It is recommended that the test earth electrode as well as H and S electrodes
should be located in one line. Due to the different field conditions it is not always
possible. On the website www.sonel.pl and in professional literature special cases
of probes location have been discussed.
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, R
E
earth electrode measurement will in-
clude an additional uncertainty. Particularly high uncertainty of measurement oc-
curs when the tested resistance is small, and the probes have a weak contact with
earth (such a situation occurs frequently when the earth electrode is well made but
the upper soil layer is dry and slightly conductive). Then, the ratio of resistance of
the probes to resistance of the tested earth electrode is very high and consequently,
uncertainty of δ measurement that depends on this ratio is also very high.
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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