|The MGA With An Attitude
Why POSITIVE EARTH? - ET-098
On 3/13/2010, Skip Scott in Walnut creek , CE wrote:
"What is the rational for a positive ground system? Is there an advantage of one system over the other? My car has a positive ground".
The short answer is, there is no functional reason why any car needs to have positive earth. The real reasons are rather twisted, based in prior historical tradition, so now you get a history lesson.
1828 first telegraph. 1876 first telephone. These devices commonly used a single wire for interconnection. These were DC battery powered systems (sometimes with a generator to recharge the batteries). At each end the circuit was grounded in the earth for the electrical return path. For the phone systems the central office ground was a relatively high current single grounding point, fairly easy to control for corrosion resistance. This was considered to be 0-volts. For station equipment (single remote telephones) it was chosen to make the earth grounding wire negative. This earth connection is then protected from cathodic corrosion because voltage is lower than the ground potential. This affects galvanic corrosion resistance between the grounding wire and mother earth (real ground).
The general use of automobiles came later. In autos there is no earth ground. When the vehicle sits on rubber tires the entire system has floating electrical system (no ground). Suppose you have a car with the positive post of a 12-volt battery connected to the car body. You can connect a single jumper cable from the negative post (the "hot" terminal) to a metal post driven into the earth. Then the "hot" side of the vehicle system becomes 0-volts in relation to earth ground, and the chassis of the vehicle becomes negative 12-volts. You could connect a 12-volt lamp between the vehicle chassis and the earth grounding post, and the lamp would light up. This is why you have to be careful using jumper cables on cars with opposite polarity electrical systems. Don't let steel bumpers touch, as it would short the batteries together in series causing catastrophically high currents.
The idea that a vehicle has an "earth" connection (or electrical grounding point) is only a traditional method of naming certain types of electrical connections (carried over from terminology used in earlier single wire electrical systems). Cars do not actually have electrical grounds. We simply call the collection of battery return path connections "earth" or "ground" as a naming convention.
In the simplest systems there may be only a single circuit from battery to ignition coil, through contact points and back to the other post of the battery. With additional circuits we typically branch the circuits on one side of the battery only, commonly called the "hot" side, so all of the return circuits can be carried on a single conductor. For a vehicle with a steel chassis it is convenient to connect the return circuits to the chassis to reduce the amount of wire used. Otherwise this is not necessary and not always true. The Lotus Elite for instance has an all fiberglass unibody with no steel frame, so all of the electrical circuits must have a copper wire conductor for return to the battery.
In a steel chassis car it is entirely arbitrary which side of the battery gets connected to the chassis, so it makes little or no difference if it is positive or negative "ground". During first half of the 20th century some manufacturers used positive ground, others used negative ground, and some switched form one to the other more than once. When car radios became a popular accessory, ignition static could be a problem, and it was good if all vehicles might have the same electrical polarity to reduce ignition noise on the radios. We still had mixed vehicles until the mid 60's. With the advent of practical portable alternators it was a convenient time to standardize, and all vehicles thereafter were built with electrical return path to the negative battery post. For a couple of years in the mid 60's there were a few positive earth alternators, but most people have never seen one.
Vehicles with "solid state" (transistorized) electronic gadgets will invariably be negative earth. The first common "solid state" electronic gadget in cars was the transistorized radio, starting in the mid 50's. Most of the early transistor car radios had a polarity plug on the back so they could be used in either positive or negative ground applications. The next common electronic gadget for cars was the triode bridge in the alternator, circa early 60's. Early alternators used silicon plate rectifiers that could be connected for either positive or negative grounding, so some early alternators were made for positive ground applications. When silicon diodes were developed with sufficient current capacity for alternator rectifiers, all alternators were quickly redesigned to use diodes. This is when all alternators became negative ground, and all cars using them were built with electrical return path connected to battery negative.
If you have an early production car with positive electrical return path (positive earth), it is generally easy to change the polarity. For my 1958 MGA it is as simple as reversing the battery, and re-polarizing the generator (a 1-second process). To be nice we also swap the primary wires on the ignition coil so the spark still jumps the same direction across the spark plug gaps (mostly for radio static reduction). When installing a modern radio in an older car it is usually easier to change system electrical polarity than to try isolating the radio cabinet or using a polarity changer for radio power input. The only good reason for retaining positive earth setup on the older car is for originality (concours show cars).