When overcurrent devices are selectively coordinated and a fault occurs somewhere on a branch circuit, what should happen?

When overcurrent devices are selectively coordinated and a fault occurs somewhere on a branch circuit, what should happen?

• A. the branch circuit OCPD that is nearest the fault should open
• B. the feeder OCPD should open
• C. both a & b
• D. neither a nor b

Answer: (A)

Selective coordination aims to isolate faults with the device closest to the problem while keeping everything upstream energized. When a fault occurs on a branch circuit, the protective device on that branch nearest the fault should trip, opening the circuit and containing the fault. This minimizes disruption, so power remains on for the rest of the system and only the affected branch is disconnected. If the feeder opened instead, a larger portion of the system would lose power, defeating the goal of selective coordination. Similarly, having both devices open would cause unnecessary loss of service. So the branch circuit protection closest to the fault is the one that should open.

Selective coordination aims to isolate faults with the device closest to the problem while keeping everything upstream energized. When a fault occurs on a branch circuit, the protective device on that branch nearest the fault should trip, opening the circuit and containing the fault. This minimizes disruption, so power remains on for the rest of the system and only the affected branch is disconnected. If the feeder opened instead, a larger portion of the system would lose power, defeating the goal of selective coordination. Similarly, having both devices open would cause unnecessary loss of service. So the branch circuit protection closest to the fault is the one that should open.