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According to answers to this question there seem to be a standardized notation on fuses. For example F8AL250V means that it's a 8A fast fuse.

On the other hand there is answers this question stating the importance of checking the datasheet for specifications on when the fuse should blow.

Pulling these answers together appears to be contradicting. While F8AL250V says 8A fast fuse you seem to have to consult the datasheet to find out what that means (which makes the notation doesn't mean anything in particular). This would also be unfortunate since it would require an end user to consult datasheets (he's not likely to understand) instead of just insisting him to use the F8AL250V fuse.

Is there a standard of requirements on fast and slow fuses regarding time-current characteristics? What does the standard say about the time before the fuse blows for various over-currents?

My specific situation is that I'm considering a power supply (Velleman FPS1320M), the load may briefely draw more than 8A. The question is if a fast 8A fuse will be adequate. For example what amount of time can I expect the fuse to withstand 16A at a minimum (I don't have the number, but I think it's just needed to last for a minute or so) and how fast can I expect it to blow (according to the PS manual it has to blow within two hours). How fast can I expect it to blow at 20 or 22A (needs to blow within 30 respectively 1 minute).

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3 Answers 3

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Individual manufacturers may have a consistent naming systems to identify slow, really slow, fast, turbo fast, or whatever, within their product line. Some smaller manufacturers may have copied parts of the designations from a dominant manufacturer, but you shouldn't count on that. Ultimately, only the datasheet tells you how fast or slow fast or slow really are.

End users shouldn't have to consult datasheets, and can't be counted onto understand them if they did. This is no different from any other component in a device. The way to handle this is to tell them the replacement part number explicitly.

For a common type of fuse, you can write something like "1A 250VAC" near the fuse holder, but that should be for convenience at best. Somewhere in your documentation it should say that the fuse is a Acme ICUB4UCME-1A, or equivalent. If they put in something else, the liability is now on them.

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  • \$\begingroup\$ Then why do those sites sell generic items like "8A fast fuse"? According to you, that description doesn't mean anything useful, a designer can't rely on that being the right part and the user can't rely on that being the correct replacement part. \$\endgroup\$
    – skyking
    Dec 28, 2016 at 7:27
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    \$\begingroup\$ @sky: "8A fast" is a rough enough spec for many cases. At the least it means a fuse that isn't deliberately manufactured to blow slowly. "Fast" could still vary across manufacturers, but fuses are so sloppy anyway that often this variation can be ignored. Whether a fuse blows in 200 ms or 500 ms, it's probably still good enough to prevent the circuit from catching on fire. \$\endgroup\$
    – Olin Lathrop
    Dec 28, 2016 at 11:51
  • \$\begingroup\$ I believe that manufacturers don't usually describe fuses as fast unless they are especially fast. For a little glass fuse, you are likely to find the boxes marked only 250 V, 8 A and 250 V, 8 A, slow blow. \$\endgroup\$
    – user80875
    Dec 28, 2016 at 12:06
  • \$\begingroup\$ @OlinLathrop For "8A fast" should be enough for any reasonable application it would need to be some specification that guarantees it to be enough. Some I've seen will stand 16A for minutes (typically 30), but without a specification there's no guarantee that it blows then. If such lack of requirement would be enough you could probably just as well replace the fuse with a 3-inch nail. \$\endgroup\$
    – skyking
    Dec 28, 2016 at 12:21
  • \$\begingroup\$ @Charles: Good point. I was just reacting to what sky said, but you're right. Generally a fuse is either "slow blow", or nothing special is specified about speed at all. No special mention of speed means "this is a fusable link designed for convenience of manufacture and cost". Slow means "we went out of our way to slow down the response of this fuse". In many cases, these catagories, while not definitive for speed, are good enough to get the right fuse. \$\endgroup\$
    – Olin Lathrop
    Dec 28, 2016 at 12:21
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Every manufacturer has their own numbering system. Some numbers tell more about the products than others. It seems fairly easy to interpret that F8AL250V means an 8 amp, 250 volt fast-blowing fuse. It is a little more difficult to tell that it is a 5 X 20 mm glass tube fuse. Littelfuse shows 217008 as a replacement and Bussmann shows S500-8-R. Bussman also offers a version that has wire leads. Both Littelfuse and Bussmann give the interrupting rating as 80 amps. It appears that the F8AL250V manufacturer number is used by ShengBaiDe Electronics of China. It is customary for a product manufacturer to mark the product or provide in the instructions brand and number of the original fuse. If the end user can not find that fuse, their best alternative is to find a cross reference chart provided by another manufacturer and use the replacement recommended there. Littelfuse and Bussmann both have online cross reference search tools. They are left with some risk that the selected fuse will not protect the equipment as well as the original fuse did.

If a designer wants to find an alternative fuse, it is best to consult the datasheets. Datasheets provide curves and other information that should be used when selecting a fuse for a new design.

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  • \$\begingroup\$ Then why do those sites sell generic items like "8A fast fuse"? According to you, that description doesn't mean anything useful, a designer can't rely on that being the right part and the user can't rely on that being the correct replacement part. \$\endgroup\$
    – skyking
    Dec 28, 2016 at 7:28
  • \$\begingroup\$ I don't believe that manufacturers make very different fuses of the same type and basic ratings for use in the same fuse holder. There are standards for fuses that may put limits on 5 X 20 mm glass tube fuses for example. Designers can rely on the manufacturer data for a given part number and match that to the original design requirements or to the data for the original fuse that was used. However, they may need to run their own tests to completely validate fuses from different manufacturers. For a given fuse holder, there will be only one or two types, a standard (fast) fuse or a slow fuse. \$\endgroup\$
    – user80875
    Dec 28, 2016 at 11:59
  • \$\begingroup\$ According to your comments on Olin Lathrop's answer there is formal standards that say something on the time-current characteristics (right?). The question was if there's such a standard and your answer is lacking that. \$\endgroup\$
    – skyking
    Dec 29, 2016 at 8:10
  • \$\begingroup\$ I read some general comments about standards that indicate some standards may say something about the maximum time a fuse can allow a certain percentage of nominal current. For a given specific situation, I don't think finding the specific standard that applies to the specific fuse will be of much use. It would be better to find the fuse curve. \$\endgroup\$
    – user80875
    Dec 29, 2016 at 15:11
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Slow is a relative term. Fast is a relative term. I wouldn't expect exact meanings for these two words.

More specific meanings of the words might be drawn from the context in which the term is used.

When using the science of electrical engineering, it is natural that the specific meaning of slow or fast should be found in a data sheet. Which would also give the conditions where the item would be slow or fast.

"require an end user to consult datasheets (he's not likely to understand)"

Thankfully there are instructors, classes, on line help (such as EE.SE), and Books to help gain understanding.

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  • \$\begingroup\$ Then why do those sites sell generic items like "8A fast fuse"? According to you, that description doesn't mean anything useful, a designer can't rely on that being the right part and the user can't rely on that being the correct replacement part. \$\endgroup\$
    – skyking
    Dec 28, 2016 at 7:27

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