What is the primary purpose of insulation-resistant testing (megger) in electrical safety?

• A. To measure insulation continuity between conductors.
• B. To test insulation strength under short-circuit conditions.
• C. To check insulation integrity between conductors and ground, identify deterioration, and verify insulation health during commissioning or maintenance.
• D. To assess the thermal rating of cables.

Answer: (C)

Insulation-resistant testing with a megger is about evaluating how well insulation keeps electrical current from leaking between conductors and from conductors to ground. It does this by applying a high DC voltage to the insulation and measuring the leakage resistance, reported in megaohms. The goal is to detect deterioration, moisture, contamination, or physical damage that would compromise insulation integrity, and to verify that the insulation is healthy during commissioning or maintenance. A healthy insulation shows a high resistance, while a drop in resistance signals potential problems that need attention before energizing or continuing service.

This method isn’t about checking whether there’s a conductive path between conductors (that’s a continuity test), nor is it testing how insulation would perform under short-circuit conditions (that would involve different hipot/withstand tests), and it doesn’t assess the cable’s or component’s thermal rating.