Would it flash

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ptonsparky

ptonsparky

Tom
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NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
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. Sound like a faulted conductor? Nothing burned thru like I would expect. Not sure of the protection abilities of the softstart but I would have expected it to fault out.

We megged it, with one set of paralells not reaching the same level as the other sets. 1780 meg vs 2G. Manager said run, so they are. I expect a large hole the next time.
 
ptonsparky said:
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. Sound like a faulted conductor? Nothing burned thru like I would expect. Not sure of the protection abilities of the softstart but I would have expected it to fault out.

We megged it, with one set of paralells not reaching the same level as the other sets. 1780 meg vs 2G. Manager said run, so they are. I expect a large hole the next time.
Click to expand...
The good/bad thing about some soft starter designs is that they may limit the current enough to prevent a trip. if there is no Bypass Contactor, and the soft starter design always has the Current Limit feature engaged, and the fault is of sufficient resistance, then it's possible to continuously limit the current in that one phase circuit. But that doesn't mean the problem isn't there. I wouldn't run it, sounds like you have a high resistance ground fault. Most newer digital soft starters would now include Residual Current GF detection, but that is by no means universal.
 
Too late to add as an edit:

If that's the case, this remains a very dangerous situation; the first person to come along and provide a lower resistance path to ground could get hurt or killed.
 
Or, there's enough carbon there to support a continuous current, which may well ramp up over time.
 
Hard to imagine a manager of any kind accepting the risk associated with a "large fireball" in their facility.






^^^^^^
That was my first thought then I remembered how the real world works.:roll:
 
Picture a 3" vertical stick of rigid conduit piercing both the floor and ceiling that are concrete. 8' max in height. Maybe one coupling visible. System is 480v wye. About 4 1/2 foot up from the floor on the back side of the conduit is a spot that apears to have gotten hot. 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?
 
A picture of the damaged spot would be helpful here.

As a WAG, I would say it is possible that the arc "welded the conduit shut" again which is why there appears to be no visible hole (or is there one?)

But it WILL fault again that much is for certain. And most likely with more violent results.
 
ptonsparky said:
Picture a 3" vertical stick of rigid conduit piercing both the floor and ceiling that are concrete. 8' max in height. Maybe one coupling visible. System is 480v wye. About 4 1/2 foot up from the floor on the back side of the conduit is a spot that apears to have gotten hot. 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?
Click to expand...

sure if it got red hot, ever put a torch to galvanized metal, before it gets to the melting point it starts throwing off white hot flashes of sparks, not sure if it is the zinc or magnesium in the galvanizing that does it, but it will do it.
 
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