How to Check a Motor With a Megger

Written by eric w. thompson
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How to Check a Motor With a Megger
A megger can test for proper grounding in electrical motors. (bio battery image by dinostock from

While the ohmmeter is popularly used in commercial electrical installations, it is not always appropriate for use in industrial installations. Instead, the megger is used in such situations, because it not only registers the subtle resistance readings common in high voltage circuits, but also generates enough voltage to properly test circuits with larger than normal conductors. Testing a motor with a megger is a very simple procedure, but you must follow the safety procedures carefully to avoid the ever present electrical danger of working on high voltage circuits.

Skill level:

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Things you need

  • Megger
  • Padlock
  • Flathead screwdriver

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  1. 1

    Shut off the power supply to the motor by flipping the breaker, then padlock the breaker to keep anyone from mistakenly turning it on while you are working on the circuit.

  2. 2

    Unscrew the wire cover on the motor using a flathead screwdriver, then pull the cover off and determinate the wires from their screws using a flathead screwdriver.

  3. 3

    Touch a megger lead to the bare end of one of the wires, connect the other lead to the motor's grounding screw, then turn the megger handle by hand and read the meter. Continue the process for each wire and motor winding. Any reading between 20 to 30 megohms is sufficient.

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