**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 = R _{10} /R*

_{1 }where

R_{10} and_{ }R_{1} is the resistance in megohms at 10 minutes and 1 minute, respectively.

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

*PI = I _{1}/I_{10}* 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 (*IR** _{1}*) is higher than 5000 MΩ, based on the magnitude of applied direct voltage, the total measured current (I

_{T}) 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.

**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:

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 *no**t** *be subjected to high-voltage testing.