Blade Fues

[Castaway]

Blew a fuse today, on the lighting circuit. When I went to replace it seems I have 1 x 7.5A and 1 x 3A in the fuse holder set up, both rated 32V? Surely this can't be right and if I'm changing them to 12V do I just multiply the Amp rating by 2.75?

[Bob-18]

No, the rating is the current the fuse is designed to blow at, not the power they are designed to cope with. The voltage rating is the "safe working voltage", above which they may not blow properly, but just arc away until something other the intended bit of the fuse fails.

Before replacing do a quick check on the loads on each fuse - the max on the 7.5A fuse should be about 90w, and the 3A fuses about 40W - if the bulbs are "normal" tungsten/low volt halogens then just add up the wattage on each bulb on the relevant circuit, its a bit harder for LEDs as there are other things to consider.

[Keeping Up]

No, replace it with the same current rating as the original.

 

The voltage rating refers only to the maximum voltage of the circuit that it may be used in. These fuses may therefore be used in circuits of up to 32 volts, and so they are fine for use in 12 volt circuitry but not in (for example) a 240v mains circuit. This does not affect their current rating at all; a 7.5 amp fuse will operate at 7.5 amps regardless of the voltage, and a 3A fuse will operate at 3 amps regardless of the voltage.

[Castaway]

Great, thanks chaps.

[DHutch]

And assuming its not blowing often, you wont have an issue. If it is, it is still not safe to increase the current (no of amps) rating to reflect the increased load, unless you are replacing all the wiring between the two or are happy that it is over specified along its whole length and hence already suitable for a large fuse, as otherwise you will then be able to overload the wiring without the fuse blowing, which isnt the point at all!

 

 

Daniel

[Chalky]

The blade fuses (littelfuse) have quite a tight tolerance on them compared to glass cartridge fuses. They are a thermal device and the blow speed depends on how much they're overloaded by and how hot the surrounding ambient is. It's possible to slightly overload them and they'll take a long time to blow, whereas a huge overload will make them blow quickly. They're also prone to ageing and can blow prematurely if they're knocked hard.

 

There are some that have a fuse wire link instead of the metal fretwork made in a single piece. From what I've heard they have different blow characteristics and are best avoided.