switchboard explosion

[jflynn]

We had a 2000amp 480v switchboard explode and basically melt away,on a project.We were un-able to open the main switch-which is a fused bolted pressure switch equipped with a GFI.Upon further review the GFI control switch was found to be in the off position,the fuses never blew,the POCO had to pull the cut outs @ the transx to kill power.We had worked in the board a number of times,when the event happened no one(thank god)was in the electric room-very scary-does anyone have any ideas on why no fuses had blown?this was a major event,thanks

 

[Cold Fusion]

The fault was on bus ahead of the fuses?

cf

 

[jflynn]

The fault was on bus ahead of the fuses?

cf

The fault was actually in the distribution section of the gear after the main fuses.From what we could see(after it was dead) and hear(while it was happening) the individual breakers in the distrbution section exploded one after the other,until there was nothing left,the explosions only stopped when the POCO killed power....

 

[whillis]

Wow, nasty!
The fuses might not have opened because the current in the arc was below their melting point. If you look at time-current curves you'll see that a fast acting fuse may carry twice it's marked rating for 2 minutes or more.

 

[charlietuna]

What came first--"the chicken or the egg"??? When you get up in size or 480 volt gear some strange things can happen in an explosion. Remember, that the argon gas produced in the explosion is conductive and can even jump across fuses during the explosion. Many explosion causes are actually never found because the cause is "gone" -- poof !! I always try to hunt it down by how bad certain parts are melted. We had one switchgear with only "fishpaper" insulation between buses??? The bus was the same phase--but one bus was service and the other was load side. When a water fault occured an explosion it caused the "fishpaper" to burn up --then the service was connected to the load side of the gear making the fuse useless!! The power company's vault was connected directly to the damaged switchgear that continued to explode for twenty minutes.. And of course sometimes we can only guess at the cause due to massive damages!!

 

[Cold Fusion]

the individual breakers in the distrbution section exploded one after the other,until there was nothing left,the explosions only stopped when the POCO killed power....

Explode translates to pieces of CB flying in random directions. What could cause individual distribution CBs to explode?

Certainly not a fault on the distribution bus.

Certainly not a fault after each distribution CB. Not only does it seem unlikely there would be a revolving fault hitting each of the distribution CB in order, but even if there was a fault on each, why would they explode? Why not just open under fault current?

You said "project". Is this new work? Or been there a while?

And the fuses didn't blow. You may wish to check the rating on the fuses. I'd probably even be checking if the incoming cabling was connected to the right spot.

And the xfm primary didn't trip. Let's see, POCO xfm, cutouts fused at maybe 600%, no differential relay. Maybe it wouldn't trip.

What's left? Even if the POCO installed a full sized xfm, that's still only a 1500kVA - probably less than 40kA SCC. A fault would have opened the fuses or distribution CB.

I'm really puzzled by this "explode" thing. If you aren't screwing with them, CBs don't just explode. Just curious, what kind of CBs were these?

How about a high voltage, internal xfm failure, on the xfm sec? I've never seen one, nor any papers on the forensics - so I can't comment on if that would cause the CB to explode.

cf

 

[don_resqcapt19]

Arcing burn downs of this type of equipment is the very reason that the code requires ground fault protection for solidly grounded wye electrical services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnect rated 1000 amperes or more. The arc can limit the current so that the standard OCPD does not open the circuit and the equipment melts down. Had the ground fault protection been turned on it should have opened the circuit and limited the damage.

 

[Besoeker]

Arcing burn downs of this type of equipment is the very reason that the code requires ground fault protection for solidly grounded wye electrical services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnect rated 1000 amperes or more.

Was there a ground fault?

 

[jflynn]

Had the ground fault protection been turned on it should have opened the circuit and limited the damage.[/QUOTE]

This is exactly what I had thought,however being it was off makes me wonder if the original install was ever commissioned?,our bad-for not requesting maintenace records to see if this gear was ever maintained.

One can only assume that like most MSB installations once there energized,they are only maintained or tested after a failure/problem...

 

[raider1]

Was there a ground fault?

I would say in an event such as this there would most likely be a ground fault at some point during the event.

Chris

 

[charlietuna]

Just consider a 480 volt-2000 amp main breaker carrying a 500 amp load when a rat jumps across the line side of the main breaker between "b" phase and "c" phase. Instantly there is a fireball as the rat turned into dust--but then the arc continues to grow as now it has flashed to the frame of the gear and now involves all three phases and is awaiting the power company's overload protection to shut down the service??? Does ground fault help out in any way???

 

[zog]

Just consider a 480 volt-2000 amp main breaker carrying a 500 amp load when a rat jumps across the line side of the main breaker between "b" phase and "c" phase. Instantly there is a fireball as the rat turned into dust--but then the arc continues to grow as now it has flashed to the frame of the gear and now involves all three phases and is awaiting the power company's overload protection to shut down the service??? Does ground fault help out in any way???

You mean like this?

 

[Cold Fusion]

Had the ground fault protection been turned on it should have opened the circuit and limited the damage.

It would be very hard for me to be that positive without seeing the coordination curves and the forensics - but still, possibly. However, even if on, the gf may have been inhibited as discussed in 230.95.A as part of the coordination.

Was there a ground fault?

I would say in an event such as this there would most likely be a ground fault at some point during the event.

From my experience and research, for solidly grounded 480V faults, they always start phase to ground - then go phase to phase very quickly (Comment limited to where there is no one in the panels nor is the panel being energized right after someone has been in it)

Should the gf have been turned on? Absolutely. However it being turned off does not explain a couple of important issues.

The switch has fuses ahead of it. It likely is series rated and one would not want it to open under a fault. 230.95.B addresses this. Those fuses should have opened. Makes one wonder if the mandated 230.95.C testing was done.

What scenerio would anyone propose that explains "the individual breakers in the distrbution section exploded one after the other,until there was nothing left,the explosions only stopped when the POCO killed power". I'm clueless as to what in a ground fault or phase-phase fault would cause that.

Also curious as to why the gf was turned off. Most I have seen don't have an easily accessible on-off switch. I'd be checking to see if it had been giving "false" trips.

Personal opinion: This system has some major design/installation issues. I'd be looking really hard at the coordination, equipment ratings, and the installation. We are not seeing all of the puzzle pieces.

(code references are 2005)

cf

 

[iwire]

The switch has fuses ahead of it. It likely is series rated and one would not want it to open under a fault. 230.95.B addresses this. Those fuses should have opened.

It is my understanding that the impedance of the arc can limit the current to below the OCPDs rating and that the faulting conductors will continue to melt back, not much different then an arc welder.

 

[Cold Fusion]

It is my understanding that the impedance of the arc can limit the current to below the OCPDs rating and that the faulting conductors will continue to melt back, not much different then an arc welder.

That meets my understanding as well. I've only been directly involved in three similar cases over the last 30 years - so not a lot of direct data. However, every one of those went phase to phase before the event was over. When you consider the gf may well be set to delay 200ms to 1000ms - that's a lot of cycles for a decent sized plasma ball to get started. I'm not a bit surprised the faults went phase to phase in that amount of time.

JAO: Designing and coordinating this kind of stuff to the NEC limits, makes for a really poorly protected system. And, I never have liked solidly grounded 480 just for this reason.

cf

 

[raider1]

It is my understanding that the impedance of the arc can limit the current to below the OCPDs rating and that the faulting conductors will continue to melt back, not much different then an arc welder.

That is my understanding as well.

Chris

 

[quogueelectric]

I have been upclose and personal in one of these explosions.
The initial arcfault welded the breaker in the closed position 600 amp 480 3ph directly off a padmount 13k/480 xformer.
I tried to time the explosions and ran back in the room to try to open the breaker with a 2x4 but it was already welded closed.
This was an older service with no ground fault and you will never ever forget the violence of these explosions.
I dont know what your gfci was set at however My suspicion would be that when the unit was initially installed there were probably nuisance trips which caused someone to disable the ground fault protection because they did not know how to do it correctly.

 

[mxslick]

We had a 2000amp 480v switchboard explode and basically melt away,on a project.We were un-able to open the main switch-which is a fused bolted pressure switch equipped with a GFI.Upon further review the GFI control switch was found to be in the off position,the fuses never blew,the POCO had to pull the cut outs @ the transx to kill power.We had worked in the board a number of times,when the event happened no one(thank god)was in the electric room-very scary-does anyone have any ideas on why no fuses had blown?this was a major event,thanks

Do you have any pics of the damage you can share? They may provide some clues.

 

[Jraef]

... My suspicion would be that when the unit was initially installed there were probably nuisance trips which caused someone to disable the ground fault protection because they did not know how to do it correctly.

I am highly suspicious of that as well. I once had a 500HP motor catch on fire and got blamed for the faulty solid state overload relay I had installed. But luckily (and unbeknown to the user) it had an event recorder function with time and date stamps of the last 99 fault events and the last 3 actions in programming, as a way to "back up" to a previous working version if you got yourself in trouble. In that programming step recorder, I found out that they had set the overload trip function to "Warn" instead of "Trip" just 2 days prior to the fire. A night shift technician confessed to doing that because he was getting nuisance trips, but he had written it in the maintenance log. Nobody had read the log.

 

[zog]

I am highly suspicious of that as well. I once had a 500HP motor catch on fire and got blamed for the faulty solid state overload relay I had installed. But luckily (and unbeknown to the user) it had an event recorder function with time and date stamps of the last 99 fault events and the last 3 actions in programming, as a way to "back up" to a previous working version if you got yourself in trouble. In that programming step recorder, I found out that they had set the overload trip function to "Warn" instead of "Trip" just 2 days prior to the fire. A night shift technician confessed to doing that because he was getting nuisance trips, but he had written it in the maintenance log. Nobody had read the log.

That must have felt good. Score one for modern electronics. And one for Jraef.