It is a ratio of the Insulation Resistance Measured for 10 minutes to the insulation resistance value measured after 1 minute. Since it is a ratio; it does not have any units.
The Polarization Index should be above 2.0 to be permissible. Machines having PI below 2.0 cannot be operated.
The Polarization Index test works on the principle that impurities in a winding act as charge carriers and are responsible for the leakage current which flows when the insulation is tested. These impurities can be polarized over a period of time. By measuring the rate of polarization, we can determine the amount of impurities in the winding and the cleanliness of the winding.
The Polarization Index does not have any significant relation with temperature upto 50 deg. C. However, the Polarization Index test should not be conducted at a temperature beyond 50 deg. C