Two windings with a similarly adequate 1-minute IR reading ā one develops a strong polarization trend over the 10-minute test and passes comfortably, the other barely rises and fails outright.
| Winding 1 ā 1-minute IR | 15 MĪ© |
| Winding 1 ā 10-minute IR | 33 MĪ© |
| Winding 2 ā 1-minute IR | 20 MĪ© |
| Winding 2 ā 10-minute IR | 25 MĪ© |
| Insulation class (both) | B/F/H (minimum PI = 2.0) |
Winding 2's 1-minute IR (20 MĪ©) is actually higher than Winding 1's ā on an absolute-IR-only view, it would look like the healthier winding.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Winding 1 PI vs. Class B/F/H minimum | ā„ 2.0 | 2.20 (Good) | ā PASS |
| Winding 2 PI vs. Class B/F/H minimum | ā„ 2.0 | 1.25 (Questionable) | ā FAIL |
Key insight: PI is deliberately a trend measurement, not an absolute one ā it exists precisely to catch windings whose absolute IR reading looks acceptable at any single instant but which fail to show the rising-resistance trend of healthy, dry insulation as dielectric absorption current decays over the test. A winding can have a perfectly adequate 1-minute IR and still fail PI, which is why IEEE 43 recommends running both checks rather than treating a single spot IR reading as sufficient on its own.
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Open Polarization Index (PI) Calculator calculator āNo ā IEEE 43 specifically defines PI using readings at 1 minute and 10 minutes because the dielectric absorption current that PI is designed to characterize decays over a timescale of minutes, not seconds; a shorter comparison window wouldn't capture enough of that decay curve to distinguish a healthy trend from a flat, contamination-affected one, which is why the standard's timing can't be substituted with a shorter test.
A PI failure with an adequate absolute IR reading typically points toward surface contamination or moisture rather than bulk insulation breakdown, so the usual next steps are cleaning and drying the winding (if accessible) and re-testing, or scheduling more frequent monitoring to watch the trend over time ā it's a flag for closer investigation rather than necessarily an immediate failure requiring replacement, though the appropriate response depends on the equipment's criticality and the technician's overall assessment.