To use a Megger insulation tester, first turn off the power that supplies the circuits to be tested. Use a voltage tester to ensure that the circuits have no power running to them. Choose the test voltage on the Megger, which, depending on the model, should be 500 or 1000 volts. Put the probes on the terminals located where the insulation is tested and turn on the Megger for at least a minute. Record the value produced.
Values should be recorded on a table. If a 110-volt circuit is used, test results should be collected from the insulation between the ground and each wire, as well as the insulation between the black and white wires. If a 220-volt circuit is in play, collect results between all three wires, as well as between the ground and said wires. If testing an appliance or circuit breaker, also gather readings from between the terminals and the terminals and the ground. In any test using the ground wire, position the Megger's positive probe on it and the negative probe on the terminal.
The values recorded on the table should be over 25 megohm. Any values that severely differ from others should be retested after the connectors are checked. Values lower than 25 megohm may be related to a malfunctioning circuit.