What device protects a circuit by melting when excess current passes through?

The device that protects a circuit by melting when excess current passes through is a fuse. A fuse contains a thin wire or metal strip that is designed to melt and break the electrical circuit when the current exceeds a certain limit. This interruption prevents overheating and potential damage to electrical devices or the wiring itself, effectively safeguarding the system from excess current.

Fuses are crucial for safety in electrical circuits, acting quickly to minimize potential hazards from overloads. Unlike circuit breakers, which can be reset after activation, a fuse needs to be replaced after it has melted. Therefore, its function is specifically geared toward one-time protection.

The other options, while related to electrical systems, serve different purposes. A circuit breaker also provides protection but does so by tripping and can be reset without replacement. A transformer is used to change the voltage of electricity, and a resistor limits the flow of current in a circuit. Understanding these distinctions is essential for grasping the role of a fuse in electrical safety systems.