What is the Polarization index value (PI value)?

What is the Polarization index ?

The Polarization Index (PI) is a measurement of an electrical apparatus’s insulating resistance, generally a motor, generator, or transformer. It is defined as the ratio of insulation resistance after 10 minutes of applying a DC voltage (often 1000 V) to insulation resistance after 1 minute of applying the same voltage.

PI is a measurement that may be used to evaluate if a motor or generator is appropriate for its intended purpose. The index is obtained through the process of computing a measurement of the winding’s (electrical) insulation resistance. The polarisation index provides information on the accumulation of dirt or moisture, the deterioration of insulation, and whether or not the motor or generator is suitable for use. An important preventative strategy for ensuring the safety of electrical equipment is to check its Polarization Index.

What is the Polarization index value (PI value)?

Polarization index value is the ratio of insulation resistance (IR) for 0min to insulation resistance for min. It varies from 5-7 for new motors and should be more than.5 for a motor in excellent condition.

The applied voltage should be maintained constant for 10 minutes. At one minute, an initial measurement of insulating resistance is obtained, followed by a second reading after ten minutes.

The resistance is measured sequentially, and further measurements beyond 10 minutes are not required. The Polarization Index value is calculated as the ratio of the 10-minute and one-minute observations.


The concept that impurities in a winding operate as charge carriers and induce current leakage is the fundamental premise upon which the Polarization Index test is derived. This equation the test’s ability to function. When the insulation is put to the test, current may be seen leaking out. These contaminants have the potential to become more polarised over time. The index gives an indicator of the number of contaminants that are present in the winding as well as the level of cleanliness that it has. While there is no correlation between the index & temperature, there are several restrictions that must be considered when trying to validate the index in conditions when the temperature is very high.


PI Value = Insulation resistance (after 10 minutes) / Insulation resistance (after 1 minute)

The PI value indicates the insulation’s quality and condition. A higher PI value suggests more insulation resistance and better insulation quality. A PI value of 2.0 (or) greater is typically regarded excellent, but less than 1.0 may signal that further inquiry or maintenance. In the electrical sector, the PI test is often used to examine the integrity of electrical equipment & identify any insulation deterioration or contamination.