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Sorry, this may be me being slow, but...

An amp-hour is 1 amp over 1 hour - simple enough. Now I have 2 fuses - one is rated in amps, the other in amp-hours. Can I I use one in place of the other? What is the ratings leeway?

In general, what should my approach be when dealing with amp-hours in design/troubleshoot terms? When should I use components rated in amp-hours in my designs? Which components are most often rated in amp-hours? When can I replace amp-hour rated components with amps?

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  • \$\begingroup\$ What are you trying to protect, and what are the two fuses you have? \$\endgroup\$ – Michael Pruitt Feb 11 '12 at 3:33
  • \$\begingroup\$ We should say, an amp-hour is an amp over the course of an hour, not \$\frac{\text A}{\text h}\$. :) \$\endgroup\$ – Kaz Jan 7 '16 at 1:08
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The short answer is that if they both have an 8A in their name then they can replace each-other. The details of the fuse are important if your supply can source very high currents(100s of amps) or if you need to deal with issues such as irregularly but short current trips that should not cause a fault.

What does the H mean?

The confusion comes from fuse nomenclature. It is not surprising either as my google fu did not easily return a list of what identifiers refer to.

This document points me to a useful table though. It seems as thought that 'H' refers to the maximum current that the fuse guarantees it can interrupt. Image from omega document that has the standard U.L. fuse ratings, basic table, Class H is 10,000 amps.

Omega's document on fuse selection that I mentioned previously has quite a large amount of information to learn from if you would like to learn more about fuses. Choosing a fuse to protect circuitry without faulting regularly for no reason is something EDN and Electronic design magazine have had good articles about.

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  • \$\begingroup\$ This is only one source I could find that was at all direct with me and after a decent bit of searching. I will spend more time looking later in the week an attempt to update my answer. \$\endgroup\$ – Kortuk Feb 11 '12 at 8:19
  • \$\begingroup\$ Woops, another source calls AH the holder code for a fuse. I will have to spend a while on this, H is definitely a fuse interrupt rating, but the AH may be referring to physical dimensions. I know that an 8AH250V fuse will be able to block 250V from going across and trip at 8As, but it will take me longer for a definitive answer. \$\endgroup\$ – Kortuk Feb 11 '12 at 8:33
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More anon, but:

Fuses are NEVER rated in amp-hours.
If one appears to be then there is some sort of confusion.

Can you provide a web link, or spec sheet or wording or labeling?

All normal fuses are rated in amps fusing current.

The closest one may come to amp-hour rating a fuse is to measure its time to blow in terms of current carried.
eg (made up example) a fuse rating may say that it will "blow within time t if it carries current I eg-
1 minute at 5 A
10 seconds at 10 A
1 second at 20A
0.01 second at 30A
Essentially instantaneously (10's of microseconds range) at 40A

The longest of these figures in amp-hours is
1 minute at 5A = 1/60 hr x 5A = 1/12th amp hour.
But that is useless as a rating as it only applies at 5A as blow time and current are not linearly related.

A circuit breaker can have more complex trip timing mechanisms but even then, amp-hours is not a sensible measure.

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    \$\begingroup\$ I would guess it is a mark for the type of fuse. \$\endgroup\$ – Kortuk Feb 11 '12 at 4:41
  • \$\begingroup\$ The rating, she say (I quote) 8AH250V. On a side note, the largest thing I have handy is 125v at 2a - is that safe? What is the tolerance? (application is computer psu) \$\endgroup\$ – Joe Stavitsky Feb 11 '12 at 5:03
  • \$\begingroup\$ @JoeStavitsky, that is a type 8A fuse. I will find some references. \$\endgroup\$ – Kortuk Feb 11 '12 at 7:55
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    \$\begingroup\$ @JoeStavitsky: Not all fuses with the same interrupt rating are interchangeable. An 8A fuses is supposed to pass through 8 amps "all day" without tripping, but some 8A fuses will by design allow e.g. 16 amps to flow for quite awhile before they pop (if one tries to put 80A through, however, they will pop very quickly). If putting 16 amps through a circuit for five seconds would cause it to catch fire, adequate protection would imply that a fuse should pop in well under 5 seconds at 16 amps. That having been said... \$\endgroup\$ – supercat Feb 11 '12 at 17:52
  • \$\begingroup\$ @JoeStavitsky: ...I would expect that for the type of situation you're looking at, any "normal" 8 amp fuse should probably be adequate, especially given that there's probably a big difference between the highest current the fuse should see in normal operation, and the level of current which would indicate a catastrophic, potentially-fire-starting, failure. \$\endgroup\$ – supercat Feb 11 '12 at 17:53

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