Read out the result.
Resistance of current electrode
Resistance of voltage electrode
The value of additional uncertainty,
generated by the resistance
of auxiliary electrodes (probes)
Measurement of resistivity earth may be carried out if voltage of interferences
does not exceed 24 V. Voltage of interferences is measured up to the level of
100 V, but above 50 V it is signalled as dangerous. The meter must not be con-
nected to voltages exceeding 100 V.
- 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 resistance using 4-pole method and
perform calculations by yourself.
- 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 be burdened with addi-
tional uncertainty. Particularly high uncertainty of measurement occurs when a small value of re-
sistance-to-earth is measured with probes that have a weak contact with earth (such a situation oc-
curs frequently when the earth electrode is well made and the upper soil layer is dry and slightly con-
ductive). In such a case, the ratio of resistance of the probes to resistance of the tested earth elec-
trode is very high and consequently, uncertainty of measurement that depends on this ratio is also
very high. Then, you can make a calculations according to the formulas given in sec. 10.2 to estimate
the influence of measurement conditions or you can use the graph also included in the appendix. You
can improve the contact between the probe and soil, for example, by dampening with water the place
where the probe is driven into earth, driving the probe into earth in a different place or using a 80 cm-
long probe. Check also the test leads for possible insulation damage and for corroded or loosened
connection between the banana plug and the test lead. In majority of cases the achieved measure-
ment accuracy is satisfactory. However, you should always be aware of the uncertainty included in
the measurement.
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