For selective coordination with an upstream fuse of larger amperage, which must clear first?

For selective coordination with an upstream fuse of larger amperage, which must clear first?

• A. Upstream fuse
• B. Downstream fuse
• C. They clear simultaneously
• D. It depends on ambient temperature

Answer: (B)

Selective coordination aims to isolate only the faulted portion of the system. The protective device closest to the fault should operate first so that the rest of the system stays energized. If the upstream device has a larger amperage rating, it is designed to allow more current and generally has a slower response for the same overcurrent than a downstream device with a smaller rating. That means the downstream fuse will reach its trip condition sooner for the same fault current and clear first, leaving the upstream supply intact for other loads. Ambient temperature can affect the fuse’s current-carrying capacity, but the basic order of operation in selective coordination is determined by the time-current characteristics of the devices.

Selective coordination aims to isolate only the faulted portion of the system. The protective device closest to the fault should operate first so that the rest of the system stays energized. If the upstream device has a larger amperage rating, it is designed to allow more current and generally has a slower response for the same overcurrent than a downstream device with a smaller rating. That means the downstream fuse will reach its trip condition sooner for the same fault current and clear first, leaving the upstream supply intact for other loads.

Ambient temperature can affect the fuse’s current-carrying capacity, but the basic order of operation in selective coordination is determined by the time-current characteristics of the devices.