This is true, NHRA does require a push pull type cutoff on the positive side, and also requires you to wire the alternator in such a way that it stops powering the car when the cutoff is switched. Lookup "alternator interupt"... The logic behind this is if the car crashes down the track, the first repsonder can run up to your car, pull the battery cutoff and shut the car down. The reasoning behind the positve side cutoff is that if sheet metal happened to crush the battery and is hitting the negative post, a negative cutoff would be inefective. My opinion why this doesn't matter on a street car, not fact... #1. You probably aren't wiring in the alternator interupt, just cutting a cable and putting the switch between them. So Neg or Pos doesn't make a difference. Without the alternator interupt, the car will still run if you take off the Pos or Neg cable. #2. If you get in a crash no one knows to run up to your car and flip your switch, and chances are you didn't put it in an easily accessible spot to quickly flip it after a crash, or it's the last thing on your mind after a crash.
I install them on the negative side to completely kill any possibility of a live wire in an accident. If the cut off switch is installed on the positive side, you will still have a live cable going from the battery to the cut off switch. Everything after the switch will be dead. Depending on how far the switch is from the battery, the live wire could be long enough to cause trouble in an accident.
I believe, technically, within the conductor (wiring) the electrons move from negative to positive. I may be wrong, though, as it has been a long time since I studied physics, and even then it was not electricity-specific. Sent from my DROID device using the TJJ mobile app
I don't think it's a matter of right or wrong. Either will work in most cases. There are uncommon instances where both could be problematic, however, for the most part, it's just a matter of preference, or rules if you are going racing. Sent from my DROID device using the TJJ mobile app
Thanx, G-son and 36roadster for the very good answers in layman's terms. It was a rhetorical question. I'd already learned that lesson in my youth in the school of hard sparks
Guys, thanks so much for all the input. The reason I asked the question is because I am of the opinion it should be in the ground side. (Pos or Neg ground car doesn't matter). The instructions that came with it say "hot" side, conforming with NHRA. Bootlegger, you are right, it is a street car, not a race car.
I've always put them on the pos. side, but if it's used for the street it will not be a NHRA tech issue. I use them for safety and security.
RE Bootlegger's comment about NHRA rules:"The reasoning behind the positive side cutoff is that if sheet metal happened to crush the battery and is hitting the negative post, a negative cutoff would be ineffective." I won't argue that it's their rule, but it seems to me if if that logic was reversed and the switch is on the positive side, and crushed metal contacts the positive post, you'd still have the very same problem. Nobody knows where that crushed sheetmetal is going in a wreck. A switch on either side will break the circuit between posts, and moving sheetmetal or other parts can certainly re-complete that circuit in a hurry. What have I missed here? I vote for the negative side to avoid the accidental wrench contact problem cited by others. Later...Oh, just thought of one more reason for negative. If NHRA is worrying about crushed sheetmetal making contact with the negative post in a wreck, consider this. Putting a cutoff on the positive side exposes two live positive posts: one on the battery, and one side of the switch, doubling the chance of a big problem: direct ground contact with positive. Switch on the negative side, any contact with the neg batt post or the batt side of the switch, worst case just regrounds the batt neg side to where it was in the first place. Not good when you want to disconnect the battery in a hurry, but far better than very quick battery meltdown or explosion.
if you think about this statement, and the flow of electricity, you have a live cable/wires/system, from the battery + all the way to the disconnect on the negative cable. this reasoning is wrong. i don't know why, but i have always been told to "switch" hot side only. in cars, boats and houses.
The house example doesn't apply because house wiring has an earth ground. Car does not. I weigh in on switching ground side. Neal
Something to think about, maybe way off topic, why do you think they ever tied + or - to a frame, firewall, or sheet metal............because it was a better conductor then copper,,,,,,,,,,,,,NO,,,, it was to save lots of money on wires.............look at the electric grid it is tied to earth ground, the earth is not a good conductor but it is big, and hopefully wet enough,.......and saved much money........... Everything would have been better with a positive and negative wire to all device's on a vehicle, then you wouldn't have to worry about all the frame ground BS............ Get you two high current contactors and a big red mechanical kill switch, kill both sides with one stone... boooooob
You are correct, as long as we're talking physics. But they had no idea about that back in the day when they first begun to figure out how electricity worked, so they decided to call one side positive, and that electricity flowed from there to negative, just as positive pressure causes a flow of gas/fluid towards negative pressure. They got it wrong and the electrons do move the opposite direction, but since it doesn't really matter for any normal use you still say that electricity/current moves from positive to negative. In science you learn that the electrons moves from negative to positive.
Model A's and other early cars used a positive ground electrical system because they did know about the movement of the electrons from negative to positive. In the end it matters not in the context of this discussion. Sent from my DROID device using the TJJ mobile app
Most times a cut off is put onto a car to 'save' the battery when it is in storage. Let's face it not many of us are really building competition cars. One thing I do with a cut off switch is fit a small inline fuse accross the two terminals. This will keep things like your clock and stereo memory alive but should a short appear while the vehicle is parked then the fuse will blow.
I always put mine on the pos. and close to the battery. Right or wrong?? been doing it that way for 49 years. Im going to bed now
You are deadass wrong my friend. If the negative side of the battery is disconnected you can feed any positive wire to any part of the car without any fireworks at all.
Right. Finally. Friends, you leave the grounded wire alone with the best contact you can get. If you switch the grounded wire (in most cases the negative pole from your battery) you leave everything powered with the possibility of inadvertently creating a completed circuit with anything that complwtes the circuit. No electrical profession would every interrupt the grounding.
FWIW, all new BMW's are shipped from Germany with a 6' disconnect on the neg side. The dealer removes these in the new car prep procedures.
This is the reason I mount the fire extinguisher and the battery kill in a position that can be reached from the drivers seat while wearing a seat belt . In a nasty crash , if you can reach them both you may stand a better chance of avoiding Frying! .
Household vs automotive - 2 different animals. Your logic on the neg is not correct. There's a 1000 x's more negative circuit "area" than there's positive. There's more of a chance that a battey could ground the pos post (even if disconnected) to a body panel in a crash.
True, but, disconnect the positive and you still have the same situation all the way to ground. It doesn't matter which side of the circuit you disconnect there will be no flow of electricity.
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