Insulation Resistance PI / DAR (Megger Test)
The polarization index (PI) and dielectric absorption ratio (DAR) from a timed insulation-resistance (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.
Formula and source
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.
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