Manual Soil Resistivity Wenner Method

Earthing Manual Section E3

Soil Resistivity Measurements

Version:

2

Date of Issue:

December 2006

Author:

Nigel Johnson

Job Title:

Earthing Specialist

Approver:

Patrick Booth

Job Title:

Asset Manager

Eathing Manual Section E3 Soil Resistivity Measurements

Revision Log Version: 2.0

Prepared by:

Nigel Johnson

Date:

December 2006

Re-branded to E.ON Central Networks. Version: 1.0 Prepared by:

Nigel Johnson

Date:

April 2006

New document issued.

Version:

2

Date of Issue:

December 2006

The master version of the this document resides in the E.ON UK Documentum database CAUTION – any other copy may be ou t of date © Central Networks plc

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CONTENTS E3.0

INTRODUCTION

4

E3.1 Wenner Method for Measuring Soil Resistivity at Primary/Grid sites E3.1.1 E3.1.2 E3.1.3 E3.1.4 E3.1.5 E3.1.6

5

Instruments Checks for other buried equipment Wenner Test Method Probes in areas of tarmac or concrete Problems with depth of probes Fluctuating results

5 5 6 8 8 8

E3.2 “One Rod Method” for Measuring Soil Resistivity at Distribution Sites

9

E3.3 Estimation of Soil Resistivity using Geological Survey Data

10

APPENDIX AE3.1 – CHART FOR “ONE ROD METHOD” RESISTIVITY TEST

12

APPENDIX AE3.2 – CHART FOR WENNER TEST

13

Appendix AE3.3 - Wenner Soil Resistivity Field Sheet

14

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E3.0 Introduction 1m cube The resistance to the earth of any earth electrode is influenced by the resistivity of the surrounding soil. This will depend to a large extent on the nature of the soil and it’s moisture content. Resistivity may change with depth, temperature, moisture content and can vary from place to place depending on the strata of the soil and rock formation. The soil resistivity figure will have a direct impact on the overall substation resistance and how much electrode is required to achieve the desired values. It will also influence separation distances between two adjacent earth systems (e.g. HV and LV earths at Hot distribution sites). The lower the resistivity, the less electrode is required to achieve the desired earth resistance value. It is an advantage to know the resistivity value at the planning Figure E3.1 Soil Resistivity stage as this gives a good indication of how much electrode is likely to be required. This section describes the different methods that can be used to determine the soil resistivity.

The resistivity of any material is defined as the electrical resistance measured between the opposite faces of a uniform 1m 3 cube (See Fig E3.1). The accepted symbol is ‘ ρ’ and is measured in ohm-meters (Ωm). Typically soils can vary from a few ohm-meters for very wet loams up to thousands of ohm-meters for granite. Table E3.2 shows the values expected for a range of soil types together with examples of the likely resistance values for two types of electrode. In practice soil is very rarely homogenous and so the values indicted should be taken as a rough guide only. The Wenner (four terminal) test is the Company approved method for determining soil resistivity at Primary/Grid sites (see Section E3.1). The soil resistivity data can influence the chosen site location as well as the decision on the best type of earthing electrode system to be installed. For example, it helps to decide if it’s an advantage to drive rods to a greater depth or whether to increase the surface area by installing more buried tape. The survey can produce considerable savings in electrode and installation costs when trying to achieve the required resistance. If the results gained from the soil resistivity survey are unclear then soil modeling can be undertaken. With up-to-date techniques a fairly good and accurate soil model can be produced. Also core drilling usually associated with a Geo-Technical survey will give an accurate soil model and can be used to check measured soil resistivity results. Soil resistivity is also important in determining the separation distances between the HV and LV electrodes at Hot distribution substations. A Wenner test could be used but a simpler procedure has been developed for use at these sites. This is known as the ‘one’ or ‘driven rod method’ and gives an average value of soil resistivity (see Section E3.2). Once an electrode system is installed then the actual resistance value must be measured and recorded. If this falls short of the design value then additional electrodes will be required to rectify the problem.

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E3.1 Wenner Method for Measuring Soil Resistivity at Primary/Grid sites E3.1.1

Instruments

A four-terminal earth tester is required (see Table E4.1), equipped with four probes and connecting leads. The latter shall be mounted on reels for easy run-out and recovery and shall be checked for continuity and condition prior to use. The calibration of the instrument shall be checked before taking any readings, using the test resistors supplied with the instrument. Provision shall also be made for a laboratory re-calibration check of the instrument every year.

E3.1.2

Checks for other buried equipment

Before carrying out any testing, checks shall be made from other utility records, our own cable records and using radio detection equipment, for the presence of any buried cables, earth conductors or other metalwork. These could adversely affect the accuracy of the readings taken, particularly if they are parallel to the measurement route. Conventional metal detectors will only locate very large pipelines or objects close to the surface, so cannot be relied upon. Location equipment should be used in the inductive mode (to locate pipes which are not connected to the earthing system), and direct mode (to locate any pipes or cables bonded to the earth grid). For the latter, the transmitter is connected to the earthing system at the substation. The routes chosen should preferably be free of long buried metal pipes or lead sheathed cables etc., but if this is not possible the measurement route should be positioned at right angles to these items wherever possible. The route chosen should not be close and parallel to an overhead line. If the line supports are earthed, then this will adversely affect the readings. If the soil resistivity measurement leads are long and in parallel with an overhead line, then an induced voltage may occur in the leads should fault current flow through the overhead line. To avoid this, measurement routes should preferably be at right angles to overhead lines. If they must be in parallel, then a separation of 20m or more from the line is required.

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E3.1.3

Wenner Test Method

Fig. E3.2 shows the general measurement setup. The four earth probes should be driven into the ground in a straight line, at a distance ‘a’ metres apart and driven to a depth of ‘P’ cm.

C1

‘a’

P1

‘a’

P2

‘a’

C2

‘x’ ‘y’ Depth ‘P’ of test robes

Make sure any links are removed between the terminals on the tester

Fig E3.2 Arrangement for the Wenner Test The maximum depth of the probe should not exceed 20cm nor exceed 1/20 th of the probe spacing distance ‘a’. A series of resistance readings are taken for various spacings of the probes. For large sites the maximum spacings are increased to enable the soil resistivity to be assessed at a greater depth. Table 3.1 shows the required spacings for various sites.

Table E3.1

Recommended probe spacing/depths for soil resistivity tests

Spacing & Probe Depths Probe Soil Dist. X to Dist. Y to Spacing Resistivity Voltage Current ‘a’ Depth ‘d’ Probe Probe (m) (3/4 of ‘a’) (m) (m) (m)

Minimum recommended range of spacings for different substation types Max. Probe depth ‘P’ (cm)

1 2

0.75 1.5

0.5 1

1.5 3

5 5

3

2.25

1.5

4.5

10

5

3.75

2.5

7.5

10

10

7.5

5

15

15

15

11.25

7.5

22.5

15

20

15

10

30

20

25

18.75

12.5

37.5

20

30

22.5

15

45

20

40

30

20

60

20

50

37.5

25

75

20

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2

Date of Issue:

Wood Pole 11kV GM Dist. Locations S/S

Small 66/33 to 11kV S/S

December 2006


Towers, 132kV S/S & large sites

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The four probes should be connected to the tester, with the outer probes connected to the C1 and C2 terminals, and the inner probes to the P1 and P2 terminals. The instrument should be kept in a central position and a series of resistance measurements made as the four electrodes are moved out in equal distances from the central point. A calculation is made to determine the average soil resistivity of all layers of soil between the surface and a depth ‘d’ which is taken to be ¾ of the separation distance ‘a’. The meter should be left on to allow the built in filters to operate and the value after 30 seconds should be taken. If the reading is varying significantly, this may be due to :-

• • • •

Electrical interference High contact resistance at the test probes Damaged test leads Reading at the lower limit of the instrument’s measuring capability

If, after investigating the above, the reading is still changing by more than 5%, then 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. The apparent soil resistivity is then given by ρ=2πaR (Ωm) where:-

ρ π a R

= ground resistivity in Ωm = 3.142 = electrode spacing in metres = measured resistance in Ω at spacing 'a'

Appendix AE3.3 can be used as a field sheet to record the results and plot the apparent resistivity as the spacing is increased. Table AE3.2 can be used to look up apparent soil resistivities and is based on the above formula. Values that do not appear in the table need to be calculated individually. It is a good to plot the results at the testing stage as any wild variations could indicate the presence of buried metalwork that is distorting the results. If this is the case then a new test route should be found. An idealised plot can be seen in Fig E3.3 and in this instance shows a site where there is a high resistivity layer above a deeper lower resistivity layer.

Fig E3.3 Typical Soil Resistivity Plot Soil resisitivity 450 400 350 300 y t i v 250 i t s i s 200 e R

150 100 50 0 0

5

10

15

20

25

30

35

40

Depth

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At least two or three series of measurements, via routes perpendicular to one another should be taken, to allow interference and small local variation effects to be balanced out. If any readings were unstable, then additional routes will be necessary, possibly further away from the site. Note that it is important to ensure that measurements are symmetrical about point X midway between the voltage probes. An Excel spreadsheet “ Soil Resistivity Tests.xls” has been developed to help interpret the results.

E3.1.4

Probes in areas of tarmac or concrete

In some cases, the required position for one of the inner ‘voltage’ probes may coincide with an area covered with tarmac or concrete. Measurements can usually still be obtained by using a flat metal plate, of approximately 10 to 15cm square, placed on a cloth soaked with saline water, instead of the driven probe. A small weight on top of the plate will help to decrease contact resistance. The usual precautions concerning buried metal structures apply and the area where the plate is used should not contain reinforced steel which runs in the same direction as the measurement route, or the reading will be adversely affected.

E3.1.5

Problems with depth of probes

Despite suggestions to the contrary in many manufacturers publications, the test probe’s depth (‘P’) need normally only need be inserted to a depth in the range of 5 to 20cm, as shown in Table E3.2. The outer (current) probes are required to have a reasonably low resistance to earth, sufficient to allow approximately 50mA to flow. However, if the surface soil is dry or frozen, the high contact resistance with the probe will restrict the flow of test current. To overcome this it is recommended that a short steel rod having a smaller radius than the test probe is driven into the soil to a depth of 20cm and removed. A weak solution of preferably warm, saline water is poured into the hole and the test probe re-inserted. If this does not provide a satisfactory reading, the probe may be driven in a little deeper. A better arrangement is a cluster of three to five probes positioned 25cm apart and connected together. Probe clusters are normally only required at long test spacings and would introduce an error if used at small spacings. It is very unusual to require probe depths of more than 30cm and precautions will be required to ensure that third party equipment or cables are not damaged if probes are driven to more than 20cm depth. Their installed depth should never exceed 1/20th of ‘a’.

E3.1.6

Fluctuating results

If there are large fluctuations in the measured values at one particular spacing, then it is likely that interference from buried cables/pipes or stray ground currents are present. Additional sets of readings must be taken at locations a few metres away. Having first discounted readings which are obviously incorrect, then the average resistivity value for each probe separation ‘a’ is used to generate the soil model. It is important to note that measurements near the site will often be subject to interference from buried structures which will result in lower apparent readings than in undisturbed soil. This is why readings cannot be taken using this method within the area of an existing substation. Software programs are available for carrying out detailed calculations, based upon data from the above readings, to provide a “best-fit”, representative soil model for the area, consisting of a number of vertical or horizontal layers having different resistivity values.

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E3.2 “One Rod Method” for Measuring Soil Resistivity at Distribution Sites At Hot distribution substations it is necessary to segregate the HV and LV earths in accordance with sections E5.10.2 and E5.10.3. In order to do this the average soil resistivity value is required. This is a simpler test than the full Wenner test described above. The test is based on measuring the resistance of a single rod that is driven into the ground for a known depth. The resistance measurement and rod dimensions are then used to calculate the average soil resistivity required to produce the measured resistance. The resistance measurement can be made by using the 61.8% method described in Section E4.0.3.5. Using the formula for calculating the resistance (R) of a rod in uniform soil, R

=

⎛ ⎛ 8 L ⎞ ⎞ ⎜ ln⎜ ⎟ − 1⎟ 2π L ⎜⎝ ⎝ d ⎠ ⎠⎟ ρ

gives

ρ

=

2π LR

⎛ ⎛ 8 L ⎞ ⎞ ⎜⎜ ln⎜ ⎟ − 1⎟⎟ ⎝ ⎝ d ⎠ ⎠

Where ‘L’ is the length and ‘d’ is the diameter of the rod, both in metres. The apparent resistivity to match the measured resistance is calculated or looked up from the table in Appendix AE3.1. For most distribution substations it is sufficient to drive the rod to a depth of 2.4m and use this reading as the average soil resistivity value. In difficult locations a 1.2m rod is acceptable but if it even this proves difficult to drive in, then it’s an early indication that there may be a high resistivity rock layer just be below the surface which could give problems achieving the desired electrode value. Once the average soil resistivity is known a separation distance between the HV and LV can be determined. It is a good idea to position the test rod so that it can be incorporated into the final earthing arrangement. Fig E3.4 shows the general arrangement for this test.

P2 31m

C2 50m

0.5m deep hole

5/8” dia 2.4m (or 1.2m) rod

Procedure:Measure rod resistance using 61.8% test (see Section E4.0.3.5) then use table in Appendix AE3.1 to look up soil resistivity value or to obtain soil resistivity value, multiply resistance reading by 2.45 for a 2.4m rod or 1.39 for a 1.2m rod

Fig E3.4 General Arrangement for “One Rod Method” at Distribution Sites A further technique would be to take a series of measurements as the rod is driven into the ground at greater and greater depths. When the results are plotted out they could be useful in the following situations:-

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(i)

to accurately locate the water table or low resistivity soil layers. The rod resistance will be seen to drop dramatically once this level is reached, assuming the surface material has a higher resistivity. This could influence the decision on whether to install deep drive electrodes or to install a horizontal electrode system.

(ii)

at primary and grid sites where readings using the Wenner method are not possible near to the substation. Rods may be driven into the ground even within the substation (once the area is proven free of buried cables/equipment). The results obtained are then used to verify or modify the soil model obtained via Wenner measurements at suitable locations outside the substation. On completion of the test the rod could be incorporated into the main earth grid if appropriate.

The earth resistance of a rod will usually reduce as its driven depth is increased. The resistance of a rod should never increase with driven depth. It is the rate at which the resistance decreases with depth that allows the soil structure and layer resistivities to be determined. Soil structure where the deeper layer has a lower resistivity than the upper, will produce sudden changes in the gradient of the rod’s resistance curve. Where the top layer has a lower resistivity than the lower layers then the structure is more difficult determine, as the test current will tend to continue to flow in the top, lower resistivity layer. The resulting low current density in the higher resistivity layer has little influence on the measured resistance of the rod. Where a very high resistivity stratum is penetrated, the rod resistance may remain virtually constant with increasing depth. If a further lower resistivity layer is penetrated beneath this, then the rod resistance will again begin to decrease with increasing depth. The technique does suffer from the fact that the rod resistance is determined by conditions close to it and there is often a wide variation between rod resistances (of the same length) obtained at different positions around a site.

E3.3 Estimation of Soil Resistivity using Geological Survey Data Whilst less accurate than direct measurement, geological data can given a reasonable indication as to the resistivity of the ground in a particular area. You may also find it useful to build up a data base of resistivity measurements from actual jobs. By using these two sources of information you can determine how much electrode to install at most job locations without the need for a preliminary survey. Of course – you MUST MEASURE the actual resistances of the earth system after installation to confirm your predictions.

In the short term the geological data can be obtained from the one inch series ‘Solid and Drift’ edition geological survey maps published by: British Geological Survey, Keyworth, Nottingham, NG12 5GG. Tel. 0115 9363100

These maps are colour coded to show different underlying geological types and surface deposits such as clay and head. The following table gives typical resistivity ranges for a number of different ground types. This method shall not be used in isolation where extensive earth installations are going to be installed such as at primary and bulk supply point substations. Accurate soil resistivity measurements and soil modelling techniques are essential to the proper design process.

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However, geological information can be useful when short-listing possible sites. It may help you to choose a site with low earth resistivity instead of a site with high resistivity. Intelligent site selection can considerably reduce the cost of earthing and also remove/reduce the implications of creating a “Hot Zone”. Where the maps indicate very high soil resistivities such as limestone, granite, gritstone etc. it is recommended that a soil resistivity test is carried out as the cost to install the earth electrode could be a significant proportion of the cost of the overall job.

Table E3.2 - Typical soil resistivity values and likely electrode resistances Soil/Ground Type

Mercia Mudstone Coal Measures Loam Alluvium Boulder Clay Keuper Marl & Waterstones Head Sand/Gravel Limestone Pebble Beds Permian Limestone & Marl Gritstone

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Typical Resistivity ( m)

For homogenous soil likely resistance of a 2.4m rod ( )

For homogenous soil likely resistance of 50m of a 70mm2 earthwire( )

20 20 25 35

8 8 10 14

0.8 0.8 0.9 1.3

50 50 70 300 300 300 400 1000

20 20 28 120 120 120 160 400

1.9 1.9 2.6 11 11 11 15 38

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Appendix AE3.1 – Chart for “One Rod Method” Resistivity Test Table to calculate soil resistivity using single rod technique described in Section E3.2 (assuming 5/8” dia. rod). Example: - 4.8m rod with resistance of 20 Ω gives average soil resistivity of 89 Ωm Length of rod Resistance (Ω)

1.2m

2.4m

3.6m

4.8m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

1 3 4 6 7 8 10 11 13 14 15 17 18 20 21 22 24 25 27 28 29 31 32 33 35 36 38 39 40 42 43 45 46 47 49 50 52 53 54 56 57 59 60 61 63 64 66 67 68 70 71 73 74 75 77

2 5 7 10 12 15 17 20 22 25 27 30 32 35 37 40 42 45 47 49 52 54 57 59 62 64 67 69 72 74 77 79 82 84 87 89 91 94 96 99 101 104 106 109 111 114 116 119 121 124 126 129 131 134 136

3 7 10 14 17 21 24 28 31 35 38 42 45 49 52 56 59 63 66 70 73 77 80 83 87 90 94 97 101 104 108 111 115 118 122 125 129 132 136 139 143 146 150 153 157 160 163 167 170 174 177 181 184 188 191

4 9 13 18 22 27 31 36 40 44 49 53 58 62 67 71 75 80 84 89 93 98 102 107 111 115 120 124 129 133 138 142 147 151 155 160 164 169 173 178 182 187 191 195 200 204 209 213 218 222 226 231 235 240 244

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6m

Length of rod esistance (Ω)

1.2m

2.4m

3.6m

4.8m

6m

5 11 16 21 27 32 38 43 48 54 59 64 70 75 81 86 91 97 102 107 113 118 124 129 134 140 145 150 156 161 167 172 177 183 188 193 199 204 210 215 220 226 231 236 242 247 253 258 263 269 274 279 285 290 296

56 57 58 59 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104 106 108 110 112 114 116 118 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220

78 80 81 82 84 86 89 92 95 98 100 103 106 109 112 114 117 120 123 126 128 131 134 137 140 142 145 148 151 153 156 159 162 165 167 174 181 188 195 202 209 216 223 230 237 244 251 258 265 272 279 286 293 300 307

138 141 143 146 148 153 158 163 168 173 178 183 188 193 198 203 208 213 218 223 228 232 237 242 247 252 257 262 267 272 277 282 287 292 297 309 321 334 346 359 371 383 396 408 420 433 445 457 470 482 495 507 519 532 544

195 198 202 205 209 216 223 230 237 243 250 257 264 271 278 285 292 299 306 313 320 327 334 341 348 355 362 369 376 383 390 397 403 410 417 435 452 470 487 504 522 539 556 574 591 609 626 643 661 678 696 713 730 748 765

249 253 258 262 266 275 284 293 302 311 320 329 338 346 355 364 373 382 391 400 409 417 426 435 444 453 462 471 480 489 497 506 515 524 533 555 577 600 622 644 666 688 711 733 755 777 799 822 844 866 888 910 933 955 977

301 306 312 317 322 333 344 355 365 376 387 398 408 419 430 441 451 462 473 484 494 505 516 527 537 548 559 570 580 591 602 613 623 634 645 672 699 726 752 779 806 833 860 887 914 941 967 994 1021 1048 1075 1102 1129 1156 1182

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Appendix AE3.2 – Chart for Wenner Test This table is to assist in determining soil resistivity using the Wenner technique in section E3.1 and is based on the formula ρ= 2 π a R. Values that fall outside this chart should be calculated individually. Example: -‘a’ spacing of 2m, with a resistance reading of 1

Ω gives a soil resistivity value of 13 Ωm

Rod Spacing ‘a’

1m

2m

3m

5m

10m

15m

20m

25m

30m

40m

50m

Measured Resistance ( ) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 150 200 300

1 1 2 3 3 4 4 5 6 6 13 19 25 31 38 44 50 57 63 69 75 82 88 94 101 107 113 119 126 157 188 220 251 283 314 346 377 408 440 471 503 534 565 597 628 942 1257 1885

1 3 4 5 6 8 9 10 11 13 25 38 50 63 75 88 101 113 126 138 151 163 176 188 201 214 226 239 251 314 377 440 503 565 628 691 754 817 880 942 1005 1068 1131 1194 1257 1885

2 4 6 8 9 11 13 15 17 19 38 57 75 94 113 132 151 170 188 207 226 245 264 283 302 320 339 358 377 471 565 660 754 848 942 1037 1131 1225 1319 1414 1508 1602 1696 1791 1885

3 6 9 13 16 19 22 25 28 31 63 94 126 157 188 220 251 283 314 346 377 408 440 471 503 534 565 597 628 785 942 1100 1257 1414 1571 1728 1885

6 13 19 25 31 38 44 50 57 63 126 188 251 314 377 440 503 565 628 691 754 817 880 942 1005 1068 1131 1194 1257 1571 1885

9 19 28 38 47 57 66 75 85 94 188 283 377 471 565 660 754 848 942 1037 1131 1225 1319 1414 1508 1602 1696 1791 1885

13 25 38 50 63 75 88 101 113 126 251 377 503 628 754 880 1005 1131 1257 1382 1508 1634 1759 1885

16 31 47 63 79 94 110 126 141 157 314 471 628 785 942 1100 1257 1414 1571 1728 1885

19 38 57 75 94 113 132 151 170 188 377 565 754 942 1131 1319 1508 1696 1885

25 50 75 101 126 151 176 201 226 251 503 754 1005 1257 1508 1759

31 63 94 126 157 188 220 251 283 314 628 942 1257 1571 1885

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Appendix AE3.3 - Wenner Soil Resistivity Field Sheet Spacing 'a' of electrodes (m)

Distance "x" to potential probes (m)

Distance "y" to current probes(m)

Max. Depth ‘P’ of probes (cm)

Resistance reading from Instrument R (Ω)

1

0.5

1.5

5

0.75

2

1

3

10

1.5

3

1.5

4.5

15

2.25

5

2.5

7.5

20

3.75

10

5

15

20

7.5

15

7.5

22.5

20

11.25

20

10

30

20

15

25

12.5

37.5

20

18.75

30

15

45

20

22.5

40

20

60

20

30

50

25

75

20

37.5

Note: -Average Soil resistivity ( ) is based on the formula: -

C1

‘a’

P1

‘a’

P2

Soil Resistivity Depth 'd' (m) (3/4 of 'a')

2

=

Average soil resistivity ‘ ’ to depth 'd' ( Ωm) (see note below)

a R (where

=

‘a’

3.142)

C2

‘x’ ‘y’

Depth ‘P’ of test probes Make sure any links are removed between the terminals on the tester

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Manual Soil Resistivity Wenner Method

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