am installing a subpanel in an outbuilding and understand that I need to use a four wire feed and keep the ground separate from the neutral at the subpanel. I know that is code, but I like to understand how things work. I am trying to conceptualize how a ground fault on a circuit in the outbuilding would trip a breaker and which would trip. There are four breakers involved. The main, the breaker supplying the subpanel, the "main" at the subpanel and the final circuit breaker. If a ground fault occurred, which one would trip? It would seem the final would need to, in order to isolate the problem. Does the surge caused by the fault travel all the way back to the main through the ground wire then back to the final breaker?
The smallest (amperage) and closest (in protection) to the fault should trip first. So if it's caused by a branch circuit in the sub, then the breaker in the sub would trip.
Think of it as the fault is working back to the main, yes, but along both lines - the ground and the hot supplying it. The hot line will reach the nearest breaker and trip.
