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Fuse or relay first in power supply circuit?

Practical Motorsport Wiring - Club Level

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In the practical wiring course clubman level.

There's an example drawing in the power supply design module (hand drawn) and has the 3 relays being fitted before the fuses to each component.

Is this the normal procedure or do the fuses come first? Or both?

Normal practice would be to have the fuse/circuit breaker as close as practical to the power supply of the circuit(s) being protected.

If you can provide a link to, or screen capture of, the circuit in question, it would help.

Thanks Gord,

Attached is the photo of the HPA power supply design diagram with the fuses further away from the power source than the relays as requested.

I'm a little confused on this, as in the videos HPA promote fuses close to the power source but in the diagram they are after relays?

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It may have been more for illustrating the wiring between the relays for the different components rather than being a definitive way to do it?

I would have expected the fuse to be used on the runs to the 30 pins, something like 10A, 30A (they won't all peak at the same time), and the fuel pump would certainly be fused before the relay, not afterwards. Excessive current isn't only a concern with wiring overheating, but can cause relays to arc the contacts, especially after some use, welding them together, or overheat the relay to the point of melting and possible ignition and/or jamming the contacts.

Basic rule is that fuses/circuit breakers can only protect the circuit against overloads/shorts from that point, so as you said, the closer to the power distribution point the better. Something to watch out for is the practice in some countries of placing the fuses at the end, which leaves the majority of the circuit vulnerable to shorts and, worse, sometimes they place switches there as well, which leaves most of the circuit 'live' when turned off - it's especially dangerous when it's used on 220-250V household appliances.

Both before and after relays are acceptable and proper. The caveat is why you are placing them before or after the relay.

In the example drawing the power being supplied by the relay is being split off to more individual circuits and the fuses are protecting each circuit, likely because the wire itself would be smaller than the supply wire (remember the fuse protects the vehicle/wiring, not the device it's powering), and having a single large fuse to protect all of the small wires isn't good practice.

I'm sure if the supply side to those relays was shown (looks like it would have been a separate diagram to reference), you would probably also find a larger value fuse there to protect the wiring leading to the relay itself.

A large fuse can be used at the source that is sized to protect the wire to the distribution box/relay bank/etc, and then smaller fuses for each circuit, it's not a one or the other deal.

just to get a grip on the right method, I used to wire the fuses after the relays. Never before the relay as I thought it doesn't matter, at worst the relay will be damaged but i'll protect the devices i'm powering. Do you mean that at least there should be one main fuse before the relay and I should put individual fuses after the relay for each circuit its powering?

I'm personally of the belief that the fuse should be BEFORE the relay. The fuse is to protect the conductor, not the device or load. Fusing after the relay leaves the relay and the conductor feeding it un fused, which could be a fire hazard.

The rule of thumb is to have the fuse 12" from the battery or source; I'd also define 'source' as a bus bar feeding a bunch of smaller fuses/relays, example is running a big cable to the trunk for a fuel system and smaller fuses for each fuel pump etc off the bus bar. In that example there should be overcurrent protection for the bus bar's feed closer to its source, which is probably the battery.

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