2005 Expedition Trophy Champion
Why is the practice to always put a fuse on the positive lead? Why not on the ground lead?
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!
It has nothing to do with electron vs. conventional flow.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.
In an isolated circuit the fuse could be on either the positive or negative lead, it makes no difference since the same current is flowing through both.
In all vehicles where the chassis is at ground potential (almost all cars) then the fuse should always be on the positive lead and as close to battery as possible. This way if any part of the positive lead after the fuse comes in contact with the grounded body the fuse will blow and protect the circuit.
If the fuse was on the negative lead and the positive lead shorted to ground it would melt the positive lead and potentially start a fire, the fuse would be useless.
This is why almost every car has a short length of wire going from the positive battery terminal directly into the main fuse block.