wenner 4 pole soil resistivity test instruction word - soil resistivity testing instruction_1pg...
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Wenner 4 Pole Soil Resistivity Test Instruction
Maximum Probe Minimum Test Probe Spacings for Different Earth Grid Systems
Spacing “a” (m) Small Earth Systems, Pole Top Subs
Kiosk, Padmount Distribution Subs
Larger Distribution Subs, Switchyards
T/L Towers, Power Stations, Solar PV
20 30 50 100
Minimum acceptable spacing set for any Wenner test (m): 0.5, 1, 1.5, 2, 3, 4, 5, 7.5, 10, 15, 20, 30 (minimum 10 points)
1. Maximum depth of test probes shall not exceed
200mm or 1/20th of probe spacing 'a', due to mutual
resistance interference affects. This is most critical
for small spacing tests (0.5m up to 2‐3m).
2. The test meter shall be kept in a central position and
a series of measurements made as two test probes
(outer current & inner voltage) are moved out in
equal distances on either side from the central point.
3. At least 2 locations should be tested near the
substation area, with 2 orthogonal test traverses at
each location. If a large variation is seen in these 2
locations, a 3rd location should be tested nearby.
4. At least 10 test points shall be made in each traverse,
in order to provide a statistically relevant data set for
software analysis of the results. The test data
provides only "apparent resistivity" results. Computer analysis is required to determine the "actual resistivities" from the
"apparent" test results.
5. The ratio of consecutive spacings should be no greater than 1.5, preferably 1.3, in order to provide a statistically relevant
data set for software analysis (1.0‐2.0 m spacings are acceptable due to practicality).
6. The minimum recommended range of spacings for the different sites listed in the table above shall be used. Sufficient test
cable lead lengths must be carried for those larger test areas.
7. Ideally, the maximum spacing distance to be used should be greater than the estimated "zone of influence" of the intended
earthing system. This is the longest diagonal of the proposed earth grid or interconnected earthing system. For large areas,
multiple test locations shall be chosen to cover the area for practicality.
8. When soil is dry and it is difficult to get readings, wetting of the "current" test electrode positions shall be done. If the stake
resistance is still too high (> 5 k) then using multiple test stakes tied together in a ring pattern, for the outer current
electrodes, is advised for the larger spacings. If high stake resistances are still impeding measurements after using the above
mitigation methods, then a higher powered meter should be used.
9. 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 not too far away. 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.
10. To check that interference is not affecting readings, "transpose" the current and voltage probes in the meter. Then repeat
the test for the suspect spacing measurement. If readings are similar, then the test is valid.