Which instrument is used to measure insulation resistance in electrical testing?

• A. Oscilloscope
• B. Megohmmeter
• C. Multimeter
• D. Thermal camera

Answer: (B)

Measuring insulation resistance relies on using an instrument that can apply a high DC voltage across insulation and accurately detect the tiny leakage current that flows. From V = I × R, the resistance is inferred, giving the insulation resistance in ohms. The device designed for this purpose is a megohmmeter (megger), which typically provides voltages in hundreds to thousands of volts and displays resistance in the megohm range. This makes it ideal for assessing insulation quality in cables, motors, panels, and other electrical equipment.

Other tools measure different things: an oscilloscope captures voltage or current waveforms but not insulation resistance; a multimeter can measure basic resistance but usually not at the high voltages or in the megohm range needed for insulation testing; a thermal camera checks temperature patterns rather than electrical leakage resistance. If insulation is compromised by moisture, cracks, or contaminants, the leakage current increases and the measured megohm value drops, signaling degraded insulation.