Electronic – Why might a multimeter ask for “wrong” fuse sizes


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.


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?

Best Answer

The actual text is:


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

Fluke Multimeter Port Labels

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.

Related Topic