View this animation to see what happens when a ground fault develops. Fault current travels on the branch circuit equipment grounding conductor to the equipment bar in the subpanel. Then it returns to the equipment bar in the service panel, over the main bonding jumper to the grounded conductor terminal, back to the utility transformer and back down the faulted phase all the way to the branch circuit breaker. If the effective ground-fault current path is a low resistance path, then the fault current will be high enough to trip the branch circuit breaker and take the faulted circuit offline.
