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View Full Version : Fuse on negative vs. positive 12v leads?



pskhaat
05-14-2009, 06:03 PM
Why is the practice to always put a fuse on the positive lead? Why not on the ground lead?

pskhaat
05-14-2009, 06:16 PM
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
05-14-2009, 06:33 PM
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
05-14-2009, 06:44 PM
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
05-14-2009, 07:01 PM
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
05-14-2009, 07:15 PM
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
05-14-2009, 07:17 PM
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
05-14-2009, 09:26 PM
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
05-14-2009, 09:32 PM
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
05-15-2009, 03:02 AM
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.

dieselcruiserhead
05-15-2009, 04:44 AM
I believe and could understand that, particularly with AC setups. But this is DC, the ground is always the smallest wire (if at all present) and when something heats or melts it is always the 12V (or 24V) positive wire and never the negative. This is why I think (and will always put unless someone reasonably convinces me otherwise) that the fuse belongs in the positive circuit. The point of fuses is not just protection of the system but particularly to prevent fires. My $.02...

slomatt
05-15-2009, 06:39 AM
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.

- Matt

Tennmogger
05-15-2009, 01:16 PM
Matt nailed it.

The fuse 'to protect the radio/load' has to be in the positive lead because of the grounding of the device chassis, and it's negative terminal. If you mount a CB to any of the vehicle's metal parts, it gets grounded. If you properly ground an antenna, the radio gets grounded through the coax braid.

The fuse in the negative lead (if to the battery neg) is to protect just the negative lead in case the vehicle's negative battery connection gets disconnected. That applies when you intentionally disconnect the negative battery terminal to work on the vehicle's electrics, too!!! Fry that lead and it could fry the vehicle. If the radio's negative lead does not go back to battery negative, the fuse is not required (but still a good idea).

Back to original query: The best source of power for any of our fun devices is the vehicle battery. The voltage will be more stable there and there'll be less electrical noise. Running a pair of parallel wires to the battery will also help reduce ignition noise pick up in the power leads.



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.

- Matt

Curmudgeon
05-15-2009, 07:35 PM
If you fuse the ground side, and a short occurs in the device you are trying to protect, the fuse will not open. You will continue feeding electicity to a dead short until something burns up; maybe even the entire truck. The fuse will only open if it is between the source and the load, or the source and the short.

pskhaat
05-15-2009, 08:21 PM
Jim, I'm confused as to your statement concerning between the "source and the load," the source in most maths senses is indeed the negative pole however I think it is an important distinction that the vast majority of the vehicle is negative ground and thus the chances for short are greatly increased without a fuse as close to the opposite pole (positive) as possible.

ntsqd
05-15-2009, 08:48 PM
The whole deal about the actual direction of electron flow means not much in a practical application. The system has an "isolated" side and "common" side. Ignore what the polarity might be labeled as it also further confuses the issue.

It is important that the fuse be as close to the "isolated" side as is reasonably possible. Matt, Jim, Dave, and myself have all said the same thing, just in different words.

In most 12VDC vehicles that means that the fuse/breaker/fusible link wants to be as close as is reasonably possible to the positive battery terminal. Putting another one in the negative wire near the grounding or common connection won't hurt, but it shouldn't be considered to be a replacement for the fuse at the positive terminal.