Why might a multimeter ask for "wrong" fuse sizes?

Question

Why might a multimeter ask for the "wrong" sizes of fuses?

On the front of my Fluke 87 multimeter, the two probe ports for amperage are labeled as fused at 10A (max) and 400mA (max). When opening the multimeter to check the fuses (both are burned out), however, the fuses are different -- 15A and 1A, respectively. Their places on the PCB are even labeled as 15A and 1A fuses.

Wh-wh-what?

Further, while looking up replacement fuses on Amazon's site, I see a number of reviews of people with Fluke 87's recommending an 11A fuse.

...?

Is there some latitude with fuse selection, where the size of fuse you pick correlates with your favorite number of the day... or something?

Answer 1

The actual text is:

10A MAX
FUSED

Maybe this should be read as "10A max" and "fused" — two different statements, not one sentence.

What I am trying to say is that you should not read the two lines on the multimeter as "fused at 10A", but as "rated (=guaranteed) to measure up to 10A" + "fused to prevent brute abuse".

Note that even a 10A fuse is not guaranteed to blow at 10A, but on the other hand it might do so. Hence, to be rated to measure 10A, the fuse must be for a higher current. Fuses are very crude things, so a 50% higher value does not seem unreasonable to me.

"Hard lines" are very rare in electronics, especially for currents: the common 7805 is over-current-protected and is rated to deliver up to 1A (or 1.5A). But the over-current protection will kick in somewhere above the rated current, but below 2.5A.

Answer 2

Something I am missing from the answers here but may still be of help to future readers is that:

Fuses are meant to avoid dangerous situations when a circuit fails, not to limit current!

The only thing of interest to the selection of a fuse is whether it does not impact the circuit in a negative way (i.e. in a multimeter: adding too much burden voltage, allowing enough current through under normal operating conditions) and that it interrupts the circuit when a dangerous amount of current flows through the circuit for too long.

It is very well possible that you have an application where typical circuit current never exceeds 10mA, but you fuse it with a 6.3A fuse. Because you will not expect dangerous situations to arise when less than something around 6.3A flows through the circuit. It's an extreme example and usually the fuse ratings are very close to max. circuit ratings, but it is very well possible.

You have to assume that when a fuse trips, the circuit is already damaged and all you are doing is avoiding a dangerous situation for the user (fire, explosion). You don't put fuses in the circuit to avoid damage to the electronics.

Answer 3

There are a few additional reasons I can think of why a higher current fuse may be specified, these are a direct quote from Fuse Characteristics, Terms and Consideration Factors from Littlefuse which is worth a further read for more background.

1. For 25ºC ambient temperatures, it is recommended that fuses be operated at no more than 75% of the nominal current rating established using the controlled test conditions.
2. The fuses under discussion are temperature-sensitive devices whose ratings have been established in a 25ºC ambient. The fuse temperature generated by the current passing through the fuse increases or decreases with ambient temperature change.
3. Most fuses are manufactured from materials which have positive temperature coefficients, and, therefore, it is common to refer to cold resistance and hot resistance (voltage drop at rated current), with actual operation being somewhere in between.

With a multimeter it wouldn't be uncommon for it to be operating with an ambient temperature well above 25ºC so some derating of the fuse's room temperature rating will occur. Also when operating at 100% the additional heat will increase the resistance of the fuse.

I'm not sure how much of a real-world effect that would have but for measurement purposes of course it's desirable to keep the resistance as low as possible. No doubt some of these recommendations would differ between manufacturers and their particular parts.