23
PM5A2
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 connecting the transformer housing to GUARD terminal. Then the circuit will be
:
In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-value resistance (the one from the micro-
ammeter), 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, because it creates an additional voltage
drop in R1 which was not regarded during equipment calibration. As regards the practical use of instrument, if R1
and R2 are higher than 100 M
Ω, any value of Rx will be measured with an insignificant error. For example: Let us
consider Rx = 3000M
Ω and R1 = R2 = 100 MΩ, the reading without using the GUARD terminal would be 187.5MΩ,
which is quite wrong. On the other hand, if the GUARD terminal is properly used, we would have 3000M
Ω.
