76244679 manual soil resistivity wenner method

8/20/2019 76244679 Manual Soil Resistivity Wenner Method

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Version: 2 Date of Issue: December 2006

Author: Nigel Johnson Job Title: Earthing Specialist

Approver: Patrick Booth Job Title: Asset Manager

Earthing Manual

Section E3

Soil Resistivity Measurements



Eathing Manual

Section E3 Soil Resistivity Measurements

Version: 2 Date of Issue: December 2006 of 14

The master version of the this document resides in the E.ON UK Documentum database

CAUTION – any other copy may be out of date

© Central Networks plc

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.



Eathing Manual






CONTENTS

E3.0 INTRODUCTION 4

E3.1 Wenner Method for Measuring Soil Resistivity at Primary/Grid sites 5 E3.1.1 Instruments 5 E3.1.2 Checks for other buried equipment 5 E3.1.3 Wenner Test Method 6 E3.1.4 Probes in areas of tarmac or concrete 8 E3.1.5 Problems with depth of probes 8 E3.1.6 Fluctuating results 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



Eathing Manual






E3.0 Introduction

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 planningstage 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 1m3 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 modeland 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.

Figure E3.1 Soil Resistivity

1m cube



Eathing Manual






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 theearthing 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.



Eathing Manual






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.

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 Minimum recommended range of spacings for

different substation types

Probe

Spacing

‘a’

(m)

Soil

Resistivity

Depth ‘d’

(3/4 of ‘a’)

(m)

Dist. X to

Voltage

Probe

(m)

Dist. Y to

Current

Probe

(m)

Max.

Probe

depth

‘P’

(cm)

Wood Pole

Locations

11kV GM Dist.

S/S

Small

66/33 to

11kV S/S

Towers,

132kV S/S &

large sites

1 0.75 0.5 1.5 5

2 1.5 1 3 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

C1 ‘a’ P1 ‘a’ P2 ‘a’ C2

Depth ‘P’ of

test robes

‘x’

‘y’

Fig E3.2 Arrangement for the Wenner Test

Make sure any links areremoved between the

terminals on the tester



Eathing Manual






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 = ground resistivity in Ωm

π = 3.142

a = electrode spacing in metres

R = 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 testroute 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

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40

Depth

R e s i s t i v i t y



Eathing Manual






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 areobviously 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.



Eathing Manual






⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −⎟

⎠

⎞⎜⎝

⎛ = 18

ln2 d

L

L R

π

ρ

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −⎟

⎠

⎞⎜⎝

⎛ =

18

ln

2

d

L

LRπ ρ

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,

gives

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.

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:-

0.5m

deep

hole

5/8” dia

2.4m

(or 1.2m)

rod

Fig E3.4 General Arrangement for “One Rod Method” at Distribution Sites

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

P2 C2

31m 50m



Eathing Manual






(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 possiblenear 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 lowerlayers 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.



Eathing Manual






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 isrecommended 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 Typical

Resistivity

( m)

For homogenous soil

likely resistance of a

2.4m rod (

)

For homogenous soil

likely resistance of 50m

of a 70mm2

earthwire(

)

Mercia Mudstone 20 8 0.8

Coal Measures 20 8 0.8

Loam 25 10 0.9

Alluvium 35 14 1.3

Boulder Clay 50 20 1.9

Keuper Marl & Waterstones 50 20 1.9

Head 70 28 2.6

Sand/Gravel 300 120 11

Limestone 300 120 11

Pebble Beds 300 120 11

Permian Limestone & Marl 400 160 15

Gritstone 1000 400 38



Eathing Manual






Appendix AE3.1 – Chart for “One Rod Method” Resistivity TestTable 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 6mLength of

rod

esistance (Ω)

1.2m 2.4m 3.6m 4.8m 6m

1 1 2 3 4 5 56 78 138 195 249 301

2 3 5 7 9 11 57 80 141 198 253 306

3 4 7 10 13 16 58 81 143 202 258 312

4 6 10 14 18 21 59 82 146 205 262 317

5 7 12 17 22 27 60 84 148 209 266 322

6 8 15 21 27 32 62 86 153 216 275 333

7 10 17 24 31 38 64 89 158 223 284 344

8 11 20 28 36 43 66 92 163 230 293 355

9 13 22 31 40 48 68 95 168 237 302 365

10 14 25 35 44 54 70 98 173 243 311 376

11 15 27 38 49 59 72 100 178 250 320 387

12 17 30 42 53 64 74 103 183 257 329 398

13 18 32 45 58 70 76 106 188 264 338 408

14 20 35 49 62 75 78 109 193 271 346 419

15 21 37 52 67 81 80 112 198 278 355 430

16 22 40 56 71 86 82 114 203 285 364 441

17 24 42 59 75 91 84 117 208 292 373 451

18 25 45 63 80 97 86 120 213 299 382 462

19 27 47 66 84 102 88 123 218 306 391 473

20 28 49 70 89 107 90 126 223 313 400 484

21 29 52 73 93 113 92 128 228 320 409 494

22 31 54 77 98 118 94 131 232 327 417 505

23 32 57 80 102 124 96 134 237 334 426 516

24 33 59 83 107 129 98 137 242 341 435 527

25 35 62 87 111 134 100 140 247 348 444 537

26 36 64 90 115 140 102 142 252 355 453 548

27 38 67 94 120 145 104 145 257 362 462 559

28 39 69 97 124 150 106 148 262 369 471 570

29 40 72 101 129 156 108 151 267 376 480 580

30 42 74 104 133 161 110 153 272 383 489 591

31 43 77 108 138 167 112 156 277 390 497 602

32 45 79 111 142 172 114 159 282 397 506 613

33 46 82 115 147 177 116 162 287 403 515 623

34 47 84 118 151 183 118 165 292 410 524 634

35 49 87 122 155 188 120 167 297 417 533 645

36 50 89 125 160 193 125 174 309 435 555 672

37 52 91 129 164 199 130 181 321 452 577 699

38 53 94 132 169 204 135 188 334 470 600 726

39 54 96 136 173 210 140 195 346 487 622 752

40 56 99 139 178 215 145 202 359 504 644 779

41 57 101 143 182 220 150 209 371 522 666 806

42 59 104 146 187 226 155 216 383 539 688 833

43 60 106 150 191 231 160 223 396 556 711 860

44 61 109 153 195 236 165 230 408 574 733 887

45 63 111 157 200 242 170 237 420 591 755 914

46 64 114 160 204 247 175 244 433 609 777 941

47 66 116 163 209 253 180 251 445 626 799 967

48 67 119 167 213 258 185 258 457 643 822 994

49 68 121 170 218 263 190 265 470 661 844 1021

50 70 124 174 222 269 195 272 482 678 866 1048

51 71 126 177 226 274 200 279 495 696 888 1075

52 73 129 181 231 279 205 286 507 713 910 1102

53 74 131 184 235 285 210 293 519 730 933 1129

54 75 134 188 240 290 215 300 532 748 955 1156

55 77 136 191 244 296 220 307 544 765 977 1182



Eathing Manual






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’

Measured

Resistance ( )

1m 2m 3m 5m 10m 15m 20m 25m 30m 40m 50m

0.1 1 1 2 3 6 9 13 16 19 25 31

0.2 1 3 4 6 13 19 25 31 38 50 63

0.3 2 4 6 9 19 28 38 47 57 75 94

0.4 3 5 8 13 25 38 50 63 75 101 126

0.5 3 6 9 16 31 47 63 79 94 126 157

0.6 4 8 11 19 38 57 75 94 113 151 188

0.7 4 9 13 22 44 66 88 110 132 176 220

0.8 5 10 15 25 50 75 101 126 151 201 251

0.96 11 17 28 57 85 113 141 170 226 283

1 6 13 19 31 63 94 126 157 188 251 314

2 13 25 38 63 126 188 251 314 377 503 628

3 19 38 57 94 188 283 377 471 565 754 942

4 25 50 75 126 251 377 503 628 754 1005 1257

5 31 63 94 157 314 471 628 785 942 1257 1571

6 38 75 113 188 377 565 754 942 1131 1508 1885

7 44 88 132 220 440 660 880 1100 1319 1759

8 50 101 151 251 503 754 1005 1257 1508

9 57 113 170 283 565 848 1131 1414 1696

10 63 126 188 314 628 942 1257 1571 1885

11 69 138 207 346 691 1037 1382 1728

12 75 151 226 377 754 1131 1508 1885 13 82 163 245 408 817 1225 1634

14 88 176 264 440 880 1319 175915 94 188 283 471 942 1414 1885

16 101 201 302 503 1005 1508

17 107 214 320 534 1068 1602

18 113 226 339 565 1131 1696

19 119 239 358 597 1194 1791

20 126 251 377 628 1257 1885

25 157 314 471 785 1571

30 188 377 565 942 1885

35 220 440 660 1100

40 251 503 754 1257

45 283 565 848 1414

50 314 628 942 1571

55346 691 1037 1728

60 377 754 1131 1885

65 408 817 1225

70 440 880 1319

75 471 942 1414

80 503 1005 1508

85 534 1068 1602

90 565 1131 1696

95 597 1194 1791

100 628 1257 1885

150 942 1885

200 1257

300 1885



Eathing Manual





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)

Soil

Resistivity

Depth 'd' (m)

(3/4 of 'a')

Average soil

resistivity ‘ ρ

’ to

depth 'd' (Ωm)

(see note below)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 3050 25 75 20 37.5

Note: -Average Soil resistivity (ρ

) is based on the formula: -ρ

=2 a R (where = 3.142)

C1 ‘a’ P1 ‘a’ P2 ‘a’ C2

Depth ‘P’ of

test probes

‘x’

‘y’

Make sure any links are

removed between the

terminals on the tester

76244679 manual soil resistivity wenner method

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