TENTECH MD1035x Manual De Uso página 27

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 the 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). For example: Let us consider Rx = 3000 MΩ and
R1 = R2 = 100 MΩ, the reading without using the GUARD terminal would
be 187.5 MΩ, which is quite wrong. The situation is changed if we
connect the transformer housing to GUARD terminal. Then the circuit will
be:
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