Fluke 1625-2 User Manual - Page 36

Notices for the setting of earth spikes, Time average measurement, If the displayed R

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1625-2 Users Manual Notices for the setting of earth spikes: Before setting the earth spikes for probe and auxiliary earth electrodes make sure that the probe is set outside the potential gradient of earth electrode and auxiliary earth electrode. Such a condition is normally reached by allowing a distance of >20 m between the earth electrode and the earth spikes as well as to the earth spikes to each other. An accuracy test of the results is made with another measurement after repositioning of auxiliary earth electrode or probe. If the result is the same, the distance is sufficient. If the measured value changes, probe or auxiliary earth electrode must be repositioned until the measured value RE remains constant. Spike wires should not run too close. 1. Apply current transformer to next pylon stub. 2. Repeat measuring sequence. Current feeding point of measuring current (alligator clip) and the polarity of the split core current transformer has to be left unchanged. After values of REi for each pylon foot are determined, the actual earth resistance RE is calculated: RE = 1 1 11 1 RE1 + R E2 + RE3 + RE4 Note If the displayed RE value is negative despite correct orientation of the current transformer, a part of the measuring current is flowing upwards into the tower body. The earthing resistance, thus coming into effect, correctly calculates if the individual equivalent resistances (under observation of their polarity) are inserted into the equation above. Time average measurement: If there is a warning "MEASURED VALUE UNSTABLE" (see Table 5) after a test sequence, most likely it is caused by strong interference signals such as unsteady noise voltage. For reliable values, the instrument offers averaging over a longer period: 1. Select a fixed frequency (see Control Loop). 2. Keep the "START TEST" button pressed until the warning "MEASURED VALUE UNSTABLE" disappears. Maximum averaging time is approximately 1 minute. 28

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1625-2
Users Manual
28
Notices for the setting of earth spikes:
Before setting the earth spikes for probe and auxiliary earth electrodes make sure that
the probe is set outside the potential gradient of earth electrode and auxiliary earth
electrode. Such a condition is normally reached by allowing a distance of >20 m between
the earth electrode and the earth spikes as well as to the earth spikes to each other. An
accuracy test of the results is made with another measurement after repositioning of
auxiliary earth electrode or probe. If the result is the same, the distance is sufficient. If the
measured value changes, probe or auxiliary earth electrode must be repositioned until
the measured value R
E
remains constant. Spike wires should not run too close.
1.
Apply current transformer to next pylon stub.
2. Repeat measuring sequence.
Current feeding point of measuring current (alligator clip) and the polarity of the split
core current transformer has to be left unchanged.
After values of R
Ei
for each pylon foot are determined, the actual earth resistance R
E
is calculated:
4
3
2
1
1
1
1
1
1
E
E
E
E
E
R
R
R
R
R
+
+
+
=
Note
If the displayed R
E
value is negative despite correct orientation of the
current transformer, a part of the measuring current is flowing upwards into
the tower body. The earthing resistance, thus coming into effect, correctly
calculates if the individual equivalent resistances (under observation of their
polarity) are inserted into the equation above.
Time average measurement:
If there is a warning "MEASURED VALUE UNSTABLE" (see Table 5) after a test
sequence, most likely it is caused by strong interference signals such as unsteady noise
voltage.
For reliable values, the instrument offers averaging over a longer period:
1.
Select a fixed frequency (see
Control Loop
).
2.
Keep the "START TEST" button pressed until the warning "MEASURED VALUE
UNSTABLE" disappears. Maximum averaging time is approximately 1 minute.