Megger testing is a method of testing electrical insulation in an electrical apparatus using an insulation tester resistance meter. It is a preventive maintenance action performed at scheduled intervals to determine the health of motors and other electrical equipment. The Megger insulation tester is a small, portable instrument that gives a direct reading of insulation resistance in ohms or megohms. The tester applies a voltage to each phase connection for the motor leads and ground, and measures the resistance between the phase windings of the motor under test and ground. The method is non-destructive and does not cause deterioration of the insulation.

During Megger testing, the item being checked is disconnected from other circuitry, if possible, before using a Megger test. The testing device only goes between 500 and 1,000 volts, which is relatively low, and generally provides information about the leakage current and whether insulation areas have excessive dirt or moisture as well as the amount of moisture, deterioration, and winding faults. The benefits of Megger testing include proactive equipment condition analysis, reduced risk of emergency power system failure, and insured availability.

However, Megger testing has limitations. It measures only resistance to ground and cannot identify insulation breakdown between sets of windings within a phase or between windings of differing phases. Therefore, it is not enough to predict all motor failures. Many safety precautions need to be followed when doing this test due to the large amount of voltage applied.