Circutor MD 5060e Manual De Uso página 69

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, since it creates an additional voltage
drop in R1 which was not regarded during equipment calibration. As
regards the practical use of the insulation tester, 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.
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