Cut off switches on positive cable

[createthis]

I've read that 12V cut off switches should always be on the negative wire. The only explanation I've read that makes sense is that these switches usually stick out and provide a potential short if the chassis touches the switch, so if the switch is on the negative wire, this is less dangerous.

I'm currently building a small ungrounded solar system in a box. The box is plastic. The chassis grounds and unconnected. Is there any reason why I can't/shouldn't put the cut off switch on the positive wire? It would be more convenient there as I need to join the inverter and charge controller positive leads and the lugs on the switch would be useful for this. My shunt is on the negative cable, so I'm using those lugs already for the negative.

Thanks.

[Sunking]

Who told you that? You switch the Ungrounded circuit conductor.

Here is the thing. Even though you do not intend to ground the system, manufactures bond the equipment chassis (controller and inverters) to the Negative Polarity. That means you must switich the positive polarities if you are going to use switches. Additionally since you do not intend to use ground reference you must install OCPD on both battery polarities.

[DanS26]

I had an old Harry Ferguson TO20 tractor that had a positive ground electrical system. All switches, fuses, etc. were on the negative lead which of course was the ungrounded circuit conductor.

I believe there is or was a solar panel manufacturer that had a positive ground.....can't remember the manufacturer off hand.

[Sunking]

I had an old Harry Ferguson TO20 tractor that had a positive ground electrical system. All switches, fuses, etc. were on the negative lead which of course was the ungrounded circuit conductor.

I believe there is or was a solar panel manufacturer that had a positive ground.....can't remember the manufacturer off hand.

Dan thing is either Positive or Negative can be grounded. If one is going to be Grounded, Negative is the one you want to use to prevent corrosion problems. Having said that the Manufactures, due to UL issues take your choices away by internal bonding either polarity which is usually Negative. That eliminates a Float System which is safer and more reliable.

[DanS26]

That old TO20 tractor had a 6V system. My research indicated that the Harry Ferguson engineers wanted the shortest, most powerful current available to the starter. Since electrons in a DC system travel from negative to positive, they connected the negative lead to the starter with the least amount of wire possible, then grounded the battery on the positive lead. It worked but it was a puny system. I converted it to a 12V system, but I kept the positive ground system. I did not see much corrosion though.

[createthis]

Who told you that? You switch the Ungrounded circuit conductor.

Here is the thing. Even though you do not intend to ground the system, manufactures bond the equipment chassis (controller and inverters) to the Negative Polarity. That means you must switich the positive polarities if you are going to use switches. Additionally since you do not intend to use ground reference you must install OCPD on both battery polarities.

Well, that works out then. Curious though, why do you switch the unbonded polarity? What happens if you switch the bonded polarity?

[Sunking]

Well, that works out then. Curious though, why do you switch the unbonded polarity? What happens if you switch the bonded polarity?

Well I think you meant: Why do you NOT Switch the Bonded Polarity?

Answer is real simple. You loose or make your Over Current Protection Device inoperable, and leaves exposed metal parts hot with voltage to touch.

[Sunking]

That old TO20 tractor had a 6V system. My research indicated that the Harry Ferguson engineers wanted the shortest, most powerful current available to the starter. Since electrons in a DC system travel from negative to positive, they connected the negative lead to the starter with the least amount of wire possible, then grounded the battery on the positive lead. It worked but it was a puny system. I converted it to a 12V system, but I kept the positive ground system. I did not see much corrosion though.

Dan no offense intended but think about what you just said. It makes no fricking difference which polarity gets bonded in terms of how much current is delivered to the load. The resistance is equal either way.

Boats, planes, and trains use the vehicle chassis as a current carrying conductor to save weight and expense. When you do that means you do not have to add the return conductor to the battery. You use the frame. That was great in the ole days with mechanical ignition systems, and no electronics. Using the frame cause all sorts or noise and interference issues. Remember the day when you heard all that alternator whine and snap crackle pop in your radio? That was caused by using the frame of the vehicle as a return conductor.

[DanS26]

Dan no offense intended but think about what you just said. It makes no fricking difference which polarity gets bonded in terms of how much current is delivered to the load. The resistance is equal either way.

Boats, planes, and trains use the vehicle chassis as a current carrying conductor to save weight and expense. When you do that means you do not have to add the return conductor to the battery. You use the frame. That was great in the ole days with mechanical ignition systems, and no electronics. Using the frame cause all sorts or noise and interference issues. Remember the day when you heard all that alternator whine and snap crackle pop in your radio? That was caused by using the frame of the vehicle as a return conductor.

I understand....so why do some solar panel manufacturers use positive ground. Surely it is not to save weight and expense?

[DanS26]

In 1935 the old codgers designing a 6V electrical system for a radical improvement in farm machinery chose a positive ground. Why did they do that?

Flip a coin....I don't think so. They had a good electrical engineering reason to do that.

When they looked at Thomas Edison's DC system in New York they probably noticed that the bulbs closest to the plant were the brightest. So design a system so that the heaviest DC loads were closest to the source of power. But then again why put your heaviest load closest to the negative side of the power source? My only conclusion is that they were looking at the direction of electron flow.

[Sunking]

In 1935 the old codgers designing a 6V electrical system for a radical improvement in farm machinery chose a positive ground. Why did they do that?

Flip a coin....I don't think so. They had a good electrical engineering reason to do that.

When they looked at Thomas Edison's DC system in New York they probably noticed that the bulbs closest to the plant were the brightest. So design a system so that the heaviest DC loads were closest to the source of power. But then again why put your heaviest load closest to the negative side of the power source? My only conclusion is that they were looking at the direction of electron flow.

Nucking Futs.

Man does not know which way current flows. Positive and Negative are Man Made Terms. There are two rules for current flow. Left Hand and Right hand. Take your pick. The two rules are:

Conventional Current current flows out of the positive terminal, through the circuit and into the negative terminal of the source. This was the convention chosen during the discovery of electricity by B Franklin.

Electron Flow flow out of the negative terminal, through the circuit and into the positive terminal of the source. Both Conventional Current and Electron Flow are used. Many textbooks are available in both formats.

Floyd, 1989, Principles of Electric Circuits, 5th edition, Conventional Current Version.
Floyd, 1990, Principles of Electric Circuits, 4th edition, Electron Flow Version.

In fact, it makes no difference which way current is flowing as long as it is used consistently. The direction of current flow does not affect what the current does. In general, high school Physics and two year technician programs use Electron Flow.

But three year technologist and university engineering programs use Conventional Current. Certain symbols (ex. diodes and transistors) and rules (ex. Right-hand rules) were created using Conventional Current. Changing from Conventional Current to Electron Flow would cause a degree of confusion for old and new students and errors would occur, so Conventional Current was kept to ensure there was no confusion with those already trained with Conventional Current. Two systems may seem confusing, but as long as usage is consistent, it really is not! You must realize what convention is being used because the rules change. Ex. Right-Hand rules in Conventional Current become Left-Hand rules in Electron Flow.

Lesson over.

[Living Large]

Flip a coin....I don't think so. They had a good electrical engineering reason to do that.

I'd be fascinated to read the explanation that you find to prove your thesis, when you locate it and post here. I'm a EE, and I don't remember any training or principle or anything I learned in practice that showed that power delivered using one direction of current flow was more efficient than if the current flowed in the other direction. To me, they would have taught us that in a basic course, right around the time we learned KCL.

[Sunking]

I'd be fascinated to read the explanation that you find to prove your thesis, when you locate it and post here. I'm a EE, and I don't remember any training or principle or anything I learned in practice that showed that power delivered using one direction of current flow was more efficient than if the current flowed in the other direction. To me, they would have taught us that in a basic course, right around the time we learned KCL.

That is pretty funny. Cause I know what the joke is. He cannot find that information from any credible source other than a blog from a shade tree mechanic.

[DanS26]

I stand corrected. They really did just flip a coin. It makes no difference.

[Sunking]

I stand corrected. They really did just flip a coin. It makes no difference.

Actually as ironic as it seems, a coin flip is about as good of an answer as any. The original Ford Model T used a negative ground. All old tractors used positive ground. You can search the web all day long looking for why some used positive ground and you are never going to find a good answer. As best as I can tell is USA used negative ground, and European companies used positive ground.

However what is factually known is positive bonded (Ground is a terrible word) vehicles suffer from frame and chassis corrosion by electrolysis. AKA electro-plating where positive is the Anode and negative is the Cathode. Pipe lines use the process to protect the pipe lines. Industry uses the process to plate things like chrome bumpers, marine industry uses it to protect the haul of a ship.