When meggering a three phase motor, could someone please help me understand the values we would expect to see for a motor with no insulation or ground faults.

From my understanding:

Phase-GND: should see a very high resistance (in MegaOhms)

Phase-Neutral: should see a very low resistance (near zero) since all the phases are tied together at the neutral

GND-Neutral: honestly I'm not sure...

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2 Answers 2


If you follow your own logic through you'll have the answer.

Since your phase to neutral resistance is close to zero then you can consider the phase and neutral as joined together (for the purpose of this exercise).

Now if phase-GND is high resistance then netural-GND must be high too.

This is correct. There should be no connection between the windings and the machine frame.

For the benefit of the unexperienced reading this post, all these measurements are carried out with the motor disconnected from the supply.

  • 1
    \$\begingroup\$ The last paragraph especially important! \$\endgroup\$
    – Solar Mike
    Sep 11, 2018 at 17:18

You do apply voltage on any phase vs. case/ground/earth. If the result is a high resistance then no winding is shorted to the case.

Makes no sense to measure phase to phase or neutral to phase. Mostly you won't even get the neutral out of the terminal box, just three phases.

However you can also check continuty for phase to phase. Mine instrument has both options. HV pulse 50V to 1kV and continutiy check with current 250mA.

Sure, the motor has to be disconnected from the frequency inverter.


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