On Sat, Jun 5, 2021 at 08:17 AM, Joe Jensen wrote:
Also learned that if you have more than 40mV of drop across fuses in a panel the fuse is bad and will fail.
I can't guess what he meant.? Fuse is "bad" normally means "blown".? ?"is bad and will fail" really doesn't make much sense when talking about fuses.
The most fundamental principle of electrical circuits, Ohm's law (V=IxR), says voltage drop across a conductor with a given Resistance (the fuse) varies directly with the current flowing, so measuring voltage drop across a hot fuse can serve as an ammeter.? Some fuse manufacturers publish fuse specs just so one can do this.? These reveal that 40mV voltage drop typically represents about 85% of the rated fuse current.
Naturally, fuses come in all sorts of varieties.? Some are rated to blow at 75% of labeled rating, some at 100% (and of course on various time scales).? ?So measuring current at 85% on a fuse that should have blown at 75% would qualify as a fuse "fail" (extremely rare). On the other hand, for a fuse that blows at 100%, measuring it at 85% might quality as "near blowing", and after blowing it would be "bad", but then of course the fuse has worked correctly, so one can't say it "failed".
