safety device used to protect an electric circuit
against an excessive current. A fuse consists essentially of a strip of low-melting alloy enclosed in a suitable housing. It is connected in series with the circuit it is to protect. Because of its electrical resistance
, the alloy strip in the fuse is heated by an electric current; if the current exceeds the safe value for which the fuse was designed, the strip melts, opening the circuit and stopping the current. The fuse housing is designed to resist the pressure generated if the overcurrent vaporizes the alloy strip, provided the voltage across the fuse does not exceed its rating. Some fuses, called slow-blow fuses, are designed to carry a small overload for a short time without opening the circuit, while others are designed to open very rapidly if the rated current is exceeded. The choice of one type or the other depends on the ruggedness of the equipment to be protected and whether large pulses of current often occur in the circuit; a slow-blow fuse is usually used to protect motors, and a fast-blow fuse to protect electronic equipment. A circuit can also be protected by a circuit breaker
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