what causes a breaker to trip

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marinesgt0411

marinesgt0411

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Naples,FL 34110-9285
on a short circuit hot terminal on receptical came in contact with metal box breaker for circuit did not trip but breaker for panel did

why did the 150 amp main trip and not the 20 amp in the panel
 
Breaker has different types according to its point of application. When breaker is equipped with metal stip for tripping, It's either thermal or magnetic, Most common nowadays is thermal-magnetic. Thermal tripping answers for overloading, and magnetic for short circuit. Most breakers in our home have no protection for human, it offers only to protect wires from overloading and short circuit so that fire will not be created whichever case may happen.

What was happen to your story was single line to ground fault, for a 20A breaker, instantaneous tripping is not possible, otherwise you have to wait for a long time before it would be able to clear the fault (depending on its characteristic curve). For you to clear this kind of fault, you have to use GFCI or ELCB breaker which can sense even milli amps and able to trip the breaker. Your 150A breaker was tripped because of the following reasons:
1. Your 150A breaker is equipped with ground fault protection or
2. its tripping curve intercept the prolonged sustained low level fault in the circuit.

I hope I have answer your question.

Charlie D.
 
Charlie,
What was happen to your story was single line to ground fault, for a 20A breaker, instantaneous tripping is not possible, otherwise you have to wait for a long time before it would be able to clear the fault (depending on its characteristic curve). For you to clear this kind of fault, you have to use GFCI or ELCB breaker which can sense even milli amps and able to trip the breaker. Your 150A breaker was tripped because of the following reasons:
1. Your 150A breaker is equipped with ground fault protection or
2. its tripping curve intercept the prolonged sustained low level fault in the circuit.
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I don't think so. These types of faults should flow enough current to opertate the breaker in the instanuous trip (magnetic trip) of the breakers. The problem is that the mag trip curves are overlapping for small breakers (under 400 amps or so) and with the overlapping curves and normal operating tolerences of the two breakers it is not uncommon that a ground fault or short circuit will cause the upstream breaker to open before the downstream one.
Don
 
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