During the Megger test, an insulation resistance meter applies a direct current voltage of 500 or 1,000 volts to electrical insulation and records data at one minute, 10 minute and intermediate intervals to determine if the insulation is adequate for continued service. Megger test results show the relative amount of moisture in the electrical insulation, current leakage where the insulation is dirty or moist and any faults or deterioration in winding. Moisture decreases insulation resistance; low resistance indicates poor insulation.
A hand crank is the most frequent method for operating the Megger insulation resistance test meter, which involves connecting one test lead to the insulation, isolating the wiring from the equipment and connecting the other test lead to the wiring. Safety considerations when using the Megger tester include: use only on high resistance measurements, do not touch the test leads while turning the hand crank, deenergize and discharge the circuit before connecting the Megger, and disconnect the equipment being tested from other circuitry prior to connecting the test meter.
Electrical insulation protects the wiring that conducts electrical current by resisting current, allowing it to flow along the electrical wiring path. If the insulation tests as having low resistance, it indicates that the insulation is not adequately protecting the flow of electricity. Insulation deteriorates over time for many reasons, such as vibration, temperature variations, corrosive vapors and humidity. These factors cause cracks or small holes that let moisture and dirt that reduce the insulation's resistance, allowing current to leak through the insulation. Periodic resistance testing and comparison of test results provides key information regarding the deterioration process.