Which protective device is non-resettable after it opens and must be replaced?

Which protective device is non-resettable after it opens and must be replaced?

• A. Fuse
• B. Circuit breaker
• C. Surge protector
• D. Resettable fuse

Answer: (A)

A fuse is a one-time protective device that opens permanently when the current exceeds its rated value. Inside a fuse is a thin conductor that melts due to heat generated by overcurrent, creating an open circuit. Once melted, it cannot be reset and must be replaced with a new fuse of the correct rating. This behavior is what makes it the non-resettable option after opening. In contrast, a circuit breaker trips but can be reset after the fault is cleared, a resettable fuse (polyfuse) returns to low resistance after cooling, and surge protectors primarily clamp voltage rather than rely on a one-time interruption.

A fuse is a one-time protective device that opens permanently when the current exceeds its rated value. Inside a fuse is a thin conductor that melts due to heat generated by overcurrent, creating an open circuit. Once melted, it cannot be reset and must be replaced with a new fuse of the correct rating. This behavior is what makes it the non-resettable option after opening. In contrast, a circuit breaker trips but can be reset after the fault is cleared, a resettable fuse (polyfuse) returns to low resistance after cooling, and surge protectors primarily clamp voltage rather than rely on a one-time interruption.