How to Use a Megger

outdated machinery image by Yali Shi from

Use a Megger to test the insulation on a grounded wire or between the conductors on a circuit board. Testing the grounding system for hospitals, business parks or any other kind of facility can help prevent electrical surges.

As with any type of current, lightning storms find the least path of resistance and always hit the ground. Learning to use a Megger continuity tester can reduce the likelihood of electrical damage to a facility.

Clamp a pair of ground testers on the grounding rod in the facility you wish to test. This is sometimes called the grounding cable. The facilities manager, architectural engineer or electrician can tell you where the grounding rod is located. Wind the handle of the generator on the Megger at about 160 turns per second. One clamp will measure for the voltage, and the other clamp will measure the current. The Megger indicator will show the ground loop resistance at the grounding rod in the facility. Usually, a technician will multiply the number of grounding rods or poles by the reading in watts for the total loop resistance.

Test a device, motor or circuit board between the circuit and the earth. Hook one terminal from the Megger to your circuit board and the other to a stake in the ground. Make sure you unplug the power on the circuit or motor that you are testing. Wind the generator with the handle on the Megger at about 160 turns per second. Note the reading on the indicator of the Megger. This is your measurement for insulation in the circuit via the earth test. Your Megger calculates the measurement for you.

Take an infinity reading. Pair each wire on the circuit board. Measure each wire to the other wire on the circuit board, and then measure each wire to the metal conduit. Wind the generator with the handle on the Megger at about 160 turns per second. The reading will appear on the indicator screen of the Megger.

Take a reading for the bus on a PCB circuit board. Follow the bus lines to their bus tangs (the ends). Take a measurement for each bus by connecting the terminals from the Megger to the bus tangs, and then wind up the generator at about 160 turns per second. Lastly, measure from the Megger to the case of the device you are testing.