1625
Users Manual
80
The Influence of Potential Gradient Areas on Earth Resis-
tance Measurement
To pick off the true voltage drop from the earthing resistance (= the resistance
between the earth electrode and the soil potential F
E
) it is to be assured that the
probe is set outside the potential gradient area of all connected earth electrodes
and the auxiliary earth electrode H.
I1
E1
U
S1
S1
U
S1
I2
E2
U
S2
U
S2
S2
I
U
G
G
U
S2
S3
F
E
U
S3
U
G
U
S4
U
S4
S4
I
H
V
V
V
V
edw059.eps
A probe positioned inside a potential gradient area leads to incorrect measuring
results. As it can be seen in the picture above, probe voltages U
S1
and U
S2
of
probes S
1
and S
2
deliver a value too low, which also means that the earthing
resistance appears to be lower than it actually is(low resistance). Probe S
4
with
U
S4
on the other hand, picks up a value too high which indicates a worse (high
resistance) earthing conditions.
Only probe S
3
picks up the correct voltage between earth electrode and soil
potential F
E
.
For that reason it is advisable to repeat each measurement with repositioned
probes and only to regard a measurement as successful and accurate if several
subsequent measurements result in the same values.
Normally, a distance of 20 m to the earth electrode and to the probes to each
other is sufficient.
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