What are PI (polarization index) and DAR (dielectric absorption ratio) values? (High Voltage Insulation Tester)

A

Used to diagnose the degree of progress of insulation deterioration, e.g. in cables and motors.
PI/DAR is one of the criteria used to judge whether insulation is good (PASS) or bad (FAIL).
The display is automatically calculated from the insulation resistance value after two specified times after the test voltage is applied, and if the PI or DAR is close to 1, it is judged that the insulation of the object under test has deteriorated.

The formula is as follows:
- PI (Polarization Index) 10 min/1 min
= Resistance 10 min. after applying voltage / Resistance 1 min. after applying voltage
- DAR (Dielectric Absorption Ratio) 1 min/30 s
= Resistance 1 min. after applying voltage / Resistance 30 sec. after applying voltage
- DAR (Dielectric Absorption Ratio) 1 min/15 s
= Resistance 1 min. after applying voltage / Resistance 15 sec. after applying voltage

Please also refer to the following example of judgment criteria.
- Example of PI (Polarization Index) Judgment Criteria
"IEEE43-2000 Recommended Practice for Testing Insulation Resistance of Rotating Machinery" recommends the criteria as shown in the table below for insulation resistance testing of a motor.

Heat resistance class	Recommended Minimum PI value
Class A	1.5 or more
Class B	2.0 or more
Class F	2.0 or more
Class H	2.0 or more