How under-rated are Fuses

No not how unsung are they from saving dopey muppets like me from electrocution...

The ST2 has an accessories socket on it, which the fuse was missing from. It was a 3 amp.

Last night I wired in the new socket with a new 3 amp fuse, and plugged in the air bed inflator, and the fuse popped, it transpires the air bed pump is 3amp.

Now I heathenistically bodged it with a wire bridging the fuse and of course it works, but how much under rated are fuses, i.e do they go at say 2.7 amp or 3.1 amp?

As I am only going to power this 3 amp fan and the sat nav from it at separate times I am more than likely just going to put a 5 amp fuse in it and let it be?

cue ducati fireworks?

[Sid Squid]

Most fuses unless they are very high quality and very closely specified have a tolerance that is surprisingly high. Usually that's above the nominal value not below.
What you need to consider is that the current is rarely constant, you were using a pump, if that's a DC motor - and it almost certainly is - at a standstill there is a dead short across the input wires = a very high starting current.
Fuses are available in varying types, such as slow action which would be useful in such an application, it won't allow a constant over current but won't pop on a transient spike.
5 amp will very probably be OK, you would need to examine the cabling and any switchgear or componentry and use a fuse that's suitable to protect that. A fuse for the individual equipment is a different matter, that may well be significantly lower rated.

[DarrenSV650S]

If the wiring could handle 5A it would have come with a 5A fuse. Can't believe you put a wire across it, oh that makes me cringe lol

It's not worth risking your bikes wiring for it. You'd be better having a look for a low amp air pump. Or just wire up a simple relay circuit for a new accessory socket with wiring that can handle more current.

Originally Posted by DarrenSV650S

If the wiring could handle 5A it would have come with a 5A fuse. Can't believe you put a wire across it, oh that makes me cringe lol

It's not worth risking your bikes wiring for it. You'd be better having a look for a low amp air pump. Or just wire up a simple relay circuit for a new accessory socket with wiring that can handle more current.

Why?

I fully understand the risk of the bikes wiring, but the appliance i was trying to run was 3amp.. the fues in the box was 3 amp but it blew..??? Hence my question which mr sid explained very nicely.

So if the appliance was pulling 3amp across a 5amp fuse the wiring would be fine.. sheeesh give me some credit, I'm not likely to plug a little kettle in and wonder why the bike went up in flames!!

And its not like the bridge is a permanent feature.

[DarrenSV650S]

Sorry didn't mean for that to sound patronising. Go for it if you want, but the fuse blew for a reason. It's not something I'd do when there are other very easy options that don't risk the wiring.

You can get a fused ciggy socket for a few quid that will bolt straight on the battery.

[Bri w]

in ye olde times, when the world was young, a 3amp fuse would blow as it approached 6 amps. Or at least that was what I was taught in my City & Guilds... double the rating.

[tigersaw]

Originally Posted by 454697819

The ST2 has an accessories socket on it, which the fuse was missing from. It was a 3 amp.

Is it a small 20mm glass fuse? You need a T3A fuse, i.e. time delay.

[embee]

Of course everything said is correct.

On the other hand, it's very unlikely that any wiring in that kind of automotive use would fry at 5A.
2mm*2 (28x0.3 stranded) is rated at 17A continuous, and IIRC down at 0,75mm*2 it is rated at 6A. and that's pretty lightweight stuff. The chances of a 5A fuse resulting in accessory circuit wiring overheating is slim, but it's your decision! If you can measure the conductor size you could put your own mind at rest.

Fuses are good but don't always protect what you want as the 200 KiloWatt motor in work found out the other day when there was a loose crimp in the panel.

Linking out fuses is a bad move because it's reliant on the next protection device in circuit which normally is a bigger fuse so more current which will most of the time cause big damage.

Its the 'spike' of a motor that caused that to blow most likely, as someone said earlier, a slight delayed fuse probably wouldnt have blown as the current draw would rise initially then settle into its normal draw, which im guessing is below 3 amps unless put under increased load.

You wouldnt believe the differences in fuses, the variants are endless