MEGABRAS MD-1035e Guia Del Usuario página 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:
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 100 MΩ,
any value of Rx will be measured with an insignificant error. For example: Let us
consider Rx = 3000 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 3000 MΩ.
Ri
A
i
Vt
26
-V
Rx
+R
R1
R2
Guard