Megger measures insulation resistance by applying high voltage?

• A. Measures low-voltage resistance.
• B. Measures current.
• C. Measures insulation resistance by applying high voltage.
• D. Measures capacitance.

Answer: (C)

A Megger tests insulation by applying a high DC voltage to the insulation and measuring how much current leaks through it, then calculating the insulation resistance from that leakage current. The idea is that good insulation will resist leakage, giving a high resistance value (often in megohms), while damaged or damp insulation will allow more current and show a lower resistance. The relationship is R = V / I, with V being the test voltage and I the leakage current. This makes the device different from an ohmmeter, which measures low-voltage resistance of conductors, and from devices that measure current or capacitance directly. After the test, the equipment should be safely discharged.