Which instrument is used to measure insulation resistance in transformers during testing?

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

Answer: (B)

Insulation resistance testing checks how well the transformer windings are insulated from each other and from the core/ground. The instrument used is a megger, an insulation resistance tester. It applies a high DC voltage to the insulation and measures leakage current, giving a resistance reading in the megohms. A healthy insulation shows very high resistance; a fault such as moisture, dirt, or aging creates leakage paths and lowers the resistance. For a transformer, you test between each winding and between windings and ground with the windings isolated, using the megger at the appropriate test voltage. The goal is to see high resistance values that remain stable over time; readings that rise with time and a good polarization index indicate solid insulation.

Other tools don’t measure insulation resistance in this context: a standard multimeter checks low-voltage DC resistance, not the high-impedance leakage resistance of insulation; a thermal camera looks for temperature patterns, not electrical leakage; an oscilloscope captures waveforms, not insulation integrity. The megger is specifically designed for this purpose and is the appropriate instrument for assessing transformer insulation health.