37
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 insulation tester calibration.
As regards the practical use of insulation tester, it shall be considered that
if R1 and R2 are higher than 100 M
Ω
, any value of Rx will be measured
with an error lower than 10%. For example: Let us consider Rx =
3,000 M
Ω
and R1 = R2 = 100 M
Ω
, the reading without using the GUARD
terminal would be 187.5 M
Ω
, which is quite wrong. On the other hand, if
the GUARD terminal is properly used, we would have 3,000 M
Ω
, with an
error lower than 10%.