided in Engineering Recommendation S34.

(iii) Obtain the appropriate total values of system earth fault current for both an
internal and external earth fault (see explanation above) and deduce the greater
value of the two following quantities of earth fault current passing through the
earth electrode system of (ii) above. Refer to Engineering Recommendation S34
for guidance on this evaluation.

(a) For an internal fault (see 9.1 (iii) (a) for non-auto-transformers), the total fault
current less that returning to any local transformer neutrals and that returning
as induced current in any earthwire or cable sheath/armour.
Technical Specification 41-24
Page 14 Issue 1
1992
Addendum, Section 15, incorporated November 2009

(b) For an external fault, that returning to local transformers less that returning as
induced current in any earthwire or cablesheath/armour.

(iv) Estimate the rise of earth potential based on the product of items (ii) and (iii)(a) or
(iii)(b) above, whichever is the greater.

If the value derived in (iv) above does not exceed the appropriate 'transferred' potential limit of
430/650 volts, then no further assessment needs to be done. The finalised design of the
earthing system can be prepared taking into account the earthing and electrode conductor
rating.

If the value derived under (iv) above exceeds the appropriate safe value of 'transferred'
potential an assessment covering 'touch' and 'step' potentials shall be made, see (v) below. If
the earthing system is safe against 'touch' potential it will inherently be safe against 'step'
potential.

(v) Estimate the 'touch' potential. Refer to Section 9.3 for guidance in this evaluation.

In general, earthing systems are complex and a rigorous evaluation of all prospective 'touch'
potential locations in a substation is impractical. Evaluations indicate that the highest 'touch'
potentials will normally arise between items of plant connected to the peripheral edge of the
earth electrode system and the ground 1 metre external to it. Using this assessment, formulae
have been derived (see Item 9.3) from which 'touch' potentials can be deduced or
alternatively an estimate obtained for the amount of buried electrode required to ensure that
the appropriate limiting value of 'touch' potential is not exceeded. The general ground voltage
profile further away from the substation can be deduced from formulae given in Engineering
Recommendation S34.

Depending on the results of the evaluation, further improvements in the design of the earth
electrode system may be necessary until the appropriate safety criteria for 'touch' and 'step'
potentials are met and any necessary isolation or additional insulation is provided to avoid
contact with 'transferred' potentials which exceed the appropriate safety limit.

Although there is no specified limit to the rise of earth potential of the substation, further
improvements may sometimes be justified to lower this value by reducing the value of the
earth electrode resistance if the ground potential outside the substation exceeds the
appropriate safe value of 'transferred potential' and if a hazardous condition might exist, for
example to communication circuits or pipelines of other authorities which pass through the
zone of high ground-potential. Even when these conditions do exist it does not necessarily
mean that the earth electrode resistance should be reduced if, as may well be the case, it is
cheaper and practical to protect the other authorities plant by isolation or additional insulation.

9.3 Assessment of 'Touch' Potential and Electrode Conductor Length

When developing formulae for calculating the value of 'touch' potentials, it is normal practice
to refer these calculations to the potential of the natural ground surface of the site. From the
safety aspect these calculated values are then compared with the appropriate safe value
given in Section 6 which takes account of any footwear or ground covering (chippings)
resistance. It is important, therefore, to appreciate that the permissible safe value of 'touch'
potential, as calculated in this Section, will differ depending on the ground covering, fault
clearance time and other factors prevailing at the site. Allowance is made for this in Figure 2.

The developed formulae are not rigorous but are based on the recognised concept of
integrating the voltage gradient, given by the product of soil resistivity and current density
through the soil, over a distance of one metre. Experience has shown that the maximum
 Technical Specification 41-24
Page 15 Issue 1
1992
Addendum, Section 15, incorporated November 2009

values of 'touch' potential normally occur at the external edges of an earth electrode. For a
grid electrode this potential is increased by the greater current density transferring from the
electrode conductors to ground around the periphery of the grid as compared with that
transferring in the more central parts. These aspects have been taken into account in the
formulae firstly for 'touch' potential and secondly for the length of electrode conductor required
to ensure a given 'touch' potential is not exceeded.

9.3.1 Substations with Separately Earthed Fence (See Figure 3)

For normal buried grid depths (typically 0.5 metres):

(i) External Touch Potential at the Edge of the Electrode (Figure 3A)

k e .k d .ρ.I k e .k d .ρ.I
E t(grid) (volts) or L (metres)
L E touch

ke is a factor which allows for the effect of a uniformly distributed electrode current over the
grid and is given by:

h = grid depth (metres)

d = equivalent diameter of conductor

circumference of conductor
(metres)
π

D = average spacing between parallel grid conductors (metres)

1
n (n A n B ) 2

where nA = number of parallel grid conductors in one direction

where nB = number of parallel grid conductors in the other direction

kd is a factor which modifies k e to allow for the non-uniform distribution of electrode current
and is given by:

L
kd 0.7 0.3
Lp

where L = total length of buried electrode conductor including rods if connected (metres)

Lp = perimeter length if buried electrode conductor including rods if connected

(metres)

ρ = soil resistivity (ohm metres)

I = total current passing to ground through electrode (amperes)

Technical Specification 41-24
Page 16 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Etouch = resulting ‘touch’ potential or, when assessing length L, the safe ‘touch’ potential
from Figure 2

(ii) External ‘Touch’ Potential at the Fence (Figure 3A).

The ground current density is significantly diminished at the fence to that at the edge of the
grid electrode. As a result, a new factor, k f, based on a two metre separation between fence
and grid electrode, is applied in place of ke in the above formulae.

Hence:

k f .k d .ρ .I k f .k d .ρ .I
E t(fence) (volts) or L (metres)
L E touch

where kf = 0.26ke

9.3.2 Substation with Integrally Earthed Fence (See Figure 3)

There are two situations to be considered here. The first assumes that the fence is situated at
the edge of the substation electrode whilst the second allows for a further peripheral electrode
conductor buried half a metre below the surface at a distance of one metre beyond the fence
and regularly bonded to it at intervals as recommended in Section 11.2.2:

(iii) External Touch Potential at Fence with No External Peripheral Electrode (Figure
3B)

Et (fence) is the same as Et (grid) in (i) above.

(iv) External Touch Potential at Fence with External Buried Peripheral Conductor 1
Metre away from Fence (Figure 3C).

k fe .kd .ρ .I k fe .k d .ρ .I
E t(fence) (volts) or L (metres)
L E touch

where

h and d as given under Section 9.3.1

S = distance between the outermost buried grid conductor and the next nearest parallel
conductor (metres).

10. INSTALLATION

10.1 Laying Conductors Forming a Horizontal Earth Mat

Buried bare copper or other approved metal conductors (aluminium shall only be used for
above-ground connections) forming part of the earth electrode system should be at about
500mm deep. This gives protection to the conductors and connections and should ensure that
they will normally be below the frost line. Such conductors laid as described under Section 7.2
will often form the basis of an adequate earthing installation. The use of this mat as a means
of providing bonds between the various items of substation plant also establishes an equal
potential between them. The arrangement of a grid type of earth mat reduces the 'step' and
'touch' potentials within the confines of the mat.
 Technical Specification 41-24
Page 17 Issue 1
1992
Addendum, Section 15, incorporated November 2009

With large substations, e.g. 400, 275, 132, 66 and 33kV it may offer economical advantage if
earthing conductors are laid at the bottom and to one side of excavations made for cable
trenches, field drains and other civil works, etc. The conductor should be surrounded by
150mm of non-corrosive soil of fine texture, firmly rammed. For practical reasons, it may
sometimes be prudent to wait until the civil works of constructing the trenches, plinths and
other structures, have largely been completed so as to avoid possible damage to these
earthing conductors.

Note: If the indigenous soil is hostile to copper, ie acidic with a pH value of less than 6
or alkaline with a pH value of more than 10, suitable surrounding soil should be
imported.

Where an adequate earth electrode installation is provided by the main earth grid, subsidiary
connections to equipment may be laid at a shallower depth, say 250mm, and by routes most
convenient to site conditions. However, due to possible wide variations in the soil resistivity
near to the ground surface, their contribution to the overall value of earth resistance should be
ignored. They do, nevertheless, serve to reduce voltage surface gradients within the
substation site.

The above recommendations deal mainly with substations on normal sites. Where ground
conditions restrict the installation depth or where the soil resistivity is very high, additional
measures may be required beyond the substation boundary to lower the overall value of
earthing resistance. It will, however, still be necessary to provide adequate bonding within the
substation to control 'step' and 'touch' potentials.

In the case of substations, for example 11kV/LV unit type substations where the concrete
plinth covers the whole site area, earth electrodes may be installed prior to construction of the
plinth and thus provision should be made to bring the connection through the concrete with
suitable protection to above plinth level.

Where bare earthing conductors cross over or are laid parallel to and touching power or
multicore cables, they should be insulated with PVC tape or sleeve to counteract possible
puncturing of cable sheaths arising from high voltage transients on the earthing conductors.

Where bare earthing conductors cross trenches it may be prudent to use one of the following
methods of providing for the crossing at the time when the trench is constructed in preference
to insulating the bare conductor subsequently.

(a) short lengths of earthing conductor laid under the trench for later connection
to the main earth grid.

(b) pipe laid under the trench, extending say 150mm beyond each side to
accommodate the earthing conductor.

Earthing conductors laid in the vicinity of drainage and soak-away pits or any other similar
civil works should maintain a separation of at least 500mm to avoid mechanical damage or
breakage during subsequent disturbance of the ground in connection with the civil works.

Where bare metal conductor connected to the earthing system is buried under metal fencing,
and the fencing is independently earthed, the conductor should be insulated by threading
through non-metallic pipe extending for at least 2m each side of the fence or alternatively
insulated conductor may be used.

When laying stranded conductor for earthing purposes, care should be taken to avoid
distorting the individual strands.
Technical Specification 41-24
Page 18 Issue 1
1992
Addendum, Section 15, incorporated November 2009

10.2 Fixing Conductors

In fixing aluminium or copper conductors to structures, etc, suitable insulated clips should be
used to prevent electrolytic action and to avoid drilling the conductors. Galvanised clips
should not be used. Fixings should be spaced not more than 1m apart.

Earthing conductors in trenches containing power and/or multicore cables should be fixed to
the walls near the top (e.g. 100mm from the top).

Copper strip conductor supported from or in contact with galvanised steel should be tinned to
prevent electrolytic action.

Sharp bends required in aluminium strip conductor should be formed by the use of a bending
machine. Unless it is protected, aluminium earthing conductor should not be laid within
250mm of ground level.

Where it can be proved that the current carrying capacity of a main aluminium or steel
member or welded sections forming a structure is at least equal to that of the required
aluminium or copper earth conductor, the structure may form part of the connection and there
is no need to fix an earth conductor along this section. Where doubt exists regarding the
effective continuity between bolted sections, bonds should be added across the joints.

Connections to metal cladding, steel structure and metal door frames and windows or any
other metallic panels should be made inside buildings.

10.3 Jointing Conductors

All crossings of earthing conductors in the main earth grid shall be jointed. Approved
compression type joints may be used for stranded conductors and when compressed firmly
on to its associated conductor the joint must conform to the requirements of BS 3288, Part 1.

No conductor strip may be drilled for a bolt having a diameter greater than one-third of the
width of the strip. If this diameter will be exceeded then a wider flag should be jointed to the
strip.

Aluminium to Aluminium:

When possible, joints on strip conductor should be welded using one of the following
methods:

(a) inert-gas tungsten arc (TIG) as stated in BS 3019, Part 1;

(b) inert-gas, metal arc (MIG) technique as stated in BS 3571, Part 1;

(c) an approved cold pressure welding technique.

Where the above type of welding is not possible or practicable then bolted type joints or
alternatively an approved explosive welding technique may be used. The bolting schedule for
bolted type joints is given in Appendix A.

When making a bolted type joint the surface of the aluminium must be cleaned thoroughly by
wire brushing and greased or an approved jointing compound applied immediately to both
mating surfaces. Bolts should then be tightened in accordance with the schedule and all
excess grease or compound wiped off and discarded.
 Technical Specification 41-24
Page 19 Issue 1
1992
Addendum, Section 15, incorporated November 2009

To ensure adequate contact pressure and avoid over-stressing, torque spanners should be
used.

Aluminium to Copper:

Joints between aluminium and copper shall be of the bolted type and be installed in the
vertical plane with the bottom of the overlap at a minimum distance of 150mm above ground
level. The fixings should be in accordance with the bolting schedules given in Appendix A.

The mating surface of the aluminium must be cleaned thoroughly by wire brushing and
greased or an approved jointing compound applied, and the copper surface should be tinned.
After bolt tightening by torque spanner, excess grease or compound must be wiped off and
discarded and the joint protected from the ingress of moisture by the application of suitable
plastic compound or insulating shrinkable sleeve with mastic lining. Alternatively the joint may
be protected by approved heavy duty bitumastic paint.

Copper to Copper:

When possible, joints on strip conductors should be brazed (using zinc-free brazing material
with a melting point of at least 600°C) or approved cold pressure welding.

Where the above type of welding is not possible or practicable then bolted type joints or
riveted and sweated joints or an approved explosive welding technique may be used. The
bolting schedule for bolted type joints is given in Appendix A.

Earthing conductor connections shall, as far as practicable, be made on to vertical surfaces

only. In the case of painted metal the paint must be carefully removed to expose clean
galvanised metal, connection made, and any unpainted surfaces should be protected by an
approved heavy duty bitumous paint.

Earthing conductor must be tinned where connected to galvanised steelwork. No connection

point shall be less than 150mm above ground level. In any position subject to corrosion the
finished joint shall be protected by an approved heavy duty bituminous paint.

10.4 Loops for Portable Earths

Loops of plain aluminium or copper should be provided on the earthing conductor at each
location where portable earthing leads may be required to be applied. Technical Specification
41-21 (portable earthing) states that earth-end clamps shall be suitable for attachment to and
removal from copper or aluminium earth conductors consisting of flat strip ranging in width
from 37mm to 50mm and in thickness from 3mm to 6.3mm. The loops should be not less than
230mm long and 75mm clear of the earth conductor, at a convenient height and be formed
separately, not by bending the earthing conductor itself. Loops shall be jointed to the earthing
conductor using the appropriate method given above.

10.5 Operating Mechanisms of HV Equipment

Operating mechanisms associated with air-break switch disconnectors, earth switches and
control kiosks of circuit breakers etc, which metallically are not fully integral with the main
equipment should be connected to the main earthing system by a branch earthing connection
entirely separate from that employed for earthing the main equipment. Reliance on an
insulated insert in the mechanism drive, as a means of preventing current from flowing in this
path, is not recommended.
Technical Specification 41-24
Page 20 Issue 1
1992
Addendum, Section 15, incorporated November 2009

10.6 Provision for Testing Earth Electrodes

Where it is proposed to segregate electrodes or earthing systems for testing, bolted links
should be fitted. Once a main earth grid has been established in a substation, it is extremely
difficult to achieve segregation of independent sections of the grid for periodic testing. Where
rod groups are employed as part of the substation earth electrode system, links may be
provided such that each group can be separately tested to determine its earthing resistance
and thus its integrity. Separate bolted links for each earth electrode of a group in close
proximity are not required. Where main earth grids on adjacent sites are connected together,
it is usually practical to insert links in the tie connections to enable segregation of the sites. It
is preferable to locate these links in a sunken concrete box, or they can be mounted on walls,
structures, posts, etc where they will be readily visible and can be suitably labelled. In the
case of indoor substations these links should be mounted at convenient points on the
substation wall.

10.7 Earth Electrodes

10.7.1 Structural Earths

Where large civil works are involved, it may often be possible to secure an effective earth
electrode by making use of deep driven sheet piling or other metalwork, for example deep
stanchions, even when encased in concrete. Steelwork associated with civil works that is near
to the surface of the soil and enclosed within the proposed main earth electrode system will
contribute little to reducing the overall earth resistance but it can reduce voltage gradients
within the site. Large and deep constructions external but connected to the main system can
provide a very effective contribution.

At any proposed substation site that will have deep civil works, the use of these works as
effective earth electrodes for incorporation into the earthing system should be considered at
the design stage.

The main steelwork of buildings should be connected to the main earth grid by bonding.
Unless such steelwork is deeply embedded it should not be considered as an additional earth
electrode.

Where sheet steel piles of the interlocking kind are utilised, the earthing connection should be
made to a steel strap welded across at least six of the piles. In addition, adjacent piles should
be welded together to a depth of 300mm for a further distance of at least 4.5m on either side
of the connection point.

10.7.2 Driven Rod Electrodes

These are generally convenient to install where the subsoil is free from boulders and rock.
Rod electrodes and their connections should be in accordance with Technical Specification
43-94. The earth resistance of a rod or group of rod electrodes may be calculated from
formulae given in Engineering Recommendation S34.

A number of rods may be connected in parallel but they should be installed with sufficient
spacing apart such that each is essentially outside the resistance area of any other. For
worthwhile results the mutual separation should be not less than the depth of the rod.

Some examples of the parallel resistance of 16mm diameter electrodes spaced at 3m and
driven to a depth of 3m below the surface in soil resistivity (ρ) have been calculated as
follows:
 Technical Specification 41-24
Page 21 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Values of Resistance in ohms (ρ) in ohms metres

Number
of Rods
In a straight line In a triangle In a square

1 0.335 (ρ) - -

2 0.144 (ρ) - -

3 0.141 (ρ) 0.147 (ρ) -

4 0.133 (ρ) - 0.188 (ρ)

Deep earth electrodes should, as far as possible, be driven into the earth vertically. They may
be particularly advantageous if the earth resistivity falls with depth. If several deep earth
electrodes are necessary in order to achieve a required parallel resistance, then, where space
is available, the mutual minimum separation could usefully be double that of the effective
length of an individual earth electrode. Figure 4 shows the method of connecting the rod to
the main earthing system.

Note: The current dissipation over a vertical rod electrode in homogeneous soil is uneven,
having a greater density with depth. Where such rods pass through a layer of higher
soil resistivity there is a much stronger tendency for current to by-pass this region,
thus limiting the effectiveness of the electrode as a whole.

Substations in large urban developments are often located below ground level in tanked
structures. In such situations special facilities for installing earth electrodes are required.
Figure 5 shows typical arrangements of water-tight chambers in which the earth electrode is
installed. These special facilities are only necessary when ground, in which earth electrodes
could be installed, is more than 4m away from the tanked structure.

10.7.3 Plate Electrodes

Where the soil resistivity is very high or in soils where it is difficult to drive rods, buried plates
are an alternative form of electrode which, because of their large surface area, ensures low
current density at the plate/soil interface.

Plates should be of cast iron not less than 12mm thick and preferably ribbed. The earth
resistance of a plate electrode may be calculated from the formula given in Engineering
Recommendation S34.

Figure 6 shows a suitable arrangement of a CI earth plate with terminal details for earth
conductor connection. The connection between the earth plate and the disconnecting link, if a
link is considered to be necessary, should be protected to reduce electrolytic action. The
plates should be set vertically and the depth of setting should be such as to ensure that the
surrounding soil is always damp. The minimum cover should be 500mm except that when the
underlying stratum is solid, e.g. chalk or sandstone, and near the surface, the top of the plate
should be level with the top of the solid stratum. Sufficient solid stratum should be removed
and replaced with fine soil or other suitable infill to ensure as low a resistance as possible.

The use of coke breeze as an infill is not recommended as it may result in rapid corrosion, not
only of the electrode itself, but also of cable sheaths etc to which it may be bonded.
Technical Specification 41-24
Page 22 Issue 1
1992
Addendum, Section 15, incorporated November 2009

For conventional sizes of place the resistance is approximately inversely proportional to the
linear dimensions, not the surface area, i.e. a 915mm x 915mm plate would have a resistance
approximately 25% higher than a 1220mm x 1220mm plate.

Where the resistance of a single plate is higher than the required value, two or more plates
may be used in parallel, and the total resistance is then inversely proportional to the number
employed, provided that each plate is installed outside the resistance area of any other. This
normally requires a separation of about 9m but, as a compromise for sizes of plates generally
employed, a separation of 2m is sufficient to ensure that the total resistance will not exceed
the ideal parallel value by more than 20%. Even at the latter spacing, it will generally be more
economical to use two plates in parallel, each of a given size, than one of twice that size. For
this and other practical reasons the size of plate is normally not greater than 1220mm x
1220mm.

11. ITEMS REQUIRING SPECIAL CONSIDERATION

11.1 Overhead Line Terminals

Where the earthwire of an incoming line ends at a terminal support, continuity shall be
provided for current in the earth wire to flow to the main earthing system.

Where the earthwire terminates on a steel tower just outside the substation fence, continuity
is provided by bonding between the earth wire and the top of the tower and between the main
earthing system and the base of the tower. These bonds must be of equal rating to the earth
wire. It is recommended that the latter bond should, for security reasons, be buried and
consist of two conductors separated from one another and must, by suitable insulation, be
kept clear of any independently earthed fence under which they may pass for a 2 metre
distance on either side.

Where the earthwire terminates a cable sealing-end compound that is well outside the
substation, continuity between the base of the tower and the main earthing system will be
provided by either the sheaths of the power cables or by a continuity conductor laid and
installed in accordance with Engineering Recommendation C55/4.

Terminal pole stay wires, if external but within 2 metres of a fence connected to the main
earthing system, should be bonded to the fence. The installation of some further buried
electrode may be necessary if touch potentials to the wires exceed the appropriate safety
limit. If the wires are close to an independently earthed fence they may still be bonded to the
fence but insulators must be inserted in such stay wires.

11.2 Fences

The continuity bonds referred to in 11.1 above which extend to terminal equipment outside
the substation are disregarded in the following situation.

11.2.1 Independent Earthing

Where the substation earthing system is effectively within the substation perimeter fence, the
fence should be separately earthed with rods 3-4 metres long at all corners and at all points
where HV overhead conductors cross the fence and with further rods at about 50 metre
intervals round the site. Gate posts should be bonded together with below ground
connections to ensure that difference potentials do not arise when the two parts are bridged
by a person opening the gates.
 Technical Specification 41-24
Page 23 Issue 1
1992
Addendum, Section 15, incorporated November 2009

The separating distance of at least 2 metres should be established between the fence and the
substation earthing system including any items connected to it.

11.2.2 Connected to the Substation Earthing System

Fences which are located within the area of ground encompassed by the substation earthing
system, or within 2 metres of it, including fences common to two or more abutting substations
having a common earthing system, should be connected to the earthing system. For long
runs of fencing such connections should be made at intervals of about 50 metres.

Where the fence which is connected to the substation earthing system is the perimeter fence,
and where the touch potential external to the fence could exceed 430/650 volts, then an
additional bare electrode conductor should be buried in the ground external to the perimeter
fence at a distance of 1 metre and at a depth of 0.5 metres. The conductor should be
connected to the fence and to the earthing system at intervals of about 50 metres such that it
becomes an integral part of the substation earthing system. Further bonds between the fence
and the earthing system should be made at all points where HV overhead conductors cross
the fence.

At locations where substation-earthed fencing abuts with independently earthed fencing the
substation earthed fencing should be completed with a 2 metre long partition. The partition
may be of insulating or conducting material but if conducting a 5 centimetre insulating gap
should be arranged each end to allow the partition to 'electrically float'.

11.2.3 Plastics Covered Chain Link Fencing

Although the insulated nature of such fencing could be said, in its new condition, to be
compatible with unearthed construction, in the long term, with subsequent wear and abrasion
experience, it has been shown that bare metal will become exposed. It is thus recommended
that such fencing should from the outset be earthed by bonding the support posts, straining
wires, barbed wire and anti-climbing device metalwork to the independent or integrated earth
electrode system as appropriate. In addition the insulation coating should be checked
regularly for damage and where necessary maintained to avoid corrosion and the exposure of
metal to 'touch' potentials.

11.3 Arc Horn Anchorage Arrangement

Where, for mechanical loading or dimensional requirements it is necessary to have an

assembly of ferrous fittings such as turn buckles, links, shackles etc between the insulators
and the earthed structure or anchor point, precautions may be required if the fault current is
very large. The earthed end arc-ring (or horn) anchorage arrangement should be attached to
the main earth connection by means of a flexible copper shunt, in order to limit the potential
which may appear across the metalwork and to prevent loss of mechanical security due to
overheating.

11.4 Water Pipes

A water supply to a substation having a potential rise in excess of the appropriate safety
criterion value should be run in a non-metallic pipe. This precaution avoids the possibility of a
substation potential rise being transferred outside the substation to the danger of other users
of the water supply system. Any metallic pipe used within the site should be bonded to the
substation earthing system.
Technical Specification 41-24
Page 24 Issue 1
1992
Addendum, Section 15, incorporated November 2009

11.5 Cable Metallic Sheath/Armour Earthing

This section covers all HV power cables contained within or entering HV substations but
excludes those HV cables which feed HV/LV transformers located in the substation where the
LV supply is exclusively for use in the substation. The requirements for these latter cables are
dealt with under Section 11.14.

11.5.1 Non-Insulated Sheath/Armour Cables

The sheath/armour of bitumen-impregnated hessian served cables when buried in the ground
should be regarded as continuously earthed. Such cables, when extended outside the
substation, significantly increase the earth electrode system and this benefit may be taken
into account when assessing the substation earth electrode resistance.

11.5.2 Insulated Sheath Cables

General Philosophy

The metallic sheath/armour of cables can, due to their inductive coupling properties, provide a
very low impedance return path for earth fault current flowing in the cable conductors, thus
greatly reducing the quantity of current that flows to ground and as a consequence reducing
the levels of inductive interference to other cables. To achieve this, however, the
sheath/armour must be earthed at least at both ends. Whilst this arrangement of earthing is
satisfactory for three-core cables it would cause a severe de-rating of single-core cables since
quite large steady-state currents could flow in the sheath/armour, giving rise to additional
heating as a result of individual coupling with the load currents. The same effects will also be
present in single-core cables if the earth path is removed but bonds between the three
sheaths/armours exist in at least two places.

As a compromise, two basic methods of installation have been developed for single-core
cables.

One of these is to earth the sheath/armours at a single point and to run an additional
continuity conductor earthed at both ends and laid close to and equally spaced with respect to
the three single-core cables. This preserves the rating of the cables but permits a voltage to
develop between the sheaths/armours and earth at the unearthed ends of the cables which
under fault conditions could, on long cable runs, exceed the safe 'transfer' potential.
Shrouding of the sheaths/armours at the unearthed ends is thus recommended.

The other method, almost exclusively restricted to non-armoured cables on 132kV systems
and above, employs sheath cross-bonding. This permits solid bonding of the sheaths to earth
at both ends of the cables to provide a return path for earth fault current in the sheaths
without permitting significant steady-state de-rating current to flow.

Details of these methods are given in Engineering Recommendation C55/4 and evaluations of
the resulting ground return currents with sheaths earthed at both ends for a range of typical
cables are given in Engineering Recommendation S34.

11.5.3 Cables Within Substations

Three-core cables will have their sheath/armour earthed at both ends.

Single-core cables will usually be short enough to allow single-point sheath/armour earthing
with continuity conductor, without causing serious induction problems. Where the
sheath/armour earth is at one end of the cable, the preferred end should be where personnel
 Technical Specification 41-24
Page 25 Issue 1
1992
Addendum, Section 15, incorporated November 2009

are most frequently present, for example at switchgear. For the higher voltage systems,
sheath voltage limiting devices (SVLs) may be installed between the sheath and earth at the
unearthed end of the cable to protect the integrity of the sheath and its terminating point
insulation against transient voltage surges on the sheath.

11.5.4 Cables Entering Substations

The sheath/armour at the substation end of the cable should be earthed to the substation
earthing system.

Three-core cables and fully cross-bonded cables will, in addition, be earthed at their remote
ends. This provides both a conductive and inductive path for fault current. With cross-bonded
single-core cables, it is the usual practice to install further additional sheath earths along the
route of the cable. Although technically this extends the earthing system of the substation, the
relatively high resistance of such earth electrodes together with the series impedance in the
sheath from the substation will normally produce an insignificant benefit, such that these
electrodes can be ignored in the assessment of the substation earth electrode resistance.

11.5.5 Outdoor Cable Sealing-Ends

Where cables terminate at outdoor sealing-ends porcelain pedestal-type insulators are fitted
to insulate the sealing-end base and gland from its support structure. If sheath earthing is
made at this location special earthing bonds are required in accordance with Technical
Specification 09-15 or Engineering Recommendation C55/4 as appropriate.

When the standing sheath-voltage at a termination can exceed 10 volts to earth, the base
metalwork of the sealing-end shall be screened against accidental contact by means of an
insulating shroud of the type illustrated in Engineering Recommendation C55/4.

Wood block sealing-end support insulators should be used only for short single-core cable
tails (3.4 metres) with an earth bond made at the trifurcating point of any three-core cable.

11.5.6 Polymeric Cables

Polymeric cables are a recent development and increasing quantities of 132kV, 33kV and
11kV cables, mostly single-core, are in service. Currently, 132kV cables have been lead
sheathed, although aluminium sheaths are an option, 33kV cables have used a copper wire
screen and 11kV cables have employed a continuity conductor placed centrally in a trefoil
arrangement of the cables. As a result of this latter arrangement economic current ratings can
be obtained without any need to single-point-earth the continuity conductor.

If single-point-earthing becomes more the norm and viewing the future where these cables
may be used to replace some of the older non-insulated sheath cables entering substations,
then a significant increase in the substation rise of earth potential could result, particularly
with small area sites, after the replacement. If the consequences of a higher rise of earth
potential are clearly hazardous and protection against it incurs a high cost penalty, then
additional measures may be necessary. These measures can include extended rod
electrodes, if practical and sufficiently beneficial, or extending the earth electrode system
outside the site perimeter, for example with buried earthing conductor in the form of radials or
by an external perimeter ring.
Technical Specification 41-24
Page 26 Issue 1
1992
Addendum, Section 15, incorporated November 2009

11.6 Metalclad Substations

Metalclad substations will normally be erected on, effectively, a single concrete raft. The
provisions for an earth electrode system in these circumstances will be similar to those
described under item 7.2.2.

The introduction of Gas Insulated Switchgear (GIS) employing single-phase enclosures to the
busbars has introduced some new aspects of earthing which need to be incorporated into the
design of the substation earthing system. Due to their very close coupling with the individual
phase conductors the enclosures receive a high level of induction. For these semi-compact
substations, steelwork is used to support the enclosures and their adjoining items of plant and
thus a closed path via this steelwork is provided in which induced inter-phase and earth
currents can flow under both steady-state and fault conditions. These currents can be very
high in the ultimate approaching the phase conductor current. Apart from the undesirable
feature of circulating large currents through steelwork with bolted interfaces, the spread of this
current renders the wiring and cabling associated with the equipment more vulnerable to
inductive interference.

A further effect of these GIS designs is to create appreciably higher surge voltages on the
enclosures and associated steelwork under switching or other sudden disturbances on the
system.

To help minimise the above effects it is recommended that a grid of earthing conductors, well
integrated in the regions close to the plant, be laid over the raft from which short spur
connections can then be taken to the specific earthing points on the equipment. To retain
current in the busbar enclosures, short circuit bonds, together with a connection to the
earthing system, should be made between the phase enclosures at all line, cable and
transformer terminations, at busbar terminations and, for long busbar runs, at approximately
20 metre intervals. Except where adjacent enclosures are insulated from each other the
interface flanges of the enclosures should have bonds across them and the integrity of bolted
joints of all bonds should be checked. As a guide the resistance of the bonded flanges should
not exceed 5 microhms. At insulated flanges consideration should be given to the installation
of non-linear resistive devices to prevent transient spark-over.

11.7 Handles of Manually Operated HV Switches and PoIe-mounted Reclosers with

Ground Level Control Boxes

Refer to Section 15.

11.8 Fault-throwing Switches

Where fault-throwing between phase and earth is used on solidly earthed 132kV systems the
associated high values of current warrant special arrangements of earthing as follows:

(i) an earthing connection should be made as directly as practicable from the switch
contact to the earthing system using the appropriate size of earthing conductor
specified in Tables 2A or 2B. Where plate or rod electrodes form part of the
earthing system they should, if practical, be sited in the immediate vicinity of the
switch;

(ii) this main earthing conductor should be lightly insulated from the switch base and
structure, by supporting bare earthing conductor on insulators or by using plastics
insulated cable. Additionally, if bare conductor is used, this should be protected
from access by an earthed screen extending to at least 2.5 metres above ground.
 Technical Specification 41-24
Page 27 Issue 1
1992
Addendum, Section 15, incorporated November 2009

For 33kV fault-throwing switches, the same conditions will apply except that, because of the
much lower value of earth-fault current, there is no need to insulate the earthing conductor.

11.9 Equipment which Passes High-frequency Current

Certain items of plant, such as surge arresters and coupling capacitors, which are connected
between line and earth, present a relatively low value of impedance to steep-fronted surges
and would, as a consequence, permit high-frequency currents to flow through them to earth.
Unless a low impedance earth connection is provided for such items of plant, the
effectiveness of the arrester could be impaired and high transient potentials could appear on
the earthing connections local to the equipment and on any other locally earthed plant. Steep
fronted surges will rapidly attenuate in the earthing system away from the source resulting in
possible large voltage differences arising between locations on the same earthing system.
Cabled circuits and their terminal equipment linking between these locations could be subject
to damage or disturbance from such high voltage stressing.

To minimise these problems the following earthing arrangements are recommended.

Surge arresters should be sited as close as is practical to the item of plant, for example
transformer or cable that they are protecting. An earthing connection should be made
between the earthing points of the plant and arrester which takes a route as short and as free
from changes of direction as is practicable. In some situations the main earthing grid
connections may, if appropriate, be used instead. If the earthing connection is relatively long,
then by routing it in the close vicinity of the line conductor between arrester and plant and by
additionally connecting the arrester earth terminal to a high-frequency earth in the immediate
vicinity, for example a rod electrode, the effectiveness of the arrester can be improved.
Provision must also be made for conducting fault current to earth in the event of a flashover of
the arrester. For this, either a separate earthing connection to the earthing system should be
made or the above earthing connection between arrester and plant may be used, provided it
effectively terminates on the earthing system and is of the appropriate current rating. For
coupling capacitors a high-frequency earth installed immediately close to the equipment is
required which may be a rod electrode (nominally 5 metres long) connected to the earth
terminal of the unit by a short connection as free from changes of direction as practicable or,
where a rod cannot be installed, between two and four (nominally 10 metres long) horizontally
buried conductors radiating from the end of the short connection in the ground. Where a rod is
used a separate fully-rated connection between the earthing system and the earth on the unit
must be provided to carry the power system fault current in the event of a flashover of the
equipment. If radiating conductors of appropriate rating are used and are connected into the
earthing system, these meet both the high-frequency and fault requirement.

11.10 Light-current Equipment Associated with External Cabling

All exposed metalwork of light current equipment shall be earthed to the main earthing
system. Where pilot or communication cables operate between two remote points and the rise
of earth potential at each end of the circuit does not exceed the appropriate CCITT limit, any
required circuit earth may be made at either end. If the rise of earth potential at either end
exceeds the appropriate CCITT limit, then protective measures shall be applied to those
circuits. The measures used will include forms of isolation, insulation and screening, the
details of which are outside the scope of this document.

11.11 Anti-climbing Precautions along the Tops of Walls

Where barbed wire or other metal anti-climbing devices are erected along the top of brick
walls etc these should be connected to earth using the same procedure as with fencing.
Technical Specification 41-24
Page 28 Issue 1
1992
Addendum, Section 15, incorporated November 2009

11.12 Earthing of Ancillary Metalwork

All normally non-current carrying metalwork associated with panels, cubicles, kiosks, LVAC
equipment lighting masts, oil tanks, transformer coolers, screens, building frames, steel
structures of all kinds etc, shall be bonded to the main earthing system in a reliable manner to
ensure that all such items are held to the same potential and, if fortuitously called upon to do
so, will carry fault currents without damage. The cross-sectional area of such bonding
connections should, where feasible, be not less than 25 x 3mm unless physical constraints
dictate otherwise.

11.13 Earthing Requirements at Substations Providing Railway Electrification

Supplies

This is a specialised subject outside the scope of this document. Refer to Engineering
Recommendation P24.

11.14 Earthing of LV System Neutral and Associated Metalwork

LV power supplies will normally be derived from MV/LV distribution transformers where
earthing of the LV neutral will be connected at the source. These transformers, either pole or
ground-mounted, may be dispersed along power lines or as all-cabled ground mounted units.
In all these arrangements a buried earth electrode system is provided to which will be
connected any associated HV and LV steelwork and the transformer tank. Where such
transformers are sited in substations the transformer tank and any associated steelwork will
be connected to the earth electrode system of that substation.

Because of the influence of ground potentials, particularly where higher voltage systems are
present, the requirements for earthing the LV system neutral and the conditions under which
an LV supply can be provided will differ. The following examples clarify the situation:

(i) where a distribution transformer is not sited in a substation containing other

higher voltage systems, the combining of the MV/LV steelwork earth and LV
neutral earth, as set out in Clause 14.1.2 of this document is permissible provided
their combined overall resistance to earth does not exceed 1 ohm. Where the
combined resistance exceeds 1 ohm then the LV neutral and any associated LV
cable sheath/armour shall be kept separate. Such separation is also required if
under HV earth fault conditions the potential of the MV earth exceeds the safety
limit of 430 volts;

(ii) where a distribution transformer is sited within a substation containing other

higher voltage systems and the rise of earth potential at that substation does not
exceed 430 (650) volts then the MV/LV steelwork and the LV neutral shall be
bonded to that substation earthing system and such installations may provide LV
supplies to both the substation auxiliaries and to external customers. Where,
however, the substation rise of earth potential exceeds 430 (650) volts the
conditions under which LV supplies may be given are restricted as follows:

(a) if the LV neutral is earthed to the substation then LV supplies can be given to
the substation auxiliaries only and not to external customers;

(b) if the LV neutral is earthed outside the 430 (650) volt ground potential contour
of the substation then LV supplies can be given to external customers only
and not to the substation auxiliaries. In addition, if the LV supplies are cabled
from the distribution transformer then any metallic shealth/armour must, at
the substation end, be isolated from the substation metalwork and be
 Technical Specification 41-24
Page 29 Issue 1
1992
Addendum, Section 15, incorporated November 2009

insulated from normal contact throughout that part of its length passing
through the high ground-potential zone of the substation.

12. TESTING

12.1 Measurement of Substation Earthing Resistance and Impedance

The main reason for making measurements is to verify the adequacy of a new earthing
system and to ascertain those additional measures, if any, that are necessary to protect
personnel and control/communication equipment.

Measurements are also recommended after major changes affecting the basic requirements
and at regular intervals (e.g. every six years) to check the condition of the earthing
installation.

Although the execution of measurements may imply some difficulties they will generally give
more reliable results than can be obtained by calculations. When the soil is non-uniform and
the earthing system large and complex, measurements to check the theoretical calculations
will always be advisable.

All methods used for measuring the resistance to earth of an electrode involve the passing of
current through that resistance and a measurement of the voltage drop across it. The
resistance is unique in one respect in that only the one terminal is positively available, the
other terminates over an infinite area in the main body of the earth. In order that current can
be made to flow through this resistance it is necessary to provide a second auxiliary electrode
that will absorb the return current from the ground. The location of this electrode needs to be
at sufficient distance from the electrode under test such that it does not distort the natural flow
of current into the earth around the test electrode. Such methods are generally described as
"fall of potential" methods.

There are several makes of commercially available composite instruments which incorporate
the above principles and give a direct reading of the resistance. Experience confirms that
instruments of this type are eminently suitable for measuring the resistance of electrodes
where the transfer of current from the electrode to soil is contained over a comparatively small
site area of, say, 10 metres square, where the influence of other electrode systems in the
near vicinity is not significant and the value of the resistance is of the order of 1 ohm or
greater. However, since these instruments are designed to measure resistance only, they are
not suitable for measuring impedance containing significant inductance as may be the case
with extended earths such as overhead line earthwires or cable sheaths connected to the
earth electrode system and acting in parallel with it. Such extended earths have reactance as
well as resistance. For earthing systems covering a larger site area, the distance to the
auxiliary current electrode may become prohibitively long and this can produce appreciable
error due to induction effects in long measuring leads.

These problems can be minimised and accuracy improved by significantly increasing the
current circulating through the electrodes. In practice this involves the use of individual current
and voltage circuits. The value of resistance or impedance is derived by Ohm's Law from the
measured quantities in these circuits. Such a measure is often referred to as the current
injection method and the basic arrangement and points of guidance are given in Engineering
Recommendation S34.
Technical Specification 41-24
Page 30 Issue 1
1992
Addendum, Section 15, incorporated November 2009

13. MAINTENANCE

Earthing systems should be inspected and maintained in accordance with ACE Report No. 80
'Maintenance of Electrical Plant and Equipment' and the work shall be undertaken in
accordance with the accepted safety procedures.

Particular attention must be paid to ensure that earthing systems designed for separate
operation remain so.

13.1 Earth Connections

Main and secondary earth connections should be inspected to ensure that all joints and
connections are sound and secure. When measuring the joint resistance of conductors of the
same size the location of the measuring probes should ideally be 25mm on either side of the
joint. For a sound joint, the measured value of resistance should not exceed the resistance of
a length of the same conductor (without joints) measured between the same probe
separation. For joints made from different conductor sizes and where the joint overlap for the
smaller conductor is greater than that for conductors of the same size, the same procedure
should apply except that the measured value for the joint should not exceed 75% of the
resistance of the smaller conductor (without joints) measured between the same probe
separation.

Where, for practical reasons, it is necessary to use very much wider probe separation, the
measured resistance including the joint should not exceed the resistance of the same length
of uniform size conductor (without joints) or the resistances of the respective lengths of
different size conductors.

13.2 Earthing Electrodes and Conductors

Termination of earthing conductors at the earth electrode(s) should be inspected for security
and, where applicable, for soundness of any protective finish. Where chemical treatment of
the ground surrounding the electrode(s) is applied, re-treatment as necessary should be
carried out.

13.3 Earthing and Bonding

Earthing and bonding connections to transformers, switchgear, cable sheaths, support

frameworks, pillars, cubicles, metalclad chambers, bases of insulators and bushing and their
associated metalwork etc should be checked to ensure that they are intact.

Flexible bonding braids or laminations should be inspected for signs of fracture and corrosion
and changed as required. A protective compound may be applied to flexible braids where
corrosive conditions exist. Earth mat connections should be verified as secure and buried
installations should be checked to ensure that they have not been disturbed.

On switchboards fitted with frame leakage protection, visual inspection should be carried out
to ensure that the insulation segregating the switchgear frame from the main earth bar and
the cable sheath is not short-circuited by spurious paths.

13.4 Neutral Connections

Neutral links should be checked to ensure that they are tight and that the neutral earth
connection is intact and, where appropriate, that the value of the resistance is correct.
 Technical Specification 41-24
Page 31 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Where liquid type neutral earthing resistors are installed, the correct electrolyte level should
be verified and the operation of the tank hearer should be checked.

In substations where the neutral connection and cable sheaths are isolated from the
substation earth, visual checks should ensure that this isolation is not short circuited.

13.5 Effectiveness of Earthing

As referred to in Section 12, the most common method used for measuring the resistance of
an earth electrode system is the "fall of potential" method. Where the earth electrode
encompasses a small area of ground or where links are provided in large area electrode
systems such that discrete electrodes, for example plates or rods or small groups of rods, can
be isolated, then the 'Megger Earth Tester' instrument is suitable for measuring these
electrodes. For earth electrodes encompassing a large area of ground, current injection
methods as described in Engineering Recommendation S34 may be required.

When carrying out these measurements the plant must either be made dead or test
procedures adopted which safeguard the operator from any rise of earth potential resulting
from a system fault occurring during the test, for example rubber gloves, mats and/or fully-
insulated test equipment.

14. EXAMPLES

The following examples have been selected to give as wide an application as practicable of
the methods of evaluation to meet the various criteria specified in this document.

14.1 Small Area Outdoor or Indoor Substations (Figure 7 Ex 1)

14.1.1 Calculations

In this example two types of substation are considered:

Figure 7 Ex 1a
2
This is a 500kVA 11kV combined substation unit (CSU) teed from two 11kV, 3-core, 240mm ,
insulated sheath/armour cables each 1km long, with one cable connected to the 11kV source
and the other feeding an open 11kV ring. A cladding enclosure surrounds the CSU and a
concrete raft covers the internal area of approximately 2.5 metres square. The declared soil
resistivity is 50 ohm m and the maximum calculated fault current is 2,700 amps for an earth
fault at the transformer on the 11kV side. For many of the older buried LV cable installations
the electrode earthing of the sheath/armour of these cables has been utilised to enhance the
substation earthing, thus reducing its resistance. In this example, polymeric LV cables are
assumed to be employed which offer no effective contribution to earthing.

The first preliminary design assumes an earth electrode comprising four rod electrodes 3
metres long, 20mm diameter, joined together and bonded to the CSU in two places.

Using the approximate assessment procedure from Section 9.1:

the resistance of the earth electrode is deducible from Engineering Recommendation S34
Table 1 as follows:
Technical Specification 41-24
Page 32 Issue 1
1992
Addendum, Section 15, incorporated November 2009

ρ 8l
R loge 1
2π l d

50 8 3
log e 1 16.15 ohms
2π 3 0.02

N=4

k = 2.6

rh ρ 50
where Col 1 rh 0.494
a 2πR 2π 16.15

and a = 2.5

0.494
0.198
2.5

16.15 1 2.6 0.198

R 6.12 ohms
4

Assessment of the rise of earth potential based simply on the product of this resistance and
the declared fault current is clearly inaccurate since the fault current alone would be reduced
by the substation resistance. Thus a more rigorous evaluation, taking into account also the
current returning in the 11kV source cable sheath, is necessary.

Assuming the source resistance to be approximately 1.0 ohm, the ground return current
associated with the source 11kV cable is, from Engineering Recommendation S34,

Figure 7 ≈ 0.035 x 2700 = 94.5 amps.

The rise of earth potential = 0.94 x 6.12 = 578 volts

This is in excess of the limit of 430 volts allowed for the local earthing of the LV neutral which
would have to be earthed 3 metres from the 11kV earth electrode and be insulated over this
distance.

The second preliminary design assumes an earth electrode comprising a bare stranded earth
conductor buried with each 11kV cable for a distance of 40 metres and connected to the CSU
equipment.

The resistance of each earth conductor from Engineering Recommendation S34, Figure 4 is:

Rh = 4.0 ohms; thus for two cables R = 2.0 ohms

The ground return current associated with this value of resistance, together with a 1.0 ohm
source, is from Engineering Recommendation S34, Figure 7 =: 0.075 x 2700 = 202 amps.

The rise of earth potential = 202 x 2.0 = 404 volts

 Technical Specification 41-24
Page 33 Issue 1
1992
Addendum, Section 15, incorporated November 2009

This is within the limit of 430 volts and thus the LV neutral can be connected to the local earth
electrode.

Figure 7 Ex 1b

This is an 11kV 250kVA distribution substation with two 11kV cables with non-insulated
sheaths/armours each 500 metres long and three LV distribution cables with non-insulated
sheaths/armours each 200 metres long. The main earth grid/earth electrode assessment
depends primarily on information ascertained for the preliminary design arrangement. The
configuration of the main earth grid is typically a length of buried conductor laid so as to form
an earth mat under the operator's feet.

The preliminary design arrangement of electrode as indicated by an examination of the

equipment layout etc, is a single 10 metre length of buried conductor connected to an earth
plate 915 x 915mm, the mean depth of this plate being 1 metre.

Using the approximate assessment procedure from Section 9.1:

(1) determine local average soil resistivity - 50 ohm metres by measurement;

(2) estimate resistance using formulae given in Engineering Recommendation S34,

Table 1.

From column 3 for buried plate:

ρ r
Rp 1
8r 2.5h r

where ρ = soil resistivity (ohm metres)

h = depth to centre of plate (metres)

1
A 2
r= (metres)
π

2
A = area of plate (metres )

1
0.92 0.92 2
r 0.52 metres
π

50 0.52
Rp 1 14.1 ohms
8 0.52 2.5 1.0 0.52

From Engineering Recommendation S34, Figure 4 for cable sheaths:

11kV cables, earthing resistance = 0.4 ohms each

LV cables, earthing resistance = 0.9 ohms each

From Figure 4 for buried conductor:

Technical Specification 41-24
Page 34 Issue 1
1992
Addendum, Section 15, incorporated November 2009

10 metres of conductor, earthing resistance = 15 ohms

Total resistance of substation:

1
1 2 3 1
Re 0.12 ohms
14.1 0.4 0.9 15

(3) determine the total earth current delivered to the substation for an HV earth fault.
This is assessed to be 2,700 amperes;

(4) determine the rise of earth potential, i.e. 2,700 x 0.12 = 324 volts.

This is below the limit of 430 volts.

There is no need to consider the alleviating factor of current returning via the cable sheaths.

In the finalised design it is required to use a buried conductor of 31.5 x 4mm (from Table 2 4 3
second rating for copper conductor based on a maximum three phase-fault current of
13.2kA).

From Figure 1 the surface current rating for this conductor is 44 amps/metre giving 44 x 10 =
440 amps for the 10 metre length. From Table 3 the current rating of the plate is 1038
amperes. Using the approximate assessment procedure the current distribution to earth
through the four electrode elements will be:

324
23 amperes through the plate
14.1

324
22 amperes through the buried conductor
15

324
810 amperes through each 11kV cable sheath
0.4

324
360 amperes through each LV cable sheath
0.9

All these currents are well within the respective electrode surface current ratings. Since the
total rise of earth potential does not exceed the limit there are no transfer, touch or step
potentials to consider. The finalised design of the earthing system can be prepared.

14.1.2 Application of General Criteria

Examples 1(a) and 1(b) show how calculations are performed to assess rise of earth
potential. In many cases it is useful to have available a few simple criteria to enable
substation earths at distribution substations to be designed so as to minimise danger without
the necessity to perform calculations. Application of these criteria will be appropriate in cases
where it has been shown such criteria produce a satisfactory earth mat, for example,
substations in urban areas connected to metallic-sheathed cables with no plastic oversheath.

The criteria are as follows:

 Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

Substation Metalwork (High Voltage) Earth

The metalwork associated with the high voltage system including the transformer tank shall
be connected to an earth electrode or system of electrodes, the value of which should be
such that the high voltage protection will operate in the event of a breakdown between either
the HV windings of the transformer or the HV line and the supporting metalwork at the
transformer position. This does not apply to a high voltage system earthed through a
continuously rated arc suppression coil equipped with earth fault alarm facilities.

Substation Neutral Conductor (Low Voltage) Earth

An earth electrode or system of electrodes shall always be installed at or near the substation
for the purpose of earthing the LV neutral. The function of this earth connection includes
protection in the event of an HV/LV interwinding fault. It is recommended that the value
should never exceed 40 ohms.

If required for testing purposes, disconnection facilities in the form of substantial bolted links
may be provided in the neutral earth lead at the substation.

Combination of HV and LV Earths

The HV and LV earths may be combined where their combined overall resistance to earth
would not exceed 1 ohm. This includes any contribution which may be obtained from the HV
and LV cable sheaths which are connected to the earthed metal of the substation equipment
and from any other electrodes which are connected to the neutral of the LV network. It is
possible to reduce the overall resistance to 1 ohm if electrically continuous non-insulated
metallic sheathed HV and/or LV cable is laid direct in soil and is included in the earthing
system. Hessian or jute serving is not considered to form an insulated sheath.

Where the combined resistance exceeds 1 ohm, the HV metalwork earth and LV earth must
be kept separate. The LV neutral should be earthed to a separate earth electrode or electrode
system at or near the substation with a resistance to earth not normally exceeding 40 ohms
and being outside the resistance area of the HV metalwork earth.

Several arrangements of HV metalwork and LV earthing arise in practice. Typical methods of

bonding and segregating are shown diagrammatically in Figures 1(c) to 1(g).

The minimum separation of the electrodes shall normally be 3 metres.

POLE TYPE SUBSTATIONS (HV/LV)

Pole Type Substation Feeding Overhead Line (Figure 7 1(c))

Segregation of the neutral earth will normally be necessary. This can most conveniently be
effected by installing the neutral earth electrode at the first LV pole, a span length away from
the transformer pole. If, however, this is not practicable, the neutral should be connected by
means of an insulated wire laid underground to a suitable earth electrode outside the
resistance area of the HV metalwork earth in an arrangement similar to that shown in Figure
1(d). The insulation in this case should be related to the high voltage supply.

Pole Type Substation Feeding Underground Cable (Figure 7 1(d))

If the combined resistance of the HV metalwork earth, cable sheath and earth electrode
connected to the neutral is more than 1 ohm segregation is required.
Technical Specification 41-24
Page 36 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Where segregation of the neutral earth is necessary, this will normally be achieved by
connecting the HV metalwork and LV neutral to their respective earth electrodes by means of
insulated wire, in accordance with Figure 1(d). Alternatively the neutral may be earthed at a
suitable joint located as close as practicable to the substation but outside the resistance area
of the HV metalwork earth.

Ground Mounted Substations (HV/LV)

Where the general ground mounted substation supplies a LV underground distribution system
containing cables with uninsulated armouring or lead sheathing, the overall earth resistance
will be likely to be less than 1 ohm. However, where the substation supplies a small
underground system or mostly on overhead system the resistance should be calculated or
measured using the methods employed in Engineering Recommendation S34. The results of
the tests will indicate the action to be taken to separate the earth electrodes.

Overall Earth Resistance 1 Ohm or Less (Figure 7 1(e))

Where the overall earth resistance is 1 ohm or less the HV and LV metalwork, cable sheaths
and LV neutral should all be bonded together in accordance with Figure 3. Modern LV feeder
pillars and fused cabinets usually have a combined neutral/earth bar and therefore will not be
two separate bars as shown in Figures 3 and 4.

Overall Earth Resistance Greater Than 1 Ohm (Figure 7 1(f))

If the earth resistance of the cable sheaths and earth electrodes combined would exceed 1
ohm the HV metalwork earth must be segregated from the LV neutral earth.

This may be achieved by keeping the HV and LV cables well apart and in cases where the
separating distance is less than 3 metres, by wrapping any non-insulated sheathed HV cables
with non-hygroscopic insulating tape.

Where LV cables with non-insulated sheaths are used between the transformer and feeder
pillar or fuseboard, it is necessary to provide insulation for the sheaths and armouring by the
use of an insulating gland or similar means, at the transformer LV cable box. The metallic
shell of the feeder pillar must also be segregated from all non-insulated sheathed cables.

All LV cables with non-insulated sheaths should also be wrapped with non-hygroscopic
insulation tape where they are within 3 metres of the foundation plinths of any equipment
connected to the HV earth electrode.

In the case of a fuse cabinet bolted directly to the transformer (Figure 1(g)), the cabinet itself
will inevitably be connected to the HV metalwork earth. The neutral busbar, earth bar if any,
and the sheath and armouring of the LV distribution cables will be insulated from the HV
metalwork earth and will be connected to the neutral earth electrode by PVC insulated cable.
This electrode must be installed at a distance of at least 3 metres from any other metalwork or
cable associated with the HV system. However, the distance will depend on the length of
electrode used to prevent overlapping. Alternatively, the neutral may be earthed at a suitable
joint located as close as practicable to the substation but outside the resistance area of the
HV metalwork earth.

14.2 Medium Area Outdoor Substation (Figure 8 Ex 2)

33kV 20kA distribution substation with two 33kV lines of unearthed construction. The 24MVA
33/11kV CER transformers have directly earthed 11kV neutrals and the 11kV cables are PVC
sheathed. The main earth grid/earth electrode assessment depends primarily on information
 Technical Specification 41-24
Page 37 Issue 1
1992
Addendum, Section 15, incorporated November 2009

ascertained for the preliminary design arrangement. The configuration of the main earth grid
is typically a rectangular loop laid so as to keep the length of connections to equipment to a
minimum or alternatively a single bar may be found more suitable.

The preliminary design arrangement of electrode as indicated by an examination of the

equipment layout etc, is a single loop of buried conductor (25 x 4mm) in the form of a
rectangle some 60 metres x 50 metres. The size and layout of the site is such that the fence
can be independently earthed.

Using the approximate assessment procedure from Section 9.1:

(1) determine local average soil resistivity - 20 ohm metres by measurement;

(2) estimate resistance using formula given in Engineering Recommendation S34,

Table 1.

From column 5 for buried grid:

where ρ = soil resistivity (ohm metres)

L= length of buried conductors (metres)

2
A = area of grid (metres )

1
A 2
r (metres)

20 20
R 0.253
1 220
4 (955) 2

(3) determine the maximum value of earth fault current passing through the substation
earth electrode system for an earth fault in the substation on the 33kV system
which is resistance earthed at source or the value of current returning to the
substation for an earth fault on the 11kV system external to the substation,
whichever is the greater. This is assessed to be 2550 amperes for an earth-fault on
the 33kV system;

(4) estimate the rise of earth potential, i.e. 2550 x 0.253 = 645 volts.

This rise of earth potential is significantly in excess of the 430 volt limit and further
consideration of options is required.

For a 33kV earth-fault in the substation the fault resistance will be very low and thus the fault
clearance time will also be very low (a small fraction of a second) if busbar protection is fitted.
For typically low fault clearance times the above value of 645 volts is well below the
permissible limits of touch and step potential derived from Figure 2. Consideration now
centres on whether BT services are brought into the substation or whether they lie within the
Technical Specification 41-24
Page 38 Issue 1
1992
Addendum, Section 15, incorporated November 2009

430 volt contour as defined in Engineering Recommendation S34. If no such services are
involved then there may be no necessity to spend more money on earthing. On the other
hand, if costs for protecting services are to be incurred then the cost of improving the earth
electrode system to remove the need for protection must be weighed against the cost of
protection.

If improvement to the earth electrode system is desired then, for this preliminary earth
electrode arrangement, the installation of rods around the periphery of the main electrode
loop is likely to prove the most cost effective solution. For a first approximation a calculation
will be made using 22 rods each 6 metres long, driven around the main electrode loop at 10
metre intervals.

From Engineering Recommendation S34 Table 1, Column 7

2
R 1 R 2 R 12
R R1 = resistance of main electrode loop
R 1 R 2 2R12

= 0.253 ohms

R2 = resistance of rod group (Column 6)

20 8 6
R' loge 1 3.6 ohms
2π 6 0.02

N = 22

20
Column 1 rh 0.0884 metres
2π 3.6

0.884
α 0.0884
10
k (Figure 18) = 6.6

ohms

20 6
R 12 0.253 log e -1 0.09 ohms
π 220 0.008

0.253 0.259 - 0.092

R 0.0173ohms
0.253 0.259 - 2 0.09
The rise of earth potential is now 0.173 x 2550 = 441 volts

Although this result is still marginally in excess of the 430 volt limit experience has shown that
calculations frequently give a pessimistic result when compared with measurement. It is
 Technical Specification 41-24
Page 39 Issue 1
1992
Addendum, Section 15, incorporated November 2009

therefore recommended that as rod installation progresses measurement of the total

electrode resistance be made at intervals until a value of say 0.17 ohms is achieved.

The permissible 3 second current rating of a 25 x 4mm conductor is, from Table 2A, 12kA.
This is well in excess of the declared earth fault value of 2550 amps and will thus be an
adequate conductor.

The 3 second electrode surface current rating can be assessed by assuming that the division
of current between the 25 x 4mm buried conductor and the rods is in inverse ratio to their
resistance.

The maximum permissible current rating for the buried conductor (Figure 1) = 56 amps/metre
and for the rods (Table 3) = 50 amps/metre.

The actual current ratings are respectively:

conductor amps/metre

rods amps/metre

Both of the above values are well below the permissible ratings and thus the finalised design
of the earthing system can be prepared.

14.3 Large Area Outdoor Substation (Figure 9 Ex 3)

400/132kV substation located in a rural area. There are two 400kV (L6) and four 132kV (L4)
double circuit overhead lines radiating from the substation which itself has a nominal
perimeter dimension of 200 x 150 metres; the 132kV and 400kV systems are connected
through auto-transformers.

The main earth grid/earth electrode assessment depends primarily on information ascertained
for the preliminary design arrangement. The configuration of the main earth grid is typically a
rectangular loop of conductor enclosing all the high voltage equipment and possibly one
cross-connection for every substation bay or pair of bays. The considerations are as follows:

(i) soil resistivity tests suggest that a buried grid will provide the most economical
way to obtain a low earth electrode resistance. For the purposes of this example
additional peripheral earth rods will be installed;

(ii) to secure minimum resistance the loop of buried conductor should be taken as
near to the perimeter fence as is practicable but still observing the 2 metre
minimum clearance. For some substation layouts (usually very large area
substations) this may involve separating it widely in places from the equipment
which requires to be connected to it. In smaller substations the designer’s
problem may be to maintain the necessary 2 metre clearances from an
independently earthed fence;

(iii) installing driven rod electrodes can, depending on subsoil conditions, be

beneficial, convenient and relatively inexpensive. However, the earth electrode
resistance is not significantly lowered by installing additional electrodes if the
existing electrodes thereby become crowded. In general, putting additional
electrodes inside the periphery of an existing electrode system is unrewarding.
Technical Specification 41-24
Page 40 Issue 1
1992
Addendum, Section 15, incorporated November 2009

The preliminary design arrangement of electrodes as indicated by an examination of the

equipment layout etc, based on a combined interconnected earthing system for the 132kV
and 400kV compound, consists of a buried 20 metre mesh grid 0.5 metres deep and made up
from 50 x 4mm copper conductor with 3 metre long rods driven around the periphery at 20
metre intervals. The size and layout of the site is such that the fence can be independently
earthed.

For this example approximate assessment procedures have shown to be inadequate and thus
the refined assessment procedure from Section 9.2 is applied as follows:

(1) determine local average soil resistivity - 100 ohm metres by measurement.

(2) estimate resistance using formulae given in Engineering Recommendation S34

Table 1.

From Column 5 for buried grid = R1 0.28Ω

From Column 6 for group of rods in hollow square = R2 1.1Ω

From Column 7 for combined grid with rods = R g 0.278Ω

It can be seen that the buried grid is by far the most effective earth electrode and that the
additional rod electrode array provides negligible improvement indicating that the grid
electrode has saturated the available ground area. The formulae assume homogeneous solid
conditions whereas in practice this is virtually never the case;

(3) determine the value of earth fault current for an earth fault in the substation and
deduct the appropriate value of earth fault current supplied by the local
transformers (see Engineering Recommendation S34 Section 5) to provide the
resulting value of earth fault current returning to remote neutrals. This value is
assessed as 30,000 amperes of which 26,000 amperes is supplied from the 400kV
system and 4,000 amperes from the 132kV system;

(4) make allowance for any fault current leaving the substation via overhead earthwires
or cablesheaths due to induction, i.e. proportion of the above current contributions
flowing to ground are from Engineering Recommendation S34 Table 2:

69.2 /179
For the 400kV lines I gr(400) 26,000 17,992 /179 amperes
100

70.8 /171
For the 132kV lines I gr(132) 4,000 2832 /171 amperes
100

Total current ground I e 17,992 /179 2,832 /171 20,800 /178 amperes

(5) make allowance for the additional shunt resistance of tower footings connected by
overhead earthwires, i.e.:

for each 400kV line with tower resistance of 5 ohms Z ch 1.22 / 46 ohms from
Engineering Recommendation S34, Figure 5;
 Technical Specification 41-24
Page 41 Issue 1
1992
Addendum, Section 15, incorporated November 2009

for each 132kV line with tower resistance of 10 ohms Z ch 1.81 / 34 ohms
from Engineering Recommendation S34, Figure 5.

To a first approximation the combined impedance is:

-1
1 1 1
Ze
Rg Zch(400) Zch(132)

-1
1 1 1
0.28 /0 1.22 /46 1.81 /34

= -0.141 /19.9º ohms

(6) calculate the rise of earth potential, i.e. 20,800 x 0.141 = 2.933 volts. This is above
the limit of 650 volts.

In order for this value of rise of earth potential to be reduced to the limit of 650 volts,
650
the substation's earth resistance would have to be lowered to = 0.03
20,800
ohms

The laying of further buried conductors within the proposed mesh would have only a small
reducing effect due to saturation of the available ground area. For a major effect the
additional conductor must be buried externally which is not possible in this case. Similarly, for
the homogeneous soil conditions assumed, the putting down of more driven rods would also
have only a limited effect. For a major effect they would have to be installed outside and well
clear of the mesh, which again is not possible in this case.

In this case the value therefore has been accepted and the consequential effects must be
evaluated.

In the finalised design it is required to use a conductor section of 50 x 4.0mm (from Table 2A,
1 second rating for copper conductor based on a maximum single phase fault current of
60kA) and a driven rod diameter of 16mm. The electrode current loading for this size of
conductor below ground is 82.0 amperes per metre (from Figure 1) and for a l6mm diameter
rod is 38.2 amperes per metre (from Table 3).

Using the above assessment the current distribution to earth through the combined grid with
2933
rods will be = 10,475 amperes.
0.28

The highest current density to be dissipated by the grid and rod electrodes will occur around
the peripheral edge of the grid. This can be determined by applying the kd factor given in
Section 9.3.1 as follows:

current density at edge of grid = average current density x kd

Technical Specification 41-24
Page 42 Issue 1
1992
Addendum, Section 15, incorporated November 2009

= 5.95 amps/metre

This current loading is well within the permissible current loadings of either the grid conductor
at 82 amperes/metre or rods at 38.2 amperes/metre.

Since the total rise of earth potential well exceeds the 'transferred' potential limit of 650 volts,
an evaluation of the 'touch' potential is necessary.

(7) Estimate the 'touch' potential, using formulae given in Section 9.3.

The 'touch' potential, at the edge of the grid (Figure 3A) is given by:

k e .k d .ρ.l
E tgrid
L

Where

3450 108
kd 0.7 0.3 2.02
700 108

The ‘touch’ potential at the separately earthed fence (Figure 3A) is given by:

k f .k d .ρ .I
E t(fence) where kf = 0.26 ke = 0.202
L

and kd = 2.02

The 400kV and 132kV systems are designated 'high-reliability' systems having fault clearance
times not exceeding 0.2 seconds. From Figure 2 the minimum safe 'touch' potential for t = 0.2
seconds is 1,750 volts which is well in excess of the above calculated values. Thus the earth
electrode satisfies all the touch and step potential safety requirements and the final design of
the earthing system can be prepared. It is noted that the rise of earth potential is 2,933 volts
and therefore precautions are required to protect staff and equipment from transferred
potentials.

14.4 The Use of Design Assessment to Achieve Acceptable 'Touch' Potentials

(Figure 10 Ex 4A and Ex 4B)

The following is a practical, although non-typical, example which demonstrates some of the
procedures and processes of evaluation.

Consider a small area 400kV substation (Figure Ex 4A) with an evaluated single-phase earth-
fault current to ground of 20,000 amperes. The outside dimensions of the earth mat are 80
 Technical Specification 41-24
Page 43 Issue 1
1992
Addendum, Section 15, incorporated November 2009

metres x 80 metres and this contains 16 meshes made up from 50mm x 4mm copper
conductor buried 0.5 metres deep.

Soil resistivity = 100 ohm metre

From Section 9.3

'touch' potential at the edge of the grid (Figure 3A) is given by:

k e .k d .ρ .I
E t(grid)
L

where

800
kd 0.7 0.3 1.45
320

0.776 1.45 100 20,000

E t(grid) 2,813 volts
800

‘touch’ potential at the separately earthed fence (Figure 3A) is given by:

k f .k d .ρ .I
E t(fence) where k f 0.26 k e 0.202
L

and kd = 1.45

0.202 1.45 100 20,000

E t(fence) 732 volts
800

The 400kV system is designated a 'high-reliability' system having a fault clearance time not
exceeding 0.2 seconds. From Figure 2 the safe 'touch' potential for t = 0.2 seconds is 1030
volts and although the 'touch' potential at the fence (732V) is well below this value the grid
'touch' potential (2183V) exceeds this value and also exceeds the higher value of 1400 volts
allowed when surface chippings are applied. Thus the earth mat of Figure Ex 4A does not
satisfy all the criteria.

To improve this situation the following options or combinations of them may be available:

(i) increase the mesh density in the existing grid;

(ii) drive ground rods around the periphery of the existing grid;

(iii) expand the area occupied by the grid by bonding-in an additional buried
peripheral conductor one metre beyond the substation fence. For this option it will
then be necessary to also bond the fence to the grid thus over-riding the normal
practice of independently earthing the fence;

(iv) expand the ground area of the substation and the grid if it is desired to retain an
independently earthed perimeter fence.
Technical Specification 41-24
Page 44 Issue 1
1992
Addendum, Section 15, incorporated November 2009

If for option (i) the grid density is doubled, i.e. the mesh size is halved to 10 metres, the new
'touch' potentials

(Figure 3A) would be:

new

1120
new k d 0.7 0.3 1.75
320

0.808 1.75 100 20,000

Et 2,525 volts
1120

0.26 0.808 1.75 100 20,000

E t(fence) 656 volts
1120

This has resulted in a relatively small decrease in the 'touch' potentials, approximately 10%,
but has not yet achieved the aim and is expensive.

If, for option (ii), soil resistivity surveys indicate the presence of low resistivity soil at reachable
depths by rods driven around the periphery of the grid then, due to their ability to distribute the
fault current deep into the soil, a significant reduction in all the potentials can often be
achieved. Evaluations of the ground voltage profile for these conditions is highly complex and
is considered impractical for presentation in this document. In such circumstances a trial and
measurement procedure is recommended.

If the soil resistivity remains sensibly constant with depth and 16 rods each 5 metres long are
installed around the periphery of the grid, the touch potentials at the grid and fence reduce to
2550 volts and 664 volts respectively. This still does not meet the safety criteria but by using
10 metre rods or doubling up the number of 5 metre rods results in acceptable values of 2235
volts and 582 volts respectively if surface chippings within the substation are also used. For
small grid areas, such as in this example, a significant reduction of potential may be practical
with moderate length rods, but for large grid areas rod lengths required can become
impractical.

Where it is impractical to drive long rods or it is desired to make a greater reduction in the grid
touch potential, option (iii) will give the following result. See Figure Ex 4B.

The new parameters for this arrangement (Figure 3C) are:

L = 1204 metres

Lp = 344 metres

1204
kd 0.7 0.3 1.75
344

0.432 1.75 100 20,000

E t(fence) 1,256 volts
1204
 Technical Specification 41-24
Page 45 Issue 1
1992
Addendum, Section 15, incorporated November 2009

This value of fence 'touch' potential is much higher than the original isolated fence value of
779 volts or of the option (i) value of 637 volts or option (ii) values of 664 volts and 582 volts
but it is well below the declared safe value of 1,750 volts and therefore is acceptable.

It is instructive to observe the resulting change in resistance and hence overall rise of station
earth potential due to the addition of the peripheral grading conductor.

From Engineering Recommendation S34 the resistance of the original grid electrode is given
by:

1
ρ ρ Area of grid 2
R where r
4r L π

(i) For the original 80 x 80 grid:

L = 800 metres

1
80 80 2
r 45.1 metres
π

100 100
R 0.679 ohms
4 45.1 800

total rise of earth potential = 20,000 x 0.679 = 13,580 volts

(ii) or the expanded 86 x 86 metres grid:

= 1204 metres

1
86 86 2
r 48.5 metres
π

100 100
R 0.599 ohms
4 48.5 1204

total rise of earth potential = 20,000 x 0.599 = 11,980 volts

Thus, although the provision of the peripheral grading conductor has reduced the maximum
touch potential considerably (2991 down to 1346 - by approximately 55%), the rise of station
earth potential has been reduced by approximately 12% only. For such unusually high values
of station rise of earth potential some further reduction in these potentials would likely be
sought. In this event rod electrodes located around the periphery of the grid electrode would
provide a significant reduction.
Technical Specification 41-24
Page 46 Issue 1
1992
Addendum, Section 15, incorporated November 2009

15. EARTHING ASSOCIATED WITH HV DISTRIBUTION OVERHEAD LINE

NETWORKS (EXCLUDING TOWER LINES & POLE TRANSFORMERS)

15.1 General Comments & Assumptions

The following earth electrode designs assume that the overhead network does not have a
return earth conductor. With this type of system the earth potential rise (RoEP) of the local
earth electrode may exceed tolerable touch, step and transfer potentials under earth fault
conditions.

Due to the possible hazardous touch potentials, earth conductors above ground shall be
suitably insulated and provided with mechanical protection for a minimum height of 3m or
above the height of the anti-climbing device, whichever is greater. In addition the main earth
conductor shall be suitably insulated for a minimum of 500mm below ground level. Where the
separation of electrodes is required guidance will be given in the relevant section.

It is not reasonably practicable to ensure in all situations that step potentials directly above an
installed earth electrode system remain below tolerable limits under earth fault conditions. It is
generally considered that the probability of an earth fault occurring whilst an individual
happens, by chance, to be walking across the earth electrode at the same time, is extremely
small. Therefore, in most circumstances no special precautions are required. However, at
1
sensitive locations that are often frequented by people, particularly children, and
concentrations of livestock in stables or pens for example, precautions may be justified to
eliminate or minimise the risk. This can usually be achieved by careful site selection or at the
time of installation by installing the earth electrode in a direction away from the area of
concern, burying the electrode as deep as practicable, and/or fencing the electrode off to
prevent access. A similar situation also applies to personnel carrying out live operations such
as HV drop-out fuse replacement, live-line tapping at earthed locations or ABSD switching
using hook stick (hot-stick or insulated rods) techniques on earthed poles.

15.2 HV Earth Electrode Value

The resistance of the HV earth electrode needs to be of a value such that the resultant earth
fault current will allow for the correct operation of the circuit protection. In general the lower
the earth electrode resistance the more earth fault current will flow, resulting in more reliable
operation of the circuit protection. On systems that employ impedance earthing of the source
transformers, an additional benefit of a low value of earth electrode resistance is to
significantly reduce its RoEP under earth fault conditions, in particular to systems that employ
impedance earthing.

Where surge arresters are used it is generally accepted that 10Ω is the preferred maximum
value of earth electrode resistance for satisfactory operation of the arrester. This is in line with
the preferred 10Ω value in BS 6651 for high frequency lightning earth electrodes.

15.3 Electrode Arrangement Selection Method

A common arrangement of rods used for earth electrodes associated with overhead line
equipment is a run of parallel rods interconnected with a horizontal conductor. The following
tables show the electrode resistance value of a range of parallel earth rods of varying depths,
spaced at 1.5 times their depth, for different values of soil resistivity. The resistance values
have been calculated using the formulae in BS 7430. The calculated values are considered
to be conservative and are based on uniform soil resistivity. Calculated resistance values for
the same rod and soil arrangements, using earthing design software are approximately 31%

1
Refer to BS EN 50341-1 clause 6.2.4.2 for definition
 Technical Specification 41-24
Page 47 Issue 1
1992
Addendum, Section 15, incorporated November 2009

lower. The tables have been included to give an understanding of the magnitude and
relationship between soil resistivity and the earth electrode resistance values obtainable.
Where the ground conditions are difficult, i.e. of high resistivity and/or rocky, the cost of
obtaining the required earth electrode resistance value may warrant carrying out a site
specific design.

No. of 2.4m Rods in Parallel, Spaced 3.6m Apart

Soil 1 2 3 4 5 6 7 8 9 10
Resistivity
(ohm m) Earth Electrode Resistance (ohms)
10 4 2.3 1.6 1.3 1.0 0.90 0.79 0.70 0.63 0.58
20 8 5 3 3 2 1.8 1.57 1.4 1.27 1.16
30 12 7 5 4 3 3 2 2 1.9 1.7
40 16 9 6 5 4 4 3 3 3 2
50 21 11 8 6 5 4 4 4 3 3
60 25 14 10 8 6 5 5 4 4 3
70 29 16 11 9 7 6 6 5 4 4
80 33 18 13 10 8 7 6 6 5 5
90 37 21 15 11 9 8 7 6 6 5
100 41 23 16 13 10 9 8 7 6 6
125 51 28 20 16 13 10 10 9 8 7
150 62 34 24 19 16 13 12 11 10 9
200 82 46 32 25 21 18 16 14 13 12
250 103 57 40 32 26 22 20 18 16 15
300 124 68 49 38 31 27 24 21 19 17
500 206 114 81 63 52 45 39 35 32 29
1000 412 228 162 127 105 90 79 70 63 58
2000 823 456 323 253 210 180 157 140 127 116

Table 15.3a Earth Electrode Resistance Values for 2.4 m Rods

 Technical Specification 41-24
Page 48 Issue 1
1992
Addendum, Section 15, incorporated November 2009

No. of 1.2m Rods in Parallel, Spaced 1.8m Apart

Soil 1 2 3 4 5 6 7 8 9 10
Resistivity
(ohm m) Earth Electrode Resistance (ohms)
10 7 4 3 2 1.9 1.64 1.44 1.29 1.17 1.07
20 15 8 6 5 4 3 3 3 2 2
30 22 12 9 7 6 5 4 4 4 3
40 29 16 12 9 8 7 6 5 5 4
50 37 20 15 12 10 8 7 6 6 5
60 44 25 18 14 11 10 9 8 7 6
70 51 29 20 16 13 11 10 9 8 7
80 59 33 23 18 15 13 12 10 9 9
90 66 37 26 21 17 15 13 12 11 10
100 73 41 29 23 19 16 14 13 12 11
125 91 51 37 29 24 21 18 16 15 13
150 110 61 44 35 29 25 22 19 18 16
200 146 82 59 46 38 33 29 26 23 21
250 183 102 73 58 48 41 36 32 29 27
300 219 123 88 69 57 49 43 39 35 32
500 366 205 146 115 96 82 72 64 58 53
1000 732 410 293 230 191 164 144 129 117 107
2000 1463 820 586 461 382 328 289 258 233 214

Table 15.3b Earth Electrode Resistance Values for 1.2m Rods

15.4 Earthed Operating Mechanisms Accessible From Ground Level

This section deals with pole mounted auto-reclosers (PMARs), sectionalisers, and air break
switch disconnectors, that are all capable of being manually operated via an earthed metallic
control box or switch mechanism. It is important to note that where a low voltage supply is
required for control circuits, the supply should be derived from a dedicated transformer whose
LV neutral is earthed directly to the installation’s main HV earth conductor.

There are several methods of minimising the risk from possibly hazardous touch and step
potentials at such installations. In selecting the most appropriate method due account should
be taken of the nature of the site, the accessibility of the equipment to third parties and the
RoEP level under fault conditions.

(1) Use of wireless remote control for a unit mounted on the pole out of reach from
ground level. With this method, an HV earth electrode system may be required
where surge arresters are fitted or where the manufacturer of the equipment
specifies. Where equipment is unearthed its mounting height shall comply with the
relevant regulations.

(2) Place the control box out of reach from ground level, access being via an insulated
ladder. Again, with this method an HV earth electrode system may be required
where surge arresters are fitted or where the manufacturer of the equipment
specifies (see section 15.6). Where equipment is unearthed its mounting height
shall comply with the relevant regulations.

(3) Install an operator’s earth mat and grading conductors to help provide an
equipotential zone for the operator. Figure 15.4a and 15.4b shows an example of
 Technical Specification 41-24
Page 49 Issue 1
1992
Addendum, Section 15, incorporated November 2009

how this may be achieved. Whilst this minimises the hazards for the operator it
requires that the installation be carried out with great diligence. It is also important
that the future integrity of the earth electrode is ensured. Misplacement of the earth
electrode conductors can result in the operator being exposed to hazardous touch
and step potentials. Consideration needs to be given to the selection of the site
prior to installation to ensure that the required earth electrode configuration can be
installed correctly, and maintained adequately into the future. Use of suitable
personal protective equipment for switching operations may also be considered as
an additional risk control measure.

Where mechanical damage is likely, for example in farmland, protective measures

need to be considered to ensure the integrity of the earth electrode and the earth
mat. An example would be to install and fix the earth mat on or in a raft of concrete
or fence off the area surrounding the earth mat.

The use of grading conductors to minimise step potentials in the immediate vicinity
of the operator’s earth mat may prove impractical in some circumstances,
particularly where there is a danger of them being damaged by ploughing. Burying
the grading conductors at a greater depth will significantly reduce their
effectiveness. Keeping step potentials within tolerable limits can be extremely
difficult and in some case impracticable. In such circumstances alternative mitigation
should be considered.

Factors such as, soil structure, operating voltage, type of HV system earthing (solid
or resistance) and system impedance all have an effect on the value of step and
touch potentials created around the earth electrode, whereas protection clearance
times will have a bearing in determining the tolerable touch and step potential limits.
At some sites it may be prudent to restrict access to the control box, for example by
use of insulating barriers or fences, so that it is not possible for third parties to touch
the control box and where operators can only touch the control box when standing
on the earth mat.

It should be noted that burying the operator’s earth mat will increase the touch
potential between the control box and the surface of the ground above the earth
mat; the greater the depth of the mat, the greater the potential difference between
the soil surface above the mat and the control box. The hazard this presents can be
managed by covering the mat with a high resistivity material which will increase the
impedance path between the hands and feet. Burying the mat will also have the
effect of reducing the step potentials for an operator stepping off the mat. However,
the prime concern is to minimise the touch potentials as these are considered to be
more hazardous than step potentials. Where the mat is buried the touch potential
and the hazard it presents will be site specific, being dependent upon the actual
RoEP and the protection clearance times for the given site, therefore a site specific
design is recommended. The surface mat shown in the figures 15.4a and 15.4b,
results in negligible touch potentials for the operator standing on the mat,
irrespective of the RoEP.

In all cases it is an option to use control measures to mitigate risk if a company

deems this is the most appropriate solution in the circumstances.
Technical Specification 41-24
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1992
Addendum, Section 15, incorporated November 2009

Potential grading
conductors
Plan max. 300mm deep
View

Operator's
access
route

Recommended pre-formed
1000mm x 1000mm metallic mesh earth mat
earth mat Min. 1000mm x 1000mm
Max. mesh size 100mm x 100mm
Extended electrode if
required to obtain correct Optional concrete
resistance value. support raft/slab
Min. depth 500mm (1000mm
in agricultural land)
Grading conductors
Soil level max. 300mm deep

2000mm
radius

NOTE: This arrangement does not exclude the use of a portable earth mat

Figure 15.4a Example 1 – Earthing Arrangement for a PMAR with Ground Level Control
Box
 Technical Specification 41-24
Page 51 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Plan
View Potential grading
conductors
Obstruction such as hedge max. 300mm deep

Operator's
access
route

Recommended pre-formed
1000mm x 1000mm metallic mesh earth mat
earth mat Min. 1000mm x 1000mm
Max. mesh size 100mm x 100mm

Optional concrete
Extended electrode if
support raft/slab
required to obtain correct
resistance value.
Min. depth 500mm (1000mm
in agricultural land) Grading conductors
Soil level max. 300mm deep

2000mm
radius

Figure 15.4b Example 2 – Alternative Earthing Arrangement for a PMAR with Ground
Level Control Box
Technical Specification 41-24
Page 52 Issue 1
1992
Addendum, Section 15, incorporated November 2009

15.5 Air Break Switch Disconnector (ABSD) with an isolated operating mechanism

There are several methods of controlling hazardous touch and step potentials, at pole
mounted ABSDs.

(1) Install an insulated rod operated ABSD at high level that does not require an earth
electrode. Where equipment is unearthed its mounting height shall comply with the
relevant regulations. This option removes the risk of the operator being exposed to
the hazard of touch and step potentials that could occur under certain earth fault
conditions when adopting method 2 below.

(2) Install an ABSD that is operated manually from ground level with a separate HV
earth electrode and operator’s earth mat. This approach relies on effective
separation of the HV earth electrode that connects the HV steelwork to earth, and
the operator’s earth mat connected to the operating handle. This arrangement is
typical of existing earthed ABSD equipment found on rural overhead line distribution
networks.

Separation is achieved by placing the HV earth electrode a minimum of 5m away

from the base of the operator’s earth mat using insulated earth conductor from the
electrode to the HV steel work, and by insulating the operating handle from the
switch mechanism using an insulating insert in the operating rod. The top of the
insert needs to be a minimum of 3m from ground level when in its lowest position.
The operating handle needs to be connected to an earth mat positioned where the
operator will stand to operate the handle. If the earth mat is installed such that it is
visible the operator can verify its existence and its connection to the handle prior to
operating the handle. The continuing effective segregation of the HV earth electrode
and the operator’s earth mat is the most important aspect of the way in which this
arrangement seeks to control the touch and step potentials around the operator’s
earth mat position. To minimise the possibility of contact between the buried
insulated earth conductor and the surrounding soil, should the earth conductor’s
insulation fail, the conductor could be installed in plastic ducting.

Where mechanical damage is possible, for example in farmland, protective

measures may need to be considered to ensure the integrity of the earth electrode
and the earth mat. An example would be to install and fix the earth mat on or in a
raft of concrete or fence off the area surrounding the earth mat using non-
conducting fencing.

Under earth fault conditions the HV earth electrode will rise in potential with respect
to remote earth. A potential gradient will be produced around the electrode; the
potentials being highest immediately above the electrode and reducing rapidly with
distance. The earth mat will be located within the potential gradient surrounding the
HV earth electrode, but due to the separation distance of 5m the potential at that
point with respect to remote earth will be relatively small. The surface level earth
mat for the operating handle and the handle itself will rise in potential but there will
be effectively no potential difference between the mat and handle.

Under earth fault conditions, assuming the correct separation distance between the
HV earth electrode and the operating handle earth mat, should the operator have
one foot on the mat and one off the mat, touch and step potentials surrounding the
earth mat should not exceed tolerable limits. However, there is a risk of hazardous
touch and step potentials arising if the HV earth electrode short circuits to the
operating handle earth mat. The risk of such a short circuit occurring is extremely
 Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

small provided that the earth installation is correctly installed, inspected and
maintained.

The actual size and shape of the earth mat shall be such as to ensure that the
operator will be standing towards its centre whilst operating the handle.
Notwithstanding this requirement the minimum size of earth mat should be 1m by
1m. Due consideration needs to be taken of the type of handle, whether it is a two
handed or single handed operation and whether the operator may be left or right
handed. A purpose made, mat is recommended in preference to a mat formed on
site out of bare conductor, as this eliminates problems of variation in shape and size
that can occur with the latter. Where a buried earth mat is used, the maximum depth
of the mat should be no greater than 300mm.

Under normal earth fault conditions the touch potential for both buried and surface
mounted scenarios will be negligible. When deciding between the use of a buried
earth mat and a surface mounted mat the following issues should be considered:

A surface mounted mat will allow the operator to visually confirm both the
position of the earth mat relative to the handle and also the integrity of the
connection between the earth mat and the handle.
A surface mounted mat will minimise any touch potentials between the soil
surface on the mat and the handle, both under normal earth fault conditions
and under second fault conditions where the handle and the earth mat
become energised although this scenario should be less likely because
effective segregation can be visually confirmed before operation.
Conversely a surface mounted mat will maximise the step potential around
the mat although this will only be an issue if the mat and handle become
energised under a second fault scenario.
A buried earth mat will not allow the operator to visually confirm either its
position relative to the handle, or the integrity of its physical connection to
the handle before operation.
Burying the earth mat will increase the value of any touch potential between
the handle and the soil above the earth mat, this potential will increase with
depth.
To maintain the same effective soil surface area with a buried earth mat for
the operator to stand on and minimise any resulting touch potentials
requires a significantly larger mat than for a surface mounted mat.
Where a second fault occurs that energises the operating handle and earth
mat, with a buried earth mat the touch potential could exceed tolerable
levels.
Conversely burying the mat will have the effect of reducing the step
potentials under such conditions for an operator stepping off the mat.
The use of suitably rated PPE in these situations would assist in minimising the risk of
exposure to possibly hazardous potentials.
Technical Specification 41-24
Page 54 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Insulated
Keep the HV and operator's
insert in
earth mat conductors as far
operating
apart as practicable on the pole
rod
(at least 1/3 of the circumference)

Insulated conductor for

min. 3000mm above
ground level Recommended pre-formed
Provide mechanical metallic mesh earth mat
protection at least as Min. 1000mm x 1000mm
high as the ACD. Max. mesh size 100mm x 100mm

Soil level
Min. 5000mm
separation
Optional concrete
support raft/slab
Insulated
conductor
HV Earth Electrode in duct
Min. depth 500mm

Deep earth (greater depth

preferable to extended
horizontal electrode)

Figure 15.5 Recommended Earthing Arrangement for an ABSD

15.6 Surge Arresters

The preferred value for the surge arrester earth electrode resistance is 10Ω or less. Ideally
this electrode system should be installed as close to the base of the pole as possible.
However, for some locations where it may be necessary for an operator to carry out switching
operations on the HV networks at that pole this may create unacceptable step potential
hazards. In such cases the HV earth electrode should be installed away from the pole at a
location where the step potential is calculated to be safe (typically 5m) for the operator to
stand when carrying out any switching operations, see section 15.8. It is preferable to have a
small number of deep earth rods rather than many shallow rods or plain horizontal conductor.
The earth conductor connecting the base of the surge arresters to the earth electrode system
should be as straight as possible, having as few bends in as is practicable.

Where other HV equipment is situated on the same pole and requires an earth electrode, only
one HV earth electrode needs to be installed. The preference is to install an earth conductor
directly from the surge arresters to the buried HV earth electrode, and then connect the earths
of the other items of HV equipment to it on the pole. At sites where switching may take place
the earth lead should be insulated to the first earth rod which should be a minimum of 5m
from the operating mat for an ABSD or 5m from the operating position for equipment that
requires the use of hot-sticks or insulated rods. Additional protection may be achieved by
placing the earth lead in ducting to that point as described in section 15.5.

15.7 Cable Terminations

Typically, cable terminations on poles are associated with surge arresters or other HV
equipment, in which case the cable sheath or screen is connected directly to the surge
 Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

arrester or HV equipment main earth conductor. In the absence of surge arresters or other
earthed HV equipment the cable will require the installation of an earth electrode.

15.8 Operations at Earthed Equipment Locations

At earthed installations fed via overhead line systems, it is essential to have robust
operational procedures to minimise the risk from the possible hazards associated with the
high rise of earth potential under earth fault conditions. It should be noted that the risk
increases during live fault switching operations. It is beyond the scope of this document to
detail such procedures but consideration should be given to the following points.

Earth systems are usually designed to minimise hazards under main protection operation.
They are not designed, unless specifically required, to minimise hazards under secondary or
backup protection conditions. This is an important point to note when developing fault
switching operational procedures. Temporarily disabling parts of the protection system or
raising protection settings to aid in fault location during fault switching can give rise to touch,
step and transfer potentials of a duration that the associated earth systems have not been
designed to take account of.

Precautions shall be taken, by virtue of the equipment design and earthing arrangements to
minimise any touch and step potential hazards. For example, where rod operated (insulated
hot sticks) equipment is used, the simplest way of minimising hazards from touch and step
potentials is by, where practicable, placing the earthing electrode, not serving as grading
conductors, away from the position where the operator will be standing. Where several people
are present during operations, any person not actively carrying out operations should stand
well clear of the installed earth electrode.

15.9 Installation

The following points should be considered when installing an earth electrode system for
overhead line equipment:

(1) Materials and jointing methods shall comply with the requirements of BS 7430.

(2) Installation teams should have a basic understanding of the functions of an earth
system, and should carry out installations to a detailed specification.

(3) Typically, installing a horizontal earth electrode system at a greater depth than
500mm will not have any significant effect on reducing the earth electrode's
resistance value. However, it is recommended that the electrode is buried as deep
as is practically possible to minimise surface potentials and the possibility of
mechanical damage. Where ploughing is a concern the electrode should be buried
at a minimum depth of 1m.

(4) Ensure maximum separation is achieved on the pole between HV earth conductors
and ABSD handle earth mat conductors.

(5) It is recommended that a test point is made available for future connection of an
earth tester above ground so that the earth electrode resistance can be measured.
This test point should be installed and constructed so as to prevent unauthorised
access, and on ABSD’s prevent possible flashover to the operator’s handle and
associated earth mat.

(6) Welded, brazed or compression connections are preferable to bolted connections

for underground joints.
Technical Specification 41-24
Page 56 Issue 1
1992
Addendum, Section 15, incorporated November 2009

(7) Corrosive materials and high resistivity materials such as sand should not be used
as a backfill immediately around the electrode.

(8) The earth resistance of the installed electrode should be measured and recorded.

(9) Where a buried operator’s earth mat has been installed, the mat should have two
connections made to the operating handle.

15.10 Inspection & Maintenance of Earth Installations

15.10.1 Items to Inspect

During routine line inspections it is recommended that the following items are visually
inspected and their condition recorded, with any defects being rectified in a timely manner:

(1) ABSD earth mat and connection to operating handle.

(2) Separation of HV and operator’s handle earth on an ABSD.

(3) Separation of HV and LV earth conductors on the pole.

(4) Check that the anti-climbing device does not compromise the separation between
the HV earth conductor and the operating handle.

(5) Insulation of HV and LV earth conductors.

(6) Mechanical protection of HV and LV earth conductors.

(7) Bonding of plant and equipment.

(8) State of connections, including any test point.

(9) Signs of possible mechanical damage to earth electrode and buried earth mats.

15.10.2 Items to Examine

Periodically examine a random sample of buried earth electrodes and buried ABSD handle
earth mats, and rectify any defects found. The examination should check for the following:

(1) Position of earth mat relative to ABSD handle and operator’s position.

(2) Insulating insert in the ABSD operating rod.

(3) State of underground connections.

(4) State of earth electrode components, particularly galvanised steel rods.

(5) State of insulation on underground earth conductors where separation of electrodes

is required.

NOTE: When carrying out this work protective measure shall be taken to ensure the safety of
personnel during fault conditions.

The results of the examinations can then be used to assist in developing ongoing inspection
and maintenance policy, and procedures.
 Technical Specification 41-24
Page 57 Issue 1
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Addendum, Section 15, incorporated November 2009

15.10.3 Items to Test

(1) Periodically test the earth electrode resistance. For the relatively small earth
systems typically associated with overhead line equipment, a small 3 terminal earth
tester is adequate. The test should be carried out in accordance with the
manufacturer’s instructions.

(2) Regularly test the continuity between operating handle and the operator’s earth mat.

(3) Regularly test the continuity of buried earth mats.

(4) Periodically test a random sample of insulating inserts used in ABSD operating
mechanisms.

Important: When carrying out these measurements the equipment should be made dead or
where this is not practicable a risk assessment should be carried out and suitable test
procedures should be adopted which safeguard the operator from any rise of earth potential.
Such procedures may for example include the use of insulating gloves, mats and / or fully
insulated test equipment.
Technical Specification 41-24
Page 58 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Appendix A

BOLTING SCHEDULES

Aluminium to Aluminium Conductor

When possible joints of strip conductor should be welded. Where this is not possible or
practicable then the following bolting schedule should be used for bolted type joints:

Bolt arrangement should be 1 or 2

Dimensions mm
Bar Length Bolt Hole Recommended Spanner
Bolt
Width Overlap Dia. Dia. Bolt Torque Length
Arrangement
mm mm mm mm Nm mm
A B

25 75 1 20 12.5 10.0 12 35 150

40 75 1 20 20.0 10.0 12 35 150
50 100 1 25 25.0 12.0 14 50 200
60 100 1 30 30.0 12.0 14 50 200
80 100 2 20 20.0 12.0 14 50 200
For tee-off connections the bolt arrangement and overlap should be the same as given above for the
same conductor size.

Bolts, nuts and washers shall be of steel, spun galvanised to BS 729. Bolts and nuts to ISO
metric strength grade designation 8.8 of BS 4190, threads conforming to BS 3643 Part 2
(coarse). Washers CS4 hard bright steel to BS 1449.

Recommended washer dimensions are as follows:

Bolt Size Washer Dimensions (mm)

(mm)
O.D. I.D. Thickness
10 25 10.5 2.4 Max
12 28 12.5 3.0 Max

Tolerance I.D. – 0/ + 0.2, O.D. - 0.2/ + 0, Thickness – 0/ + 0.2

 Technical Specification 41-24
Page 59 Issue 1
1992
Addendum, Section 15, incorporated November 2009

Appendix B

REFERENCES

IEC 364 Electrical installations of buildings.

IEC 479 - 1(1984) Effects of current passing through the human body.

Guide to the short-circuit temperature limits of electric cables with a rated voltage
IEC 724 (1984)
not exceeding 0.6/1.0kV.
Directives concerning the protection of telecommunication lines against harmful
CCITT effects from electricity lines. The International Telecommunications Union,
Geneva.
BS 3019: Pt 1
Specification for TIG welding of aluminium, magnesium and their alloys.
(1984)
BS 3228: Pt 1 Specification for insulator and conductor fittings for overhead power lines:
(1973) (1979) Performance and General Requirements
BS 3571: Pt 1
MIG welding: Specification for MIG welding of aluminium and aluminium alloys.
(1985)
BS 7430 (1991) Earthing.

Jointing instructions and procedures for mass impregnated and mass

TS 09-15
impregnated non-draining paper insulated 19/33kV cables, Issue 2, 1989.
Portable earthing equipment for open type hv apparatus in substations, Issue 1,
TS 41-21
1985.
TS 43-94 Earth rods and their connectors, Issue 3, 1990.

ER C55/4 (1984) Insulated sheath power cable systems.

National code of practice on the application of protective multiple earthing to low

ER G12/2 (1982)
voltage networks.

ER P24 (1984) AC traction supplies to British Rail.

ER S34 (1986) A guide for assessing the rise of earth potentials at substation sites.

ACE Report No. 80 Maintenance of electrical plant and equipment.

IEEE No. 80 (1986) Guide for safety in ac substation grounding.

Tag G F Earth Resistances. Publishers George Newnes Ltd, London.

Technical Specification 41-24
Page 60 Issue 1
1992
Addendum, Section 15, incorporated November 2009

TABLE 1: TYPICAL SOIL RESISTIVITY VALUES

SOIL RESISTIVITY (ohm metres)

Loams, garden soils, etc 5 – 50
Clays 10 – 100
Chalk 30 – 100
Clay, sand and gravel mixture 40 – 250
Marsh, peat 150 – 300
Sand 250 – 500
Slates and slatey shales 300 – 3,000
Rock 1,000 – 10,000
 Technical Specification 41-24
Page 61 Issue 1
1992
Addendum, Section 15, incorporated November 2009

TABLE 2A: CONDUCTOR RATINGS (COPPER)

These copper sizes are based on a temperature rise of 375°C occurring in 3 seconds
and 1 second above an ambient temperature of 30°C with the currents in columns 1(a)
and 1(b) respectively applied to the conductors. For each substation it will be
necessary to specify whether column 1(a) and 1(b) should apply. The temperature rise
is related to the requirement in Section 8.3.1.
Fault Current (kA) Stranded Copper Conductor
Copper Strip (mm)
Not Exceeding (mm)
1 (a) 1 (b) 2 3 4 5
Single Duplicate or Duplicate or
(3 Single (spur)
(1 sec) (spur) Loop Loop
secs) Connections
Connections Connections Connections
4 25 x 4 25 x 4 19/1.53 19/1.53
8 25 x 4 25 x 4 19/2.14 19/1.78
12 25 x 4 25 x 4 37/1.78 19/2.14
13.2 31.5 x 4 25 x 4 37/2.03 19/2.14
18.5 40 x 4 25 x 4 37/2.25 37/1.78
22 50 x 4 31.5 x 4 37/2.03
26.8 40 x 6.3 40 x 4 37/2.25
40 50 x 4 31.5 x 4
60 50 x 6.3 50 x 4
Technical Specification 41-24
Page 62 Issue 1
1992
Addendum, Section 15, incorporated November 2009

TABLE 2B: CONDUCTOR RATINGS (ALUMINIUM)

These aluminium sizes are based on a temperature rise of 295°C occurring in 3

seconds and 1 second above an ambient temperature of 30°C with the currents in
columns 1(a) and 1(b) respectively applied to the conductors. For each substation it
will be necessary to specify whether column 1(a) and 1(b) should apply. The
temperature rise is related to the requirement in Section 6.3.
Fault Current (kA) Stranded Aluminium Conductor
Aluminium Strip (mm)
Not Exceeding (mm)
1 (a) 1 (b) 2 3 4 5
Single Duplicate or Duplicate or
Single (spur)
(3 secs) (1 sec) (spur) Loop Loop
Connections
Connections Connections Connections
4 20 x 4 20 x 2.5 19/2.14 19/1.53
7.5 25 x 4 20 x 4 37/2.03 19/2.14
12 40 x 4 25 x 4 37/2.03
13.2 50 x 4 25 x 4 37/2.03
18.5 40 x 6 40 x 4 37/2.25
22 50 x 6 50 x 4
26.8 60 x 6 40 x 6
40 50 x 6 50 x 4
60 80 x 6 50 x 6
Duplicate or loop connections have been rated to carry 60 per cent of the full fault current.
 Technical Specification 41-24
Page 63 Issue 1
1992
Addendum, Section 15, incorporated November 2009

TABLE 3: MAXIMUM CURRENT RATING OF ROD AND PLATE ELECTRODES

3 - Second Current Rating 1 - Second Current Rating

Soil
Resistivity Rod
Ohm - 16mm Plate 915 x Plate 1220 x Rod 16mm Dia. Plate 915 x Plate 1220 x
Metres Dia. 915mm A 1220mm A A/metre 915mm A 1220mm A
A/metre
10 69.7 2322 3135 120.7 4022 6979
30 40.2 1340 2217 69.7 2322 4128
40 34.9 1161 1568 60.4 2011 3575
50 31.2 1038 1402 54 1799 3197
60 28.4 948 1280 49.3 1642 2919
70 26.3 878 1185 45.6 1520 2702
80 24.6 821 1108 42.7 1422 2528
100 22 734 991 38.2 1272 2261
150 18 600 810 31.2 1038 1846
200 15.6 519 701 27 899 1599
250 13.9 464 627 24.1 804 1430
300 12.7 424 572 22 734 1305
Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

TO USE THIS NOMOGRAM LAY A STRAIGHT EDGE TO CROSS THE RESISTIVITY AND
CONDUCTOR PERIMETER SCALES AT APPROPRIATE VALUES THEN READ CURRENT
RATING AT CROSSING OF CURRENT SCALE.

EXAMPLE:- SUBSTATION SOIL RESISTIVITY - 30 OHM. METRES

ELECTRODE CONDUCTOR PERIMETER - 58mm (25 X 4 mm)
ELECTRODE RATING - 80.4A PER METER (1 SEC)
46.4A PER METER (3 SEC)

Figure 1 Maximum Surface Current Rating of Electrode Conductors

 Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

Figure 2: Curves of Statistically Safe Step and Touch Potentials

Technical Specification 41-24
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Addendum, Section 15, incorporated November 2009

Figure 3: Touch Potentials for Typical Fence Earthing Arrangements

 Technical Specification 41-24
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Technical Specification 41-24
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Figure 5: Installation of Watertight Chamber

 Technical Specification 41-24
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Technical Specification 41-24
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 Technical Specification 41-24
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Note *: Superseded by the requirements of Section 15.

Technical Specification 41-24
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**

NOTE **: Superseded by the requirements of Section 15.

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