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Earth/Ground Tester
Procedure of Measurements
39
reliable values, the instrument offers the possibility of averaging over a longer
period.
1. Select a fixed frequency (see "Control loop", Operation)
2. Keep the "START TEST" button pressed until the warning "measured
value unstable" disappears. Max. averaging time is approx. 1 min.
Measurements on High Voltage Pylons
Measuring the Earthing Resistance without Disengaging
the Overhead Earth Wire Using the Selective Clamp
Method
The measurement of the earth resistance of a single high voltage pylon usually
requires the overhead earth wire to be disengaged (lifted off) or the seperation
of the earthing system from the pylon construction. Otherwise false reading of
the resistance of the pylon earth electrode are liable to occur because of the
parallel circuit of the other pylons connected to each other by an overhead
earth wire.
The new measuring method employed in this instrument - with its external
current transformer to measure the true current flowing through the earth elec-
trode - allows measurements of earth electrode resistances without disconnec-
tion of the earthing system or disengaging the overhead earth wire.
As all four pylon stubs are connected to the foundation earth of the pylon, the
measuring current I
meas
is divided into five components according to the pre-
sent resistances involved.
One part flows via pylon construction to the overhead earth wire and further to
the parallelly circuited pylon earthing resistances.
The other four current components (I
1
... I
4
) flow via the individual pylon foots.
The addition of all currents result in a current I
E
going through the earthing
resistance, i.e. the resistance of the "composite"earth electrode to the soil.
If the current transformer is fixed to each pylon stub, one after the other, four
resistances have to be measured which show a behaviour inversely propor-
tional to the corresponding current components I
1
... I
4
. The feeding point of
the measuring current is to be left unchanged to avoid a change in the current
distribution.