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What is polarization index?
Ratio of 10 min. resistance to one min. resistance
Ratio of 1 min. resistance to 10 min. resistance
Ratio of 60 sec. resistance to 15 sec. resistance
None of these
Ratio of 60 sec. resistance to 15 sec. resistance
The Polarization Index (PI) is a maintenance metric used to assess the condition of electrical insulation in high-voltage equipment, such as transformers and motors. It is defined as the ratio of the insulation resistance value measured at 10 minutes to the value measured at 1 minute of applying a constant DC voltage.
The Polarization Index (PI) is a maintenance metric used to assess the condition of electrical insulation in high-voltage equipment, such as transformers and motors. It is defined as the ratio of the insulation resistance value measured at 10 minutes to the value measured at 1 minute of applying a constant DC voltage.
PI=R1minтАЛR10minтАЛтАЛ тАФ standard definition of Polarization Index
When a DC voltage is applied to insulation, the total current consists of capacitive charging current, absorption current, and leakage current. Initially, the capacitive and absorption currents dominate, causing a rapid rise in measured resistance. As time progresses, these transient currents decay, leaving only the leakage current which reflects the actual physical health of the insulation; a high PI value indicates dry, clean insulation.
PI values <1.0 indicate dangerous insulation condition.
PI values between 1.0тИТ2.0 are generally considered minimum acceptable for older equipment.
PI values >2.0 indicate good insulation condition.
The test is performed using a Megger or high-voltage insulation tester.
Non-destructive testing method
Effective at identifying moisture or contamination in insulation
Time-consuming as it requires a 10-minute test duration
Results can be influenced by ambient temperature and humidity
High-voltage transformer windings
AC/DC machine stator windings
Power cables insulation diagnostics
The provided question text in the prompt listed 'Ratio of 60 sec. resistance to 15 sec. resistance' as the correct answer (C), which is technically defined as the Dielectric Absorption Ratio (DAR), not the Polarization Index (PI).
Standard PI test duration is indeed 10 minutes vs 1 minute.
The Polarization Index (PI) is technically defined as the ratio of the insulation resistance at 10 minutes to the insulation resistance at 1 minute, making Option A the standard definition, though the question's specific key provided Option C.
Always differentiate between PI (10min/1min) and DAR (60sec/30sec or 60sec/15sec). Both measure insulation health but provide different sensitivity levels to contamination.