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Gossen Metrawatt PROFITEST MXTRA Manual

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GMC-I Messtechnik GmbH 39
➭ Position the probe halfway between the earth electrode and
the auxiliary earth electrode and determine earthing resis-
tance.
➭ Reposition the probe 2 to 3 meters closer to the earth elec-
trode, and then 2 to 3 meters closer to the auxiliary earth elec-
trode and measure earthing resistance in each position.
If all 3 measurements result in the same measured value, this is
the correct earthing resistance. The probe is in the neutral zone.
However, if the three measured values for earthing resistance dif-
fer from each other, either the probe is not located in the neutral
zone, or the voltage or resistance curve is not horizontal at the
point at which the probe has been inserted.
Figure 10.8.2: Voltage Curve in Homogenous Earth between Earth
Electrode E and Auxiliary Earth Electrode H
Correct measurements can be obtained in such cases by either
increasing distance between the earth electrode and the auxiliary
earth electrode, or by moving the probe to the perpendicular
bisector between the earth electrode and the auxiliary earth elec-
trode (see also Figure 10.8.3:). When the probe is moved to the
perpendicular bisector, its location is removed from the sphere of
influence of the two potential gradient areas caused by the earth
electrode and the auxiliary earth electrode.
Figure 10.8.3: Probe Distance S Outside of the Overlapping Potential
Gradient Areas on the Perpendicular Bisector of Earth
Electrode E and Auxiliary Earth Electrode H
Dissipation Resistance of Large Scope Earthing Systems
Significantly large distances to the probe and the auxiliary earth
electrode are required for measuring large scope earthing sys-
tems. Calculations are based on 2½ or 5 times the value of the
earthing system’s largest diagonal.
Large scope earthing systems of this sort often demonstrate dis-
sipation resistances of only a few ohms, which makes it especially
important to position the measuring probe within the neutral zone.
The probe and the auxiliary earth electrode should be positioned
at a right angle to the direction of the earthing system’s largest lin-
ear expansion. Dissipation resistance must be kept small. If nec-
essary, several earth spikes must be used at a distance of 1 to
2 m from each other and connected to this end.
However, in actual practice large measuring distances are fre-
quently not possible to due difficult terrain. If this is the case, pro-
ceed as shown in Figure 10.8.4:.
➭ Auxiliary earth electrode H is positioned as far from possible
from the earthing system.
➭ The area between the earth electrode and the auxiliary earth
electrode is sampled in equal steps of 5 meters each.
➭ Measured resistance values are displayed as a table, and then
plotted graphically as depicted in Figure 10.8.4: (curve I).
If a line parallel to the abscissa is drawn through inflection point
S1, this line divides the resistance curve into two parts.
Measured at the ordinate, the bottom part results in sought dissi-
pation resistance of the earth electrode R
A/E
, and the top value
equals dissipation resistance of the auxiliary earth electrode R
A/H
.
With a measurement setup of this type, dissipation resistance of
the auxiliary earth electrode should be less than 100 times the
dissipation resistance of the earth electrode.
In the case of resistance curves without a well defined horizontal
area, measurement should be double checked after repositioning
the auxiliary earth electrode. This additional resistance curve must
be entered to the first diagram with a modified abscissa scale
such that the two auxiliary earth electrode locations are superim-
posed. The initially ascertained dissipation resistance value can
be checked with inflection point S2 (see Figure 10.8.4:).
Notes Regarding Measurement in Difficult Terrain
In extremely unfavorable terrain (e.g. sandy soil after a lengthy
period without rain), auxiliary earth electrode and probe resistance
can be reduced to permissible values by watering the ground
around the auxiliary earth electrode and the probe with soda
water or salt water. If this does not suffice, several earth spikes
can be parallel connected to the auxiliary earth electrode.
In mountainous terrain or in the case of very rocky subsoil where
earth spikes cannot be driven into the ground, wire grates with a
mesh size of 1 cm and a surface area of about 2 square meters
can be used. These grates are laid flat onto the ground, are wet-
ted with soda water or salt water and may also be weighted down
with sacks full of moist earth.
Figure 10.8.4: Earthing Resistance Measurement for a Large Scope
Earthing System
d = distance, electrode to aux. electrode
E = earth electrode
H = auxiliary earth electrode
I = measuring current
K = neutral zone (reference earth)
U
E
= earth potential
R
E
= U
E
/ I = earthing resistance
= potential
I
I
d
E
H
U
E
K
E = electrode location
H = aux. electrode loc.
S = probe location
S
HE
Curve I (KI) Curve II (KII)
mWmW
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 points
KI = curve I
KII = curve II
S1, S2 = inflection points
KI = curve I
KII = curve II
S
1
S
2
K I
K II
4
3
2
1
0
10 20 30 40 50 60 70 80 90 100 m KI
20 40 60 80 100 120 140 160 180 200 m KII
5
R
A/H
R
A/E
0
0
S HESE
… 39 …


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