In order to measure the ground resistivity to predetermine the ground dispersion resistance or grounding impedance, the “Four Probe Method” (e.g. Wenner Method), which enables determination of this resistance for various depths, must be used.
Regulation Annex-N 2.2.1 can be referred to for the distances between probes. Special devices have been designed to implement this method.
Measurement Principle:
A voltage with a frequency of up to 150Hz is applied between the external electrodes. The voltage between the internal probes is measured. Special devices provide the resistance value directly. The frequency of the voltage to be applied must be varied to eliminate the effect of other currents in the ground at the measurement location.
a > 20 m. e a / 20 and E = 2..a.R. R (ohm) measured resistance; E (ohm.m) is in a and e (m).
Ground Resistivity:
Ground type Ground resistivity rE (ohm.m)
Swamp 5-40
Mud, clay, humus 20-200
Sand 200-2500
Gravel 2000-3000
Stone decomposed due to the effect of air, mostly <1000
Sandstone 2000-3000
Granite >50000
Moraine >30000
Cement (pure) 50
1xCement+3xSand 50-300
The differing resistivity of layers at various depths affects the resistivity of the ground.