Circutor MI-15KVe Guia Del Usuario página 29

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 of the microammeter) therefore its influence is minimized 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 considered that if R1 and
R2 are higher than 100 MΩ, any value of Rx will be measured with an error lower
than 10%. For example: Let us consider Rx = 3.000 MΩ and R1 = R2 = 100 MΩ,
R R R
1 x 2
A B
Fig. 2
Fig. 3
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