BFL13 wrote:
I feel that my world has turned upside down!
The fuse that "protects the positive wire" (from "becoming the fuse") between the battery and the converter, which fuse is to be mounted near the battery post, is "after" the battery, not before it. The threat being the battery shorting, not the converter shorting.
The short wire between the battery and that fuse is not "protected" but the long length "after" that going to the converter is protected.
All that indicated to me up till now, that the fuse came "after" the short so it would blow and protect whatever came next "after" the fuse.
The main threat being protected by the large fuse at the battery is the wiring shorting. The battery can supply a very large current (it's not good for it, of course, but it can), hundreds of amps and more than any wire commonly used in an RV could safely carry.
Usually a converter would itself limit the charge current in the event of a shorted cell or even shorted battery to somewhere around maybe 40-100A, depending on the converter's specification. Usually the wiring between the converter and the battery would be sufficiently sized to carry this worst-case current; the fuse isn't particularly to protect a battery that is itself internally shorted. Unfortunately we sometimes say "shorted battery" to mean a battery connected to a short circuit and sometimes use the same words to mean a battery or at least a cell that is internally shorted (that presents a short circuit to whatever is connected to it), and the two are rather different situations.
More generally, a fuse or circuit breaker can only respond to current that actually flows through it. If the fault current does not flow through the fuse or circuit breaker, it'll just sit there...and even if it blew it would not stop the fault current anyhow, any more than a water valve would control the flow of water that doesn't go through the valve.
