I'm going to embarrass myself now.
I've been thinking about a problem too much, and now can't see the wood for the trees.
Can you just check this for me - I've been thinking about this too much, and got myself confused.
Let's ignore, for the moment, that many fuses will allow more than the rated current to flow through them.
I think that a fuse limits the amount of *current* through it. It does this buy popping when the power (I^2.R) exceeds a limiting factor - since the resistance of the wire in the fuse is constant, the factor is the square of the current. This means that the fuse will pop at a given current, regardless of the voltage. Despite this, fuses are rated in terms of voltage and current. I assume that the voltage rating is a safety thing - correct?. So, a fuse rated at 250V/2A will pop whenever >2A crosses it, regardless of the voltage - correct?
Where I got myself confused (in two different ways!) was here:
- A fuse (say) 3A rated at 230V will allow 690W of power to be drawn before it pops. But 690W @ 115V is 6A. Therefore a 3A / 230V fuse is the same as a 6A / 115V fuse; and
- A fuse (say) 3A rated at 230V, implies the fuse has a resistance of
I sort of know (err hope) that 1 and 2 are wrong but am having difficulty explaining to myself why! Can someone (gently) put me straight.