Insulation Resistance and Polarization Index Testing
Insulation Resistance and Polarization Index Testing
This domain tests IEEE 43 methods: selecting DC test voltage, connecting guard, correcting for temperature, and interpreting IR, DAR, and PI.
Core rules
- Select test voltage by winding rating: commonly 500 V DC for under 1000 V, 500-1000 V for 1000-2500 V, and higher voltages for medium-voltage apparatus; never exceed the winding rating.
- Record IR at 1 minute and 10 minutes. PI = R10 min / R1 min; DAR = R60 s / R30 s.
- Typical guidance: PI at or above 2.0 is good; 1.5-2.0 is questionable; below 1.5 requires investigation. Interpret with historical data and temperature.
- Use the guard terminal to divert surface leakage. Discharge the winding after testing.
- IR roughly halves for each 10 C rise, so compare readings corrected to a common temperature.
Worked example
A 5 kV motor winding reads 500 Mohm at 1 minute and 1,200 Mohm at 10 minutes. PI = 1,200/500 = 2.4, acceptable. If the same winding read 800 Mohm at 1 minute and 900 Mohm at 10 minutes, PI = 1.125, indicating moisture, contamination, or insulation damage; investigate before energizing.
Traps
- Not discharging the winding and receiving a shock.
- Letting surface leakage dominate without a guard connection.
- Comparing readings taken at different temperatures.
- Applying PI limits to every insulation system without manufacturer guidance.