Fuse

Fuse, safety device used to protect an electrical circuit from the effect of excessive current. Its essential component is usually a strip of metal that will melt at a given temperature. A fuse is so designed that the strip of metal can easily be placed in the electric circuit. If the current in the circuit exceeds a predetermined value, the fusible metal will melt and thus break, or open, the circuit. Devices used to detonate explosives are also called fuses (see Explosives).

Two types of fuses are commonly used, cylindrical fuses and plug fuses. A cylindrical fuse consists of a ribbon of fusible metal enclosed in a ceramic or fiber cylinder. Metal end caps fastened over the cylinder make contact with the metal ribbon. This type of fuse is placed in an electric circuit so that the current must flow through the metal strip to complete the circuit. If excess current surges through the circuit, the metal link will heat to its melting point and break. This action will open the circuit, stop the current flow, and thus protect the circuit. The cylindrical type of fuse is used mostly to protect electrical equipment and appliances.

Plug fuses are commonly used to protect electric wiring in homes. This type also consists of a fusible metal strip through which the current must flow to complete the circuit. The strip is, however, enclosed in a plug that can be screwed into an ordinary electric receptacle or light socket. Plug fuses usually have a mica window at the base of the plug so that the condition of the metal strip can be seen at a glance.

Recent fuse developments include types that will permit a momentary overload without breaking the circuit. These are necessary for circuits that are used to power air conditioners or electric broilers, because initial surges of power can be expected with such appliances. Another recently developed type of fuse contains several links that can be selected by the flip of a switch. If the fuse is blown, another link can be switched in without replacing the fuse.

In high-voltage circuits, subject to frequent interruptions, and increasingly in residential wiring, protection is provided by circuit breakers instead of fuses. See Electric Power Systems.