Jon Elson wrote:


How large a fuse would have to be in
the circuit to allow the traces to burn up?


The problem is, except for very special energy-limiting fuses, the fust does
NOT limit the current before it blows. It allows whatever current the unit
will draw for some amount of time before blowing. So, it is possible for
even a 1A fuse to allow fairly thick circuit traces to burn through when the
device has a dead short.

** The thin wire of a 1A fast fuse would normally melt and open well before a PCB track a few mm wide can do so.

It would not be unusual for a 1 A fuse to allow
maybe 100 A in a dead short situation, for a few milliseconds.

** What happens with large overloads, like 100 times, is the fuse wire vaporises and initiates an arc from end to end of the fuse. Such arcs show negative resistance and so are very destructive - substantial copper tracks and
wires vanish instantly until another fuse link or breaker trips.

Of course, special fuses types exist that can handle fault currents of thousands of amps without arcing - microwave ovens and Fluke DMMs use them, but not small appliances.


.... Phil