A fuse acts as the weakest link in an electrical circuit, designed to permanently open when current exceeds a specified rating for a certain duration. The internal element heats up and vaporizes under fault conditions, extinguishing the resulting arc within a filler material like silica sand. Once operated, it must be physically replaced.
We lost phase B because the 400 amp fuse cleared a downstream fault on the feeder.
Always check the breaking capacity (kA rating) of a fuse, not just its normal current rating. A fuse installed where the fault current exceeds its breaking capacity can explode instead of clearing the fault.
Do not mix it up with
- Circuit breaker: A fuse is a one-time sacrificial device, whereas a circuit breaker can be reset after tripping.
Related terms
Blow a fuse
To draw enough overcurrent to melt the sacrificial element in a fuse and break the circuit.
Circuit breaker
A mechanical switching device capable of making, carrying and breaking currents under normal and fault conditions.
Overcurrent protective device
A component designed to interrupt an electrical circuit when the current exceeds a predetermined safe limit.
Fuse wire
A bare, easily melted metallic strand used as the replaceable sacrificial element in older rewirable protective devices.