Circutor MI-20KVe Guia Del Usuario página 23

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 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 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, 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, with an error lower than 10%.
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