Placement of fuses / relays

[Grumpy]

Im busy wiring up my relay and fusebox station and building wiring harnasses but have a query regarding placement of fuses in the system.

Basically all wiring diagrams shows a fuse get installed between the battery positive supply (hot wire) and the relay - pin 30. But, is this the best place to put a fuse? Quite often one see / hear relay stations being damaged by an overheated relay / pin and sometimes relay destroyed and this made me wonder.

Why not put the fuse on the accessory wire feed (pin 87) between the relay and accessories let say for example radiator fan motor? If the fan develops a problem drawing an overload of amps or short circuited out, the fuse will pop and save the relay. Or am I missing something?

Your input would be greatly appreciated, thanks

[Tom17]

In a circuit where you have a power source (Battery), a fuse, a relay and a load(fan motor etc), it does not make any electrical difference where the fuse is placed. If it is a 10 amp fuse, every part of that circuit will be protected by it regardless of where in the circuit the fuse is. It could even be in the ground wire for the fan if that was practical (which is usually is not).

If the fan in your example, develops a fault and starts drawing more than, for the sake of argument, 10A then a 10A fuse will pop wherever it is located in that circuit.

However, that said, I would think that putting the fuse BEFORE the relay would be a good idea. It could be conceivable that the relay could get an internal fault and short the live feed to ground. If you put the fuse after the relay, then that internal short would mean that the live wire from the relay to the battery is not protected. It could then melt and cause a potential fire risk.

I'm not sure if an internal short to ground could ever be possible in a relay, I guess it depends on the design, but that's one scenario I can think of where the fuse before the relay is desirable.

I cannot think of an advantage to having the fuse between the relay and the load.

If relays are overheating it is because they are being used at a higher amperage than their rating allows or the contacts inside are corroded and have a high resistance.

[rx7locost]

Location of the fuse depends on the purpose for the fuse.

If you are trying to protect from an "overload" condition, then anywhere in the circuit will work. An example of this is a 10A fused accessory outlet intended for a radio or a GPS. Normally, everything would work fine. When you plug in an electrical defroster that draws ,say 20A, the fuse will blow and protect the wiring from overheating.

If you are trying to protect from an accidental "fault", then the closer the fuse is to the source (i.e. battery or higher current fuse feeding several smaller fuses) the better. For instance, you may pinch a wire going to the relay or accidentally place a screwdriver across a live circuit. The shorter the unfused wire from the battery to the fuse usually means less opportunity to cause damage further upstream.

Puting the fuse closest to the source does both. It protects better against accidental faults and protects from overloading. So this is usually the choice made.

[Grumpy]

Thanks for the advice guys

My box has 10 relays for circuits and 20 fuses for overload protection. Since I already have the fuses wiring cut and soldered to the accessory supply end of the relays I will add additional fuses to the hot wire side of the relays. I bit of an overkill but surely very safe at the end of the day.

[KB58]

In general, a fuse limits the potential fire or short to the wiring upstream of it. In other words, only the unfused portion of the harness can be harmed, so if the fuse is right next to the battery, nothing much happens. But if the fuse is next to the load at the far end of a 10-ft wire, a short anywhere along the harness from the battery to the fuse could start a fire, or at least melt the harness. You always want circuit protection as far upstream as possible. I work on military avionics where that sort of thing is kind of important.

[Grumpy]

IOW, I can leave the fuses out on the supply side (hot end) of the individual relays and just install a fuse at / near battery connection of hot wire feeding the relays.
On the accessories supply feed from the relays the fuses are already protecting for overload/short that side and the fuse at the battery for the hot wire supply to relays will take care of that side of the relays and system should be well protected.

[tibimakai]

Sorry to bring back this old thread, but I'm confused about this as well.
If I want to add relays to almost everything, shouldn't I have a fuse on the low current circuit of the relay(coil side to ecu) and another one for the load side?