What is a Slow-blow Fuse?

A 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.

A circuit designer 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.

As an example, let's say that a circuit is powered by 15 volts. The circuit needs this 15 volts in order for certain parts of it to work. The voltage is allocated where a DC motor receives 12 volts in order to run while a fan inside of it only needs 3 volts. Normally, the circuit would be wired so that the motor receives the 12 volts and the fan receives 3 volts. But, as happens many times with circuits (unintended), the wires in the circuit or traces of copper wire may make undesired connections. It is possible that the 3 volts can make connection with the 12V voltage line or 15 volts. If this is the case and a slow-blow fuse is connected to it, the fuse will blow to protect against this short circuit. Thus, the fan may be saved. Many times electronic components can receive more voltage than rated for and still survive if not under these conditions for too long. So slow-blow fuses can protect electronic circuitry in this way. The same can be applied to the DC motor. It may make unintended, sustained connection with the 15V line. When this is the case, it will eventually blow so that current will no longer flow to the motor. This, too, may be a protective mechanism which can save the motor for sustained elevated current levels.

And this is the type of applications that these fuses have. They can protect electronic circuitry and save them, since they cut off sustained current when short circuits do occur. They do not protect against momentary surges (ordinary ones do), but they do stop sustained surges.

And just like ordinary fuses, they are given standard current ratings. Thus, there are 1 amp, 2 amp, etc. slow-blow fuses. A 1 amp fuse would be able to tolerate momentary spikes above 1 amp without blowing and opening up. However, if it received above 1 amp for a period of time, it would give way and blow.

Related Resources

How Fuses Work

How to Test a Fuse