What device is commonly used to protect circuits by melting under excessive current?

The device that is commonly used to protect circuits by melting under excessive current is a fuse. Fuses are designed as safety devices to prevent excessive current from flowing through a circuit. When the current exceeds a specific threshold, the fuse element, typically made of metal, heats up and eventually melts, breaking the circuit. This action stops the flow of current, thereby protecting other components in the system from potential damage due to overheating or electrical fires.

Fuses are critical in electrical systems because they provide a simple, reliable means of overcurrent protection. Once a fuse has blown, it must be replaced in order for the circuit to function again, serving as both a protective measure and a signal that an overcurrent condition occurred. This contrasts with other devices such as capacitors, transformers, and resistors, which serve different functions in electrical circuits and do not provide the same level of protection against excessive current.