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I have an old ( 8y+) submersible pump motor that had been disconnected for more than 6 months. I wanted to check and see if it tested ok and so did an insulation resistance test between phase and earth. I was expecting to get a reading either good or bad, however something unexpected happened – the tester immediately commenced with a low reading (0.4M ohm) and then started climbing up to around 5M Ohm. I removed the test lead and then re-applied it – the meter showed that it could not apply a test because there was a voltage of 54V present. This had to be discharged before the test could be reapplied (with similar results).

All in all it tests more like I’d expect from a capacitor and less like a motor. Has anyone come across this before and found an explanation for it? What would you say about the motor condition? The fact that it stores charge suggests to me that the insulation is fine, however because of the low initial reading I'd expect high inrush currents that may be problematic.

Any thoughts suggestions would be appreciated

Presumably you disconnected both the active and neutral before trying the IR test?
Did you try testing to earth with A&N connected together?

The motor is a 37kW internally connected (delta) with three active conductors only (no neutral). Hence the motor lead on which the tests were done has three active conductors and earth. The test showed the same pattern regardless of whether it was conducted between red and earth, white and earth, or blue and earth.

is the test done on a long length of cable? if not then the motor is no good, sounds like moisture that is evaporating slowly when IR tested, if it's on a long length of cable, it will be acting like a capacitor an charging up and holding the charge.

Thanks for your thoughts/advice Gregmcc - it is a short length of cable 2 or 3m long