MEGABRAS MD-5060x Guia Del Usuario página 25

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 disre-
garded, Rx appears to be in parallel with (R1 + R2). The situation is changed if we con-
nect the transformer housing to GUARD terminal. Then the circuit will be:
Fig. 3
In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-value resistance (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 megohmmeter, it shall be con-
sidered that if R1 and R2 are higher than 100 MΩ, any value of Rx will be measured with
an insignificant error. For example: Let us consider Rx = 3.000 MΩ and R1 = R2 = 100
MΩ, the reading without using the GUARD terminal would be 187.5 MΩ, which is quite
wrong. On the other hand, if the GUARD terminal is properly used, we would have
3.000 MΩ.
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