Fuses - over different voltages

Reesy

I was in a motor factors (Mitchels, Capel St, D1) the other day buying 10A fuses for the car, and the guy behind the counter told me that the ampereage rating of the fuse doesn't change if you change the voltage, i.e. if I found a 10A domestic fuse (for use on a 240V circuit), I could use that in my 12V car.

On the one hand the guy seemed very knowledgeable, but on the other it seems a bit counterintuitive. Unfortunately I can't remember much from my physics 'O' level days!

Thanks...

mikhail

I don't think it's right. Your basic fuse literally burns out - the conductive bit in the middle melts and the connection is broken. That's a heating thing, and that's power related. Power depends on current and voltage.

Michael Collins

Nope, it doesn't really matter in most cases.

The wire inside a fuse is designed to melt at a certain temperature, which depends only on the current through the fuse. So the voltage in the exterior circuit will have no effect.

Having said this, it wouldn't be good practise to use a fuse designed for low voltage circuits, say for 12 volts, in a significantly higher voltage circuit, say 1000 volts. This is due to other effects such as arcing and insulation issues. There are shock hazards also with doing this - car fuses tend to be kinda exposed. I see no problem going the other way around though - from high voltage to low voltage.

Bullzi

The man is correct. The amps rating of your fuse is a fixed value. It doesn't change ever. The rating as you probably know is a limit value, if the amps exceed the rating then the fuse will 'blow' as a safety device. The amps your device pulls is proportional to the load resistance and voltage at the load, ie with your stereo the resistance is usually fixed on your speakers at around 8 ohms but as you increase the volume the voltage increases. if the voltage increases to a value which when divided by your resistance is greater than your rated amps it will blow the fuse to protect your speakers from damage.

mikhail

Michael Collins wrote: »

The wire inside a fuse is designed to melt at a certain temperature, which depends only on the current through the fuse. So the voltage in the exterior circuit will have no effect.

Actually, come to think of it, you're sort of right.

P = V I = I^2 R. So you design the fuse with a given R, and if I exceeds some threshold, the wire will melt. Of course, that'll only happen if the voltage spikes, so the voltage written on the fuse doesn't mean much.

SOL

mikhail wrote: »

Actually, come to think of it, you're sort of right.

P = V I = I^2 R. So you design the fuse with a given R, and if I exceeds some threshold, the wire will melt. Of course, that'll only happen if the voltage spikes, so the voltage written on the fuse doesn't mean much.

Or if the resistance of the total circuit drops...

Gurgle

Reesy wrote: »

if I found a 10A domestic fuse (for use on a 240V circuit), I could use that in my 12V car.

Essentially correct, but the rating for a given fuse will be different for 250V AC to 12V DC.

If you take a 13A fuse out of your toaster's plug and run 10A DC through it, I expect it will burn out pretty quick.

Michael Collins

Gurgle wrote: »

Essentially correct, but the rating for a given fuse will be different for 250V AC to 12V DC.

If you take a 13A fuse out of your toaster's plug and run 10A DC through it, I expect it will burn out pretty quick.

Why do you say this? If you have 13 amps (r.m.s.) at AC is effectively the same as 13 amps at DC. The fuse doesn't know the difference!