You guys have me smiling. Just for fun, I'm gonna throw my hat inta the ring with yuns.
The BATTERY cutoff switch is only a battery cutoff switch. If the car's generator or alternator is putting out adequate voltage, meaning the engine is running, when you open your BATTERY cutoff switch, you'll need to turn the ignition key off to open the primary ignition circuit. Otherwise the alternator / generator will continue to feed the flames / juice to any circuit that is completed, by its switch being closed or by any short/s that complete a circuit back to the generator/alternator.
The battery is the voltage regulator for the stock Model A GENERATOR. I'm uncertain what failing to turn the ignition key off will do to your generator when you turn the BATTERY cutoff switch off but allow the engine to continue to run. I think the voltage will go thru the roof? I think you will see some smoke fairly quickly. Current does not flow if a circuit is not complete. For the alternator/generator to recognize/charge the battery, both battery posts must be connected. So, no matter what side the BATTERY cutoff switch is wired into, if the cutoff is open, the generator / alternator can't make use of the battery.
The reason the BATTERY cutoff is better placed in the ground side is that if the ground cable falls off the frame or the battery, no dangerous short can be created. Not so, if the cutoff is wired into the hot / negative cable. A frayed or broken negative cable flopping against the frame will absolutely create a zinger of a short. A short could occur inside the cutoff switch also.
I will wire my cutoff into the ground side. If the 1929 November wiring change has been applied to the car and accessories are sourced from the yellow/black stripe charge side of the ammeter, most shorts will show on the ammeter. However, a stuck closed starter switch will not. If the starter is stuck, I'll know it as soon as I take my foot off the button. In either case, time is of the essence. I want to reach between my legs and open all the battery circuits at once. I will then turn the ignition off if the engine is running.
ROTORWRENCH, where are ya, buddy.
