Would it flash

[Jraef]

... Would the heating of this conduit during a brief fault cause the conduit to get hot and bright enough to cause the flash they saw?

In a word, no.

An insulation failure and arcing fault to ground in one conductor inside that conduit, maybe. Hence the concern we all express. Problems like that don't just go away, they get worse.

Look, you megged the cable and found an anomaly, as distasteful as it sounds you have to get to the root of the problem and most likely replace that cable. MAYBE if you can pull it back a short distance to a J-Box somewhere you might be able to get a wrap-around repair sleeve on it. But that opens up the risk of nicking it somewhere else, or another cable with it. Plus are there other conductors in that conduit? If so, they were most likely damaged as well. Cable insulation has a much lower temperature damage threshold than RGC, if that burn mark came from the inside, there is major damage in there.

 

[Mark_H.@Barretts]

Tom, I would go with at least 250 or 300 with that long of pull. Just a suggestion.

 

[zog]

Call from customer today stating they witnessed a large flash or ball of fire early this morning from behind a three inch rigid conduit. 480v to 250hp motor via softstart. Nothing tripped. Ok. Sure enough there is an obvious spot on the conduit.

Obviously there is an issue here.

We megged it, with one set of paralells not reaching the same level as the other sets. 1780 meg vs 2G.

Phase to ground or phase to phase?

Manager said run, so they are. I expect a large hole the next time.

Is this manager taking responsibility when (not if) this thing fails again? Or is he going to say "Ptonsparky said it was all good"?

 

[ptonsparky]

Phase to ground were the readings. Phase to Phase is good. I believe it was a phase to ground fault in the conduit. Mice are not out of the question. There really is not much for visible damage to the conduit other than the spot.

The manager knows what is going on and no, they won't come back. He has the opinion they have a potential problem as well.

That is 6 runs of 3/0 for a total of 700 feet. Not one way.

Option of jbox is good and likely to be at point of fault.

Thanks all.

 

[cdcengineer]

Sounds like a damaged spot to the insulation of one of the phase conductors. We had a very similar situation in a cable tray over a year ago. It was a portable 2Meg gen-set temporarily feeding an EM switchboard. Same description, and nothing tripped.

Please keep us posted.