Insulation Resistance and Polarization Index Testing
Insulation Resistance and Polarization Index Testing checks whether a winding or cable insulation system is clean, dry, and sound. The megohmmeter applies DC voltage and measures leakage current; the displayed resistance is voltage divided by current. A single IR reading is affected by temperature, humidity, test voltage, and surface contamination, so records and trends matter.
Core rules
- Isolate and lock out the equipment, disconnect sensitive electronics and surge paths, and discharge the test specimen after testing.
- Select test voltage by equipment rating and standard or manufacturer guidance; 500 V DC is common for 480 V motors, while higher-voltage apparatus may require higher test voltages.
- Use the guard terminal to divert surface leakage when testing bushings, cables, or dirty windings.
- Polarization Index is PI = R10min / R1min. A PI of 2.0 or greater is commonly accepted as good for many rotating-machine windings; below about 1.5 needs investigation.
- Record temperature, humidity, test voltage, duration, and discharge time.
Worked example
A 480 V motor winding reads 200 MΩ at 1 minute and 600 MΩ at 10 minutes. PI = 600 / 200 = 3.0, which indicates a healthy polarization response. The technician also compares the 1-minute value with prior records and the manufacturer minimum. A high PI does not excuse a low absolute IR value, and a low PI does not prove failure without considering temperature, moisture, and contamination. The reasoning is trend plus absolute value plus PI.
Traps
Not discharging after the test; touching leads during the test; testing a winding full of condensation; ignoring surface leakage; using the wrong test voltage; and treating PI as valid when the 1-minute value is unstable or the test was too short.