I think this depends. An alternator needs a voltage supply to start generating electricity, but once powered, it will continue to generate power without a battery as long as you keep it turning. If the alternator ties back to the battery and the switch is after this junction, then yes opening the switch will kill the engine. However, if the alternator ties in after the switch, opening the switch only takes out the battery, and the alternator will continue to power and run the engine (assuming engine is running when switch is opened).

I'm thinking the alternator wire does tie directly to the battery terminal, so yes the switch would be after the tie point.

Are we in agreement?

O.D.- Agreed- depends on many other factors. For example a brainbox is usially wired directly off a battery. So removing the battery out of the loop will kill the engine because the brainbox died. In days of old the hot side of the coil was switched, but still directly tied to the alternator because positives were tied together prior to the battery. As stated before. It all depends on many things. As for this thread I still say the negitive side is the way to safety switch usially. But that's not to say there is not compromise going that way.

This turned into a hot topic while I was away from my computer before hitting the post button on my last post.

When the car is running the alternator provides the current needed for running the engine, headlights, radio and seat heaters etc. At the same time it charges the battery if needed.
The alternator indeed must be grounded in order to work (like the battery, it is a complete circuit).
Depending on how your ignition and alternator are wired, If you disconnect the battery (positive lead or negative lead), the car may very well keep running on the alternators output. This is very bad for electronic items in the car, as the battery is needed to help regulate the voltage, and to some extent filter out the AC ripple that the diode bridge in the alternator doesn't completely remove.

The reason you need a very high current switch for your battery disconnect, regardless of which lead you switch is because the battery alone turns the starter over, which is where all the heavy lifting is done in the electrical system.

I was just outside taking some measurements with a clamp on DC Ammeter (a Fluke i1010 used with a Fluke 83 multimeter).
- A word of warning here. Most multi-meters are only rated to measure 10 amps or so, and measuring high current with them generally requires the use of a clamp on ammeter attatchment where the meter is actually measuring voltage output from the clamp. Measuring DC current requires a specialized clamp on meter which has it's own power source. The i1010 I used is rated to measure up to 1000 amps DC.

On startup in 50 degree weather, using the min/max function on the meter, my Cobra drew a maximum of 368 amps measured on the negative lead of the battery, and was within 5 amps of that (363 amps max) when measuring at startup on the positive lead.

The reason the current changed was as much because the oil was stirred up as anything, plus the alternator had run for 30 seconds or so between measurements which raised the battery voltage slightly. If you understand ohms law, when your dealing with 360 amps, a very small rise in voltage will make a measurable difference in current measured.

On a colder day the current draw at startup would have been much higher.

But you can see from these measurements why you need a very high current rated disconnect no matter which lead you disconnect with it.

Yetiman - We are ALL in agreement that you need a high rated safety switch regardless of which side you put the darn thing on. But you are missing something here. For example the starter is a very good example of something that can and should be bypassed wired, esp. on a Cobra w/a dash starter switch. Don't forget the function of this switch is for emergency shutoff per the NHRA or or whatever, or can be used as just a simple ignition kill. In a sense it's not about voltage draw so much as it is about how you wire the car and how you plan to use it.

Back in the 50's a LOT of English cars (and perhaps others) were positive ground systems, many of them 6 volt at that. Positive ground 'didn't catch on' because most everybody in the world was using negative ground.

I would assume from that a (EDIT: "typo" I first said positive, but I mean...) NEGATIVE ground system works better, unless of course your pretty sure Lucas Wiring was onto something. Lucas? Had a better idea? Not likely! :D :D

This is complicated enough without screwing up positive and negative when your typing! :LOL:

This is interesting discussion. As an electrical worker, I've always been taught to never interrupt a ground cuircuit. So I wired my cutoff switch to the positive side with that frame of mind. Works fine for what I use it for.

But, have you ever noticed that any automotive book or instruction manual tells you to disconnect the negative battery terminal before you do any work that involves the electrical system? Hmmm..... more food for thought.

Sydney

Sydney - good point, never thought of the battery disconnect thing. Could it be that you get a varying shutdown voltage spikes on the positive side if you do it that way. Could be important to sensitive micro processors in late model cars. Could it be that the engineers are smarter than we are?

I was by no means saying that a positive ground system was better. But if it were the standard that everyone followed, it wouldn't be so bad. The problems arose when people unknowingly used components designed for negative ground in those vehicles.
The main problem wasn't inherant to the positive ground system.

6 volt electrical systems were a terrible idea because everything has to draw twice the amount of current, and the wiring was rarely up to the load over the lifetime of the car.

The reason I replied to this thread in the first place was because multiple people had stated that you could use a switch with a lower current rating if you switched the negative lead (which is false).


The reason books always tell you to disconnect the negative first is because if you touch the chassis with the wrench it won't arc, and once the negative lead is disconnected, if you touch the chassis with the wrench while disconnecting the positive terminal, it also won't arc (no circuit to complete). It comes down to being a slightly safer procedure, and there are lawyers in the world.

Yetiman - I'll buy everthing you just said. But I still think the negitive side makes more sense in a steel body car. May just be the way that I've always done things. That said, my Cobra is glass.
Later

Let me throw this question out

how are full out GT comp cars wired for shut-off? Anyone know?

When buying cutoff switches, easy enough to ask for ratings, Use one with a 1000 amp instanteous and at least 180/200 amps continous rating. Anything higher is better.
Don't go lower. Isn't the four post cutoff switch designed to cut power from the battery and the alternator ? A two post works fine for the negative side.

I took electronics my last 2 yrs of high school - a long time ago. I have been involved in industrial electricity to a minor extent over the last 29 years - mostly process control, but I digress. There actually is a technical reason that positive ground went the way of the dinosaur. Seems like something to do with direction of electron flow and dissimilar metals, but I cannot remember what the heck it was. A good electrical engineer might know.

They were harder on something - points or generator or something growing green corrosion seems to fuzzily float around in my head.

Taking the discussion to the next level, how to keep the engine from running from the alternators power when disconnecting the switch...

If you put the disconnect on the battery's negative lead or on the positive lead, the engine will most likely continue to run if you turn the disconnect off while the engine is running. The alternator will continue to power the ignition untill you turn the ignition switch off.

I will also point out that SCCA and NHRA require the disconnect on the outside of the car to be connected to the positive side of the electrical system. Switching the negative won't pass tech.

There are a couple ways to wire your disconnect, and they depend on what you want to achieve.

On my own car the disconnect is on the positive lead from the battery. This allows me to kill power to the car quickly and easily while working on the car, and disconnecting the negative would work the same way and just as well. I am aware my car would continue to run if disconnected while running, but that is not a concern of mine.

If your number one concern is allow someone to shut off the vehicle from outside the car or to have A hidden security disconnect, simply wire it to the power source for your ignition. For this you can even use a low current (20 amp capacity) switch. You will still have 12 volts under the hood, but the engine will shut down and won't start again till you reset the disconnect.

If you want to have a foolproof way to kill power to the whole car beyond the switch And have the engine shut off when disconnecting you need a four pole disconnect (versions of which are readily available from most suppliers).
They have a pair of high current contacts for the battery wire (you could even wire this to the positive OR negative battery lead as you prefer) and a second set of low current contacts for the ignition system, disconnecting both circuits at the same time with one flip of the disconnect. This is the "right" way to do it.