25
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 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 considered that if R1 and R2 are higher than 100M
Ω
, any value of Rx will be
measured with an insignificant error. For example: Let us consider Rx = 3.000M
Ω
and R1 = R2 = 100M
Ω
, the reading without using the GUARD terminal would be
187.5M
Ω
, which is quite wrong. On the other hand, if the GUARD terminal is properly
used, we would have 3.000M
Ω
.
