Posted by Gerald J. on January 26, 2010 at 09:23:08 from (70.219.126.63):
In Reply to: Positive Ground posted by mss3020 on January 26, 2010 at 07:08:34:
The question of polarity was debated from the beginning of electric starters until some time in the 1960s when SAE put out a new standard that required negative ground. Search out posts on this topic and you'll find where I posted the number of that SAE standard. I'm still wanting to look up more history and I have some older SAE standards books but some have been packed up and moved already. Some haven't appeared while packing yet.
I think the corrosion arguments came from the corrosion experiences with water pipes and DC trolley systems. Turns out that no matter how big a grounded conductor you use, so long as you have contacts to earth the large cross section of earth, though not a great conductor, will carry a significant portion of the power (something called Carson's theory). In a DC trolley feed, then depending polarity of the trolley wire and distance from the power plant a metal water pipe will either be eroded or will have metal added. One polarity is better for pipes near the power plant but worse for pipes away. So the trolley runs with the polarity that gets the fewest water company gripes.
Fact is, on a vehicle, either polarity works for starting and ignition, and in the early solid state days radios came with a way to set the polarity to match the power source. Reversed polarity usually means destruction of the solid state device. Often with striking visual results (a cloud of smoke)! Alternators are among those very polarity sensitive products and for convenience is probably why the SAE demanded negative ground, long about 1968.
Deere fuel gauges in the '20 era are polarity sensitive, why I don't know.
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