Amusing cable question

[holy god of nil]

I have mixed up the power cables from my monitor and pc while they've been packed up to move, the monitor is a Dell 2209wa and the psu is a corsair hx450w, one cable is fused at 5amp and the other at 13, which is which? Does it matter?

[Barry]

Chances are that your monitor is the 5 amp, worst that will happen if the wrong fuse is used is that it will just blow.

[Rave]

TBH I'd be inclined to put a 5A fuse in the PC lead as well, 5A at 240V is 1200W, which should be far more than a 450W power supply can suck up.

[holy god of nil]

Actually it looks rather like the 13a came with the monitor and the 5a with the psu!

[mac124]

Not a lot uses 13A in the home the problem is most items seem to come with a 13a fuse fitted, probably because its all made in China or somewhere like that.

[Agent]

Chances are that your monitor is the 5 amp, worst that will happen if the wrong fuse is used is that it will just blow.

Actually much worse can happen sadly

A 13A fuse will let a lot (well, 8 Amps ) more current through before blowing. This means if you have a higher fuse rating in a device than you should, if anything does go wrong, you risk a lot more damage happening before (and if) the fuse blows.

Fuse ratings are way more important than people give credit for!

[peterb]

And there is still more to it than that because you need to consider the switch on surge when the PSU is switched on.

But - in the UK, the plug top fuse is there to protect the connecting cable. The equipment itself should have its own internal fuse to provide protection for the equipment. So if you have a connecting lead rated at 13Amps, then a 13Amp fuse is perfectly safe provided an appropriate fuse is fitted in the equipment.

This does not usually apply to white goods, such as kettle or washing machines, which have captive mains connections (that is, no plug/socket on the appliance.

So a computer PSU should have a correctly rated fuse internally - and by correctly rated, I mean that the current carrying capacity and its fusing characteristics, surge rating etc should be chosen for the application. While the better PSU's might (and it could be an internal fuse) I'm not convinced that lesser ones do.

So if it doesn't...

The problem is with sealed 'brick type PSUS - used for things like monitors - as there is no way to tell if they are internally fused or not. In that case the plug top fuse may also be used to protect against equipoment failure as well as lead failure.

A 450 W output PSU will be drawing roughly 600Watts at full poer (very roughly) which would be around 2.5 Amps. That assumes it is running at unity power factor - but in practice it will be slightly less than that, so the actual currevnt will be slightly higher - again taking worst case it will be three amps. The switch on surge is harder to estimate - rule of thumb - allow double, so that is 6 Amps - but a fuse will not blow instantaneously at a 20% overload, so the 5Amp fuse will probably be sufficient.

For the monitor, taking (say) 60 watts, then a 3A fuse would be adequate.

More to a fuse than meets the eye!

So much for the theory. In practice there are thousands - millions - of IEC leads in use, powering computers across the country, all fitted with 13A fuses, with their owners oblivious to the fact. Are they in any danger? No. If the equipment fails catastrophically and applies a dead short on the mains lead, the fault current will be so high that even a 13A fuse will rupture within a couple of cycles of mains power - which means within 20 to 40 milliseconds.