How many fuses?

[Walrus]

I've been wondering... if I add a relay for something (lights, heater etc) should I put a fuse in the power side or the switch side, or both - or am I getting OCD in my old age?

[itchyfeet]

Fuses should always be as close to battery as reasonably possible i.e straight to fuse before anything else
only fuse positive side
fuse rating to be smaller than wire rating in Amps so if shorts it wont melt the wire.

[Walrus]

Ok... so should a relay be fused like:
Switch side: Battery+ > Fuse > Switch > Relay > Earth (back to battery- ). Light current, e.g. 5A Fuse
Application side: Battery+ > Fuse > Relay > Application > Earth (back to battery- ). Heavy current e.g. 40A Fuse

My point being that a short could develop on either side of the relay - if only one side is fused sods law says the short/wire melting/fire will happen on the other side... so should both sides be fused seperately?

[nicq]

It doesn't mater if the circuit is fused at the bat side as long as the fuse is smaller than the current that the wire can take the whole circuit is protected.

[Walrus]

Ok... still not answering the question!

A relay is a way to switch a high-power circuit on using a low power circuit. The two circuits are effectively isolated from each other, so that the high-power circuit does not draw electricity from the low-power one. If both circuits are connected to the battery - one for the switch, one for the appliance, should both circuits be fused? Yes? No? It depends?

[CJH]

Both circuits should be fused.

As noted, the fuse is there to protect the wiring in case of a short - if a wire is connected to the positive terminal of the battery it should always have a fuse as close to the battery as possible. The section of wire between the battery and the fuse is not protected - if you slide your seat over that bit of wire and short it to earth, you'll melt the wire and maybe start a fire, even if there's a fuse further down the wire. Only the bit of wire beyond the fuse is protected.

The lower power switching circuit may only need thin wire, but it's still connected to that high energy power source, so if it shorts to earth it's just as likely to get red hot and start a fire as the fatter high power circuit.

[ghost123uk]

Ok... so should a relay be fused like:
Switch side: Battery+ > Fuse > Switch > Relay > Earth (back to battery- ). Light current, e.g. 5A Fuse
Application side: Battery+ > Fuse > Relay > Application > Earth (back to battery- ). Heavy current e.g. 40A Fuse

Is correct (though the fuse needs to only be just a bit over what the Application uses for top safety, i.e. say a water pump that draws 6 Amps, then a 10 Amp fuse (nearest rating above) would be correct).

[Walrus]

Cool, thanks - that answers my question!

[itchyfeet]

Cool, thanks - that answers my question!

Sorry misunderstood your question I thought you were asking if you needed multiple fuses in a circuit.

A relay is two circuits the trigger or coil low power circuit and the relay swithed power circuit, yes both need to be fused.

I will take this opertunity to harp on about the unfused circuits many of us have thanks to VW
Petrol vans don't have ignition fused, or the carb heater, can't see why more vand don't catch fire, I have seen burnt wiring looms

add fuses in black box is my reccomendation.

[Walrus]

Thought I'd check - upgrading some wiring so thought I'd take the opportunity to upgrade fuses too.