Megger the Motor: Insulation Resistance Testing

If you work in a refinery, you will probably never hear someone say "let's use the megohmmeter to test the motor insulation." What you will hear is "let's megger the motor." A megger is a megohmmeter, a tool used to identify damaged electrical insulation by measuring resistance.

How the Test Works

One lead is connected to the pump casing and the other to the motor windings. The megohmmeter generates a voltage and measures the resistance between the two leads. One megohm equals 10^6 ohms. By measuring resistance, the instrument can detect very low current flows that would otherwise be invisible.

What the Results Mean

Motors have insulation to prevent current from flowing between the windings and the casing. If the resistance reading is low, current is leaking across that insulation, which means the insulation is damaged. A low reading means the motor needs to be repaired or replaced, depending on its size and cost. A high resistance reading means the insulation is intact and the motor is safe to run.