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Earth/Ground Tester
Advanced Operation
33
4. Read out measured value R
E
.
From the indicated resistance value R
E
, the soil resistivity calculates according to the
equation:
EE
Ra..2
π
ρ
=
ρ
E
...... mean value of soil resistivity (Ωm)
R
E
...... measured resistance (Ω)
a ...... probe distance (m)
The measuring method according to Wenner determines the soil resistivity down to a
depth of approximately the distance "a" between two earth spikes. By increasing "a",
deeper strata can be measured and checked for homogeneity. By changing "a" several
times, a profile can be measured from which a suitable earth electrode can be
determined.
According to the depth to be measured, "a" is selected between 2 m and 30 m. This
procedure results in curves depicted in the graph below.
Distances of Probe
Soil restivity ρE
A
2
1
3
edw021.eps
Curve 1: As ρE decreases only deeper down, a deep earth electrode is advisable.
Curve 2: As ρE decreases only down to point A, an increase in the depth deeper
than A does not improve the values.
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