Insulation resistance test

An insulation-resistance test, the IR test or megger test, measures the resistance between an electrical conductor and earth to confirm the insulation has not degraded, expressed in megohms. The engineer applies a DC test voltage, typically 500 or 1000 volts, with an insulation tester across motor windings, cables, and switchboard busbars, and compares the reading against the minimum acceptable value, since low insulation resistance signals moisture ingress, contamination, or insulation breakdown that can flash to an earth fault or fire. It is a routine planned-maintenance-system task on shipboard machinery and a checked item at electrical surveys. A reading at or near zero condemns the equipment until the fault is found.