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#7416 02/05/02 01:34 PM
Joined: Feb 2002
Posts: 3
D
drtopp Offline OP
Junior Member
I have a question concerning an electrical blast we had at our plant. We have two MCCs rated with 1200A mains in a room. All three of the vertical busbars in two of the columns of one MCC appears to have arced to the bottom horizontal ground bar. A total of seven vertical bars blew about an inch of copper off the bottom portion of the bar. The main did not trip. The 30A on the primary side of tranformer did not blow but one 80A up the line blew. The cutout did not fall but the link pulled loose from the locknut. Can you give any insight which end this incident occured from? This is a pet food plant with some dust build-up.

Thank you,
Daniel Topping

#7417 02/05/02 04:06 PM
Joined: Nov 2000
Posts: 2,148
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Member
What is the system voltage and is it a grounded or ungrounded system? If grounded, is it solidly grounded or impedance grounded?
Don(resqcapt19)


Don(resqcapt19)
#7418 02/05/02 04:37 PM
Joined: Feb 2002
Posts: 3
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drtopp Offline OP
Junior Member
The primary is an 13200V delta to an 277V/480V wye secondary and solidly grounded.

#7419 02/05/02 04:56 PM
Joined: May 2001
Posts: 717
G
Member
Daniel,
Thorny problem. Need pics and schematic to do more than guess.

These vertical bussbars, are they line or load side? Line side, the fault came from upstream, load side, from one of the breakers feeding downstream.

Your best friend is a megger, or hy-pot here, if all are clear, you had a problem with the utility.

The 80 amp fuse that blew before the 30, is it on the upstream or downstream side of the transformer you speak of? What is the voltage and kw rating of the trans? Impedance might help too!

Dust that is normally not conductive will often reveal itself otherwise under a hy-pot test. We had a sub-station (12,500) next to a coal burning boiler house, had to constantly watch for little signs like a burned cob web that was a sign of "tracking". We saw tracking, we only had a little time to clean the gear, or we'd lose it. Your dust could be your enemy here (actually, it always is your enemy) during humid conditions, it will track and cause a mess.

Once an arc establishes, it doesn't go away until it has destroyed the path, you may be lucky you did not have a MAJOR fire.

I'd hy-pot every path, or at least a 5kv megger, put it back together, do it again, the first test is for your knowledge, you've already had the problem, just real handy to know where it came from. Might wanna do a TTR on your transformer as well. That will tell you if your trans is still in good shape, you did not say if it was yours or the utilities.

Good luck.

#7420 02/05/02 10:52 PM
Joined: Nov 2000
Posts: 2,148
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Member
Sounds like an arcing ground fault. Once something starts an arcing fault in that voltage system, the MCC or switchgear can sometimes completely melt down without tripping the OCPD on the line side of the fault. The fault could have been initiated by a small animal or build up of damp dust. This arcing burn down problem on 480/277 volt systems is the reason that ground fault protection of equipment is required when the main OCPD is rated 1000 amps or more, however the problem can occur on smaller systems.
Don(resqcapt19)


Don(resqcapt19)
#7421 02/06/02 12:00 AM
Joined: Dec 2001
Posts: 270
E
Member
Most likely an arcing ground fault, that caused a secondary arcing ground fault on an adjacent phase, of the supply bus bars in the MCC.
I agree with Don. Any problem upstream wouldn't initiate an arcing problem in the MCC, considering the normal clearances and insulation associated with MCC bus bars. It would take one heck of a high voltage spike to initiate the fault. It might be possible that the 13200 momentarily popped over to the load side and initiated the fault, so George's suggestion of a megger test makes sense. Still need to know if there are any other loads on the transformer, and distance to MCC, before guessing about weird circumstances. If there was a better chance for an momentary over-voltage to fault to ground in some other gear than in the MCC, but it didn't, then most likely it was just something local to the MCC.
As Don said, maybe some critter pushed a wire into the bus. You could look at the gear and see if the dust or some loose wire might have been involved..tho I suppose you already did that.
Scary problem. I certainly wouldn't want to re-energize with out meggering everything.

#7422 02/06/02 09:18 AM
Joined: Feb 2002
Posts: 3
D
drtopp Offline OP
Junior Member
A little more info. - And yes, I was a tad bit scared re-energizing the MCC. Only information about the tranformer I can locate is that it is an 1000kva Westinghouse. The 30A primary cutouts are about 25ft from transformer and the 80A cutouts are about 300ft upstream. The 80A feed three other small loads. At the time of the blast no more than an 50A load was on the MCC. I was informed that the lights dimmed at the other buildings about the same time. There was no evidence of any critters or bare wire. The plant has been running since the re-energize. Could the 80A fuse link slipping from the locknut create an inrush?

I would just like to thank everyone for their input. This is a problem I have never experienced and this forum is the place for answers.

#7423 02/06/02 10:31 AM
Joined: May 2001
Posts: 717
G
Member
Glad everything worked out for you, I'd still test the snot out of that circuit(s). Your answer is if it is Delta/Delta and it probably is, yes it can.

Let us know how it all works out.

Just reread your original. Delta/Wye usually does not cause that kind of problem. Suggest you check arrestors on the line side, and look REAL closely at the ground path for the transformer.

[This message has been edited by George Corron (edited 02-06-2002).]

#7424 02/06/02 05:04 PM
Joined: Oct 2000
Posts: 5,392
S
Member
I'd follow thru with George's suggestion, with one suspicion added...check for any faulty motor disco's, contactors, etc. I've seen damage due to decaying magnetic flux & a 3 ph disco that is NFG... [Linked Image]

#7425 02/06/02 07:42 PM
Joined: Dec 2000
Posts: 4,291
Member
You've also got a problem with selectivity.
If your fault was the only cause, then why didn't the 30A ocpd work?

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