PI, Polarization Index Test

PI is a ratio of the megohms after 10 minutes divided by the megohms after 1 minute. This test is mainly used on form wound motors and generators. Megohm data is recorded over the 10 minutes, and the resulting graph and PI ratio can provide additional information about the winding insulation beyond the megohm number itself.

The PI test, like any resistance measurement, is the measurement of voltage and currents, and in reality PI is the ratio of leakage currents assuming constant voltage:

PI = R10 /Rwhere

R10 and  R1 is the resistance in megohms at 10 minutes and 1 minute, respectively.

R = V/I  so the PI formula reduces to:

PI = I1/I10  the current at 1 min divided by the current at 10 min.

Because the changes in currents may be very small, the IEEE 43-2013 states:

“When the insulation resistance reading obtained after the voltage has been applied for 1 min (IR1) is higher than 5000 MΩ, based on the magnitude of applied direct voltage, the total measured current (IT) can be in the sub microampere range. At this level of required test instrument sensitivity, small changes in the supply voltage, ambient humidity, test connections, and other non-related components can greatly affect the total current measured during the 1 min –10 min interval required for a PI test. Because of these phenomena, when the IR is higher than 5000 MΩ, the PI may or may not be an indication of the insulation condition and is therefore not recommended as an assessment tool.”

Note that for machines with no or little absorption current where the total leakage current stabilizes within 1 minute, PI values will be close to or equal to 1, and PI should not be used as an evaluation tool. This is often the case for random wound rotating equipment.

The table below shows minimum IEEE 43 PI ratings.

Thermal Class Rating

DA or DAR test

Dielectric Absorption Ratio is the ratio of the megohms at 1 minute divided by the megohms at 30 seconds.  It is typically used when the leakage current measured stabilizes within 1 minute. If this happens, the 10 minutes PI test is useless, the ratio becomes 1, and the DAR should be used instead if a ratio test is required or useful.

Values for DAR and PI commonly used in the literature and by manufacturers of test equipment for assessing the insulation conditions are:

Insulation Condition as a function of DAR and PI

With DAR and PI, it is not necessary to make a temperature correction since they both are ratios. 

It is recommended that machines with low IR readings not be subjected to high-voltage testing.