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#127093 - 04/26/01 06:55 PM Dead Shorts
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Had a 200A 3 ph 208Y120V main blow today due to a dead short in a 20A circuit.

The spike must have had sufficent duration to do this

But the 20A breaker never tripped, and i am at a loss as to explain this


any input apprectiated

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#127094 - 04/26/01 09:02 PM Re: Dead Shorts
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Sparky,

I've seen the same thing happen. I always thought that maybe the main was more sensitive somehow to the characteristics of a dead short.

Bill

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#127095 - 04/26/01 10:44 PM Re: Dead Shorts
Anonymous
Unregistered


You said the main blew. I think we are to assume that it was a non-GFI circuit breaker that opened with no damage.

Did both have the same interruptor rating?

If the load through the main was already running 200 A before the fault occurred, it is imaginable. The 20 A CB was cold and the 200 A CB was already warmed up.

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#127096 - 04/26/01 11:56 PM Re: Dead Shorts
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Sparky,

Did it happen when you/someone turned on the 20A breaker?

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#127097 - 04/27/01 03:03 AM Re: Dead Shorts
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Dspark, Bill,
It happened when the 20A was turned on, the panel is new, not loaded up as yet, did not amp it out but should be about 40A usage give or take.

The interupter rating should be more for the main, however, as you pointed out, there was a hand holding the 20A !

So, somehow the main saw enough AIC ?

Does this make sense?


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#127098 - 04/27/01 07:32 AM Re: Dead Shorts
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
I had an apprentice miswire a switched leg... wired it like a switch loop... dead short. I should have been watching him closer...

New House, very little load on the main.
The SQ D 100A main tripped before the 15A lighting CB. I replaced the SQ D 15A breaker while the appy rewired the switch (correctly this time).

I never had a good explanation either, but I didn't take any chances... Breakers can be bad out of the box... Even SQ D
_________________________
-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

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#127099 - 04/27/01 03:42 PM Re: Dead Shorts
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Sparky,

That's what I was wondering about. Maybe the hand holding it had something to do with the Main going before the 20A.
Just fishin'


Bill

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#127100 - 04/27/01 06:17 PM Re: Dead Shorts
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
I see what Bill's getting at... in fact, yes, being a dead short, my hand was forcing the CB closed at the time...
_________________________
-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

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#127101 - 04/27/01 08:02 PM Re: Dead Shorts
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
It is my understanding that the short circuit trip curves for most breakers 200 amps and less are almost identical and with the normal manufacturing tolerances it is not unusual for a main to trip out before a smaller branch breaker on a short circuit. Holding the handle of a breaker has nothing to do with its trip time. The breaker will open in the same time even if the exterior handle does not move.
Don(resqcapt19)
_________________________
Don(resqcapt19)

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#127102 - 04/28/01 06:25 PM Re: Dead Shorts
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Don,
thanks, i was wondering about those breaker locks on the fire breakers....

this has me wondering about series fault..


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