A fuse is a device used to limit the current in a circuit. it consists of a thin conductor which has a slight resistance; when the current flow becomes too large, the energy dumped into the resistor will raise the temperature of the conductor so that it fails. Although they are very simple, and generally reliable, they have the disadvatage that, once actuated, they have to be replaced; they cannot be repaired. Although they were once common, they have generally been replaced with circuit breakers (which perform exactly the same function); although they are still to be met with in special circumstances - e.g. if there is not enough room (e.g. on a PCB) for a circuit breaker.