LEM A188506510, Руководство по эксплуатации

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37
Apply current transformer to next pylon stub.
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 of all pylon foots are determined, the actual earth resistance RE
has to be calculated :
R
E
R R R R
E E E E
=
+ + +
1
1
1
1
2
1
3
1
4
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 8. "Description of displays")
after a test sequence, most likely it is caused by strong interference signals ( e.g.
unsteady noise voltage).
Nevertheless, to get reliable values, the instrument offers the possibility of averanging
over a longer period.
Select a fixed frequency (see 8.1.2 "Control loop")
Keep the "START" button pressed until the warning "measured value unstable"
disappears. Max. averaging time is approx. 1. min.
8.4.2.2 Measuring earthing impedance with 55Hz (R*)
For the calculation of short circuit currents in power supply plants, the complex
earthing impedance is important. Direct measurement is possible under the following
conditions:
Phase angle at 50 Hz:
30o... 60o inductive
auxiliary earth electrode (ohmic) : > 100 · ZE
Measuring process:
The measurement of the earthing impedance (R*) is only possible if it is activated by
putting in a personalized user's code (see 8.7 "change of setup data with
personalized code"). If this measuring function is activated, in every measurement of
the four RE positions, the earthing impedance R* is displayed before all other
measured values.