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#127093 04/26/01 08:55 PM
Joined: Oct 2000
Posts: 5,392
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sparky Offline OP
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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
[Linked Image]

any input apprectiated

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#127094 04/26/01 11:02 PM
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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


Bill
#127095 04/27/01 12:44 AM
A
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.

#127096 04/27/01 01:56 AM
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Sparky,

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


Bill
#127097 04/27/01 05:03 AM
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sparky Offline OP
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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?

[Linked Image]

#127098 04/27/01 09:32 AM
Joined: Nov 2000
Posts: 2,236
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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
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#127099 04/27/01 05:42 PM
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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'

[Linked Image]
Bill


Bill
#127100 04/27/01 08:17 PM
Joined: Nov 2000
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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
#127101 04/27/01 10:02 PM
Joined: Nov 2000
Posts: 2,148
R
Member
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)
#127102 04/28/01 08:25 PM
Joined: Oct 2000
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sparky Offline OP
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Don,
thanks, i was wondering about those breaker locks on the fire breakers....

this has me wondering about series fault..

[Linked Image]

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