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1625
Users Manual
Measuring Method
The current voltage measuring method is based on the block diagram circuit
shown in the figure below.
G
I
E
E
R
E
U
E
An AC generator G feeds current I via earth electrode E (earth electrode resis-
tance R
) and auxiliary earth electrode H (auxiliary earth electrode resistance
E
R
).
H
Voltage U
drops on earthing resistance R
E
age is picked up and measured by probe S. With the so called three wire cir-
cuit, the instrument sockets E and ES are connected to each other. In a four
wire circuit an separate cable is used to connect socket ES with the earth elec-
trode.
With that, the voltage drop of the cable between socket E and earth electrode is
not measured. As the voltage measuring circuit has such a high impedance, the
influence by the probe resistance R
Thus the earthing resistance evolves
U
Meas
R
E
1
and is independent from the resistance of the auxiliary earth electrode R
generator runs at a frequency between 70 and 140 Hz.
It has to hold a minimum distance of 5 Hz to one of the nominal frequencies
between 16 2/3, 50 or 60 Hz and their harmonic waves. A frequency selective
filter adjusted to the generator frequency is inserted.
78
U meas.
V
ES
S
S
S
R
S
(U
proportional to R
E
E
is neglectable within certain limits.
I
H
H
R
H
edw057.eps
.) This volt-
E
. The
H
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