ecs 06-0024 earthing testing and measurements · ecs 06-0024 earthing testing and measurements ......

Document Number: ECS 06-0024

Version: 2.0

Date: 27/07/2015

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ENGINEERING CONSTRUCTION STANDARD

ECS 06-0024

EARTHING TESTING AND MEASUREMENTS

Network(s): EPN, LPN, SPN

Summary: This standard provides practical guidance for field staff on earthing testing and measurements.

Owner: Allan Boardman Date: 27/07/2015

Approved By: Steve Mockford Approved Date: 29/07/2015

This document forms part of the Company’s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue.

Circulation

UK Power Networks External

All UK Power Networks G81 Website

Asset Management Contractors

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Connections Meter Operators

HSS&TT

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Earthing Testing and Measurements Document Number: ECS 06-0024

Version: 2.0

Date: 27/07/2015

© UK Power Networks 2015 All rights reserved 2 of 43

Revision Record

Version 2.0 Review Date 27/07/2019

Date 27/07/2015 Author Stephen Tucker

Why has the document been updated: Periodic review.

What has changed:

All sections fully revised in line with current earthing measurement practices.

Earth resistance measurements at small substations included (Section 6.3).

Tower measurements added (Section 9).

Measurement certificates added (Appendix C)

Version 1.2 Review Date

Date 28/09/2011 Author Stephen Tucker

Reclassification and reformatting of document from Earthing Construction Manual Section 4

Version 1.1 Review Date

Date 22/03/2011 Author CDL

Rebranded

Version 1.0 Review Date 31/03/2013

Date 31/03/2008 Author Bob Higgins

Original


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Date: 27/07/2015


Contents

1 Introduction ............................................................................................................. 5

2 Scope ....................................................................................................................... 5

3 Abbreviations and Glossary ................................................................................... 6

4 General Safety Requirements ................................................................................. 7

5 Soil Resistivity Measurement ................................................................................. 8

6 Earth Resistance/Impedance Measurements ...................................................... 15

7 Earth Conductor Joint Resistance Measurements ............................................. 26

8 Earth Connection Resistance Measurements (Equipment Bonding Tests) ...... 28

9 Terminal Tower Earth Continuity Measurement .................................................. 30

10 Earth Electrode Separation Test .......................................................................... 33

11 Touch, Step and Transfer Voltage Measurement ................................................ 35

12 HOT Zone Plotting ................................................................................................. 37

13 Buried Earth Electrode Location .......................................................................... 39

14 Earthing System Records and Earthing Database .............................................. 40

15 Instrumentation and Equipment ........................................................................... 40

16 References ............................................................................................................. 41

Appendix A – Earthing Test and Measurement Equipment ........................................... 42

Appendix B – Training ...................................................................................................... 42

Appendix C – Measurement Certificate Proforma .......................................................... 43


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Figures

Figure 5-1 – Typical Soil Resistivity Measurement Routes at an Existing Site ....................... 9

Figure 5-2 – The Wenner Soil Resistivity Measurement Array ............................................ 11

Figure 5-3 – Example of an Apparent Resistivity against Wenner Rod Spacing Plot with an ‘Outlier’ Data Point ........................................................................................ 12

Figure 5-4 – Example of a Soil Resistivity Sounding Adversely Affected by a Buried Metallic Structure ............................................................................................... 13

Figure 6-1 – Fall-of-Potential Measurement Equipment Connection .................................... 17

Figure 6-2 – Typical Fall-of-Potential Curve ........................................................................ 18

Figure 6-3 – Potential Probe Position against Slope Coefficient .......................................... 19

Figure 6-4 – Earth Resistance Measurement of a Small Electrode System ......................... 22

Figure 6-5 – Earth Resistance Measurement using the Comparative Method and a Clamp Meter (Electrode under Test Connected) ........................................................... 24

Figure 6-6 – Earth Resistance Measurement using the Comparative Method and a Four-terminal Earth Tester (Electrode under Test Disconnected) ............................... 24

Figure 7-1 – Connections for Earth Conductor Joint Resistance Measurements ................. 26

Figure 8-1 – Connections for Earth Bonding Conductor Resistance Measurements ........... 28

Figure 9-1 – Terminal Tower Current Measurement ............................................................ 31

Figure 9-2 – Terminal Tower Potential Difference Measurement ......................................... 32

Figure 12-1 – HOT Zone Plot Measurement Location Examples ......................................... 38

Figure 12-2 – HOT Zone Plot .............................................................................................. 38

Tables

Table 5-1 – Soil Resistivity Rod Spacing, Rod Depth and Locations for Different Substation Types ............................................................................................... 11

Table 6-1 – Typical Separation between Substation Earthing System and Remote Current Probe (C2) ......................................................................................................... 17

Table 7-1 – Typical Resistance Values for Various Joints ................................................... 27

Table 8-1 – Acceptable Values for Measure Resistance ..................................................... 29


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1 Introduction

This standard provides guidance on earthing measurements and testing and includes the most common measurements used during the design, commissioning or maintenance of a substation earthing system. The following measurements are included:

Soil resistivity measurement.

Overall earthing system resistance/impedance measurements.

Individual electrode resistance measurements using the comparative method.

Earth conductor joint resistance measurements.

Equipment and structure bonding testing.

Electrode separation test (fence or independent HV and LV electrodes).

Touch, step and transfer voltage measurements.

Hot zone plotting.

Buried electrode location.

Each measurement is covered in a separate section and includes guidance on safety, test equipment, application, method, interpretation and sources of error.

The majority of earthing measurements especially those at grid and primary substations are usually carried out by earthing contractors working for UK Power Networks or third parties. However earth resistance measurements at secondary substations are carried out by UK Power Networks and are described in Section 6.3 which builds on the measurements section in ECS 06-0023.

Appendix A contains a list of instruments approved for carrying out earthing measurements and also those available from UK Power Networks stores.

Appendix B provides an outline of the training requirements.

2 Scope

This standard applies to earthing testing and measurements at all substations and overhead lines at all voltages.


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3 Abbreviations and Glossary

Term Definition

CDEGS Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis. Industry standard earthing software package

COLD Site A COLD site is a grid, primary or secondary substation where the earth potential rise is less than 430V or 650V (for high reliability protection with a fault clearance time less than 200ms)

EPR Earth potential rise. EPR is the potential (voltage) rise that occurs on any metalwork due to the current that flows through the ground when an earth fault occurs. Historically this has also been known as rise of earth potential (ROEP)

Grid Substation A substation with a primary operating voltage of 132kV or 66kV and may include transformation to 33kV, 11kV or 6.6kV

HOT Site A HOT site is a grid, primary or secondary substation where the earth potential rise is greater than 430V or 650V (for high reliability protection with a fault clearance time less than 200ms)

ITU International Telecommunication Union. ITU directives prescribe the limits for induced or impressed voltages derived from HV supply networks on telecommunication equipment and are used to define the criteria for COLD and HOT sites – see below

PPE Personal Protective Equipment

Primary Substation A substation with a primary operating voltage of 33kV and may include transformation to 11kV,6.6kV or 400V

Secondary Substation A substation with a primary operating voltage of 11kV or 6.6kV and may include transformation to 400V

Step Voltage The potential difference between a person’s feet assumed to be 1 metre apart

Touch Voltage The potential difference between a person’s hands and feet when standing up to 1 metre away from any earthed metalwork they are touching


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4 General Safety Requirements

The earthing measurements described in this standard are potentially hazardous and the guidance below is provided to supplement the requirements of the Distribution Safety Rules.

All measurements shall be carried out by competent staff with appropriate training using safe procedures following a thorough site specific risk assessment. The risk assessment should include, but not be limited to, consideration of the following aspects and the necessary control measures implemented as necessary (e.g. personal protective equipment, special procedures or other operational controls):

1. Potential differences that may occur during earth fault conditions between the substation earthing system and test leads connected to remote test probes. The likelihood of an earth fault occurring should be part of this assessment, e.g. not allowing testing to proceed during lightning conditions or planned switching operations.

2. Potential differences that may occur between different earthing systems or different parts of the same earthing system. In particular, approved safe methods shall be used when disconnecting earth electrodes for testing and making or breaking any connections to earth conductors which have not been proven to be effectively connected to earth.

3. Potential differences occurring as a result of induced voltage across test leads which are in parallel with a high-voltage overhead line or underground cable.

4. Environmental hazards of working in a live substation or a construction site as governed by the Distribution Safety Rules or the CDM regulations as applicable.

5. Use of test equipment.

6. Use of long test leads over large distances in surrounding land.

Each individual involved in carrying out earthing related measurements shall wear suitable personal protective equipment (PPE) in accordance with UK Power Networks health and safety policy. Where non-standard PPE is required this is included in the relevant measurement section.

In addition to the above each individual measurement section contains specific safety control measures.


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5 Soil Resistivity Measurement

5.1 Application

A site specific soil resistivity measurement is used to determine the resistivity of the materials (soil, drift, bedrock etc.) that make up the ground where earth electrode is to be installed. The results are used to design earth electrode systems for new and existing substations therefore it is essential that the measurements are accurate.

The earthing database (refer to EDS 06-0002) contains all existing soil resistivity measurements and may be used for design purposes provided the location of the measurement is applicable.

The earthing maps available in NetMap (refer to EDS 06-0018) also contain soil resistivity values and may be used for initial earthing assessments or preliminary (feasibility study) design calculations but site specific measurements are required for detailed earthing design.

5.2 Equipment

A suitable four-terminal composite earth tester with sufficient range.

Four leads to connect the earth tester to each probe. These should be fitted with suitable connectors and coiled onto a suitable reel/frame for ease of use. To improve site efficiency and reduce error, leads can be pre-measured and labelled to suit the Wenner spacings detailed in Table 5-1.

Four copper-clad steel rods (probes) of 0.3 metre length.

Mallets for driving in probes in areas of hard ground.

Note: Where numerous measurements are required further efficiency may be achieved by using a set of switched multicore leads and series of probes.

5.3 Safety Requirements

No measurements shall be carried out during lightning conditions in the immediate vicinity.

No measurements shall be taken within 20 metres of a grid/primary substation boundary, overhead line, underground cable or other metallic buried service. Where near to overhead lines, test leads should be run at 90 degrees to the line where possible to avoid induced voltage.

5.4 Method

5.4.1 Overview

Soil resistivity measurements should be taken as early as possible during the feasibility/design stages as final earthing design calculations cannot be prepared until they are available.

The measurements should be taken directly on the area of the proposed substation where practicable or as close to the substation as possible in open areas that are free from interference from earthing and/or buried metallic services. If there is not sufficient space on the site to achieve the Wenner array spacing recommended in Table 5-1 the measurements shall be supplemented by additional measurements at the nearest representative location.


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A typical plan of proposed measurement locations is shown in Figure 5-1 where SR 1 to 4 indicate locations for soil resistivity measurements. For large substations it is important to take measurements at a number of different locations around the site so that an average may be used. Sets of orthogonal measurements can help identify the adverse effects of buried metallic services and should be used where this is suspected. Locations chosen should have preferably similar properties to the site, i.e. similar elevation and soil type/structure. If they do not, the equivalent soil model will need to take this into account.

In urban substations where no suitable measurement areas exist in the immediate vicinity of the substation, traverses should be taken in the nearest open areas, e.g. parks, playing fields etc., on at least two sides of the substation. An average soil model can then be derived and applied to the substation earthing calculations. Even if the measurements are taken 500 to 1000 metres away from the substation, they are still representative because it is likely that the cable network connected to the substation extends over a similar area.

It is also important to ensure that the route is not close or parallel to overhead lines. To avoid induced voltage, measurement routes should preferably be at right angles to overhead lines or separated by 20 metres.

Figure 5-1 – Typical Soil Resistivity Measurement Routes at an Existing Site

There are a number of available measurement techniques which involve passing current through an array of small probes inserted into the surface of the soil and measuring the resulting potentials at specified points. Using Ohm’s law a resistance can be calculated which is related to the apparent resistivity at a particular depth using suitable formulae. Varying the positions of the probes, and hence forcing the current to flow along different paths, allows the apparent resistivity at different depths to be measured. The most commonly used arrangement for earthing purposes is the Wenner Array and this is described below.

PRIMARY

SUBSTATION

SR 1

SR 2

SR 3

SR 4


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5.4.2 Wenner Array Procedure

1. Before starting work check the route is clear of any buried cables, earthing and pipes etc. using utility records and above ground detection equipment.

2. Drive four earth rods into the ground in a straight line at a distance 'a' metres apart and a depth of 'd' metres using the required spacing and depth from Table 5-1.

Note: If the position of one of the inner voltage rods coincides with an area covered with tarmac or concrete then measurements may be obtained using a flat metal plate (approximately 200mm x 200mm), placed on a cloth soaked with saline water, instead of the rod. The area should not contain reinforced steel that runs in the same direction as the measurement traverse as the reading could be adversely affected.

3. Connect the rods to a four-terminal earth tester as shown in Figure 5-2, with the outer rods connected to the C1 and C2 terminals, and the inner ones to the P1 and P2 terminals.

4. Turn on the earth tester and allow the meter to settle for 30 seconds before recording the

resistance (R). The apparent soil resistivity () is given by 2aR in ohm-metres.

Note: If the reading is varying significantly, this may be due to interference, high contact resistance at the test rods, a damaged test lead or the reading being at the lower limit than the instruments measuring capability. If, after investigating the above, the reading is still changing by more than 5%, record a series of ten consecutive readings over an interval of few minutes, calculate the average and then proceed with the rest of the measurements.

5. Repeat the measurement for all relevant spacing and depth from Table 5-1.

6. Repeat the measurements using a second traverse which is perpendicular to the first to allow interference and small local variation effects to be identified. If any readings are unstable then additional traverses may be necessary, possibly further away from the site.

Note: It is important to ensure that measurements are symmetrical about point X (Figure 5 2), midway between the voltage rods.


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Figure 5-2 – The Wenner Soil Resistivity Measurement Array

Table 5-1 – Soil Resistivity Rod Spacing, Rod Depth and Locations for Different Substation Types

Minimum Recommended Wenner Array Spacings

Spacing a (m)

Rod Depth α (mm)

Pole Type Small Ground Type

11/6.6kV to 33kV

132kV and Large Sites

1.0 50

1.5 50

2.0 50

3.0 100

4.5 100

6.0 100

9.0 150

13.5 150

18.0 150

27.0 200

36.0 200

54.0 200

Suggested Number of Measurement Locations

1 to 2 1 to 2 2 to 3 3 to 4


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5.5 Interpretation of Results

The design of the substation earthing systems is dependent on detailed knowledge of the soil resistivity and how this varies within the various soil layers, therefore it is important for it to be as accurate as possible.

It is difficult to interpret measurement results by inspection other than for a uniform or two-layer soil model. A uniform soil resistivity value is suitable for simple earthing design formulae but more accurate design calculations carried out using software requires a multi-layered horizontal soil model.

Detailed resistivity models can also be created using commercially available earthing modelling software based on a curve-fitting approach. This can be supplemented with geo-technical information such as borehole records where available to reduce the uncertainty in the model by indicating layer boundary depths, materials, water table height, bedrock depth, etc. The more detailed analysis is important at grid and primary substations to allow the earth electrode system to be optimised. For example, vertical rods are better suited to a soil with a high resistivity surface layer and low resistivity material beneath. Conversely, where there is low resistivity material at the surface with underlying rock then extended horizontal electrodes will be more effective.

A curve of apparent resistivity against separation distance ('a') should be drawn during the measurement exercise so that obvious errors can be identified and measurements repeated if necessary. Figure 5-3 shows an example where the resistivity value at one particular spacing (20 metres) seems to be too high and is evident as an ‘outlier’ on the otherwise smooth set of data points. This reading is typical of a poor connection on one of the voltage rods.

Figure 5-3 – Example of an Apparent Resistivity against Wenner Rod Spacing Plot with an ‘Outlier’ Data Point


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5.6 Sources of Error

There are a number of sources of measurement error which should be considered when planning and carrying out these measurements. These include, but are not limited to:

1. Influence of buried metallic structures such as bare cable armouring/sheaths, earth electrodes, pipes, etc. Measurements taken above or near buried metallic services will indicate lower resistivity values than actually exists. This can lead to under-designed earthing systems which may be costly to rectify. Measurement locations shall be carefully planned to avoid interference from metallic structures by consulting service records and, where there remains uncertainty, the use of scanning methods on site. It is also important that measurements are taken at a number of different locations (minimum of two) around the site of interest so that any influenced results become apparent in comparison to unaffected results. Two orthogonal sets of measurements can also help to indicate an error. An example is shown in Figure 5-4 where the data sets SR1 and SR3 can be seen to be in close agreement but SR2 exhibits an obvious depression between the spacings of 3 and 13 metres. All measured values are generally lower than observed in sets SR1 and SR3.

Figure 5-4 – Example of a Soil Resistivity Sounding Adversely Affected by a Buried Metallic Structure

2. Interference from stray voltages in the soil or induction from nearby electrical systems

may adversely affect measurement results, normally evident as an unstable reading on the instrument or unexpectedly high readings. This may be reduced by avoiding test leads running in parallel with high voltage power lines/cables or near other potential sources of interference, e.g. electric traction systems.

3. The Wenner Array spacings used shall be appropriate for the size of the earthing system and recommended spacings are provided in Table 5-1. If the spacings are too short the lower layer resistivity layers may not be correctly identified which can introduce large positive or negative error into design calculations.

4. Low resistivity soils, especially at long Wenner Spacings, require relatively small resistances to be measured at the surface. Instrumentation with an inadequate lower range may reach its limit and incorrectly indicate higher resistivity values than exist.


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5. Care shall be taken in interpreting the measurement data. If using computer software tools, it should be remembered that the result is a ‘model’ of the soil conditions which is largely determined by automatic curve-fitting routines or user judgement. To increase confidence it is good practice to ‘test’ the model by comparing it to other geological data available for the site and the expected range of resistivity values for the materials known to be present. Measured resistances of vertical rods installed at the site can also be compared to calculated values obtained using the soil model to increase confidence. It should be recognised that the soil resistivity model may need to be refined throughout the project as more supporting information becomes available.

6. Adequate test lead insulation is important as inadvertent contact (e.g. where insulation damage allows bare wires to become in contact with wet ground) will introduce error into the measurement results.

5.7 Alternative Method

The driven rod method is an alternative to the Wenner Array method and is particularly useful in built-up urban areas where there is inadequate open land to run out test leads. This method should be used with caution and precautions are required to avoid the possibility of damage to buried services, in particular HV cables. Where the absence of buried services cannot be established, rods shall not be driven. An earth rod is driven vertically into the ground and its earth resistance measured as each section is installed using either of the methods from Sections 6. Using the simple equation provided below or computer simulation (for multi-layer analysis) the soil resistivity may be deduced from the measured rod resistance and its length in contact with the soil.

ρ = 2πR

(ln (8𝑙𝑑

) − 1)

where = uniform soil resistivity, R = rod resistance, l = rod length, d = rod diameter.

This method can be cost-effective as the rods can be used as part of the earthing installation. Where possible the results from driven rods at a number of locations around the site should be used together with any available Wenner Array method data to improve confidence in the derived soil resistivity model.


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6 Earth Resistance/Impedance Measurements

6.1 Overview

Earth resistance measurements are used to determine the overall resistance of a substation earthing system or individual earth electrodes during commissioning of a substation and at maintenance intervals. The overall substation earth resistance is used with the ground return current to calculate the earth potential rise (EPR).

There are various ways to measure the earth resistance of individual earth electrodes and complete substation earthing systems. The method used will depend on the size of the system and the availability of suitable measurement routes. This section includes the following measurement methods:

Fall-of-potential method using an earth tester.

Fall-of-potential method for smaller substations and pole-mounted sites.

Comparative method using an earth tester and clamp meter.

6.2 Fall-of-Potential Method

6.2.1 Application

The fall-of-potential method is used to measure the substation earth resistance or impedance. The measurement will include all earthing components connected at the time of the test (substation earth grid, power cable sheaths, structural steelwork etc.).

This method may also be used to measure the earth resistance or impedance of individual electrodes, tower footings or tower line chain impedances.

If there is no immediate adjacent land to run test leads the following options are available:

1. Carry out a fall-of-potential measurement from the nearest secondary substation or terminal pole (connected via cable sheath/screens) where there is adjacent open ground and use calculations that account for the sheath impedance to extrapolate the resistance at the target substation.

2. Measure the resistance of individual electrodes installed at the substation using the comparative method (Section 6.4) and compare these to calculated values for similar components. Good agreement will lend greater confidence to overall resistance/impedance calculations.

6.2.2 Equipment

A four-terminal composite earth tester.

Suitable test leads (up to 2 x 1000 metre in length for large substations – refer to Table 6-1) stored on reels for ease of use.

An earth rod cluster (e.g. 4 x 0.5 metre copper-clad-steel earth rods) for the remote current probe and a single 0.5 metre rod for the voltage probe.

A short lead with suitable earth clamps (ideally with a screw action to allow penetration of any surface oxidation, dirt or paint) to provide a low resistance connection to the earthing system under test.

A suitable equipotential mat (where the earth tester is located outside of a known earthing system and where there is likely to be a high EPR, e.g. a transmission tower).

Communication equipment.

Class 1 HV insulated gloves and dielectric footwear.


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6.2.3 Safety Requirements

A minimum of two people are required to carry out a fall-of-potential test.

Work should not proceed where there is an increased likelihood of an earth fault e.g. lightning activity (in the vicinity of the substation or the overhead lines connected to it) or planned switching.

Class 1 HV insulated gloves and dielectric footwear shall be worn for the connection and disconnection of all test leads and screw clamps to any earth electrode or earthing system.

The test equipment should be set up within the substation earth grid, as this will reduce possible touch voltages. If situated outside the earth grid, the equipment should be positioned on an equipotential mat, large enough for both the equipment and the operator, which is connected to the substation earthing system.

The test connection point should be part of the above-ground earthing system which connects directly to the substation electrode/grid as close to the ground as possible

The test route shall be selected to be as straight as practically possible, whilst minimising any risks. Test leads shall not run in parallel to overhead lines with earthed steel towers. They should preferably not be run parallel with, wood-pole unearthed construction lines for any significant length (otherwise a separation of at least 20 metres is required).

The operator shall remain in communication with those who are placing, connecting or disconnecting test leads remote from the testing point. During the test, remote staff shall only touch the current or voltage rods or leads when specifically directed to do so by the person in charge, i.e. after they have safely disconnected and insulated the test lead connections at the substation end.

The remote leads and probes shall not be left unattended at any time.

6.2.4 Method

The most commonly used method for measuring substation earth resistance or impedance is the fall-of-potential method. The method injects a small current into the substation earth system using a standard four-terminal earth tester. The current return is via a test probe located at a distance from substation as detailed in Table 6-1. A voltage gradient is set up around the test probe which is measured by a second potential probe connected to the earth tester. The connections are shown in Figure 6-1.

1. Select a suitable test route free of buried metallic cables and pipes. Measurements may be taken along any route but traverses that are parallel or orthogonal to the current lead are most commonly used and are more readily interpreted using standard methods

2. Connect terminals C1 and P1 of a four-terminal earth tester to the earthing system under test.

3. Place the C2 current probe away from the earthing system under test using the distances specified in Table 6-1 and connect to the earth tester.

4. Place the P2 probe at a distance of 80% of the defined C2 distance, connect the lead to the earth tester and record the resistance.

5. Disconnect the P2 lead from the earth tester.

6. Take further resistance measurements at 70%, 65%, 60%, 55%, 50%, 40%, 30% and 20%.


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Note: The P2 lead shall be disconnected from the earth tester when moving the P2 probe and the distance between the C2 and P2 leads should be maintained at around 300mm with no crossings.

7. Plot the results as a curve of resistance against distance for each route as shown in Figure 6-2. The actual value of resistance can then be determined using the ’61.8%’ or ‘slope’ method described in Section 6.2.5.

8. Record the test route.

Figure 6-1 – Fall-of-Potential Measurement Equipment Connection

Table 6-1 – Typical Separation between Substation Earthing System and Remote Current Probe (C2)

Substation Earth Electrode Type Remote Current Probe (C2) Distance

20kV, 11kV, or 6.6kV secondary substation or pole-mounted site – local earth rods or horizontal electrode <10m

50m

20kV, 11kV, or 6.6kV secondary substation or pole-mounted site – horizontal electrode >10m

100m (in opposite direction, or minimum of

90 to the horizontal earth electrode)

33kV or 66kV primary substation 400m

132kV grid substation 600m

Very large earthing system, e.g. shared site with a large generator or national grid

1000m

Earth Grid


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6.2.5 Interpretation of Results

Earth resistance or impedance measurement results are normally in the form of a series of points on a curve which need to be interpreted mathematically to obtain the actual resistance value; however care is required in selecting a suitable method and its limitations understood.

1. The most common method is the 61.8% rule. As shown in Figure 6-2, the electrode resistance theoretically occurs on the resistance curve at a distance from the substation electrode corresponding to 61.8% of the distance to the current probe. This is an approximate method but provides reasonable results providing the remote current probe is located sufficiently far away from the electrode under test. If it is located too close (e.g. due to limited available land) the interpreted result will generally be higher than the true value.

P2 C2

Figure 6-2 – Typical Fall-of-Potential Curve

2. An alternative method is the ‘Slope’ method which checks that the measured resistance

curve gradient is valid and provides an indication of when a larger electrode to current probe separation is required.

The earth resistance measurements at the 20%, 40% and 60% distances are used to

calculate the slope coefficient (), where:

μ = R60% − R40%

R40% − R20%

The slope coefficient gives a measure of how the measured fall-of-potential curve differs from the ideal curve and should fall within the range 0.1 to 2. Slope coefficients outside of this range are invalid, indicating that the assumptions have not been satisfied. If an invalid slope coefficient is obtained for a set of measurements, the remote current probe should be positioned further away from the earthing system and the test repeated. If the slope coefficient is still out of range, then it is likely that the soil structure is highly non-uniform.


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If a valid slope coefficient is obtained it can be used in conjunction with the graph shown in Figure 6-3 to calculate the required potential probe position for a correct earth resistance measurement, i.e. the value obtained from Figure 6-3 is used instead of 61.8% to obtain the earth resistance from the measured data curve.

Figure 6-3 – Potential Probe Position against Slope Coefficient

3. The final method involves the use of specialist simulation software such as CDEGS to

interpret the measured values.

6.2.6 Sources of Error

There are a number of sources of measurement error which should be considered when planning and carrying out these measurements. These include, but are not limited to:

1. Influence of buried metallic structures such as bare cable armouring/sheaths, earth electrodes, pipes, etc. Measurements taken above or near buried metallic services will generally underestimate the substation resistance. Measurement locations shall be carefully planned to avoid interference from metallic structures by consulting service records and, where there remains uncertainty, the use of scanning methods on site. Measurement results that have been influenced by a parallel buried metallic structure will typically be lower than expected and the resistance curve will be flat. A metallic structure crossing the measurement traverse at right-angles will result in a depression in the resistance curve. If interference is suspected the measurement should be repeated along a different route or an alternative method used.

2. The distance between the substation and the remote current probe is important to the accuracy of the measurement. The theoretical recommended distance is between five and ten times the maximum dimension of the earth electrode with the larger separations required where there is underlying rock. In practice, where there is insufficient land to achieve this, the current probe should be located as far away from the substation as possible. Measurements taken using relatively short distances between the substation and return electrode may not be accurately interpreted using standard methods and require analysis using more advanced methods. Typical distances used range from 400 metres for standard 33/11kV primary substations up to 1000 metres or greater for grid substations, refer to Table 6-1.

Percentage Probe Position against Slope Coefficient

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.2 0.4 0.6 0.8 1

Percentage Probe Position (x100%)

Slo

pe

Co

effi

cien

t

0.2,0.4,0.6


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3. Interference caused by standing voltage ‘noise’ on a substation earthing system may result in standard earth testers failing to produce satisfactory results. This is normally evident as fluctuating readings, reduced resolution or via a warning/error message. Typical environments where this may be experienced include transmission substations (275kV and 400kV), railway supply substations or substations supplying large industrial processes such as arc furnaces or smelters. Results shall be interpreted using an appropriate method and compared to calculations. Where there is significant difference further investigation is required.

4. Most commercially available earth testers use a switched DC square wave signal. Where it is possible to select a very low switching frequency (below 5Hz) the measured values will approach the DC resistance which will be accurate for small earth electrode systems in medium to high soil resistivity. When higher switching frequencies are used (128Hz is common) inductive effects may be evident in the results. Where an appreciable inductive component is expected and long parallel test leads are used it is advisable to use an AC waveform so that mutual coupling between the test lead may be subtracted and a true AC impedance obtained. Due to the appreciable standing voltage commonly found on live substation earth electrodes AC test signals are normally selected to avoid the fundamental and harmonic frequencies. For the most accurate results, measurements should be taken using frequencies either side of the power frequency to allow interpolation. Where it is considered necessary to undertake an AC earth impedance test further guidance should be sought from an earthing specialist.

5. Use of a three-pole earth tester is acceptable where the resistance of the single lead connecting the instrument to the electrode is insignificant compared to the electrode resistance. These instruments are generally suitable only for measuring small electrode components such as rods or a small group of rods in medium to high soil resistivity soils. For larger substations or low resistance electrodes a four-terminal instrument is essential to eliminate the connecting lead resistances which would otherwise introduce a significant error.


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6.3 Fall-of-Potential Method for Smaller Sites

6.3.1 Application

The earth resistance measurement of small earth electrode systems associated with secondary substations and pole-mounted equipment can be carried using only three measurements.

If the electrode system is extensive the full fall-of-potential test should be used. As a rule the test leads need to be ten times the length of the buried earth electrode system.

This method is only applicable for earth resistance measurements at sites with a small electrode system.

6.3.2 Equipment

A four-terminal composite earth tester.

Two 50 metre test leads (one lead should be marked at 25, 31 and 35 metres).

Short rods for the remote current and voltage probes.

Two short leads and suitable earth clamps to provide low resistance connections to the earthing system under test.

Communication equipment.





The test connection point should be part of the above-ground earthing system which connects directly to the substation electrode/grid as close to the ground as possible.


The operator shall remain in communication with those who are placing, connecting or disconnecting test leads remote from the testing point. During the test, remote staff shall only touch the current or voltage rods or leads when specifically directed to do so by the person in charge, i.e. after they have safely disconnected and insulated the test lead connections at the substation end.

6.3.4 Method

1. Connect terminals C1 and P1 to the HV or LV earthing system under test.

2. Place the C2 current probe 50 metres away from the substation or pole.


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3. Take three measurements by placing the P2 potential probe in line with the C2 probe at 25 metres (50%), 31 metres (62%) and 35 metres (70%) away from substation or pole the as shown below1.

C1 P1 P2 C2

FOUR-TERMINAL

EARTH TESTER

50% 62% 70% 100%

25m 31m50m

35mP1

P2

C1

C2

Disconnect P2 terminal

when moving P2 probe

Operator to wear HV

rubber gloves

Figure 6-4 – Earth Resistance Measurement of a Small Electrode System

6.3.5 Interpretation of Results

If the measured values are within 5% of the middle (31 metres) value and do not decrease with distance, the value at 31 metres is the overall earth resistance.

If there is more than 5% difference between the measurements (see examples below) the test should be repeated using a different transverse, i.e. relocate the C2 probe at 90 degrees to the first test and measure the potential using P2 along the new transverse. If this does not provide a satisfactory value the P2 probe spacing should be doubled to 50, 62, 70 metres and C2 probe placed at 100 metres and the test repeated.

Example 1:

9.6Ω measured at 25m



10Ω x 0.95 (-5%) = 9.5Ω

10Ω x 1.05 (+5%) = 10.5Ω

The readings are within the range 9.5-10.5, therefore the resistance of 10 ohms is valid.

Example 2:




12Ω x 0.95 (-5%) = 11.4Ω

12Ω x 1.05 (+5%) = 12.6Ω

The readings are outside the range 11.4 to 12.6, therefore the resistance of 12.0 ohms is not valid and the test should be repeated.

1 Longer distances may be used provided the percentage distances are maintained.


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6.4 Comparative Method

6.4.1 Application

The comparative method is used to measure the earth resistance of small individual electrode components within a large interconnected earthing system. This method is most effective where a relatively high resistance electrode is measured in comparison to a ‘reference earthing system’ which has a much lower resistance.

6.4.2 Equipment

A four-terminal composite earth tester, connecting leads and connectors or

a clamp type earth resistance meter.


Insulated tools if opening earth electrode test links via an approved method.



Note: Disconnection of earth electrodes as required in one of the test methods shall only be carried out during commissioning of a new or refurbished substation prior to energisation.

6.4.4 Methods

6.4.4.1 Method 1 – Clamp Meter

The first method uses a clamp meter and is the preferred method as the earth electrodes can be tested without disconnection.

1. Place a clamp meter around the connection to the electrode under test as shown in Figure 6-5.

2. The clamp meter generates and measures the current and voltage in the electrode loop and displays the ‘loop resistance’. If the reference earth resistance is sufficiently low relative to the electrode resistance the measured value will approach the electrode resistance.


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R1

If R(Parallel) << R1 the

measured ‘earth loop’

resistance [R(Parallel)+R1]

approaches R1

PARALLEL NETWORK OF

ELECTRODES

(REFERENCE/GLOBAL EARTH)

SUBSTATION

ELECTRODE UNDER

TEST

(CONNECTED)

R (PARALLEL) CLAMP TYPE

EARTH TESTER

Figure 6-5 – Earth Resistance Measurement using the Comparative Method and a Clamp Meter (Electrode under Test Connected)

6.4.4.2 Method 2 – Four-terminal Earth Tester

The second method uses a four-terminal earth tester and requires the earth electrode under test to be disconnected from the remainder of the substation earthing system. Therefore this method of test shall only be used prior to energisation during commissioning of new or refurbished substations.

1. Connect terminals C1 and P1 of a four-terminal earth tester to the earth electrode under test.

2. Connect terminals C2 and P2 to reference earth the as shown in Figure 6-6.

3. A current is circulated around the earth loop containing the electrode and the reference earth resistances and the voltage developed across them is measured. Ohm’s Law is used to calculate the series ‘loop resistance’ and if the reference earth resistance is sufficiently low relative to the electrode resistance the measured value will approach the electrode resistance.

R1

If R(Parallel) << R1 the

measured ‘earth loop’

resistance [R(Parallel)+R1]

approaches R1

C1 P1 P2 C2

FOUR-TERMINAL

EARTH TESTER

PARALLEL NETWORK OF

ELECTRODES

(REFERENCE/GLOBAL EARTH)

SUBSTATION

ELECTRODE UNDER

TEST

(DISCONNECTED)

R (PARALLEL)

Figure 6-6 – Earth Resistance Measurement using the Comparative Method and a Four-terminal Earth Tester (Electrode under Test Disconnected)


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6.4.5 Interpretation of Results and Sources of Error

In order to accurately measure an electrode resistance using the comparative method it is necessary to have a very low reference earth resistance compared to the electrode resistance (10% or lower is recommended). It is also necessary to have a reasonable physical separation between the electrode and reference earth to reduce mutual coupling through the soil.

If the reference earth resistance is too high the measured result will be significantly higher than the electrode resistance (if it is known it can be subtracted). If the electrode and reference earths are too close together then a value lower than the electrode resistance may be measured. These errors may be acceptable if the purpose of the measurement is a maintenance check where it is only necessary to compare periodic readings with historical results to identify unexpected increases, e.g. due to corrosion or theft.

If several different electrodes are tested with respect to the same reference earth more detailed interpretation methods may be developed to increase confidence in the individual electrode resistances and may also allow the reference earth resistance to be deduced.

Note: This method cannot be directly used to measure the overall substation earth resistance which requires the use of the fall-of-potential method described in Section 6.2.


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7 Earth Conductor Joint Resistance Measurements

7.1 Application

This measurement is used to measure the resistance across an earth joint to check its electrical integrity. This test should be carried out across every joint created at a new substation prior to backfilling and also at a sample of above-ground joints during periodic maintenance assessments.

The method described here may be used for testing all types (bolted, brazed, welded) of earth conductor joints at any type of substation.

7.2 Equipment

A four-terminal micro-ohmmeter.

Connecting leads and suitable earth clamps.

Class 1 HV insulated gloves.



7.4 Method

The method uses a micro-ohmmeter to measure electrical resistance across a joint using the connection arrangement shown in Figure 7-1.

1. Connect terminals C1 and P1 of the micro-ohmmeter to one side of the joint using earth clamps with sharp pins that can penetrate through paint or surface corrosion to reach the metal underneath. Connect terminals C2 and P2 of the micro-ohmmeter to the other side of the joint. Ideally, the connectors should be no more than 25mm either side of the joint.

2. Select a suitable scale on the micro-ohmmeter (normally a minimum current of 10A is required to measure in the micro-ohm range) and record the average value after the test polarity has been reversed.

3. Finally give the joint a firm tap with a steel hammer to ensure it is mechanically robust.

EARTH

CONDUCTOR

JOINT

C1 P1 P2 C2

FOUR-TERMINAL

MICRO-OHMMETER

Figure 7-1 – Connections for Earth Conductor Joint Resistance Measurements


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The measured resistance should not significantly exceed that of an equivalent length of conductor without a joint – typical values are given in Table 7-1. Joints which exceed this by more than 50% shall be remade. Where different sized tapes are involved, the threshold value used should be that of the smaller tape.

At new installations it is recommended that a few sample joints are made under controlled conditions (e.g. in a workshop), their resistance measured and the median of these values used as the benchmark for all other similar joints made at the installation. Alternatively measure the resistance across 1 metre of sample conductor and use it as the benchmark.

Where sample measurements cast doubt over the quality of the installation additional measurements shall be carried out. If these also reveal high values the affected joints shall be replaced.

Table 7-1 – Typical Resistance Values for Various Joints

Joint Resistance ()

Bolted joint copper/copper 5

Bolted joint aluminium/aluminium 10 to 40

Bolted joint aluminium/copper 10 to 40

Welded or brazed joint copper/copper 2

Welded joint aluminium 5

Existing Hepworth type clamp 13 to 20

Tinned copper tape to aluminium structure leg 10


It is imperative that four separate test leads are used to connect the four terminals on the micro-ohmmeter to locations either side of the joint. This will avoid introduction of test lead resistance into the measured result.

The test points either side of the joint under test shall be free of dirt or grease to ensure good contact with the instrument probes/connectors.


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8 Earth Connection Resistance Measurements (Equipment Bonding Tests)

8.1 Application

This measurement is used to measure the resistance between an item of equipment and the main substation earthing system to check bonding adequacy. This test should be carried out during commissioning of a new substation to confirm that each item of equipment is effectively connected to the main earthing system. It is also useful as an on-going maintenance check and for operational procedures, e.g. during post-theft surveys.

The method described here may be used for testing equipment connections at any type of substation. Refer to Section 9 for terminal tower testing.

8.2 Equipment

A four-terminal micro-ohmmeter.

Four connecting leads and suitable earth clamps.

Class 1 HV insulated gloves.



The probable path of the injected current shall be considered and where the substation uses a bus-zone protection scheme care shall be taken to ensure that any test current does not produce enough current to operate protection systems.

8.4 Method

The method is based upon the principle of measuring the resistance between a set point (or points) on the main electrode system and individual items of earthed equipment. A micro-ohmmeter is used and the connection arrangement is illustrated in Figure 8-1. Measurements can be taken from one central point (such as the switchgear earth bar) or, to avoid the use of unduly long leads, once a point is confirmed as being adequately connected, it can be used as a reference point for the next test and so on.

C1 P1 P2 C2

FOUR-TERMINAL

MICRO-OHMMETER

EARTH

CONNECTION 2

EARTH

CONNECTION 1

Figure 8-1 – Connections for Earth Bonding Conductor Resistance Measurements


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1. Connect terminals C1 and P1 of the micro-ohmmeter to the substation earth grid using suitable earth clamps.

2. Connect terminals C2 and P2 of the micro-ohmmeter to the earth terminal or structure of the equipment under test.

3. Select a suitable scale on the micro-ohmmeter (an injection current of at least 100mA is recommended).


The measured resistance between the two connection points will depend on the length, cross-sectional area, material and number of earth conductors between them – typical values are given in Table 8-1. Based on a maximum distance of 50 metres between connection points a threshold value of 20mΩ will provide a good indication of whether further investigation is required.

If the measured value is near or above the acceptable limit, then the most likely reason is a badly made or corroded joint. If so, this will need to be shorted out.

Table 8-1 – Acceptable Values for Measure Resistance

Equipment Acceptable Upper Limit Between Equipment and

Switchgear Earth Bar ()

Transformers and switchgear 50

Bus bar supports, operating mats and cable glands

100

Metal outer cubicle 200

Metal doors and ancillary equipment 300


The measured resistance between the two connection points will depend on the length of earth tape between them. If an unacceptably high resistance is measured further measurements should be taken using a different reference point (closer to the suspect item of plant) to confirm (or otherwise) the poor connection.

When relatively long test leads are used, e.g. 10, 20 metres or more, the resistance introduced into the test current circuit may limit the available current and reduce the accuracy available on the instrumentation. Furthermore, induced voltages in longer test leads can also interfere with the micro-ohmmeter.


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9 Terminal Tower Earth Continuity Measurement

9.1 Application

This measurement is used to determine the resistance between a tower and the main substation earthing system. This test should be carried out during commissioning of a new substation and during maintenance or site assessment to determine if the tower is effectively bonded to the substation earthing system.

9.2 Equipment

Multi-meter rated to 1000V (e.g. Fluke 177)

LEM-Flex box.

AC flexible current probe (e.g. LEM-Flex RR3030).

HV probe with 1000:1 ratio (e.g. Tenma 72-3040).

A four-terminal earth tester.

Four long connecting leads and suitable earth clamps.




Note: Special procedures are required when checking the connection between the substation earthing system and a terminal tower. If the bond is ineffective or missing a potential difference may exist and may pose a shock hazard to the test operator or damage to a test instrument. The additional procedure involves checking the current flow in the terminal tower legs and checking the voltage using insulated probes before the continuity measurement is carried out.

9.4 Method

This method first uses an insulated current (Part 1) and voltage probe (Part 2) to safely determine whether the tower is connected to the substation earthing system before measuring the tower resistance (Part 3).

9.4.1 Part 1 – Terminal Tower Current Measurement

Where the earth connection point to the terminal tower is not visible, it is necessary to determine which leg is bonded to earth. The following procedure allows safe measurement of the current flowing through each leg of the tower (the leg with the greatest current flow is assumed to be bonded to earth).

1. Connect the current probe test box leads to a multi-meter, ensuring polarity is observed.

2. Set multi-meter to measure AC volts.

3. Place the current probe around the base of tower leg.

4. Set the current probe range to 3000A (1mV/A) and observe reading displayed on multi-meter. If the value is in range the tower is connected, note reading and proceed to Part 2 (Section 9.4.2). If multi-meter is out of range, proceed to Step 5.


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5. Set the current probe range to 300A (10mV/A) and observe reading displayed on multi-meter. If the value is in range the tower is connected, note reading and proceed to Part 2 (Section 9.4.2). If multi-meter is out of range, proceed to Step 6.

6. Set the current probe range to 30A (100mV/A) and note reading displayed on multi-meter.

7. Repeat test for next tower leg.

8. If the results from all four of the tower legs are similar, it can be assumed that there is no connection to the substation earth grid and parts 2 and 3 of this procedure shall not be carried out. The tower should be reported so that it can be safely connected to the substation earthing system.

OFF

HzV ~

LEM-Flex 3000A

Off

3000A (1mV/A) 300A (10mV/A) 30A (100mV/A)

Low Battery

RR3030

Figure 9-1 – Terminal Tower Current Measurement

9.4.2 Part 2 – Terminal Tower Potential Difference Measurement

1. Determine a suitable connection point on the substation earthing system by testing to another point to check continuity.

2. Run out the test leads from the substation earthing system towards the terminal tower, but do not connect to the earth grid or terminal tower.

3. Connect the red and black HV probe leads to the multi-meter and the remaining lead to the test leads.

4. Connect the test leads to the substation earthing system.

5. Set multi-meter to read AC volts, touch the tower leg with the HV probe and observe the reading on multi-meter. If the reading is greater than 1V (indicating greater than 1000V at the tip of the probe), further testing shall not be carried out, otherwise proceed to Step 6

6. Disconnect the test lead from the substation earthing system and disconnect the HV probe from the multi-meter.

7. Connect the standard multi-meter test probe to the positive (red) terminal of the multi-meter.


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8. Re-connect the test leads to the substation earthing system.

9. Set multi-meter to read AC Volts, touch the tower leg with the test probe and observe the reading on multi-meter. If the reading is greater than 25V, further testing shall not be carried out, otherwise proceed to Part 3 (Section 9.4.3).

OFF

HzV ~

0.0V

Figure 9-2 – Terminal Tower Potential Difference Measurement

9.4.3 Part 3 – Terminal Tower Earth Resistance Measurement

Providing the reading at the end of Part 2 is less than 25V, proceed with the tower earth resistance measurement as follows.

1. Disconnect the test lead from the substation earthing system and multi-meter.

2. Connect two earth clamps to a suitable point on the terminal tower.

3. Connect one test lead to terminals C1 and P1 on the four-terminal earth tester.

4. Connect another test lead to terminals C2 and P2 on the four-terminal earth tester.

5. Connect one test lead to the substation earthing system and the other to the terminal tower.

6. Carry out a test and record the value.


The measured resistance should be below 20mΩ. If the measured value is near or slightly above the acceptable limit, then the most likely reason is a bad connection or a corroded joint.


The measured resistance between the two connection points will depend on the length of earth tape between them. If an unacceptably high resistance is measured further measurements should be taken using a different reference point (closer to the terminal tower) to confirm (or otherwise) the poor connection.


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10 Earth Electrode Separation Test

10.1 Application

This test is used to assess the electrical separation of two electrodes in the soil by measurement, e.g. segregated HV and LV electrodes at an 11kV secondary distribution substation or a separately earthed fence at a grid or primary substation.

10.2 Equipment

A four-terminal earth tester.

Four connecting leads and suitable connectors.



Class 1 HV insulated gloves shall be worn when carrying out these measurements at a live site to protect against potential differences across open-circuit or high resistance connections.

10.4 Method

1. Independently measure the earth resistance of each electrode (R1 and R2) using the fall-of-potential method described in Section 6.

2. Measure the ‘earth loop’ resistance (R3) of the two electrodes via the ground using a similar test to the earth connection resistance measurements detailed in Section 8 and Figure 8-1.


If two electrodes R1 and R2 are separated by a large distance then the R3 will approach the series resistance of R1 + R2. Lower measured values of R3 indicate a degree of conductive coupling through the soil.

10.5.1 Separate HV/LV Earths

Generally, for the purposes of checking satisfactory segregation of an HV and LV earth electrode at a secondary or pole-mounted substation the following test is used:

RS > 0.9 (RHV+RLV)

where:

RHV = measured HV earth resistance. RLV = measured LV earth resistance. RS = measured resistance between HV and LV earth.

A value lower than 0.9(RHV+RLV) may indicate inadequate separation and further investigation is required.


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10.5.2 Separately Earthed Fence

The segregation between the fence and the electrode system can be verified as acceptable if:

RSeparation > 0.8 (RGrid + RFence).

where:

RGrid = measured earth grid resistance. RFence = measured fence earth resistance. RSeparation = measured resistance between HV earth and fence.

Note: The value of 0.8 is based on assumption and experience but it could be measured for a number of substations to check that 0.8 works in practice.

Generally a separate fence will measure greater than 1 and a connected fence will give a measurement in the order of milli-ohms. If the system is not satisfactorily segregated, this will normally be due to one or more of the following:

Hessian served cables not being insulated 2 metres either side of the fence.

Fence earth rods being installed too far inside, i.e. close to the HV electrode system.

Unintentional connection between the fence and earthed equipment inside the substation.

Main HV earth grid electrode being within 2 metres of the fence.

The reason for the unsatisfactory value should be identified and the cause corrected. If this is difficult and/or expensive, then the option of bonding the fence to the earth grid and installing an external perimeter grading electrode should be considered.


As a prerequisite of this method adequate measured earth resistances are required for the two individual electrodes via the fall-of-potential method. The sources of error relating to these types of measurements are included in Section 6.2.6.


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11 Touch, Step and Transfer Voltage Measurement

11.1 Application

This measurement is used to measure touch, step and transfer voltages for comparison with calculated values. These measurements may be required to confirm that the installed design complies with the safety limits. Advanced techniques and equipment are required to perform these measurements at live substations and should be carried out by an earthing specialist.

These measurements may be carried out at any type of substation but the greater expense/complexity may only justify them in rare occasions where touch and step voltage measurements are critical to reduce uncertainty in calculated values.

11.2 Equipment

A four-terminal composite earth tester can be used but the resistances being measured are often below the tolerance of standard units. More sophisticated instrumentation is often necessary which can inject larger test signals or measure smaller voltages, in the presence of substation electrical noise.

Suitable test leads (up to 2 x 800 metres in length for large substations) stored on reels for ease of use.

An earth rod cluster (e.g. 4 x 0.5 metre earth rods) for the remote current probe.

A single 0.5 metre rod for the voltage probe.

A suitable equipotential mat where the test meter is located outside of a known earthing system.

Communication equipment (walkie-talkies or mobile telephones).

A means of providing a low resistance connection to the earthing system to be measured, normally a two core lead several metres long and suitable connectors, ideally with a screw action to allow penetration of any surface oxidation, dirt or paint.




Class 1 HV insulated gloves and dielectric footwear shall be worn when making and breaking connections to the earthing system and when in contact with the remote earth connections.

The test equipment should be set up within the substation earth grid, as this will reduce possible touch voltages. If situated outside the earth grid, the equipment should be positioned on an equipotential mat, large enough for both the equipment and the operator, which is connected to the substation earthing system.

The test connection point should be part of the above-ground earthing system which connects directly to the substation electrode/grid as close to the ground as possible.


The operator shall remain in communication with those who are placing, connecting or disconnecting test leads remote from the testing point. During the test, remote staff shall only touch the current or voltage rods or leads when specifically directed to do so by the person in charge, i.e. after they have been safely disconnected.


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Care should be taken when inserting test probes inside the substation to measure the surface potential due to the risk of inadvertent contact with buried structures. Use of a small plate electrode placed on the surface is an effective and safer alternative.

11.4 Method

These measurements use advanced techniques and instrumentation and should only be attempted by earthing specialists. An overview of the general method is given below. Further guidance on the different methods available can be found in IEEE 81.

Earth potentials may be measured by injecting a current into the substation electrode so that it returns through a remote electrode via a connecting conductor. The return electrode may be another substation electrode connected via a de-energised power line or a temporary test lead and set of probes. Providing the return electrode is located at a large enough distance from the substation (relative to the size of the substation electrode) a potential profile will be set up around the substation proportional to that which would exist during fault conditions. The voltage between the substation electrode and different points on the surface can then be measured and related to touch voltage. Step voltage can also be determined from measurements of the potential difference between points on the surface which are 1 metre apart. In both cases the actual voltages can be found by scaling in the ratio of the test current and fault current. Measurements should be concentrated in the areas where the highest touch and step voltages would be expected to occur, e.g. around the corners of the electrode.

Measurements may also be carried out to determine the voltage transferred from a substation electrode to a nearby metallic structure, e.g. a steel pipe or the earthing system associated with a different electrical system.

Measurements to determine the actual touch or step voltage that a human would be exposed to can also be carried out by passing a current through the earthing system and measuring it at a remote electrode. The prospective touch voltage across the area of the substation can be measured using a small earth rod as the potential probe to simulate the human foot.


The measurement results should be interpreted and compared to calculated values. It is recommended that a series of measurements are taken at a number of locations around the substation where high touch or step voltages are expected (normally at the corners or in areas where the electrode mesh is less dense). This will enable the trends in the voltage gradients to be assessed to identify spurious data points. Where the return electrode is not located sufficiently far away from the test electrode large errors may be introduced. These errors may be corrected using a detailed computer model or by averaging the measurements obtained using different current return electrode locations.


The same errors may be introduced as described in Section 6.2.6.

These measurements use advanced techniques and instrumentation and should only be attempted by earthing specialists.


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12 HOT Zone Plotting

12.1 Application

This measurement is used to estimate the area where the earth potential rise exceeds the ITU limits and to determine the HOT zone.

These measurements may be carried out at any type of substation but the greater expense/complexity may only justify them in rare occasions where HOT zone measurements are critical to reduce uncertainty in calculated values, e.g. before committing expenditure to mitigation works.

12.2 Equipment

Refer to Section 6.2.2.


Refer to Section 6.2.3.

12.4 Method

1. Calculate the ‘target contour resistance’ (ZX) for the required voltage contour level (X) using the calculated earth potential rise (EPR) and ground return current (Igr) using the formula:

gr

XI

XEPRZ

For example if the 650V contour is required, the EPR is 2000V and the ground return current is 1000A then:

35.11000

5060002Z650

2. Use the fall-of-potential method described in Section 6 to determine the distance of ZX in relation to substation fence. The potential probe should be moved in steps (as with a normal FOP) and the resistance measured until ZX is found.

3. Repeat the measurement in other directions by leaving the current probe (and connecting lead) in the same place and moving the potential probe along different routes, normally at 90 degrees (two directions if possible) and at 180 degrees as shown in Figure 12-1.

4. Once the distance to the contour resistance has been obtained in multiple directions around the substation they can be joined together using a circle to estimate the contour as shown in Figure 12-2.

5. If further contours are required repeat the test again for the new contour value.

Note: Results from the measurement in parallel with the current circuit need to be corrected for mutual coupling, whereas measurements taken at right angles do not.


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Figure 12-1 – HOT Zone Plot Measurement Location Examples

Figure 12-2 – HOT Zone Plot


Refer to Section 6.2.5 for interpretation of fall-of-potential results.


The same errors may be introduced as described in Section 6.2.6.

A more accurate plot can be obtained using earthing modelling software (e.g. CDEGS).


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13 Buried Earth Electrode Location

13.1 Application

Earth electrode location is used to locate buried electrode at existing sites to allow earthing drawings and records to be updated. The record should include the position of the electrode, its burial depth, material, size and installation method (e.g. above ground, ducts, buried etc.).

Buried electrode location may be used at any type of substation.

13.2 Equipment

Radio-frequency signal generator and receiver (ideally with a conductor depth measurement facility).


Care shall be taken when applying a test signal to the earthing system when using an earth rod as the signal return electrode.

Normal precautions should be taken to avoid inadvertent contact with buried services.

13.4 Method

Underground earth conductors can be located from the service using standard buried metallic service locators in used for locating cables and pipes etc. The method for using these devices is well documented elsewhere and operators will be expected to have training and experience which is outside the scope of this document.

Ground penetrating radar can also be used and the results plotted on a computer. Again a description of its use is outside the scope of this document.

If these methods cannot give a sufficiently clear plot of the earthing system then it will be necessary to dig trial holes.


This procedure should be carried out by a person who has received training in the operation of the location equipment. It can be difficult to discriminate between buried earth electrodes and multicore/power cables as the signal will also be injected onto the cable screens. Use of earth return signal injection modes will result in more current flowing through bare conductors and can be useful where they run close to cables. Where there remains uncertainty some trial excavations maybe necessary.


A cable screen may be mistaken for an earth electrode (see above).


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14 Earthing System Records and Earthing Database

A copy of the earthing drawing and test results shall be left on-site.

A copy of all drawings, test forms and any earthing reports (design, survey, measurement etc.) shall be sent to [email protected] for loading into the earthing database (refer to EDS 06-0002) and the document management system

15 Instrumentation and Equipment

It is imperative that measurements are taken using the most suitable instrumentation for the required task, which are in good working order and have a valid calibration certificate. The instrumentation will be used for field measurements in all weather conditions; it shall therefore be robust, have a good level of water resistance and be suitably protected from electrical transients (e.g. by fuses) and shielded for use in high-voltage installations. A list of recommended equipment is given in Appendix A

UK Power Networks’ current list of approved test equipment is available in the document management system.

Instruments shall be calibrated annually as a minimum to a traceable national standard. However heavily used instruments should be checked more frequently, e.g. against other calibrated instruments or standard resistors, between formal calibration periods.

Many of the measurements require ancillary equipment such as test leads, earth rods, connection clamps, etc. and it is equally important that these are also fit-for-purpose and well maintained.

mailto:[email protected]


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16 References

EDS 06-0001 Earthing Standard

EDS 06-0002 HOT Site Requirements

EDS 06-0012 Earthing Design Criteria

EDS 06-0018 NetMap Earthing Information System

ECS 06-0022 Grid and Primary Substation Earthing Construction Standard

ECS 06-0023 Secondary Distribution Network Earthing Construction Standard

ECP 11-0406 Grid and Primary Substation Earthing Test Form

ECP 11-0503 Secondary Substation Earthing Commissioning Procedure

Distribution Safety Rules

IEEE 81 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System Part 1: Normal Measurements


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Appendix A – Earthing Test and Measurement Equipment

Measurement Site Type Instrument Material Code2

Substation Earth Resistance

All Substations AVO Megger DET 2/2

Chauvin Arnoux 6470N/Terca 3

IRIS Syscal Junior

Substation Earth Resistance

Small Substations AVO Megger DET 4TD Series 65346A

Lead Kit with Markings 65306Q

Fluke 1625

Soil Resistivity All Substations AVO Megger DET 2/2

Chauvin Arnoux 6470N/Terca 3

Chauvin Arnoux 6460

Chauvin Arnoux 6462

IRIS Syscal Junior

Small Substations AVO Megger DET 4TD Series 65346A

Fluke 1625

Individual Electrodes Resistance

All Substations Chauvin Arnoux CA6410 65439D

Chauvin Arnoux CA6412

Chauvin Arnoux CA6415

Fluke 1625

AVO Megger DET 10C/20C

AVO Megger DET 14C/24C

Railway Installations Chauvin Arnoux CA6415R

Bonding/Joint Resistance

All AVO Ducter DLRO 10

AVO Ducter DLRO 10X

Chauvin Arnoux 6240 Micro-ohmmeter

Cropico D07 Plus Digital Ohmmeter

Appendix B – Training

All of the measurements detailed in this document require specific training and measurement equipment and are hence carried out by a specialist earthing contractor. However a course on earth resistance measurements at secondary substations (SEG042) is available via the UK Power Networks Learning Management System (LMS).

For specific course details refer to the ‘Courses and Training’ section of the Earthing Intranet page http://powernet/intranet/asset-management/engineering-standards/earthing/.

2 UK Power Networks material code.

http://powernet/intranet/asset-management/engineering-standards/earthing/


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Appendix C – Measurement Certificate Proforma

Measurements certificates should be succinct (i.e. no more than one or two pages) and contain the following information:

1. Test certificate number

2. Substation name and address

3. Date of test

4. Test method – fall-of-potential, clamp meter, comparative method

Fall-of-potential method – test route and test point

Comparative method – reference point and reference earth value (if known)

Clamp meter method – reference earth value (if known)

5. Earth rod number/position/location, length, type and measured resistance

Electrode Number / Reference

Position / Location

Type (Rod, Plate, Nest, Pile)

Length Measured Resistance

Measured Current

6. Overall earth resistance and interpretation method (standard method, computer software

etc.)

7. Test equipment manufacturer, type, calibration date and expiry

8. Test operative company, name and signature

ecs 06-0024 earthing testing and measurements · ecs 06-0024 earthing testing and measurements ......

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