Polarization Index, or PI, is a ratio of the megohms after 10 minutes divided by the megohms after 1 minute. This test is primarily conducted on form wound motors and generators. An instrument records megohm data 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. PI is the ratio of leakage currents assuming constant voltage:
PI = R10 /R1 where
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 minute (IR1) is higher than 5000 MΩ, based on the magnitude of applied direct voltage, the total measured current (IT) can be in the sub micro-ampere 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.”
In summary, for motors with no or little absorption current where the total leakage current stabilizes within 1 minute, PI values are close to or equal to 1. In this case, PI is not a proper evaluation tool. This is often the case for random wound rotating equipment.
The table below shows minimum IEEE 43-2013 PI ratings.
The DA Test, or Dielectric Absorption Ratio (DAR), is the ratio of the megohms at 1 minute divided by the megohms at 30 seconds. When the measured leakage current stabilizes within 1 minute, operators typically use the DA Test. If this happens, the 10 minute PI test is useless because the ratio is 1.
Values for DAR and PI commonly used in the literature and by manufacturers of test equipment for assessing the insulation conditions are:
It is not necessary to make a temperature correction since both DAR and PI are ratios. It is recommended that motors with low insulation resistance readings not be subjected to high-voltage testing.