Insulation Resistance PI / DAR (Megger Test)

The ratios that tell whether a motor, generator, or transformer winding is clean and dry or wet and contaminated. Good insulation keeps absorbing charge, so its resistance keeps RISING over the test; wet or dirty insulation stops rising or falls. DAR = IR(60 s)/IR(30 s); PI = IR(10 min)/IR(1 min) (the 1-minute and 60-second readings are the same). By IEEE 43, PI below 1 is dangerous, 1-2 questionable, 2-4 good, above 4 excellent; DAR below 1.25 is marginal, 1.4+ good. An 800/1,040/4,160 Mohm test gives DAR 1.30, PI 4.0. A trend/screen, not a pass/fail: temperature-correct to a common base first, and on very-high-IR epoxy windings PI loses meaning per IEEE 43. The baseline trend and OEM criteria govern.

dar = IR(60 s) / IR(30 s); polarization_index = IR(10 min) / IR(1 min). IEEE 43 bands: PI < 1 dangerous, 1-2 questionable, 2-4 good, > 4 excellent; DAR < 1.25 marginal, 1.4+ good.

IEEE 43 (Recommended Practice for Testing Insulation Resistance of Electric Machinery) polarization index and dielectric absorption ratio, by name; the machine baseline trend and the OEM acceptance criteria govern.

The PI and DAR are public ratios of timed insulation-resistance readings; the interpretation bands are the IEEE 43 guidance, and the readings come from the megger test.

Estimate. AHJ and licensed professional govern.

Field names used by the API: ir_30s_mohm, ir_1min_mohm, ir_10min_mohm, dar, polarization_index

• PI and DAR ratios DAR = IR(60s)/IR(30s); PI = IR(10min)/IR(1min); rising IR = good insulation; IEEE 43 interpretation bandsIEEE 43
• Trend, not pass/fail temperature-correct to a common base (~halve IR per 10 C); PI loses meaning above ~5000 Mohm at 1 min; the baseline trend and OEM criteria governIEEE 43 practice