R
o
R
x
C.T.
1:n
Oscillator
Synchronous
rectifier
Figure 2 Measurement Principle (2-pole method)
(2) Simplified measurement (2-pole method)
Figure 2 shows the basic circuit principle for a
simplified earthing resistance measurement
using an existing earthing body.
If the earthing resistance of the existing
earthing body is taken as Ro and the earthing
resistance of the measurement object as Rx, the
same equation as for the 3-pole method
applies:
R
x+
R
o=
R
s/n
Therefore, the earthing resistance can be found
by adding the earthing resistance of the
existing earthing body (Ro) to the earthing
resistance of the measurement object (Rx).
The 3151 uses a very low measurement
current, so that the leakage current circuit
breaker of a household power supply will not
be tripped when the grounded side of an AC
outlet is used as existing earthing body.
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