Which protective device melts its fusible link when current exceeds safe levels?

• A. Resistor
• B. Capacitor
• C. Circuit Breaker
• D. Fuse

Answer: (D)

Overcurrent protection that relies on a fusible element works by sacrificing the device to open the circuit when the current is too high. Inside a fuse is a thin metal link that melts when it overheats from excess current, creating an open circuit and stopping the flow to prevent damage. That self-destruct mechanism is the defining feature of a fuse, making it the correct choice here.

A circuit breaker, while also protecting against overloads, does so by tripping a mechanism (thermal or magnetic) and can be reset after the fault, rather than melting a link. Resistors and capacitors are not protective devices that disconnect circuits; a resistor limits current and dissipates heat, while a capacitor stores energy and can fail in other ways, but neither opens the circuit on overload like a fuse.