Two related IEEE 43 condition-assessment checks on the same winding — a minimum-IR pass/fail and a polarization index that clears its minimum by a much narrower margin.
| Motor rated voltage | 6.6 kV |
| Winding type | Form-wound stator, built after 1970 |
| 1-minute IR reading | 150 MΩ |
| 10-minute IR reading | 330 MΩ |
| Test temperature | 40°C (reference temperature — no correction needed) |
| Insulation class | B/F/H |
IEEE 43-2013 sets the minimum 1-minute IR at 40°C by winding type: 100 MΩ for most ac windings built after about 1970 (form-wound coils), 5 MΩ for random-wound and sub-1 kV form-wound coils, and kV + 1 MΩ only for windings made before about 1970 and field windings.
The widely used rule of thumb is that IR roughly halves for every 10°C above 40°C, or doubles for every 10°C below — since this test was already run at exactly 40°C, no correction is applied.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 1-minute IR (corrected to 40°C) vs. IEEE 43 Table 3 | ≥ 100 MΩ | 150 MΩ | ✓ PASS |
| Polarization Index vs. Class B/F/H minimum | ≥ 2.0 | 2.20 | ✓ PASS |
Key insight: IR and PI test two different things and can disagree — a winding can pass a healthy absolute IR reading while its PI trend (how much IR rises over the 10-minute test as the insulation polarizes) is comparatively weak, or vice versa. PI specifically screens for moisture or contamination, which affects how the reading develops over time rather than its absolute level at any one instant, which is why both checks are run together rather than relying on either alone.
Every input in this example is editable in the live calculator — free, no signup.
Open Insulation Resistance Checker calculator →A PI close to 1.0 despite an adequate absolute IR value can indicate surface contamination, moisture absorption, or leakage current paths that don't fully 'polarize' the way clean, dry insulation does — the IR reading barely climbs from 1 minute to 10 minutes because the current flowing is dominated by a steady leakage/surface-conduction component rather than the capacitive charging and dielectric absorption current that should decay over the test.
Class A insulation (the oldest, least thermally robust classification) has a lower IEEE 43 minimum PI of 1.5, while the more common modern Class B/F/H insulation systems are held to a stricter 2.0 minimum — reflecting that better-quality modern insulation systems are expected to show a stronger polarization trend when healthy, so a weaker trend is a more meaningful red flag for them.