A single insulation resistance reading — apply a DC voltage, read the megohms — tells you the insulation isn't currently shorted. It tells you very little about whether that insulation is contaminated, damp, or aging toward failure, because raw insulation resistance varies with temperature, humidity, and test duration. Polarization Index (PI) and Dielectric Absorption Ratio (DAR) exist specifically to solve this: both are ratio measurements that cancel out much of that variability.
Why a Single IR Reading Falls Short
When DC voltage is first applied, current flowing through insulation isn't constant — it starts relatively high and decays over time as internal capacitance charges and polarization effects settle. Good insulation shows resistance climbing steadily; insulation that's contaminated or damp reaches a stable (low) reading quickly and shows little further change. PI and DAR exploit exactly this difference in shape, not just magnitude.
Dielectric Absorption Ratio (DAR)
DAR is the ratio of insulation resistance at 60 seconds to resistance at 30 seconds: DAR = R(60s) / R(30s). A DAR value above roughly 1.2 generally indicates healthy insulation; values approaching 1.0 suggest contamination or moisture.
Polarization Index (PI)
PI extends the same logic over a longer window: PI = R(10min) / R(1min). This makes PI more sensitive for larger insulation systems — motor and generator windings, large transformers. A PI value above 2.0 is generally considered acceptable per IEEE 43.
| PI value (approximate, per IEEE 43) | Interpretation |
|---|---|
| Below 1.0 | Dangerous — do not energize without further investigation |
| 1.0 – 2.0 | Questionable |
| 2.0 – 4.0 | Good |
| Above 4.0 | Excellent |
Why These Ratios Work When a Single Reading Doesn't
PI and DAR are temperature-independent in a way absolute IR readings are not, since any temperature effect cancels out in the division. They assess whether insulation is exhibiting healthy polarization behaviour over time — not a direct measurement of dielectric strength. They're most valuable when tracked over successive maintenance cycles rather than read in isolation.
Practical Testing Notes
Use a dual-voltage capable instrument for larger or older insulation systems — covering the relevant voltage range (commonly 500V through 5kV or higher) from one instrument.
When DAR alone is sufficient. If a 60-second reading already shows a high, stable resistance, a full 10-minute PI test adds little additional value.
Record trends, not just pass/fail. Maintenance programs that log these values over time get substantially more diagnostic value.