Tripped main

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JdoubleU

Senior Member
I was on vacation when I recieved a call about a main breaker tripping. The LMEs were replacing a switch and the hot conductor touched the metal box. Instead of the 20 amp single pole breaker tripping, the main breaker in the main electrical room that feeds two panels tripped. Why wouldn't the 20 amp breaker trip first?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Jake,
When you are working with molded case breakers where the fault current is in the instantaneous trip range of both the branch and feeder breaker, either one or both of the breakers may trip.
Don
 

brian john

Senior Member
Location
Leesburg, VA
In addition was the Main we are talking about protected with GFP? It trips for the same reason.

Your electricians had NO BUSINESS replacing a switch hot in this circumstance. A misunderstanding of the consequences involved in working with energized conductors.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Tom,
I was told by a fuse rep that fuses in the same frame size and type may not blow as we would expect.
Fuses have a selectivity ratio that you can find in the manufacturer's literature If you have two fuses in series and the are sized per the ratio then the one closest to the fault will always open first. Example if the ratio is 2:1 and you have a 20 amp backed up by a 40 amp, the 20 will open with out opening the 40. If you have the 20 backed up by a 30, then the 30 may open. Note that the selectivity ratio only applies within the same brand of fuses.
Don
 

JdoubleU

Senior Member
brian john said:
In addition was the Main we are talking about protected with GFP? It trips for the same reason.

Your electricians had NO BUSINESS replacing a switch hot in this circumstance. A misunderstanding of the consequences involved in working with energized conductors.

I agree, the LMEs know not to and this will be adressed. Thank you for your concern. I will look into the GFP part.
 

Energy-Miser

Senior Member
Location
Maryland
don_resqcapt19 said:
Tom,

Fuses have a selectivity ratio that you can find in the manufacturer's literature If you have two fuses in series and the are sized per the ratio then the one closest to the fault will always open first. Example if the ratio is 2:1 and you have a 20 amp backed up by a 40 amp, the 20 will open with out opening the 40. If you have the 20 backed up by a 30, then the 30 may open. Note that the selectivity ratio only applies within the same brand of fuses.
Don
Good point about selectivity. Here however, assuming a 200 amp main breaker (since it feeds two panels, 200 amp is probably reasonable to assume), then your selectivity must be over 10 (200 : 20) to cause the main to open before the 20 amp breaker. I don't know if selectivity of 10 is reasonable. Could it be that the 20 amp breaker malfunctioned and developed a bolted short internally, causing the main to trip instead?
 

brian john

Senior Member
Location
Leesburg, VA
Commercial facilty with maintenance men I'd almost put my money on 480/277 with GFP. But I do not gamble other that driving the DC beltway everyday and then there is the being in business stuff.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
energy-miser,
Good point about selectivity. Here however, assuming a 200 amp main breaker (since it feeds two panels, 200 amp is probably reasonable to assume), then your selectivity must be over 10 (200 : 20) to cause the main to open before the 20 amp breaker. I don't know if selectivity of 10 is reasonable. Could it be that the 20 amp breaker malfunctioned and developed a bolted short internally, causing the main to trip instead?
Breakers don't work the same way as fuses. If the fault current at the point of the fault is in the instantaneous trip range of the branch breaker and the main breaker then either one or both of the breakers will trip. It really doesn't matter what the ratio of sizes is. It is not uncommon to have a fault on the load side of a 20 amp breaker take out a 400 amp breaker when the system is supplied by a source that has a large available fault current.
Don
 

ELA

Senior Member
I have experienced this when the "local" -lower amperage branch circuit breaker was had a long time curve ( installed due to nuisance trips) due to a device with a large inrush demand. The larger amperage main breakers instantaneous curve was near the same or faster.
 
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