Thread: Master power switch. Electrical GURUS needed

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Electrical GURU'S needed!

I am installing a keyed, master power switch, but I need someone with good electrical knowledge to confirm it. My main reason for the switch is for security, but I also want one in case of emergencys. I also plan on having a fused keep alive wire, across the switch for computer memory.

Questions;

1) If one used the negative battery cable wired to the battery switch, (rather than a positive one), would it work as effectively in case of a short, or an accident? Pros/Cons?

2) Would a fused keep alive wire allow enough of a ground circuit, that the switch wouldn't kill all power in an emergency situation, and perhaps allow a short to continue shorting? (If so, I could wire a toggle switch to break the keep alive circuit as well)

3) Will shutting off the master power switch, while the engine is running not harm the alternator, or voltage regulator diodes? I seem to remember reading about a warning never to disconnect your battery while running.


The reason I am considering the ground cable being used rather than the traditional way of wiring it is two fold.

One, I can easily wire the ground cable to the location I intend to install it, where the positive cable would be more complicated.

Two, I read of a recent incident where a fellow had a master power switch break off at the threaded post that his positive cable was attached to. This allowed a HOT wire to dangle under the car, near the fuel tank, and frame members, until he got the car to a stop. Could have been a bad situation. (This would not have been a problem with a negative cable set up)

Answers, suggestions, pros/cons appreciated!!
Thanks Bill

[QUOTE=

Could have been a bad situation. (This would not have been a problem with a negative cable set up)

Answers, suggestions, pros/cons appreciated!!
Thanks Bill[/QUOTE]

I strongly disagree! A wire carrying current when it completes a circuit will arc, positive or negative! I don't see any benefit other than the ease of installation. And, I don't think there would be any issue to break the ground side of the circuit. I think you should be fine.

Originally Posted by 34_40

I strongly disagree! A wire carrying current when it completes a circuit will arc, positive or negative! I don't see any benefit other than the ease of installation. And, I don't think there would be any issue to break the ground side of the circuit. I think you should be fine.

In this case, if the power switch had been wired to the ground, and the battery ground cable broke off the switch and dangled, there would be no complete circuit, simply ground to ground. The postive cable would be grounding out every time it hit a frame part, or fuel tank, and arcing like crazy.

MOTER, that is exactly what I planned on doing. There would be a negative cable leading to one pole on the switch, then a nother cable from the other pole to a frame grounding point.

MRJB1929- The "keep alive" wire is a small gauge wire, ( inline fused ), that would jump across the two terminals of the switch, thus completeing a circuit, even when the switch was off. The large cable from the battery, and the other going to frame ground, would be switched. The idea of this "keep alive wire" being fused is that if one tried to start the vehicle with the master switch off, the low amp fuse, (which is all that would be required for computer memory) would blow from the high amps the starter would draw. This would prevent a complete circuit of any sort so the starter wouldn't turn.

Being the switch is the keyed type, I hoped this would perhaps foil thieves, if they tried to hot wire the ignition switch. Being it would be a ground kill switch, would perhaps lessen the chance of them figuring it out quickly. A smart thief coulkd try and jump the switch posts, but they would need some heavy gauge wire in their pocket to do so..

Your idea should work just fine,except instead of the fuse I would use an automotive circuit breaker,sooner or later your gonna forget about having the switch turned off and will pop the fuse. The circuit breaker will reset itself in a matter minutes. G.M.used them for power seats and wiper motors for years. Any Auto Parts store should have them in stock.

Great idea! So do you feel the ground "keep alive" would trip a breaker, or blow a fuse, if a short occurred, or if the starter was cranked (without the switch turned on) as well as the positive wired set up would?

For some reason I keep thinking that a smaller gauge ground wire (keep alive), even fused with a low amp fuse/breaker, would still allow enough of a ground to crank the engine, or allow a short to occur, where as the positive cable would not. I don't know why this is stuck in my head. Am I totally wrong about this concern? I think I am, just want confirmation from someone who knows more than me about 12v electrical systems..

What of my concerns re the alternator/regulator, if one switched off while the engine was running?

I read one comment on another forum that said, "regardless of the switch wiring at the switch, (- or +), the engine would not stop running unless the alternator exciter wire was also disconnected"..I don't know what to think of that..but I seem to recall that a vehicle can continue to run, with battery disconnected. Perhaps this is where I heard of the alternator/regulator issues occurring. If that is the case how is the master switch a benefit at a race track. Car hits the wall, throttle stuck wide open, driver unconscious, track Marshall turns off the master switch, engine keeps screaming..I don't get it? I suppose the fuel pump would be switched off, so that would be a plus, as well as other electrics, that might short and cause a fire..

I suppose in an emergency, where the driver is still aware, you would turn off the ignition switch, then turn off the master switch..

My opinion is to not to try to reinvent the wheel on this one. There is a reason every boat that uses a master battery switch is wired on the positive side, with the switch as close to the battery as possible. That reason is you want to be able to disconnect any potential spark source in case a wire comes loose or touches ground. If you do it on the negative side you still have a long run of positive cable just laying there waiting to touch metal and become a dead short.

Now, I know we are talking about a car vs a boat, but same principle applies. If you want something like memory to stay hot all the time, simply connect that accessory in on the positive side of your disconnect side. Everything beyond that is protected when the battery switch is turned off.

Don

Originally Posted by Chev malibu

In this case, if the power switch had been wired to the ground, and the battery ground cable broke off the switch and dangled, there would be no complete circuit, simply ground to ground. The postive cable would be grounding out every time it hit a frame part, or fuel tank, and arcing like crazy.

My point exacty! Every time the circuit becomes complete an arc will occur! It doesn't matter if it's + or - ! Consider an arc welder, it can be positive ground or negative ground. There will still be an arc when the circuit becomes complete! Don't believe me? Fire up you stick welder, clip the electrode holder to the steel and clamp the ground to a rod, yes, you can still weld with it! the circuit will be completed! And don't ask me how I know this! I guess I can be silly to prove a point!

If you plan for the circuit to stay isolated you will be prepared! I like the breaker idea too!

Originally Posted by 34_40

My point exacty! Every time the circuit becomes complete an arc will occur! It doesn't matter if it's + or - ! Consider an arc welder, it can be positive ground or negative ground. There will still be an arc when the circuit becomes complete! Don't believe me? Fire up you stick welder, clip the electrode holder to the steel and clamp the ground to a rod, yes, you can still weld with it! the circuit will be completed! And don't ask me how I know this! I guess I can be silly to prove a point!

If you plan for the circuit to stay isolated you will be prepared! I like the breaker idea too!

I must be getting brain farts..are you saying that if I went out and disconnected my negative battery cable from where it is mounted to the frame, and then tapped the frame with it, I would still get an arc? I would have thought that I would have to actually touch a positive wire to accomplish that. I can't see how a circuit would be completed, with a ground to ground contact.

Now on the other hand, if I disconnected my positive cable say from the starter, and then started tapping the frame, or any other grounded point, I know I would get arcing.

Originally Posted by Chev malibu

I must be getting brain farts..are you saying that if I went out and disconnected my negative battery cable from where it is mounted to the frame, and then tapped the frame with it, I would still get an arc? I would have thought that I would have to actually touch a positive wire to accomplish that. I can't see how a circuit would be completed, with a ground to ground contact.

Now on the other hand, if I disconnected my positive cable say from the starter, and then started tapping the frame, or any other grounded point, I know I would get arcing.

Yes! Go ahead and remove the ground connection. Now turn on the headlights then touch the side of the connector to the post and watch for a spark. it'll be there as the current has to flow when the circuit is completed.

install the switch between the negative battery cable and the system.

So let me ask this question. If you are using a master switch to control the ground, how are the memory items going to get their ground connection??? If the ground is completely disconnected from the battery through the master switch, your memory won't work either as there won't be any residual ground bleed.

If there is a switch between the negative battery post and ground and it is turned off there is not a live positive wire anywhere on the vehicle that will arc if you touch it to metal because negative is disconnected from ground and therefore, ground metal is dead and cannot be used to complete a circuit.

Originally Posted by AzDon

If there is a switch between the negative battery post and ground and it is turned off there is not a live positive wire anywhere on the vehicle that will arc if you touch it to metal because negative is disconnected from ground and therefore, ground metal is dead and cannot be used to complete a circuit.

Exactly! The first thing any manual tells you before starting most work on a vehicle is. remove the ground (negative) cable from the battery. It makes sense to me that my switch wired this way, will function as I desire it to.

Now my engine is EFI, has a computer, and electric fuel pump, so I'm thinking that in an emergency situation, IE; smoke, fire, sparks, or accident, I should turn off the ignition switch, then turn off the battery switch. Engine is off, Fuel pump is off, any shorts will stop immediately.. makes sense to me. Oh and perhaps I should add grab fire extinguisher..lol

Denny, now see what you started...

Originally Posted by Chev malibu

Exactly! The first thing any manual tells you before starting most work on a vehicle is. remove the ground (negative) cable from the battery. It makes sense to me that my switch wired this way, will function as I desire it to.
That's because of the potential spark that could ignite battery fumes if you remove the positive cable first.


Now my engine is EFI, has a computer, and electric fuel pump, so I'm thinking that in an emergency situation, IE; smoke, fire, sparks, or accident, I should turn off the ignition switch, then turn off the battery switch. Engine is off, Fuel pump is off, any shorts will stop immediately.. makes sense to me. Oh and perhaps I should add grab fire extinguisher..lol
Not necessarily so, any capacitance in the circuit could fuel the fire until it is totally discharged.
Denny, now see what you started...

And the NHRA rulebook states that a battery disconnect will interrupt the positive side per techinspector1

Originally Posted by DennyW

Just look at all the fun we just had though.

It WAS a "lively" ... discussion! Wasn't it?

It has always been an unwritten rule when working around batteries to disconnect the negative FIRST! The reason for that is, should your wrench touch the body, you won't have a spark when you touch the grounded metal and possibly cause the battery gases to explode.
(when I was in vocational high our teachers would insist that we place a folded shop towel over the positive terminal to further prevent any possible connection of positive & negative! It's a practice I still use!)

I don't believe it has anything to do with your switch manufacturer intending anyone to insert the switch into that side of the circuit.

Everyone is correct tho' in the most common and some sanctioning bodies rules is to require the Positive side to be interupted with the switch.