Gossen MetraWatt GEOHM C Mode D'emploi page 56

If the same measured value results from all 3 measurements, the correct
earthing resistance value has been obtained and the probe is in the
ground reference plane.
However, if these 3 measured values differ from one another, either the
probe is not in the ground reference plane, or the characteristic resistance
curve is not horizontal at the point at which the probe has been driven into
the ground.
I
a
E
Φ
U
E
K
a = Distance from earth electrode to auxiliary earth
electrode
E = Earth electrode
H = Auxiliary earth electrode
I = Measuring current
K = Ground reference plane
U = Earth potential
E
R = U / I = Earthing resistance
E E
Φ = Potential
Figure 4 Characteristic Voltage Curve in
Homogenous Soil between Earth
Electrode E and Auxiliary Earth Electrode H
20
I
H
E H
E = Earth electrode
S
H = Auxiliary earth electrode
S = Probe
Figure 5 Probe S lies outside of the overlapping potential gradients
on a vertical line between earth electrode E and auxiliary earth
electrode H.
Dissipation Resistance of Earth Electrodes with Large Spread
Significantly greater distances to the probe and the auxiliary earth
electrode are required for measurements performed at larger earthing
systems. Distances of 2.5 and 5 times the largest diagonal included in the
earthing system are used.
It is especially important to position the probe within the ground reference
plane for measurements at large earthing systems, because they fre-
quently demonstrate dissipation resistances of only a few ohms or less.
The probe and the auxiliary earth electrode should be positioned at a right
angle to the largest longitudinal extension of the earthing system. Dissipa-
tion resistance must be kept to a minimum, and several earth spikes
should be used if necessary and connected to each other (at a distance of
1 to 2 meters from one another).
However, larger distances to the probe are frequently impossible in
practice due to physical obstacles.
If this is the case, proceed as shown in Figure 6 on page 21.
Ð
Auxiliary earth electrode H is positioned as far as possible from the
earthing system.
Ð
The area between the earth electrode and the auxiliary earth electrode
is measured at equidistant points (in steps of approx. 5 meters).
In such cases, correct measure-
ment results can be obtained
either by increasing the distance
between the earth electrode and
the auxiliary earth electrode, or by
moving the probe along the vertical
line between the earth electrode
and the auxiliary earth electrode
(see also Figure 5 on page 20).
Moving the probe along this verti-
cal line removes it from the zones
of influence of the earth electrode
and the auxiliary earth electrode.
GMC-I Messtechnik GmbH