IEEE
IEEE 43
Recommended Practice for Testing Insulation Resistance of Electric Machinery
How to measure and interpret insulation resistance and the polarisation index on motor and generator windings, including the temperature correction without which a reading means very little and the minimum acceptable values by winding voltage. Its most useful point for an integrity programme is that the absolute number matters less than the trend on a consistently corrected basis, and that a high resistance with a poor polarisation index tells a different story from a low resistance with a good one.
This is an explanation, not the standard. PipingInspector is not affiliated with IEEE and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.