Which device is used to measure insulation resistance?

• A. Multimeter
• B. Ohmmeter
• C. Megohmmeter (Megger)
• D. Oscilloscope

Answer: (C)

Measuring insulation resistance requires a device that can apply a high DC voltage and read very large resistance values. The megohmmeter (often called a megger) is designed for this purpose. It delivers a known high voltage to the insulation under test and measures the leakage current that flows, then uses Ohm’s law (R = V/I) to compute the insulation resistance, typically expressed in megohms. High insulation resistance indicates good insulation; lower readings reveal deterioration, moisture, or contamination.

A standard multimeter or ohmmeter is built for much lower resistance ranges and voltages, so they aren’t suitable for accurately assessing insulation resistance. An oscilloscope is used to observe signals and waveforms, not to measure insulation resistance directly.