Compressor insulation testing: is one megohm reading enough?
A compressor’s insulation resistance might be above the manufacturer’s minimum, but what if it has dropped significantly since the previous service?
Would that change your assessment?
A spot insulation-resistance test provides one piece of information. Where recommended by the manufacturer, a polarisation index (PI) test adds another by measuring how insulation resistance changes during a continuous test.
The compressor remains switched off and electrically isolated throughout. The ten minutes refers to the insulation tester running—not the compressor.
The insulation tester applies a steady DC test voltage to the winding insulation:
After 1 minute, record the insulation resistance.
Keep the same test running continuously.
After 10 minutes, record the insulation resistance again.
Then calculate:
PI = 10-minute resistance ÷ 1-minute resistance
For example, 100 MΩ at one minute and 160 MΩ at ten minutes gives a PI of 1.6. PI is a ratio, not a percentage.
Many insulation systems gradually draw less current while the test voltage is applied, causing the resistance reading to rise. PI measures that change.
However, 1.6 is not a universal pass or fail. Interpretation depends on the insulation system and the compressor manufacturer’s guidance.
Refrigerant and oil conditions, terminal contamination and connected electrical components can also influence insulation-resistance readings. Comparing historical results requires consistent test conditions, including temperature.
Testing should follow the manufacturer’s procedure, including electrical isolation, disconnection of relevant electronics, the correct test voltage and safe discharge afterwards.
Chiller and refrigeration techs: do you routinely record PI alongside megohm readings? What findings would prompt you to investigate further?
This is great information, usually I just megga the windings and don't perform a time based test