MEGABRAS MD-5060x Guia Del Usuario página 28

A typical example of this situation is when the insulation resistance between pri-
mary 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 megohmmeter (terminals Vt 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 cir-
cuit will be:
In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-value resis-
tance (the one from the microammeter) therefore its influence is reduced during
reading.
Through resistance R2 circulates a current which is not passing through the meter
and consequently does not affect the reading. In fact, current through R2 originates
a certain error, since it creates an additional voltage drop in R1 which was not
regarded during megohmmeter calibration. As regards the practical use of me-
gohmmeter, it shall be considered that if R1 and R2 are higher than 100 MΩ, any
R R R
1 x 2
A B
Fig. 2
Fig. 3
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