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Not your typical hobbyist component, the fuse is nonetheless important in understanding some basic electrical and electronic properties. Although originally named for their round, cylindrical shape, fuses have been heavily modified since their 19th century invention for various industrial uses. Types include AGC, cartridge (ferrule), plug, ANL, Lucas, and others. Perhaps the most distinguished is the blade-type fuse, seen above, found in nearly every modern automotive fuse box.

the IEEE/ANSI fuse circuit symbol.

The common auto fuse package is described as “Regular,” the others being Mini and Maxi. Again, these merely indicate form-factor, and were standardized around 1976 as more electronic components started making their way into consumer vehicles (the “Mini” fuse was developed in the 90s as both a space-saving and cost-cutting measure). Designed to be both accessible and expendable, this part will cease the flow of electricity should any excess electrical charge beyond the rating of the fuse pass through.

Without getting too complicated (while trusting in Ohm’s Law), that rating is measured in amperes. Remember electrical resistance, or “an object’s opposition to the flow of electricity,” ultimately generates heat. We also know that one ampere is equal to one coulumb/second which is equal to 6.241 × 1018 electrons/second. So, a 3A fuse is rated to pass up to 18,723,000,000,000,000,000 electrons per second! Even one electron more and the fuse will “blow” or melt in order to protect the circuit. The whole process happens quite quickly like this:

3A fuse blow

That backwards-S shape in the fuse package is called the element. The element, itself made of zinc, copper, silver, aluminum, or alloys, is manufactured very precisely to conform to the rating of the fuse. The shape and thickness of the element also relate to its rating. And yes, as with most standards, the color packaging of blade-type fuses also correspond to their numerical value, should their imprint be removed or otherwise illegible. For example, the 3A fuse is violet, 10A is red, and 40A is orange.

Vehicles typically have two fuse boxes, one in the engine compartment and one inside the passenger compartment. The latter is typically located either near the driver’s knees or underneath/around the glove box. Basically anything electrical has a fuse to protect it. And no matter how much I think I understand the awesomeness of this universe, I still get giddy at the thought of those quintillions upon quintillions of electrons flowing through your typical fuse box at any given second! All that charge translates to power locks, dome lights, windshield wipers, power for the radio, and increasingly, embedded systems such as GPS and other networked processes.

Update: The phrase “one electron more” used above has been misunderstood by some, who feel an understanding of how fuses work was falsely presented. Readers took issue with the hyperbole of the phrase, and interpreted it very literally. My point about being “giddy” at the number of electrons in a circuit (and the universe around us) was also missed, and instead the technical specificity of the number and the phrase, contested. Please read the comments below for further insight.


Nick Normal

Nick Normal


  • miroslava von schlochbaum

    So, a 3A fuse is rated to resist up to 18,723,000,000,000,000,000 electrons per second! Even one electron more and the fuse will “blow” or melt in order to protect the circuit.

    …someone’s going to point out that this statement, if held to ridiculous (yet stated) accuracy, would be beyond an engineering feat of epic proportion particularly in a 75cent item, let alone quantum mechanics; and i guess it’ll be stupid ol’ me. otherwise, nice article/summary!

    (i just realized that one can tolerably insert the phrase “let alone quantum mechanics” into nearly any phrase. “you gonna eat that last wing?..let alone quantum mechanics…?”)

    • Nick Normal

      hi miroslava, Obviously a bit of hyperbole never hurt the comedian… let alone quantum mechanics.

      • Erik Walthinsen

        Except that it’s wildly inaccurate and only vaguely touches on how a fuse actually works. It “blows” because a sustained amount of *over*-current causes the filament to overheat and eventually melt, thus breaking the circuit. The actual number of electrons only has the most tenuous of connections to what’s going on.

        • Nick Normal

          hi Erik, The animated GIF you see is in realtime, those are individual frame grabs slowed down and stretched out for visual clarity. There is no long drawn-out amount of current needed, rather the electrons flow that fast. And the “filament” (element) does what I described, “the fuse will “blow” or melt in order to protect the circuit.”

          • miroslava von schlochbaum

            aw you shoulda held at your first reply. it was endearing and disarming. now it looks like you’re attempting to defend provably inaccurate statements. time to move on to zener diodes ;)

          • omegacs

            Yes, you describe the mechanism fairly well in general, but as soon as you talk about “one electron too many” you’re strongly implying that the fuse *counts* electrons and trips at a specific point. I would have no issue at all with the rest of your description if you just plain left that part out.

            As far as the duration, there are fast- and slow-blow fuses, with different ratings for the current-over-time requirements before it will blow. Any fuse can take more than its rated current for an amount of time inversely proportional to the amount of overcurrent. In some cases they will explicitly spec what kind of pulsed overcurrent will *not* blow the fuse.

            It all has to do with temperature rise in the filament before melting point. I couldn’t tell you the exact equations, but basically Ohm’s law gives you watts dissipated by the filament as a function of current, which due to the relative thermal isolation inside the casing will build up the filament temperature until it melts. Pulsed currents don’t cause the filament to heat up enough before the current drops and thus temperature drops back down again. A high ambient temperature would change that point, for example.

        • Nick Normal

          hi Erik, Also, for example, you could fill the fuse package with sand, to dissipate heat – the point here is there are ways to cheat every “law”. I think scrutinizing the “one electron more” phrase is a stretch on your part. Thanks for reading!

          • omegacs

            I’m not trying to over-scrutinize anything, and I’m fully aware that there are many additional aspects to the physics involved. I went into the macro-scale physics of fuses in my replies because there *are* several major factors in why fuses blow when they do. Filling them with sand has nothing to do with their functioning except making them *not* do what they’re supposed to.

            I take issue with the “one electron more” phrase because the post purports to describe fuses to people who presumably don’t understand how they work. As such they are unable to differentiate between the facts and “poetic license”, and thus the hyperbole does these people a disservice. I’d like to think that’s supposed to be the difference between MAKE et al and mass-market “journalism”.

        • Nick Normal

          hi Erik, Well I still think that’s a bit of a stretch, but thanks for the exchange! And just so you know, filling the fuse with sand is exactly, to me, what maker culture is about. And again, like I said via email, I invite you to write a blog that I will link to or submit for publication here, an article about how fuses actually work and their mechanism as you see it. Cheers!

          • omegacs

            I guess I wasn’t clear enough: Fuses and their details have nothing at all to do with my problem with “one extra electron”. I don’t care about fuses.

            My problem is with the use of such extreme hyperbole that is indistinguishable from fact by people who don’t know the topic, in the middle of a piece that’s clearly targeted at such people. I expect that from a newspaper or TV new show, I was hoping not to see it on MAKE.

  • Jason

    As others have stated, the description of how and why a fuse blows is wildly inaccurate.

    Fuses blow due to heat, not “one extra electron.” There are fast blow fuses and slow blow fuses, and even fast blow fuses will more than likely never blow with just “one extra electron” over their rated current. Every type of fuse should have a spec sheet along with it indicating the time it takes to blow at various current levels, good datasheets will also have derating information based on ambient temperatures.

    While I have no doubt the fuse in the animated GIF blew so suddenly, somehow I doubt it was being fed with just “one extra electron.” More than likely, it was a straight short from a battery producing more than enough current to make it blow immediately.

    For example, that 3A fuse won’t blow at all at exactly 3A, at 3.3A it may still never blow, at 4A it may take as much ten minutes before it blows (or as little as .750 seconds, Littelfuse’s datasheet gives a very wide range for such little overcurrent).

    Look over this datasheet, and read a lot more about fuses:

    Frankly, I’m disappointed. I expect a MUCH higher quality of reporting out of Make.

    • Nick Normal

      hi Jason, I’m sorry that you are disappointed. I would also invite you, as I have invited others, to write an article explaining the technical function of fuses and how their Opening Times, Cold Resistance, l2t, and Ambient Temperatures relate to their overall rating and ability to process current. For example what variables would cause the same 3A fuse to blow after 10 minutes or .750 seconds? An educational understanding of this condition would be extremely helpful for everyone.

      • Jason

        It’d be much easier for you simply do your due diligence before posting things like this… or in this case, editing them to reflect the actual facts. It wouldn’t have taken much on your part to get this somewhere in the ballpark of being correct.

        I generally don’t consider Make to be on par with Yahoo Answers, but this post most certainly is.

        • Nick Normal

          The invitation remains open. Thank you.

          • Jason

            Instead of suggesting, daring, others to write their own tutorial, you would do better to accept responsibility for your poor work and fix it.

  • m000z0rz

    I think omegacs worded this perfectly:


    I felt like it was presented as a nifty science fact instead of an exaggeration, which I fear is misleading. Fuses work much fuzzier than that.

    Other than that, great article. The .gif of the fuse blowing is excellent!

  • jamesbx

    The photos make fuse testing look so clinical and efficient. My grim reality has included checking fuses in the dark, with a dim flashlight, on the side of the road, in the rain, in a bad neighborhood, that I drove into with no headlights, when the wiring on trailer I was towing shorted to ground. I cut the supply to the trailer lights, and swapped a less critical fuse, then drove it on home. Since then, I supply trailer lights with a 12v circuit breaker, not a fuse.

    And isn’t there is some number of electrons, that when exceeded by one, will cause the metal in the fuse to undergo a phase state transformation? I thought the molecular lattice can accommodate some number of electrons, and when that value is exceed, I’m pretty sure a plasma briefly forms and things push apart. It is different than something like nichrome wire, that gets progressively hotter as the electron flow increases. The The 3A fuse rating, as described as a rate of electron flow in the article, only implies five significant figures of accuracy, not 20. So within that value, it is perfectly reasonable that there is some exact value, that when exceeded by one, will blow a fuse.

  • Chris

    Like others, I take issue with the “one electron more” phrase in an otherwise good article.

    I’m also disappointed that Nick has not taken previous comments on this issue as constructive criticism. Instead, he points out that at least he wrote something; then challenges us to write our own, better tutorial, if we don’t like his. In essence, he is telling us to “put up or shut up”.

    But we don’t really need another fuse tutorial. There are plenty of good ones out there already, ranging from beginner to advanced. While more are still welcome, writing a misleading tutorial is worse than writing nothing at all.

    Stamping out false information is best done immediately and at the source, before it gets ingrained; or worse, repeated to others. Thus we’ve done the best we can do, by commenting here.

    Of course, it would be better if Nick were to simply correct the article. So on behalf of myself and the rest of the commenters (since all have taken issue with this phrase, without exception), I hereby challenge Nick to do so.

  • R. Mark Adams

    Best animated GIF ever!

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