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I just got a classic Ford 8N tractor running again. Someone trying to be helpful flipped the battery around, connecting negative to ground. This made the engine crank backwards! So I had spark, compression, and fuel but it wouldn't start. This got me thinking:

Positive ground was very common, especially on 6V system on work machines. Today almost all vehicles are negative ground.

Is this just a matter of convention, or are there reasons to choose one over the other?

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3 Answers

up vote 4 down vote accepted

It is partly a matter of convention and standards, and partly because positive earth encourages galvanic corrosion* of the surrounding bodywork - whearas with negative earth is is the much-easier-to-replace electrical contacts that corrode.

*Think back to school physics lessons with anodes and cathodes...

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Hooray for Wikipedia:

Originally, the voltages on the wires were positive with respect to earth. This is called negative ground, since the negative side of the battery is grounded to earth. Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage on the wires, in respect to earth, (called positive ground) the copper is protected from corrosion. This is referred to as cathodic protection.

Clearly, there is a trade-off. You're going to have electrically-motivated corrosion on one set of components or another. As Nick points out, corrosion of electrical components (e.g., wires and connections) is much easier to replace on a vehicle than the bodywork or frame itself. Combine this with the fact that, frankly, we're better at protecting wires than we used to be and it makes a lot of sense to run a negative ground.

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There is also a safety factor.

If you have the positive terminal of your battery connected to the frame of the car, then it is very easy to make new circuits, through your body for instance.

There are a lot more negative ground sources out in the open then positive ones.

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I'm not convinced. Even if you touch 2 vehicles with different grounds, you won't complete a circuit, as the non-ground side of the battery is isolated. Right? –  Jay Bazuzi Sep 16 '11 at 0:02
    
Could be wrong, not an electrician, but connecting positive of the battery to any negative ground sure seems like a circuit to me. With a vehicle it has to wrap back to the battery because the whole vehicle is isolated from another ground by the tires. We are talking about the flow of electrons, they want to move from the positive to the negative to create balance, they don't care if that the negative is on the other side of the battery, the ground beneath your feet or anywhere else. –  ManiacZX Sep 16 '11 at 7:14
    
I just thought of what might be a great example of what I'm talking about. A Lightning strike! When you get a strong enough positive charge in the sky it jumps to an available ground, in this case the earth. This isn't a cyclical circuit where it leaves the cloud to somewhere else and then goes back to the cloud. It leaves the cloud and goes into the earth, from a positive to a negative. –  ManiacZX Sep 16 '11 at 7:23
    
Static electricity such as lightning, at tens of thousands of volts, is likely not a relevant example for what happens in dynamic electricity at the low tens of volts. Though I can't explain why fully. I imagine it has to do with the potential, the positive side of a battery doesn't have high potential relative to earth ground, it has high potential relative to the negative terminal of that same battery. An example of this is that you can't take two batteries and ONLY connect, say, a light bulb to the positive terminal of one and the negative of the other and have it work. –  Sean Reifschneider Sep 23 '11 at 4:42
    
Agreed lightning isn't a perfect example, talking about way higher voltages then we are dealing with, but I thought it would be close enough and a universally understood example. The light bulb and two batteries example doesn't seem good to me as I can see the positive side of a battery working connected somewhere else but not necessarily the negative. Out flow of electricity from the positive seems a lot more plausible then in flow of a limited capacity device. –  ManiacZX Sep 23 '11 at 19:09
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