Polarization Index
Polarization Index is the ratio of insulation resistance measured after ten minutes to the resistance measured after one minute during a timed insulation test.
What this term means in maintenance
Polarization Index is the ratio of insulation resistance measured after ten minutes to the resistance measured after one minute during a timed insulation test.
Formula
PI = 10-minute insulation resistance ÷ 1-minute insulation resistance
Practical example
A winding measures 500 megohms at one minute and 1,000 megohms at ten minutes.
PI = 2.0
What PI can indicate
The result may provide information about insulation cleanliness, moisture, contamination, and condition.
Interpretation
Acceptance depends on equipment type, insulation system, temperature, history, and applicable guidance.
Limitations
Some modern insulation systems may produce different behavior, and one result should not be interpreted without context.
Common mistake
Using one universal PI limit for every machine and insulation type can be misleading.
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Glossary FAQs
- What does PI stand for?
PI stands for Polarization Index.
- How is PI calculated?
Ten-minute insulation resistance divided by one-minute insulation resistance.
- Can one PI limit apply to every machine?
No. Equipment type, insulation system, temperature, history, and guidance matter.