A single insulation reading can deceive you: damp insulation may give an instantaneously acceptable number. The Polarization Index (PI) adds the dimension of time to the test, distinguishing sound dry insulation from deteriorated damp insulation through a simple ratio between two readings.
How Is PI Calculated?
The megger voltage is applied continuously for ten minutes, and the resistance reading is recorded at the first minute and at the tenth minute; the ratio between them is the polarization index.
Why Does Resistance Rise Over Time in Good Insulation?
When voltage is applied, charging and polarization currents flow through the insulation and gradually fade away, leaving only the leakage current over time. In dry insulation, the leakage is minimal, so the reading rises noticeably over time (high PI). In damp or contaminated insulation, a large constant leakage current dominates, so the reading remains nearly constant (low PI).
Common Guideline Interpretation
| PI Value | Guideline Significance |
|---|---|
| Below 1.0 | Dangerous — deteriorated or very damp insulation |
| 1.0 – 1.5 | Poor, usually needs investigation and drying |
| 1.5 – 2.0 | Marginally acceptable depending on the specification |
| 2.0 and above | Good — usually sound, dry insulation |
These remain guideline values; the manufacturer's specification and the adopted standard (such as IEEE) are the reference, and some very modern insulation systems give a low PI despite being sound, because the polarization current is inherently low when the absolute resistance is already very high.
Sample answer: PI is the ratio of the insulation resistance measured after 10 minutes to that measured after 1 minute of continuously applying the megger voltage. In dry insulation, the reading rises over time, giving a PI of around 2 or higher, while a low value (close to 1) indicates the dominance of a constant leakage current caused by moisture, contamination, or insulation deterioration, requiring drying or investigation before operation.
Applying PI limits blindly to any insulation. If the absolute resistance is already very high (thousands of megohms), the PI may naturally be low, and conversely, a good PI with a low absolute resistance doesn't guarantee soundness. Read both numbers together.
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