Slow-blow fuse is a type of fuse which can handle a temporary surge current which exceeds the current rating of the fuse. A slow-blow fuse does not blow for a temporary surge current; it must receive a sustained elevated current above its current rating for a period of time before it blows. This is why it is called a slow-blow fuse.
A typical fuse will blow right away once it receives current above its current rating. This means that any momentary surge will make the fuse blow. Thus, the fuse will break open and the circuit will no longer work. A slow-blow fuse, though, is constructed different internally. It has an element with a coiled construction and is designed to open only on a continued overload such as a short circuit.
Circuit designers would use a slow-blow fuse when it is desired for a circuit to continue to work even after temporary spikes of current received such as a circuit being hit by lightning. Slow-blow fuses only blow when it receives an elevated sustained current spike over a period of time, such as a short circuit. Thus, slow-blow fuses are not concerned with momentary spikes of current but only continued overloads.
Being that they work in this fashion, slow-blow fuses are perfect devices to protect electronic circuitry caused by a short circuit. A short circuit is a circuit where an unintended connection is made where a part of the circuit may receive more voltage than it should.
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