What does insulation resistance testing measure and what information does it provide?

• A. It measures resistance to leakage between live conductors and earth/neutral, indicating insulation integrity.
• B. It measures continuity of conductors to verify connections.
• C. It measures resistance to voltage drop under load.
• D. It measures earth impedance to touch.

Answer: (A)

Insulation resistance testing checks how well the insulation between live conductors and earth (or neutral) resists leakage current. By applying a high DC voltage with a megohmmeter, it measures the leakage resistance of the insulation path. A large resistance, typically in the megohm range, indicates solid insulation and low leakage; a low resistance reveals leakage paths caused by moisture, contamination, damage, or aging, signaling degraded insulation and potential safety risks. This information helps determine the overall safety and reliability of an installation and guides maintenance decisions. The test is done with the system de-energized and with any capacitive charge discharged. The result is distinct from continuity testing (which checks for a complete low-resistance path in conductors), voltage drop under load (which examines performance under operation), or earth impedance to touch (a separate touch-potential measurement).