Why is the fuse on the positive side?

[Elkins45]

Electricity flows from the negative terminal of the battery to the positive, yet the fuse on negative ground cars is on the positive side of the circuit. That means even if a fuse blows there’s still a potential applied to the load. What’s the logic of that? If the fuse were on the negative side of the circuit then a blown fuse would mean no potential anywhere downstream, which seems like a better arrangement.

My dad’s 1953 Farmall tractor was a 6V system with a positive ground. IMO positive ground just seems like a better idea but if it exists on any modern vehicles I’m unaware of it.

 

[Jdeere562]

If I'm not mistaken...it flows from positive to negative.

 

[nomas]

Current flows from positive to negative . Electrons flow from negative to positive .
https://www.allaboutcircuits.com/textbook/direct-current/chpt-1/conventional-versus-electron-flow/

 

[KJSmith]

If I'm not mistaken...it flows from positive to negative.

IIRC, the Navy teaches it negative to positive.

 

[zrxkawboy]

It doesn't matter which way the current flows....a blown fuse breaks the circuit.

 

[KJSmith]

If you go positive to negative, the blown fuse can prevent a short.
If you go negative to positive and blow a fuse, there is no "potential difference" and that prevents a short.

Potential difference meaning there is nowhere for the flow to go.

 

[nomas]

I read that post four times and I still have no idea what you said .

 

[KJSmith]

The positive side is isolated from the chassis until after the load.
Blown fuse, no flow.

The chassis and negative side are the same.
If there is a short, there is nowhere for the 12volts to flow.


Make more better sense?

 

[Timmastertech]

Because its far far far more likely for power to short to ground than for a ground circuit to short to power

 

[Elkins45]

Current flows from positive to negative . Electrons flow from negative to positive .
https://www.allaboutcircuits.com/textbook/direct-current/chpt-1/conventional-versus-electron-flow/

"Current" in this case means "conventional current" which isn't real.

If I'm not mistaken...it flows from positive to negative.

Physics established the idea of "conventional current" before we actually knew which way it flowed. In physics class they still teach conventional current, but electricity actually flows from negative to positive. There's something called the "right hand rule" that states if you cup your right hand around a wire so that your fingers curve in the direction of the surrounding magnetic field then your thumb points in the direction of current flow...but when you test it experimentally it actually works for your left hand.

There's a weak movement among physics teachers to abandon the teaching of conventional current because it is at odds with observed reality. Unfortunately all of the old literature used it so the idea probably won't gain much traction until we stop using old printed texts.

 

[JimPghPA]

If a positive side wire providing power to something is fused and then becomes damaged so it can electrically connect to the chassie ( ground ), the fuse will blow and prevent extreme amounts of current flow that otherwise could possibly get the wire hot enough to cause a fire. Therefore, it makes sense to fuse the positive wires on a vehicles with the metal chassis tied to negative (ground).

There is no need to fuse wires of the same polarity as the metal chassis because if they end up electrically touching the metal chassis nothing happens.

--------------------

I heard some British vehicles used positive to the chassis years ago, (positive ground), but I don't know if any are still made that way.

If a vehicle has positive tied to the chassis, then all wires supplying negative should be fused because if one of those becomes damaged and electrically touches the positive metal chassis it could overheat with too much current and start a fire.

BTW, positive ground may somewhat reduce the rate of rust, but that's a DEPENDER, as it depends on if it can actually interact with the flow of ambient air electrons.

 

[KJSmith]

I heard some British vehicles used positive to the chassis years ago, (positive ground), but I don't know if any are still made that way.

That could make door dings with "normal" cars a lot more exciting.

 

[JimPghPA]

That could make door dings with "normal" cars a lot more exciting.

Actually, no. There's not a complete path for the current.

 

[KJSmith]

LOL.
Missed it by that much.

 

[redbone3]

In the world of alternating current the fuse is always in the "High" circuit. The same principles applies to DC circuits, the fuse is in the high (not ground) circuit.

 

[supton]

Wouldn't matter which side, as current flows in a loop. Break the loop and nothing flows. Doesn't matter which side has the fuse.

Thing is, a car is a specific use case. The battery negative terminal is "strongly" bonded to the car's chassis. The car itself is the return path. To use fuses in the low side would require one to run lots of negative wires in addition to positive wires--or to switch to a positive ground.

In household wiring, the neutral wire is referenced to earth ground. If neutral was fused instead of hot, and the fuse blew, you could still get zapped by standing on anything that was earthed while touching the "dead" circuit. It's much like our automotive case, where the "low" side is bonded to everything that isn't meant to be hot. [Yes AC goes above and below ground potential. Using common vernacular here.]

While electron flow is from negative to positive, you can easily analyze circuits using current as flowing from positive to negative. And in solid state electronics, we talk about positive and negative current flow--positive current flow is "holes" flowing in the opposite direction as electrons. [It's been sadly too many years since I studied it in college, fascinating stuff, what goes on in PN junctions.]

 

[ka9mnx]

If the negative side of a faulty circuit blows there is still a voltage potential at the faulty circuit. If the fuse is on the positive side and the fuse blows, voltage potential is broken to the faulty circuit.

In many circuits the negative side of circuits has many return paths to the power source. Equipment chassis, vehicle chassis are usually tied back to the negative side of the source. If a starter or alternator blows with the negative side fuse, the alternator or starter still has a negative path to the source through it's body.

This is how ground fault interrupters (bad choice of words) in house wiring works. If there is a fault and the hot side touches the chassis current flows through the third wire and trips the interrupter. The GFCI monitors the difference in current between the white and green wires.

 

[eljefino]

Tradition.

First we need to separate car wiring from house wiring in our minds. House wiring has a hot and a neutral, with neutral grounded to actual earth ground for safety. So if you have a tool that goes berzerk and makes its metal case hot in relation to ground, that current goes to neutral through a low-resistance ground wire not your medium-resistance body. Ok, forget anything more, we're not talking about this.

Cars use the chassis for ground because they're cheap and can run half the wire. This industry standard was only fairly recent in the scheme of things. We fuse as near to the battery as we can so the most of the circuit can be protected as possible. There is an unfused circuit-- the starter motor! The alternator also has a fusible link but it's so big you could start a decent fire before it pops. Car fuseboxes (plural) are set up the way they are, so you have an "engine fusebox" that likely stays with the engine harness until it plops into the car. You'll have a big feed to and from the ignition switch but it's otherwise self contained. Then you have a "body fusebox" for the lights and creature comforts.

 

[jhellwig]

It doesn’t matter what way the current flows or what side of the power source is referenced. You put the circuit protection on the unreferenced side of you power source in any system. It is to stop the unreferenced side from becoming referenced and blowing things up.

 

[jhellwig]

This is how ground fault interrupters (bad choice of words) in house wiring works. If there is a fault and the hot side touches the chassis current flows through the third wire and trips the interrupter. The GFCI monitors the difference in current between the white and green wires.

No. That is incorrect. A gfci monitors the differential of current on the source(“hot, black”, ungrounded conductor) and return (“neutral, white,” grounded conductor). Doesn’t matter what side faults to ground.