What polarization index is used for in insulation testing?

• A. To trend insulation aging by comparing insulation resistance over time.
• B. The ratio of voltage to current.
• C. The measure of inductance.
• D. The ratio of generated emf to terminal voltage.

Answer: (A)

Polarization index looks at how insulation resistance grows when DC voltage is applied, which reflects the insulation’s ability to polarize ions inside it. To compute it, you measure the insulation resistance after a short time (often 1 minute) and again after a longer interval (typically 10 minutes) with the same applied voltage, then take the ratio R10/R1. A higher index means more polarization has occurred and generally indicates healthier, drier insulation; a lower index suggests aging, moisture, or contamination. By tracking PI over time, you can trend insulation aging and detect deterioration before problems develop. This concept is about resistance changing with time under voltage, not about voltage/current ratio, inductance, or emf-to-terminal voltage ratios.