Which testing method detects a ground fault by measuring insulation resistance?

• A. Capacitance testing.
• B. Continuity testing.
• C. Insulation resistance testing.
• D. Voltage drop testing.

Answer: (C)

Insulation resistance testing focuses on how well insulation blocks leakage current to ground. By applying a high DC voltage between a conductor and earth and measuring the leakage current (or the resistance), you get a value that indicates how well the insulation is resisting leakage. A very high resistance means the insulation is good and there’s minimal leakage; a lower resistance shows current slipping to ground, signaling a ground fault or degraded insulation.

Capacitance testing checks dielectric properties by measuring how much charge the insulation stores, which helps detect moisture or deterioration but isn’t a direct measure of leakage to ground. Continuity testing confirms a conductor is electrically connected end-to-end, not the insulation’s ability to resist leakage. Voltage drop testing looks at voltage loss under load, not insulation integrity to ground.