In general, insulators that are in good condition will show a “high” polarization index
while insulators that are damaged will not. IEEE 43 recommends minimum acceptable
values for the various thermal classes of motor insulation:
NEMA Class A 1.5
NEMA Class B 2.0
NEMA Class F 2.0
NEMA Class H 2.0
The Baker tester will automatically calculate the PI value at the end of a 10-minute HiPot
test. At the test’s conclusion, the PI value may be stored in one of the 10 memory
locations in the Baker tester for later recall.
Note: Some insulating materials developed in recent years for wire insulation do not
readily polarize. For example the newer inverter grade wires do not significantly polarize.
As recommended in IEEE 43, if the one minute insulation resistance is greater than
5000Mohms, the PI measurement may not be meaningful. In these situations the leakage
current is often very low – almost zero. Such low leakage currents are difficult to
accurately measure and as a result, instrument errors become very evident. However, the
operator must use judgement before declaring the PI test to be meaningless. The
indication of damaged insulation based on the PI test can be a very low leakage current
and a low PI value.