KIll Switches - power or earth through switch

[peemohr]

I have always belive the power cable should run through the kill switch but have just come across professional restoration on a F/Ford that has it using with the earth lead.

I figure that in this scenario one could disconnect the starter power lead, drop it on to the frame and bingo, you have Guy Fawkes Day in your shop ...

From experience, using the positive (assuming neg to earth) through the switch you are protected at all time ... or have i been playing Rusian roulette all these years?

Thanks in advance for your ideas and theories

Pete M

[Brian]

Electrically it makes no difference = positive and negative are just conventions used for convenience (except on a molecular level). Cars I've seen have it wired about 50 - 50 from the factory. and it depends on which is easiest. The Pi dash manual I've been reading lately suggests strongly that the convention is to interrupt ground from the battery. If you have an alternator, it's a second source of power whenever it's turning so you need to think about interrupting it as well, and there are field coil issues with alternators.

In your scenario, if the kill switch was open, you could certainly connect the power lead to the chassis with no issues since the negative side of the battery would not be connected to the chassis.

Bottom line is that the risk of an incident is equal either way you do it, no difference. You always need to cover the side of the switch that's connected to the battery, even on a ground-switched car, and both sides of the switch on a positive switched car.

Brian

[rickb99]

Assuming there are NO electronics in the car that strongly suggest one specific configuration:

To the best of my recollection (and that's bad at best), the majority of the 50's and 60's cars (vintage) had the kill switch on the hot side of the battery (if they had a kill switch). Why? No clue maybe it just seemed logical.

But there may be a valid safety reason for switching the ground.

If you have the hot lead switched and the cable TOO the switch gets shorted to the frame in a shunt, you have a path back to the battery even if the switch is in the OFF position. Therefore, sparks and super heating.

If the switch is in the negative side and the cable TO the battery from the switch gets shorted to the frame in a shunt the switch will NOT interupt the ground path to the battery. HOWEVER, for it to cause sparks and and an ignition source for a fire, some POSITIVE cable must have also been shorted to the frame during the shunt. But, all electrical stuff like fuel pump and ignition will remain on with this configuration. With a direct short in the ground lead back to the battery from the switch, it takes a double problem (positive side short also) to cause a direct battery short out through the frame.

On the other hand if the kill switch is in the positive side and the cable from the battery to the switch gets shorted to the frame you have the potential for sparks flying. But, anything that's fed FROM that switch such as ignition, fuel pumps, electronics, etc will be OFF with the switch in the off position regardless of the positive cable before the switch being shorted to the frame in a shunt.

Thus, the dilema.

Assuming no damage to the cable the switch is in (from the battery too the switch, either is fine (assuming some fancy electronics in the car doesn't want it a particular way).

If you have an alternator, this kill switch provides alternator protection.

http://www.pegasusautoracing.com/pro...asp?RecId=1464

[Dragnmorad]

I dont know if it is still the case but at one time the GCR required the kill swithch on the positive side. I know because I had a Ralt RT-4 that came with a factory ground kill switch in a very awkward place. To add insult to injury the kill switch was also part of the fire system... dont ask!! it was ingenious but yuck!!!! So I couldnt relocate the kill switch (it was buried deep in the car & controlled by a cable pull) I had to rewire the car & get the positive lead down to this switch & back to the battery. I thought it odd that they wouldnt allow a system that had been on the car for over 4 yrs at the time.

[Charles Warner]

To add insult to injury the kill switch was also part of the fire system...

FIA requirement at the time. One pull does it all.