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Fuses placed in series with the path of some current flow are protective of excessive current flow taking place in that current path. Although fuses are rated for clearing in terms of “I²t” where “I” is in amperes and “t” is time, my personal view is that such ratings are of dubious value from a protective calculation standpoint. To me, a fuse is either a “fast blow” device or a “slow blow” device at whatever amperage applies and which type of fuse to select isn’t always cut and dried, straight forward, or unambiguously obvious.
Some fuses contain their innards inside of glass which allows you to see the current carrying element and some do not. Where glass lets you see that element, actual fuse blowouts can be instructive in terms of the overload condition that led to that blowout and can perhaps lead you to reselecting that fuse’s rating or to fixing some other problem.
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Figure 1 An intact fuse and two blown fuses: one from a moderate current overload and one from a massive current overload.
The middle case in the above figure is a fuse that is probably underrated for whatever application it has been serving. Using a somewhat higher I²t device might be a good idea.
However, the lower case shows a fuse that got hit with a blast of overload current that was way, way, way beyond reason and something elsewhere in the system in which this fuse played a role had just plain better be corrected.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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