Phenomenon: Half-Blown Fuses
A question that we get not-infrequently, when we mention to customers that they need to visually inspect their fuses rather than just continuity-testing them to make sure they're not "half-blown" (i.e. degraded) is:
"Is it really possible for a fuse to half-blow? I've never heard of that!"
The answer, though it may surprise many, is a resounding YES. The attached photo is of a 3.15A fuse from a Cerbo GX power supply wire, and it has "half-blown", and what we mean by that is: under 0A load, the fuse will pass full vbatt (13.6v in this case), with no voltage drop (at least, none within a hundredth of a volt).
As soon as even the smallest load (~5ma) is applied, however, voltage through the visibly melted fuse filament drops to roughly half vbatt, and the voltage drop gets worse as the load is increased. At ~2.5A, voltage through the filament has dropped to ~1.6v.
The lesson here is simply to understand that a cursory visual inspection of a glass fuse such as this (or a BMV, or CCGX, or any number of other products that use glass fuses) is not always enough to determine whether or not the fuse is good; indeed, even a basic continuity test will not always reveal this condition, as -depending on how the filament melted- the fuse may present perfect or near-perfect continuity when tested with a multimeter.
As such, when experiencing power issues with any device using this type of fuse, we always suggest replacing the fuse first, even if it seems to look okay, because looks can be deceiving when it comes to these.
While replacing the fuse, of course also visually inspect the contacts inside the fuseholder, as indeed even the smallest amount of corrosion or other contaminant present at the contacts can also lead to erratic behavior including sporadic loss of power under certain circumstances and/or total loss of power altogether.
It's further worth noting that we've seen similar occurrences in MRBF fuses, ANL fuses, and others as well, so never assume!