How do you use a fuse with a rating of Amp-Hours?

Question

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?

Answer 1

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. .

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.

Answer 2

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.

Answer 3

In the case of fuses I suspect amp-hour is current x time. If you look at some datasheets for fuses, the time it takes to blow is a function of current and temperature.

Answer 4

An Amp-Hour is the same as a Coulomb. It is the amount of charge something has.

For example, a battery can "hold" so many electrons. A 1 Amp-Hour battery contains 3600C. A Coulomb is approximately 6*10^18 electrons.

You can think of the amp-hour as the "size" of the container. The larger the size the more electrons there are for use in doing useful things.

It makes no sense to have a fuse rated in terms of Amp-Hours since the only way to blow the fuse would be to push all those electrons in it. But things burn up not due to how many electrons total but how many at localized instant.

For example, you can run a small 5V computer fan at 0.25A almost indefinitely. It may use 1000000000000000C's. But you can burn the fan up by just passing 1C through it if you do it fast enough. (coulumbs/s = A)

Fuses work by burning up when so many amps pass through them and have no "memory"(for the most part). So we could pass 100000000000000C through a fuse without any problem is we do it over a long enough time frame... but 1C could easily blow most fuses.

If 1 Amp is flowing through a fuse then that is 1C in one second. So a small 0.25A fast fuse will probably blow. If we only allow 0.1Amps to flow through the fuse then in one month the fuse would have passed about 250kC.

When dealing with columbs you are generally talking about how many electrons. Amps both how many and how fast. You can have a lot of electrons moving very slow which could give a low current or a few moving very fast which can give a high current.

99.99% of the time when you are worried about something burning up you are dealing with amps and voltage. Fuses should be rated in terms of Watts which is the unit of power and related to temperature. But since fuses generally have a fixed resistance and P = I^2 R we can simplify and talk about amps instead(in this case since R is constant and I = sqrt(P)/R).

To directly answer your question there is not much of a directly relation between amps and amp-hours. They are no interchangeable. One deals with charge/second and the other with charge. It is analogous to speed and distance. They are related but not interchangeable.

I doubt you will ever find a fuse rated in terms of AH's. Chances are you read it wrong or there was a quirky symbol on the fuse(some fuses have weird symbols as they are "Codes" rather than units).

Answer 5

AH stands for Amps High. Fuse fails instantly if current reaches 8 amps.