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A typical example of this situation is when the insulation resistance
between primary and secondary windings of a transformer mounted
inside a metal housing is to be measured.
Rx
: Insulation resistance between primary and secondary winding.
R1
: Insulation resistance between primary winding and housing.
R2
: Insulation resistance between secondary winding and housing.
If insulation tester (terminals
V
and
R
) is connected to transformer
terminals A and B, and considering that the resistance of the coils on
each side of the transformer may be disregarded, Rx appears to be in
parallel with (R1 + R2). The situation is changed if we connect the
transformer housing to GUARD terminal. Then the circuit will be:
In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-
value resistance (the one from the micro-ammeter) therefore its influence
is reduced during reading.