Taping up a damaged wire, then meggering it

[zappy]

So one of our VFD's were fried. We meggered the load side wires to the motor, and the motor itself. One of the wires were grounding out in the condulet. Wire had a bad nick on it. We taped it up really good with some 33. Then we megged it out. It megged out at over 11M ohms. Before it was under 1M ohm. Should we just replace the wire, or do you think this is acceptable? What is a Acceptable range? Thank you for your help.

 

[Little Bill]

I think 11Mohm would be ok.

 

[ptonsparky]

dry conduit, 11 Meg probably ok. Any chance of it having water in it, No. What do the other two measure?

I would not have removed wire, taped it and reinstallEd it.

 

[zappy]

dry conduit, 11 Meg probably ok. Any chance of it having water in it, No. What do the other two measure?

I would not have removed wire, taped it and reinstallEd it.

Damp conditions when it rains. There shouldn't be water in the conduit.

 

[TwoBlocked]

Recently I was the second electrician to look at a motor lead problem with a VFD. Was actually there for another reason, but was asked for a second opinion. One of the leads was marked "bad." It meggered ok, but not great like the other leads. (Sorry, I don't remember the numbers, but could look back in the service records if it's important.) Plenty good enough according to the 1 meg plus 1 meg per 1000 volts rule (1.5M for a 480 motor). Yet was told the VFD had been tripping out with some kind of a motor fault. Got looking up in a pull box and found the lead was pinched into the end of a conduit where it left the box - too short of a radius at the inside of the 90 degree turn. Motor lead was in a horseshoe shape if that makes sense. Anyhoo, used a strap to flatten the horseshoe and relieve the pinch. Megger reading improved tremendously. I'll be back with a replacement breaker, which was good but was used to replace another breaker that went bad. I expect everything will work fine. If so, I saved them $40K. I think each phase had 2-5000MCM about 200ft long, it was a distracting day...

Sooooo, can a compromised motor lead cause a VFD to fault even if the megger reading is ok, but not perfect? I think it can. A megger uses DC, but the motor uses AC. In the case of the motor lead I described, there must have been some leakage when pinched, otherwise it would not have improved when the pinch was relieved. But I think we must also consider capacitance, which would not theoretically show up by looking at the max megger reading. And with the high frequency of a VFD carrier frequency, the effect of capacitance is much greater, causing a greater ground fault current, than an across the line starter with only 60hz. I believe that is why the VFD was tripping even though the motor lead megger reading was in the "OK' range.

If it's not a huge problem to do, I'd replace the motor leads.

 

[GoldDigger]

One consideration is that the voltage waveform generated by the VFD has very sharp, high frequency leading and trailing edges. That sharp voltage rise can cause greater stress to insulation than DC at the peak voltage.
A second consideration is that if the length of the lead from the VFD to the motor is long enough there can be standing waves along the run which can have a peak voltage as much as twice the nominal output voltage.
Be sure that you are using enough layers of tape to withstand the higher voltage and that you are using a high enough voltage on the Megger to provide a valid test.

 

[Fred B]

Would heat shrink tube be better at making insulation more closely approximating to original wiring insulation vs taping? Would heat shrink or can it maintain the moisture resistance that would be of the same level as the original wiring? Another words, would thwn-2 still maintain a water resistance and temperature rating that it originally had with use of properly applied heat shrink tube?

Edit: I know they make and have used a listed splice kit for underground wire repair that utilizes heat shrink.