A Fuse is a Fuse? – Not Quite!

The purpose of fuses is relatively easy to grasp. A fuse interrupts a circuit (turns off) if the current flow (amps) is more than expected or more than the circuit components including wires etc. can safely handle. High current is usually the result of a short circuit, the failure of a device connected to the circuit (as an appliance) or overloading (as in the case of too many devices connected to a single circuit in your home). At first it would seem that fuses and circuit breakers are pretty straight forward. In its simplest form, a fuse is no more than a piece of wire that will melt or “blow” if the current flow exceeds the fuse rating. Discussion over? Not quite!

Response Time

There are many different types and ratings of fuses – many more than I can address here but let’s hit one of the highlights.  One of the characteristics of a fuse is how long it will take it to blow in an overcurrent situation.  The terms generally applied to fuses are fast blow, medium and slow blow.  Each is designed differently to produce the desired time response to a particular level of overcurrent.


Fuses can be designed to respond differently under various overcurrent conditions. The above is an example and should not be used for specification purposes under any conditions.

Slow Blow

One typical use for a slow blow fuse is an application which requires a high current for a short period of time and a lesser amount of current for an extended time.  The starting of a powerful motor or the impulse of a resistance welder would be a good examples here.  A current limiting device for an application such as this must allow high current for a short period of time but prevent long term overcurrent which might indicate a failure or malfunction. In many cases the short term current rating may need to be several times that of the long term current.  In addition, however, there may be a current limit in the same application that results from a catastrophic condition where current must be removed immediately (such as a short circuit). Now, instead of just sensing a long term elevated current, the appropriate current limiting device must sense two different current levels providing a suitable, and perhaps different, response to each.  This could be accomplished using two fuses in series as shown below, but many individual fuses accomplish this in a single device.

Fast Blow

There are also applications where even the shortest duration overcurrent indicates a problem requiring shut down. One example of this would be a partial short circuit caused by intermittent arcing between wires with each instance of arcing too short blow a medium response fuse.  At the same time, repeated arcing might ignite a fire.  A good example of this is kind of failure is a heating pad or electric blanket in a home.  An intermittent arc could start a fire even though the current draw is not sufficient or sustained for a long enough period to cause a fuse with a slow response time to interrupt the current.

The response time is far from the only variable characteristic of fuses that must be considered in their selection.  I’ll try to discuss some of these in upcoming blogs.  In response to these and other variations in need, many special purpose current limiting devices have been developed. In the design of electrical equipment, designers take all things into consideration when specifying current limiting devices. When replacing them, maintenance personnel should be careful that the replacement has the same properties as the devices being replaced. A “fast blow” fuse, for example should not replaced with a “slow blow” fuse. A device rated for high short circuit current flow or open circuit voltage should not be replaced with one without an equivalent rating.  Gone are the days when dad went down to the fuse box and put a penny behind the blown fuse when mom wanted to use the iron and the washing machine at the same time!

 –  JF  –

Leave a Reply