Megging wires in a conduit. The megger fluctuate between 100 megaohms to 300 megohms, but it doesn't stay still, it's constantly moving.

[zappy]

What does this mean? Thank you for your help. BTW, I separated the wires from the VFD, separated the wires from the motor. Have the wires capped off on one end, meg wire to wire, meg ground to wire. It does the same thing every time it fluctuates constantly.

 

[synchro]

Have the wires capped off on one end, meg wire to wire, meg ground to wire. It does the same thing every time it fluctuates constantly.

Since it fluctuates the same way no matter which conductors you are probing, I suspect that some kind of interference is happening and not a defect in the insulation. How long are the wires?

 

[TwoBlocked]

Is the motor turning? It can easily happen if it is a fan resulting in a fluctuating reading. Another thing to watch out for is polarity in case there are damp, exposed conductors. Galvanic action can cause a DC voltage between the conductors and ground. One polarity can give a higher reading than the other. When in doubt, test with positive and then with negative to ground.

 

[ptonsparky]

Is the motor turning? It can easily happen if it is a fan resulting in a fluctuating reading. Another thing to watch out for is polarity in case there are damp, exposed conductors. Galvanic action can cause a DC voltage between the conductors and ground. One polarity can give a higher reading than the other. When in doubt, test with positive and then with negative to ground.

I believe the motor has been disconnected, but yes, I've had fan motors turning in the wind do that.

Check conductor to conductor, not just to ground or earth.

 

[TwoBlocked]

What does this mean? Thank you for your help. BTW, I separated the wires from the VFD, separated the wires from the motor. Have the wires capped off on one end, meg wire to wire, meg ground to wire. It does the same thing every time it fluctuates constantly.

Let's look at this again. First, you only need 2 megs to pass on 600vac wire. So this is not a show stopper. Second, gotta wonder if it is a instrumentation problem. Got fresh batteries in the megger? Got another megger you can verify with. Third, what else could be going on? Are the ends of the conductors nice and clean? (damhik...). Could the conduit be part of a ground loop, causing a voltage to be induced in the conductors? Is there nearby equipment that could be turned off to see if there is interference? Are there other conductors in the conduit? Is the ground conductor disconnected? (Sorry, had to ask.) Lastly, might be one of those eternal mysteries.

 

[ptonsparky]

Look for what would be cycling at the rate you meter fluctuates.
I've had fencers interfere while doing first checks with testers/meters.

 

[Opie11]

What level are you testing at? I have seen weird things happen testing at higher voltages.

Not sure what value you are looking for or expecting but the values you state are GTG in my book.

 

[TwoBlocked]

Look for what would be cycling at the rate you meter fluctuates.
I've had fencers interfere while doing first checks with testers/meters.

Like these guys?

 

[ptonsparky]

My Fluke 1507 gives me a voltage at which time the resistance is given.
2 meg at a voltage of 800 volt would be Ok for 480v systems.
2 meg at 24 volt would not.