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Positive Ground Electrical System

Can anyone explain to me why cars utilized a positive ground system in the good ole days. Wouldn't the chasis be like a live wire? I don't understand it at all.


  • edited December 2009
    Not really. Your assumption is that the circuits in the negative side of the uncompleted circuits would take on a negative charge. essentially fill with the excess free electrons from the battery's plates, waiting for some poor soul to create a connection to the relatively surplus-electron-free positive side of the circuit. It doesn't really work that way. An open circuit will have no current flow, a closed one will, the only change is in the direction of flow.

    But in truth, if some poor soul completes a circuit with enough voltage to go through their body?s resistance the hard way it doesn't matter which side is the surplus electron side (negative) and which is the electron deficient side (positive). He/she is gonna get zapped.

    Realize too that people generally get injured not from the zap, but from the being startled and impaling one's head on the hood latch. Being electrocuted actually requires that the current be through a path that interferes with a person's heart rhythm or other critical autonomic nerve signal. Even lightening rarely kills, as it generally follows a path to ground that does not disable the person's critical systems. That?s why you?ll sometimes read of someone getting struck and their watch will be melted and their wrist burned, but they?re otherwise unharmed. The current will have passed (generally) over the surface of their body and out through their Rolex (all golfers wear Rolexes) to ground.

    In summary,
    1) the circuits don't store the electrons. The battery plates do.
    2) all golfers wear Rolexes.
  • edited December 2009
    Just out of curiosity:

    What would happen if a positive-ground car wrecks into a negative-ground car?
    (Besides the obvious.)
  • edited December 2009
    You'd get a Yugo.

    Seriously, nothing strange. If they crashed such that a circuit was completed, you'd have simply negative to positive to negative to positive....a circuit with 24 volts, if you will. Sort of like having a 24 volt battery. Current would depend on resistance in the path.
  • edited December 2009
    Nothing more than if both cars had the same terminal grounded to the chassis. The difference in potential exists only between the terminal posts of the individual battery. If the batteries aren't interconnected, there is no difference in potential.

    The actual current flow, I believe,is from the negative post to the positive post. However, it makes no difference as to which terminal the engineers design to be the "ground" terminal. I bought a 1948 Dodge and the son of the owner had installed the battery backward. The ammeter read discharge when the car was running and would indicate a charge when the engine was off and the headlights were switched on. I discharged the battery and recharged it the correct way and then repolarized the generator by momentarily bridging the armature terminal of the regulator to the battery terminal. This car had a positive ground and the battery was installed backward. We didn't have semiconductors in those days that would be ruined by a reverse flow of current. As I remember, the radio didn't work when the battery was installed with the negative terminal grounded, but worked fine after I corrected the problem.

    As I remember, Ford and Chrysler products had a 6 volt positive ground system through 1955. Some General Motors cars in the 1930's may have had a positve ground. GM then went to a 6 volt negative ground and phased in 12 volt systems beginning in 1953 with the Cadillac, Buick Super, and Oldsmobile. The Buick Special adopted the 12 volt system in 1954 and all GM cars became 12 volt in 1955. Tbe Imperial introduced by Chrysler in 1955 had a 12 volt negative ground system. In 1956 all U.S. cars went to the 12 volt negative ground system.
  • edited December 2009

    I used to own some positive-ground cars ('54 Ford, two British sports cars from the '60s). It makes no difference which terminal is grounded. Battery DC does not work the same as household AC.

    Household electricity uses only one "hot" wire. The other carries no current at all until it completes a circuit. So only one wire is dangerous to a clumsy electrician.

    But for a DC battery, both terminals are always "live." The only difference polarity makes is that some motors and gauges will run backwards if installed in a car of opposite polarity. The auto industry finally decided to standardize; it could have gone either way.

    A wreck between a positive- and negative-ground car? Probably nothing significant on dry pavement. There should not be any complete circuit. On wet pavement, however, both batteries might slowly discharge each other using the road surface to complete a circuit.
  • edited December 2009
    Household electricity uses only "one" hot wire. The other carries no current at all until it completes a circuit.

    Actually, at the transformer, the center tap of the secondary coil goes to the earth. The voltage between the outer taps is 240 volts. The voltage between each of the outer taps and the center tap is 120 volts. The earth acts as a giant capacitor or reservoir. If you touch the grounded or neutral wire you won't experience any shock because you are at the same potential as the earth. If you touch the ungrounded or hot wire, you will receive a shock--the severity depends upon the resistance between you and the earth.

    With an automobile, the connection of the battery terminal isn't made to the earth, but to the chassis of the car.
  • edited December 2009
    Current flow is always from negative to positive. The electron is a negatively charged particle, and the side with surplus free electrons will always be negative relative to the side with a relative definciency of free electrons.

    In the scenerio I suggested, "such that a circuit was completed" my assumption intended was that the circuit would put both battery circuits in series. I was trying to dream up a "worst case scenerio". The worst that could happen is a 24 volt DC circuit. No big deal. Perhaps a toasted item or two. But I neglected to envision a possible parallel circuit....

    I was just doing brain excercises. In truth you're right, nothing spectacular would happen. And I'm sure there have been many crashes in the past with positive and negaively grounded vehicles. And I'm sure nothing spectacular ever happened.

    I liked your stories, by the way. Yeah, gas tubed radios don't work when reverse biased..............
  • edited December 2009
    So here's a question for everyone - why are cars now all negative ground? For standardization? Or is there some other reason? I sure don't know.
  • edited December 2009
    Frankly it makes no difference. If you remove the negative wire from the battery everything will stop, if you remove the positive, the exact same thing will happen. There are a few electronic parts that can tell the difference in the direction of flow but most in a car, can not.

    Being negative or positive is only meaningful in relation to a circuit. Any break in a circuit and nothing happens. For example if you remove either prong in an electrical cord and plug it in, it can not function. However keep in mind that there may be a back door.
  • edited December 2009
    I too have no clue why this is.

    Then again, why is it that tires' sectional widths are stated in millimeters, the sidewalls in a ratio, and the wheel diameters in inches?
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