Circutor MD5060e Manual De Uso página 66

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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 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). The situation is changed if we connect the
transformer housing to GUARD terminal. Then the circuit will be:
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