How under-rated are Fuses

[454697819]

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]

#-o 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 :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.

[454697819]

#-o 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 :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.

:smt101

[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]

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.

[thefallenangel]

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.

[Specialone]

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 :(

[thefallenangel]

go to rswww.com and look up fuse and see how much things come up