When an high voltage system insulation test value is suspect or recorded during an annual survey, a polarization index test is performed. What is the polarization index?

Correct Answer: Option C — The polarization index is the ratio of the insulation resistance taken at ten minutes to the insulation resistance taken at one minute.

The polarization index is a diagnostic ratio comparing the insulation resistance measured at ten minutes to that at one minute during a sustained megger test. A low ratio indicates moisture or contamination, complying with IEEE 45 testing standards for rotating machinery insulation.