Wear your PPE

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knoppdude

Senior Member
Location
Sacramento,ca
A friend of mine just experienced an arc blast when re-energizing a 480 volt main for a decrepit MCC. It exploded, but he did not get burned because he was wearing his 15 cal/sq cm suit. These things are life savers, and he said he would have been burned without it. I had opened and closed this same breaker a month ago, and was wearing a 40 cal suit. The fact is, it can happen if conditions are such that an uncontrolled arc will occur. If I can get pictures of this MCC abomination, I will try to post them. The violations are many.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
A friend of mine just experienced an arc blast when re-energizing a 480 volt main for a decrepit MCC. It exploded, but he did not get burned because he was wearing his 15 cal/sq cm suit. These things are life savers, and he said he would have been burned without it. I had opened and closed this same breaker a month ago, and was wearing a 40 cal suit. The fact is, it can happen if conditions are such that an uncontrolled arc will occur. If I can get pictures of this MCC abomination, I will try to post them. The violations are many.

Glad to hear that your friend is OK. I am also glad to hear a tale of safety equipment working to the extent described. I do find it a bit disconcerting that the mere act of closing a breaker could result in such a disaster.

I would be nice to know what brand, model, etc. we are talking about.
 
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hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
They are like bulletproof vests though, don't think your invincible when wearing one, we had an arc flash accident about a year ago, the 15 cal jacket was nothing but rags afterwards, but it did reduce the amount of injury that could have been sustained. The faceshield did protect his eyes, but the rubber gloves resulted in 3rd degree burns on the wrists. Make sure the jacket sleeves cover your wrists under the gloves. Most arc flash incidents are preventable, but it looks like the one you have described was mechanical failure, not operator error.
 

knoppdude

Senior Member
Location
Sacramento,ca
I will post the information on this breaker monday. I am supposed to help replace it, if the switchgear is still workable. I was told by the manager that power just stopped to the plant, and the company electrician checked line voltage, which was present, then charged and closed the breaker when it arced. Although I haven't seen the switchgear, I doubt it is unscathed. This may be a problem even if a new breaker is available. We will work this de-energized.
 

eric7379

Member
Location
IL
I will post the information on this breaker monday. I am supposed to help replace it, if the switchgear is still workable. I was told by the manager that power just stopped to the plant, and the company electrician checked line voltage, which was present, then charged and closed the breaker when it arced. Although I haven't seen the switchgear, I doubt it is unscathed. This may be a problem even if a new breaker is available. We will work this de-energized.

First off, I am glad to hear that your friend is OK.

Secodnly, with this bit of information above, it almost sounds as though there is a fault downstream of this main breaker. Knowing this info, I would check out the loads downstream of this main before I even thought about just closing it. A simple, quick check is to put your meter on the load side conductors. If anything even looks remotely suspicious, I would open all discos donwstream, then megger the load side conductors of the main that faulted. If that turns out OK, then start closing the disconnects one at a time until you find the fault.

Actually, now that I think about it, I would megger out the conductors no matter what your meter said.

Without knowing anything about the plant that this happened in, I suspect that there is more to this story than just the main disco/breaker blowing up. I could be wrong, though!
 

knoppdude

Senior Member
Location
Sacramento,ca
This is what we found in the MCC when we started to clean things up; the arc occured on the secondary side of the breaker, and burned up two termial blocks, and the breaker terminals. We performed insulation tests on all of the motor branch circuits, and found two that failed. In addition, we found that dirt and build up behind the breakers causes low readings to ground so we cleaned each of these until we had good readings on all of the feeders to ground. In addition, there was a branch circuit breaker that had fused closed. The area electrician is of the opinion that the connections at the terminal blocks were loose, or worn, and overheated to the point that they ionized. The main breaker was used several times a day to lock out equipment for maintenance, and even though we were told nothing was wrong with it, I don't think it was meant to be used like a light switch. I also believe that some of the wiring on the secondary side of the main to the terminals were aluminum, and if I can find these on tuesday, this could have added to the problem. At any rate, the plant is ready to run, and I will be there for start up. I am just happy no one got hurt or killed.
 
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