The calculations assume that the distance between the measuring electrodes are
equal (Wenner method). If it is not the case, perform the measurement of earth re-
sistance using 4-pole method and calculate resistivity from the formula:
ρ = 2πLR
E
where:
L – distance between the electrodes
R
E
– measured resistance
Particular attention should be paid to quality of connection between the object being
tested and the test lead – the contact area must be free from paint, rust, etc.
If resistance of test probes is too high, resistivity measurement will include an
additional uncertainty. A particularly large measurement uncertainty arises when a
small resistance value is measured with probes that have weak contact with the
ground. Then, the ratio of resistance of the probes to measured resistance as a re-
sistivity formula component is very high and consequently, uncertainty of measure-
ment that depends on this ratio is also very high. Then, in accordance with the for-
mulas from section 10.4.4, calculations can be made to estimate the influence of
measurement conditions.
To reduce the uncertainty of the δ, measurement, the contact of the probe with
earth may be improved, for example, by:
o moistening the spot where the probe is driven with water,
o driving the probe in a different location,
o applying an 80 cm probe.
Also, test the test leads for:
o whether their insulation is not defective
o whether the lead – banana plug – probe contact areas are not corroded or loos-
ened.
In majority of cases the achieved measurement accuracy is satisfactory. However,
you should always take account of the uncertainty included in the measurement.
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