Current-limiting fuses contain a strip of metal within them that melts and quickly opens the circuit when the current in the circuit exceeds a rated value. The opening of the fuse is a protection mechanism meant to protect either sensitive electrical components or the wiring in the circuit, depending on the type of fuse. An open fuse means something is wrong that should be corrected before the current is restored.
The two types of current-limiting fuses are cylindrical and plug fuses. Cylindrical fuses have a ribbon of metal that is enclosed in a fiber cylinder and is placed directly in the circuit to protect electrical equipment. Plug fuses are screwed or spring-held into an electrical panel and are used to protect the wiring in homes. The heat that melts the metal link is generated from the current flowing through the resistive element; the heat increases as the current increases. The metal link is sized so that the heat produced by the normal current does not melt it. Although blown fuses need to be replaced, which makes them less convenient than circuit breakers, they are simpler and less costly, and they are more likely to discourage people from ignoring faults in a circuit. They also need little maintenance to operate properly.