Kill Switch (Page 1 of 2)

Here's a link to what  the rule is referring to.  There are many variations of this switch.  Some incorporate a resister to absorb voltage spikes when you shutoff the switch with the engine running.  Prices will vary widely.  USA built vrs. Chinese, etc.

https://www.pegasusautoracing.com/produ … ecID=10194

Your local parts store has them too.  They are cheaper but tend to not last as long.

I was curious about this rule too. Does it have to be the style where you insert the big red twisty guy and rotate it 90 degrees? Or could it just be like a little toggle switch?

Uh, no.

This switch needs to be able to safely pass current from the battery to the electrical system in the car, including the STARTER, which can draw well over 100 amps. The best toggle switch you are likely to find will only be able to handle 50 amps or so.

When you turn the kill switch to the 'off' position, there are actually 2 (or 3) switches inside which are actuated simultaneously, performing the following functions:

Switch #1. Interrupt the power going to the ignition system (this is what actually stops the car from running).

Switch #2. Disconnect the battery from the electrical system (extra safety, removing all sources of DC power from the system). Don't want electricity available around the engine if you have a gas leak, etc.

Switch #3. (optional) this switch connects the alternator to ground through a 3 ohm resistor. If you have a super simple electrical system and the only loads on the alternator are the ignition system and the battery, then the kill switch will cause an open circuit condition on the alternator output, which is not good for the alternator diodes. You can damage the alternator if you remove all loads while it is running. The 3 ohm resistor will provide a roughly 5 amp load to the alternator so it won't be damaged by the shutdown and to ensure that you have a load to dissipate any residual voltage coming from the alternator as it spins down. The resistor doesn't actually 'short out' the alternator output to ground as some people indicate - it could drive a 5 amp load all day long. However, the 3 ohm resistor is only sized to take the load for a few seconds, its not designed to be a continuous load. Most are 11 Watts, as compared to the 65 Watts that will be generated by applying 14V across 3 ohms (P = V * V / R). If your resistor gets super hot while the car is running - you have it installed incorrectly.

RSB, do you have a resistor carrying current while your car is running during the race? Or just when you shut off the kill switch? If it's constantly carrying current, I'm guessing you don't have the Pyrotech switch sold on this site, correct?

So you didn't read this yet in the rules:

3.G.1 Master Electrical Kill Switch. All cars must have a racing-type master electrical kill switch easily turned both off and on by the belted-in driver. The control for this switch should be red; the OFF position should be clearly indicated; the switch should be easily accessible from outside the car; and the switch should be clearly marked by a three-inch or larger lightning-bolt symbol. All electricity, including the battery, charging, and ignition circuits, must be interrupted by the kill switch. Don’t put the switch where it’s likely to be hit by another car in traffic or crushed in an accident. All exposed posts and connections must be insulated with electrical tape, rubber caps, or other nonconductive material.

Thanks for the diagram RSB, that makes things clear.

Why not alternator and battery on one pole and everything else on the other?

Because ideally the ignition kill should open a few degrees of rotation before the battery kill. And also for battery draw simplicity unless you have a fully gutted electrical system. the switch just goes inline between the battery and the main positive distribution jobby.

Otherwise either way is Electrically identical, but one minimizes the amount of giant expensive cable.

B.L.U.F.:
It makes more sense to me from a safety perspective to disconnect a circuit at the main trunk line closer to the higher potential and _before_ multiple conductors distribute that potential throughout a system (the car), rather than at the main trunk line closer to the lower potential.

[Disclaimer: I'm not an electrician or electrical engineer, I'm just trying to learn about best practices]

This is my overly simplistic belief as to the reasons for this rule [I'd be happy to have a better understanding if someone can share]:

I see the purpose of this as primarily interrupting two sources of electrical potential (plus the ignition circuit by definition) to prevent sparks - and fuel ignition (inside or outside the engine cylinders) - in the case of emergency.

I see there's the stored potential in the battery, and the continually produced potential (current) from the alternator.

Any conductors attached to the positive terminal of the battery carry that potential until an interruption of the circuit. Same for the charging wires from the alternator.

A switch can be anywhere in a circuit and still interrupt the circuit. Switches can be bypassed. Switches can fail.

[As an aside, I don't understand this ^ in relation to "... all electricity ... must be interrupted ...". I'm not following your explanation, probably because I'm not familiar with what you're describing.]

I too don't get the switched negative is much safer approach.  I don't know of a car that only has one ground conductor.  Well, my British cars tend to have few grounds because they go bad a lot but I digress.  Let's say I crash hard enough to have the car body crush into the fuel pump body or negative terminal and ground it while it's still getting 12v even though my negative ground kill switch is off.  I'd consider that to be bad.

If  you break the positive (somewhere other than right next to the battery), then you will have a length of wire that is always hot with 12V. In our car, the battery is in the trunk and  the kill switch is next to the gear lever (we use a remote pull to pass the rule about the kill switch being accessible from outside the car). So, the length of wire from the trunk to the gear lever is ALWAYS HOT. There is no way to remove voltage from that wire. In the event of a crash that pinches that wire against the chassis, there will be a dead short across the battery that can't be removed. That's a fire hazard. We insulate the hell out of that 'feeder' wire from the battery to the kill switch, I have it encased in a length of heater hose that adds a ton of  insulation/dielectric to it so its less likely to get shorted to chassis during a crash or other stupid things that we might do.

If you break the negative, then the entire length of positive wire is now dead (relative to the chassis), and it doesn't matter if its pinched against the chassis, there is no current loop and there is no short circuit.

Current flows in a closed loop. Break the loop and you have no current and any component that  WAS in that loop will no longer function. Doesn't matter if its the battery positive or negative that is isolated, the loop is broken when you throw the kill switch. Since each component has its own ground (a wire or hard mount to chassis) then there are multiple parallel current loops in the car.

Again, the key item here is that battery isolation is only a secondary safety issue. The car will run if you only disconnect the battery, because the alternator is still connected and will power the car. You have to disconnect the ignition or ECU or fuel pump to get the car to stop running (ideally, all three).