Camille Bauer GOSSEN METRAWATT GEOHM C, Operating Instructions Manual

Page: 57 / 236

GMC-I Messtechnik GmbH 21
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Measured resistance values are subsequently displayed as a table,
and then as a graphic representation (curve I in Figure 6 on page 21).
If a parallel line is drawn through inflection point S1 to the abscissa, the
resistance curve is divided into two parts by this line.
Measured from the ordinate, the bottom portion results in the sought after
dissemination resistance of the earth electrode R
A/E
. The upper value
represents dissemination resistance of the auxiliary earth electrode R
A/H
.
The dissemination resistance of the auxiliary earth electrode should be
less than 100 times the dissemination resistance of the earth electrode
with a test setup of this type.
If the characteristic resistance curve does not include a well defined hori-
zontal section, measurement should also be performed after changing the
position of the auxiliary earth electrode. The new characteristic resistance
curve must then be entered to the first diagram and the abscissa scale
must be changed such that both auxiliary earth electrode positions are
identical. The originally determined dissemination resistance value can be
double-checked with deflection point S2 (see also Figure 6 on page 21).
Instructions for Measurements on Unfavorable Terrain
In the event of extremely unfavorable terrain (e.g. sandy ground after a
lengthy dry period), auxiliary earth electrode and probe resistance values
can be reduced to allowable values by watering the ground around the
auxiliary earth electrode and the probe.
If this watering is not sufficient, several earth spikes can be connected to
the auxiliary earth electrode in parallel.
In mountainous terrain or if the ground is quite rocky and spikes cannot be
driven into the earth, a wire mesh with a mesh size of 1 cm and a surface
area of approximately 2 square meters can be used. The wire mesh must
be laid flat onto the ground and doused with soda water or saltwater, and
may also be weighted down with sacks full of moist soil.
Figure 6 Measuring Earthing Resistance at a
Large Earthing System
Curve I (CI) Curve II (CII)
m
Ω
m
Ω
5
10
15
20
25
30
40
60
80
100
0.9
1.28
1.62
1.82
1.99
2.12
2.36
2.84
3.68
200
10
20
40
60
80
100
120
140
160
200
0.8
0.98
1.60
1.82
2.00
2.05
2.13
2.44
2.80
100
S1, S2 = Inflection point
CI
= Curve I
CII = Curve II
S1, S2 = Inflection point
CI
= Curve I
CII = Curve II
S
1
S
2
CI
CII
Ω
4
3
2
1
0
10 20 30 40 50 60 70 80 90 100 m CI
20 40 60 80 100 120 140 160 180 200 m CII
5
R
A/H
R
A/E
0
0
S H ES E