Fuse on negative vs. positive 12v leads?

[pskhaat]

Why is the practice to always put a fuse on the positive lead? Why not on the ground lead?

[pskhaat]

So what about this:

Do I need a fuse on the negative wire to my radio?

It won't hurt if you install a fuse on the negative lead, and the wiring harness that ships with most Amateur mobiles has two fuses.
The fuse on the negative (ground) lead is only really required on installations that connect directly to the vehicle battery.
The reason for the fuse, is that if the vehicle grounding from the battery to the car frame/engine should fail, either partially or completely, the radios negative lead "ground", in some cases, become the return path to the battery for the vehicle, causing the wire to burn.
This is not an issue for radios connected to power supplied, the vehicle chassis, or batteries not in a vehicle.
ALWAYS FUSE THE POSITIVE LEAD!

http://www.emrg.ca/fuses.htm

[ntsqd]

Consistency asks that they always be in one particular location of any circuit.
Technically a fuse anywhere in the circuit will work. The best location is at the start of the circuit since it protects all of the circuit's length from shorting. Since commonly the + is the insulated conductor it is the logical place for the fuse to be. Were we talking about a Positive Earthed vehicle then the - would be the most logical place for the fuse.

[spencyg]

Some highly sensitive electronics actually have a fuse on each side of the circuit, both positive and negative. Either side would work, but for the sake of standard electrical practice the leg with high potential (positive) is protected from overcurrent. You could certainly add a fuse to the ground leg instead and it would work fine, but if you ever sold the rig or had somebody working on it not familier with your "unique" strategy, they could find themselves in a bit of trouble. Best to keep with the standards. ABYC is the governing body with yacht electrical systems and they plainly state that fused circuit protection should be maintained on the positive side of the circuit.

Spence

[dieselcruiserhead]

My belief on the fuse being on the positive side is the positive side is the one that does damage. The negative relay would also stop it but if it didn't pop and the positive was hard wired something could melt or burn. I once put a 30 amp fuse inline in a pinch on an e-locker that was acting up. It almost burned down my truck one evening, the 30 amp wouldn't pop. Similar potential scenario, I would say..

[eugene]

It doesn't matter which lead, its just done on the positive for standardization. In electronics engineering courses we get into history of current flow and how it was believed to flow positive to negative but later it was learned that electrons flow negative to positive and the holes left by their flow go from positive to negative so you have 'flow' both directions. positive to negative is officially termed 'conventional current flow' and negative to positive is termed 'electron flow'. It really doesn't matter which but fuses are generally placed opposite the common or ground, but weather positive or negative is ground is just a standard for the sake of having a standard.

[pskhaat]

My issue is I have an OEM fusebox/relay/fuse setup on one side of the vehicle and both the load and battery on the other side. It would be SO much cleaner to use the existing fuse on the negative and series the load into the battery on the same side. I'm trying to stick to OEM design without ghetto wiring like inline fuses sticking off the battery and I can't figure out how they would'a done it.

This is for an LC80 CDS fan BTW.

[ntsqd]

I'm guessing that you would be tapping into an existing blank fuse spot?
If so then more than likely one side is already hot with either keyed or direct battery power.

Ideal circuit protection places the fuse nearest the source and furthest away from the common. That protects all of the supply wire to the load, the load itself, and the ground wire.

Placing the fuse after the load protects the load from an internal short, but does not protect the supply wire from the source through the load to the fuse. If anywhere in that stretch shorts to the ground/common there would be no fuse in the circuit to blow and protect the truck.

[pskhaat]

Nope, the fusebox has a blank into which I will be adding a high-amp fuse (and clips) from a factory auxiliiary battery. JDM/non-US had this setup. I guess the right thing to do here is to ghetto fuse the positive lead to my CDS fan load BUT I want to keep the relay on the same fuse side, maybe that's not as much of an issue.

[DaveInDenver]

I'm guessing that you would be tapping into an existing blank fuse spot?
If so then more than likely one side is already hot with either keyed or direct battery power.

Ideal circuit protection places the fuse nearest the source and furthest away from the common. That protects all of the supply wire to the load, the load itself, and the ground wire.

Placing the fuse after the load protects the load from an internal short, but does not protect the supply wire from the source through the load to the fuse. If anywhere in that stretch shorts to the ground/common there would be no fuse in the circuit to blow and protect the truck.

It has nothing to do with electron vs. conventional flow.

ntsqd's advice here is spot on, the location of the fuse is to protect the circuit as best as possible. While a fuse would open on the negative size just the same as on the positive side due to current flow, the position of the fuse is to maximize protection of the feed, load and connections. If the positive feed abraded just after the battery having a fuses on the return wire to the chassis wouldn't be much use.

BTW, Scott, the factory might have used a fusible link someplace rather than a mini fuse in the box. Or they might have put the fuse and relay a second aux box under the hood. My truck has an orphaned connector behind the AFM box on the left side under the hood that doesn't seem to serve much purpose for USA trucks but is the connection for the second battery on 24V systems, which would not have the air box in that spot.