When working with heavy-duty, high voltage electrical or electronic systems, the megger, more properly called a "megohmmeter," is used to test continuity and the insulation qualities of conductors in that circuitry environment. "Megging" a circuit will tell you if there's a short circuit or an open circuit in much the same way that a pocket-sized volt-ohm meter can alert you to interrupted circuits, or circuits shorted to ground in a smaller piece of electrical or electronic equipment.
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Use the voltage selector switch on the megger to set the megger to the appropriate test voltage, usually the next highest voltage above the voltage rating of the piece of equipment being tested. The voltage range for the megger may be as low as 250 VAC, depending on the model of megger, and as high as 1,000 VAC.
To find the location of a short or open circuit, connect the megger leads to the megger, then connect one lead to the circuit and the other to a reliable ground. All leads must be connected before the "Test" button is pressed.
Turn the megger on by pressing the "Test" button. Hold the "Test" button down for one minute, then end the test by releasing the button. If the circuit is intact, the resistance reading will remain steady or decrease.
Remove the test leads from the tested circuit only after the voltage has dropped to zero. Some circuits may be capacitive and retain a significant charge after the test has ended.
Tips and warnings
- Gross differences in the normal system voltage and the megger's range can damage the equipment you're testing. While you'll need to test 600 VAC circuitry at 750 VAC, you can't expect 110 VAC circuits to survive being tested at the same level.
- Never touch the megger's leads or terminals during testing. Always wait until the voltage reads zero before touching a terminal or a lead.
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