ECS EARTHING TESTING AND MEASUREMENTS

Transcription

1 THIS IS AN UNCONTROLLED DOCUMENT, THE READER SHALL CONFIRM ITS VALIDITY BEFORE USE Document Number: ECS ENGINEERING CONSTRUCTION STANDARD ECS 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 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 All UK Power Networks Asset Management Capital Programme Connections External G81 Website Contractors ICPs/IDNOs Meter Operators HSS&TT Network Operations UK Power Networks Services Other

2 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 UK Power Networks 2015 All rights reserved 2 of 43

3 Contents 1 Introduction Scope Abbreviations and Glossary General Safety Requirements Soil Resistivity Measurement Earth Resistance/Impedance Measurements Earth Conductor Joint Resistance Measurements Earth Connection Resistance Measurements (Equipment Bonding Tests) Terminal Tower Earth Continuity Measurement Earth Electrode Separation Test Touch, Step and Transfer Voltage Measurement HOT Zone Plotting Buried Earth Electrode Location Earthing System Records and Earthing Database Instrumentation and Equipment References Appendix A Earthing Test and Measurement Equipment Appendix B Training Appendix C Measurement Certificate Proforma UK Power Networks 2015 All rights reserved 3 of 43

4 Figures Figure 5-1 Typical Soil Resistivity Measurement Routes at an Existing Site... 9 Figure 5-2 The Wenner Soil Resistivity Measurement Array Figure 5-3 Example of an Apparent Resistivity against Wenner Rod Spacing Plot with an Outlier Data Point Figure 5-4 Example of a Soil Resistivity Sounding Adversely Affected by a Buried Metallic Structure Figure 6-1 Fall-of-Potential Measurement Equipment Connection Figure 6-2 Typical Fall-of-Potential Curve Figure 6-3 Potential Probe Position against Slope Coefficient Figure 6-4 Earth Resistance Measurement of a Small Electrode System Figure 6-5 Earth Resistance Measurement using the Comparative Method and a Clamp Meter (Electrode under Test Connected) Figure 6-6 Earth Resistance Measurement using the Comparative Method and a Fourterminal Earth Tester (Electrode under Test Disconnected) Figure 7-1 Connections for Earth Conductor Joint Resistance Measurements Figure 8-1 Connections for Earth Bonding Conductor Resistance Measurements Figure 9-1 Terminal Tower Current Measurement Figure 9-2 Terminal Tower Potential Difference Measurement Figure 12-1 HOT Zone Plot Measurement Location Examples Figure 12-2 HOT Zone Plot Tables Table 5-1 Soil Resistivity Rod Spacing, Rod Depth and Locations for Different Substation Types Table 6-1 Typical Separation between Substation Earthing System and Remote Current Probe (C2) Table 7-1 Typical Resistance Values for Various Joints Table 8-1 Acceptable Values for Measure Resistance UK Power Networks 2015 All rights reserved 4 of 43

5 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 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. UK Power Networks 2015 All rights reserved 5 of 43

6 3 Abbreviations and Glossary Term Definition CDEGS COLD Site EPR Grid Substation HOT Site ITU PPE Primary Substation Secondary Substation Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis. Industry standard earthing software package 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) 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) A substation with a primary operating voltage of 132kV or 66kV and may include transformation to 33kV, 11kV or 6.6kV 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) 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 Personal Protective Equipment A substation with a primary operating voltage of 33kV and may include transformation to 11kV,6.6kV or 400V 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 UK Power Networks 2015 All rights reserved 6 of 43

7 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. UK Power Networks 2015 All rights reserved 7 of 43

8 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 ) 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 ) 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 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. UK Power Networks 2015 All rights reserved 8 of 43

9 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. SR 2 SR 3 SR 1 PRIMARY SUBSTATION SR 4 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. UK Power Networks 2015 All rights reserved 9 of 43

10 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 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. UK Power Networks 2015 All rights reserved 10 of 43

11 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 Suggested Number of Measurement Locations 1 to 2 1 to 2 2 to 3 3 to 4 UK Power Networks 2015 All rights reserved 11 of 43

12 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 twolayer 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 multilayered 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 geotechnical 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 UK Power Networks 2015 All rights reserved 12 of 43

13 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. UK Power Networks 2015 All rights reserved 13 of 43

14 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 ( 8l d ) 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. UK Power Networks 2015 All rights reserved 14 of 43

15 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 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 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. UK Power Networks 2015 All rights reserved 15 of 43

16 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 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 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%. UK Power Networks 2015 All rights reserved 16 of 43

17 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 Record the test route. Earth Grid 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 20kV, 11kV, or 6.6kV secondary substation or polemounted site local earth rods or horizontal electrode <10m 20kV, 11kV, or 6.6kV secondary substation or polemounted site horizontal electrode >10m 33kV or 66kV primary substation 132kV grid substation Very large earthing system, e.g. shared site with a large generator or national grid Remote Current Probe (C2) Distance 50m 100m (in opposite direction, or minimum of 90 to the horizontal earth electrode) 400m 600m 1000m UK Power Networks 2015 All rights reserved 17 of 43

18 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 = R 60% R 40% R 40% R 20% 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 nonuniform. UK Power Networks 2015 All rights reserved 18 of 43

19 Slope Coefficient Earthing Testing and Measurements Document Number: ECS 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 Percentage Probe Position (x100%) 0.2,0.4,0.6 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 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. UK Power Networks 2015 All rights reserved 19 of 43

20 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. UK Power Networks 2015 All rights reserved 20 of 43

21 6.3 Fall-of-Potential Method for Smaller Sites 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 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. Class 1 HV insulated gloves and dielectric footwear Safety Requirements 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 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 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. UK Power Networks 2015 All rights reserved 21 of 43

22 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 below 1. 50% 62% 70% 100% P1 25m 31m 35m 50m C1 P2 C2 C1 P1 P2 C2 FOUR-TERMINAL EARTH TESTER Disconnect P2 terminal when moving P2 probe Operator to wear HV rubber gloves Figure 6-4 Earth Resistance Measurement of a Small Electrode System 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: Example 2: 9.6Ω measured at 25m 10.0Ω measured at 31m 10.4Ω measured at 35m 10.3Ω measured at 25m 12.0Ω measured at 31m 13.9Ω measured at 35m 10Ω x 0.95 (-5%) = 9.5Ω 10Ω x 1.05 (+5%) = 10.5Ω The readings are within the range , therefore the resistance of 10 ohms is valid. 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. UK Power Networks 2015 All rights reserved 22 of 43

23 6.4 Comparative Method 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 Equipment A four-terminal composite earth tester, connecting leads and connectors or a clamp type earth resistance meter. Class 1 HV insulated gloves and dielectric footwear. Insulated tools if opening earth electrode test links via an approved method Safety Requirements 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. 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 Methods 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 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. UK Power Networks 2015 All rights reserved 23 of 43

24 R (PARALLEL) CLAMP TYPE EARTH TESTER R1 PARALLEL NETWORK OF ELECTRODES (REFERENCE/GLOBAL EARTH) SUBSTATION ELECTRODE UNDER TEST (CONNECTED) If R(Parallel) << R1 the measured earth loop resistance [R(Parallel)+R1] approaches R1 Figure 6-5 Earth Resistance Measurement using the Comparative Method and a Clamp Meter (Electrode under Test Connected) 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 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. R (PARALLEL) PARALLEL NETWORK OF ELECTRODES (REFERENCE/GLOBAL EARTH) R1 SUBSTATION ELECTRODE UNDER TEST (DISCONNECTED) C1 P1 P2 C2 FOUR-TERMINAL EARTH TESTER If R(Parallel) << R1 the measured earth loop resistance [R(Parallel)+R1] approaches R1 Figure 6-6 Earth Resistance Measurement using the Comparative Method and a Four-terminal Earth Tester (Electrode under Test Disconnected) UK Power Networks 2015 All rights reserved 24 of 43

25 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. UK Power Networks 2015 All rights reserved 25 of 43

26 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. 7.3 Safety Requirements 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. 7.4 Method The method uses a micro-ohmmeter to measure electrical resistance across a joint using the connection arrangement shown in Figure 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 UK Power Networks 2015 All rights reserved 26 of 43

27 7.5 Interpretation of Results 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 Sources of Error 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. UK Power Networks 2015 All rights reserved 27 of 43

28 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. 8.3 Safety Requirements 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 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 microohmmeter 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. EARTH CONNECTION 2 EARTH CONNECTION 1 C1 P1 P2 C2 FOUR-TERMINAL MICRO-OHMMETER Figure 8-1 Connections for Earth Bonding Conductor Resistance Measurements UK Power Networks 2015 All rights reserved 28 of 43