Tripped main

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jsilva

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Yesterday a friend of mine was changing a light switch, when the sw. ground out to the steel box and tripped a 100 amp 3 pole breaker in a distribuition panel and a 150 amp 3 pole main that feeds the distribuition panel. -----The light switch is fed from a 20 amp 1 pole c/b from a sub-panel where the feed comes from the 100 amp c/b.----This 20 amp c/b did not trip. In another word both the 100 and 150 amp tripped, but not the 20 amp 1 pole. Does any of you have any idea why this occurred.
Thanks Guys.
 
There are several possibilities, but number one may be that the 20 amp is defective.

Additionally a 100 and a 150 protecting a 20 amp circuit may be to close in size and the time current curves may overlap.
 
This could be caused by the lack of selective coordination as listed in several locations in the code. Essentialy some overcurrent devices respond faster so a short as you described it may peak out at amperages to trip all the breakers in the circuit but only the ones that are designed to trip faster will have the chance to respond. I had a similar condition at a school where a 20 amp lighting circuit was shorted and a four hundred amp breaker tripped. The 20 amp breaker was tested and found to be fine.
 
Another possibility is that the 100 A and 150 A tripped on electronic ground fault.

It is strange that both tripped but again wehave to go to the co-ordination study.

Jim
 
This very common with 277/480 volt systems, as others have said, it can be the coordination of the breakers. Not very common though for 120/208 volt systems to do this though. You did not metion the system voltage though.
 
As brian john said It's not that unusual an the fault current can be of such a magnetude that it falls within the trip curve of both breakers. Remeber that faults are instantaneous and are not base upon time and as such it is pot luck which breaker will trip should the fault fall within the trip range of bothe breakers.
 
I have seen this many times. I usually just reset the tripped breaker and not think to much about it.
 
The amount of current available to the fault will make a difference in how much current flows. This will vary as the length of the circuit to the point of the fault changes.

If you have 20 amp circuit with a fault 5 feet from the overcurrent device you will have a higher fault current than if the fault were 100 feet from the OC device. If the panel with the 20 amp OC device is located at the service equipment vs at the end of a long feeder will also result in different fault current.

All of the fault current regardless of what magnitude will flow through service, and feeders ahead of the final OC device and if high enough magnitude to cause trip will trip.

If it is important to not have service or feeders trip then careful selection of devices is necessary or designs that create enough distance to reduce the available fault current can work also.
 
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