3 phase explosion

[mark82jd]

A weird one but here we go.

I was asked to add a new 3ph circuit into an existing 6 way 3 phase dorma smith D.B. The board had to remain up and running ( not quite meeting the EAWR I know) so with the singles now in all I had to do was slide the 3ph breaker in the spare space 6L1-6L3.... This was where the explosion happened.

Everything was clear of the terminals, the new breaker was, just in case, switched off. As I pushed it onto the bare copper something caused 2 phases to come together resulting in me with temporary blindness, my hand completely black and ending up with me on the floor on the other side of the switch room.

The board and myself were in a bad way but once I was ok I began looking for explanations. The breaker wasn't bridged out which I had expected and nothing obvious stood out in the smoking D.B.
My foreman couldn't explain it either but he had heard of it happening before because of the air in a live board becoming 'ionized' therefore becoming conductive. Any spark caused by the breaker touching the busbar would have been enough to jump through the now conductive ionised air and bridge out to the next phase.

Has anyone else heard or come across this as I'm struggling to find a possible cause

Mark

 

[Geordie Spark]

I'm thinking that had it been me I wouldn't have dared tell anybody - never mind post it on a public forum, but there you go ... each to his own.

 

[malcolmsanford]

Well can't see where this "spark" would have come from without a load on it, and to be honest if there was a build up off anything be it 'ionization' or what, surely the cover would have been open and the air replaced.

No mate a more simpler explanation is that you somehow shorted the phases out. If your absolutely certain you never somehow went across them with a tool or something then for me a dodgy MCB with phases joined internally

 

[stef]

More than likely he pushed the busbars together whilst ' pushing the breaker in ', shame on his black piano fingers, him laying on the floor.

 

[mark82jd]

There is absolutely no chance the busbars pushed together, there was also nothing in my hand other than the breaker.
Are you guys seriously telling me you have never added a new breaker to a live board?? No established electrician around can say they have ever done this.

I have looked up ionisation and this seems plausable, just wanted to hear if this was possible....

 

[DurhamSparky]

again ionization will only happen under load...!!! so you eith had the breaker on with load or your telling porkies!!

the pics below are a true 3 phase explosion!

 

[Geordie Spark]

There is absolutely no chance the busbars pushed together, there was also nothing in my hand other than the breaker.
Are you guys seriously telling me you have never added a new breaker to a live board?? No established electrician around can say they have ever done this.

I have looked up ionisation and this seems plausable, just wanted to hear if this was possible....

Yes !! :yes: :smug: :vanish:

 

[stef]

There is absolutely no chance the busbars pushed together, there was also nothing in my hand other than the breaker.
Are you guys seriously telling me you have never added a new breaker to a live board?? No established electrician around can say they have ever done this.

I have looked up ionisation and this seems plausable, just wanted to hear if this was possible....

Frankly, no, I haven't. There is some sort a code of practise and a certainn H&S standard which I comply to, for the sake of my own and others well being. And no, no established sparky has ever done this. I once saw a training video when I was an apprentice ( decades ago) where a dummy via remote dropped a 17mm spanner into a 400A bus bar, the spanner vopurized in a fraction of a second, melting itself through the dummies visor burnt off the top layer of it's head, the bang it made is something I will never forget. So, no, I definately haven't.

 

[plugsandsparks]

The MG isobar boards are the only ones i would consider adding a breaker to whilst powered up. Some of the cheap stuff on the market is way to scary with all those prongs just sitting there waiting to grab you when you least expect it. No idea why it blew up but highly likely to be something directly across the busbars.
Even Sp Dbs are an accident waiting to happen simply adding a breaker for a shower etc.
Suggest put it down to experience, **** happens with this stuff , doesn't take much current really to make a melt down and we can get a bit blase about 40A ccts, 200Amp cicuits whatever

 

[Knobhead]

Ionised air degrades from O3 to O2 rapidly so it wouldn’t be present before work started. To create O3 either a high voltage corona discharge or a high energy arc is required.
You shorted the bus-bars, no ifs, no buts. Once the arc is struck O3 is generated propagating the growth of the arc.

Sorry to be blunt, you brought this on yourself. “The board had to remain up and running”. I’ll bet they had to manage without it afterwards.

I have looked up ionisation and this seems plausable, just wanted to hear

if this was possible....

Start looking for another excuse.
The HSE are going to have a field day with you, your boss, the company…….

 

[Guest123]

I have come across this before but only where the busbar bar connection prongs are of the fork type rather than one solid stub.

With a TP MCB, it is possible to get one half of the fork of one phase in the same connection terminal as one half of the fork of another phase, consequences are BOOM!


I've only ever known older ABB/wylex/crabtree DB's to have this fork style prong in them, not any other though.

 

[Guest123]

Similar to this.....

 

[Guest123]

Sorry to keep going on but, having had a think about it, Dorman Smith DB's do have this fork style arrangement in them also.

Believe it or not, it is quite easily done. I would be 99% certain this is what happened.

 

[Knobhead]

Qed . . . .

 

[Bobster]

Even in HV situations where I could hear the air ionising (winding ocbs back in). It would take a long time for a dangerous amount of O3 to build up, it turns back to O2 fairly quickly at sea level so I can't see the board being filled with it.

 

[Knobhead]

Why won't this board allow all capitalised words?

 

[DurhamSparky]

beacuse its a sign of shouting and been a hush hush, nancy pancy forum now shouting is banned...!

bit like plastic knives on aeroplanes!

 

[Guest123]

It's a rubbish File but, if you scroll down to "fitting and removal of MCB's" I think you'll catch my drift about the fork issue.

 

[Knobhead]

Well our OP has just looked back in. Didn't seem inclined to reply.

A question to the OP, "did you seriously think we would defend your actions?"

You took the chance!

 

[shagbite]

yep. dorman have twin pronged terminals and this bloke has definitely straddled two phases. i bet his ears are still ringing.