Negative Ground vs Positive Ground History Question.

[Diesel Bill]

My long gone 1965 MGB roadster was 12 volt positive ground. was a pain in the rear to put in a newfangled AM-FM Cassette player. I used a small square of plywood under the dash to mount it with negative ground.

[Lepke]

If your boat with a positive ground has one improperly wired item it can eat the underwater metal in a very short time. Commercial fishing trollers were commonly wired with a positive ground before many electronics were common. The reason was to put a +0.5 volt charge on the fishing lines. It was known to better attract fish.
I had a fishing friend who installed a new radio and wired it backwards. Everything ran ok. One day, three weeks later, backing down, the blades on his 44" prop came off.

[Snowpetrel]

Originally Posted by Lepke

If your boat with a positive ground has one improperly wired item it can eat the underwater metal in a very short time. Commercial fishing trollers were commonly wired with a positive ground before many electronics were common. The reason was to put a +0.5 volt charge on the fishing lines. It was known to better attract fish.
I had a fishing friend who installed a new radio and wired it backwards. Everything ran ok. One day, three weeks later, backing down, the blades on his 44" prop came off.

Yeah, this was my understanding as well, but I dont have the technical understanding to back it up with anything substantial.

For what its worth one old steel boat engineer told me the big plus to a negative earth was it would potentially trip the circutbreaker of any positive feed that leaked to the hull. Preventing this kind of rapid electrolytic corrosion. He reckoned wiring the whole boat above earth was a recipe for problems, as a positive leak could do real damage before it was spotted. I am still not sure if the negitive earth protecting the hull is an urban myth, or real? But the reading I have done seems to support the idea.

This seems to make the idea of a positive earth pretty bad from a corrosion point of view?

[Wotname]

Originally Posted by Snowpetrel

Yeah, this was my understanding as well, but I dont have the technical understanding to back it up with anything substantial.

For what its worth one old steel boat engineer told me the big plus to a negative earth was it would potentially trip the circutbreaker of any positive feed that leaked to the hull. Preventing this kind of rapid electrolytic corrosion. He reckoned wiring the whole boat above earth was a recipe for problems, as a positive leak could do real damage before it was spotted. I am still not sure if the negitive earth protecting the hull is an urban myth, or real? But the reading I have done seems to support the idea.

This seems to make the idea of a positive earth pretty bad from a corrosion point of view?

Yes, this seems to a common POV. I wonder if it is myth or not???????

Presumably we are talking corrosion of the chassis / hull etc rather than corrosion of the hot side wiring?

[Snowpetrel]

^^ my thoughts are that it has to be the case, look at how impressed current systems work, they drive the hull into the negative voltage, effectively pumping it full of electrons to prevent corrosion (at least thats my non technical mental picture of what happens...)

Now I am figuring cars and other stuff like planes or electrical gear really do not have the same issues, because generally speaking cars and such aren't continuously immersed in an electrolyte soup like our boat is, so nothing particularly bad happens most of the time. Just as an impressed current system and anodes don't work very well above the water(though galvanising does...)

I think it takes more than just a positive earth to cause a problem. There has to be a volatage difference somewhere. If the whole hull is at +12 or -12v there is no issue, but if one part is at +12v and another part is at -12v and they are connected electrically and are also floating in the same electolyte bad things can happen to the positive part. This is my very fuzzy understanding. And it is probably wrong, hence posting here to hopefully have the errors of my ways corrected.

[senormechanico]

Originally Posted by Viking Sailor

It had to do with the evolution of the vacuum tube. As the complexity of the vacuum tube and the surrounding circuit design increased it became more efficient to use a negative ground and a positive plate supply. When tube based radios were installed in cars it was much simpler to tie the negative battery terminal to the chassis then to "float" the radio on a positive ground.

Could be a contributing factor, but the old tube car radios power supply went from the DC input through a vibrator and a transformer to supply 150 volts to the tubes.
The transformer secondary was isolated from the primary, so the input voltage could have been either positive or negative ground.
Actually, the radio worked without regard do the DC input polarity.
Input polarity only became an issue in about 1963 when GM cars had a DS105 transistor for the output stage, and the tubes were designed for 12 volts operation.
GM went from generators to alternators in 1964.
Before that with generators, you could get them to charge with either battery polarity if you "flashed' the field winding.
As there were no solid state items in the car before that, everything worked without regard to polarity of the battery.

Back in the day ('60's) the shop I worked at used to repair car radios IN THE CAR for $1.50 + parts. If it couldn't be repaired in the car, we removed the radio, fixed it on the bench (usually while the customer waited for it), reinstalled it in the car, and charged $1.50 each way + $1.50 + parts.
Believe it or not, we made a living doing that.


Edit: I see Wotname beat me to the generator flashing bit.

[Bycrick]

At least with vacune tube circuits, there were all kinds of advantages to keeping the heater/cathode and control grid circuits at close to "ground" potential. The plates and other grids were usually at a high positive voltage relative to everything else.

On the other hand, some magnetrons in radar or microwave systems, apparently for ease of connection of metal waveguides, did run with the "plate" connected to ground and the cathode/heater at a high negative voltage.

It would be interesting to actually KNOW what the reasoning was, if only for historical reasons.

[Rapanui]

The answer I got when I asked this in the 60s of a motor mechanic who lived next door was that it was to minimise spark erosion of the spark plug centre pin. The problem certainly occurred with mechanical distributors with contact breaker points where one point would erode and the other develop a 'tit', which would necessitate regular fettling and the gap resetting. Whether it was a real or theoretical risk for plugs, I don't know. Never noticed a problem when the practice reversed to negative earth, but then it also coincided with the advent of electronic ignition systems.

[bdgWesternMass]

I believe with positive ground one can encourage galvanic corrosion to happen on the zinc side of the circuit with the ground.
A circuit with negative ground is more susceptible galvanic corrosion happening anywhere on the logic side of the circuit.