Circutor MI20KVe Manual De Uso página 37

A typical example of this situation is when the insulation resistance
between primary and secondary windings of a transformer mounted in-
side 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 termin-
als 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.
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