What signs of insulation deterioration can megger testing reveal?

• A. Increased insulation resistance with applied voltage.
• B. Decreased insulation resistance between conductors/ground, unusual readings, or rapid drop in insulation resistance with applied voltage.
• C. No change in readings with voltage.
• D. Only visual signs.

Answer: (B)

Megger testing reveals insulation deterioration by showing how the insulation leaks current under a high DC voltage. When insulation is healthy, resistance between conductors and to ground stays high. If insulation is deteriorating, leakage paths (through moisture, dirt, or damaged material) cause the resistance to fall, and you may see readings that are unusually low, drift over time, or drop sharply as voltage is applied. This rapid drop in resistance under test voltage is a clear sign of compromised insulation. Reading that stays high or shows no change suggests good insulation, while relying on visual signs alone isn’t enough because megger results reflect electrical performance, not just appearance.