Master ''Kill'' Switch.

[GermanGorilla]

Hi,

I am in the process of fitting an FIA Master Kill Switch.
It is the version with the relay so that the Ignition Coil is
terminated as well.

Can I run the Battery main live, straight to the Starter Motor,
then loop back from the Starter motor to the on/live terminal on the
Master kill switch, so that when turned off the power is cut to the main
fuse board/Ignition/ etc ?
But the Starter Motor would in effect still be ''live'' ?

Or do I have to run the Battery main live through the Master Kill switch,
then on to Starter Motor, looping back to the same terminal, so that when
the Master Switch is terminated, its all dead ?

What is the thinest cable/wire I can use for the starter live return back
to the battery live ?

Many thanks,

Regards,

The Gorilla.

[LuckyP]

This is my first post, and it's pretty long one.
Bear with me - it might make sense if you read it twice...


In theory it doesn't matter which side of the battery you fit the kill switch to, as long as you make an exclusive connection from one of the battery teminals to one side of your kill switch - i.e. if you hook it up on the negative side, make sure there are no other cables connecting the -ve battery terminal directly to any part of the car that don't go through the kill switch first.

If you isolate the negative battery terminal, then the previously "live" circuits are no longer live. This is because voltage is not an absolute measurement, it is a relative measurement - it quantifies the difference in potential between two points. If you isolate the negative side of your battery, then everything connected to the positive side of your battery is no longer "at 12 volts", as there is now nothing to compare the "12 volts" to.

Lots of circuits on most cars are switched on the negative side.
Old positive earth cars have everything switched on the negative side.

*** Hovever, if you're going to be competing in the car (which is likely, given the forum your post is in) then you'll need to check the regs in the MSA Blue Book and any additional regs for the particular event or championship you enter. ***



The correct cable size depends on the engine size, compression ratio, starter motor efficiency, length of cable etc etc etc.

I happen to make custom battery / starter cables on the side, so perhaps I can offer some advice here too.

Standard cables tend to be around 16 square mm, which is fine for most cars. Bigger cables should be used on large capacity and/or high compression engines. They give your battery and sarter motor an easier time of it as there is less voltage drop across the cables, meaning less current is required. On the downside they cost more and weigh more.

If you tell me your engine spec and battery location, I could make a recommendation.

[GermanGorilla]

Hi,

LuckP- Many thanks for the response, and it all makes sense.

I have gone an un-conventional route, as most, as you say terminate
the Battery Earth as a means of killing the electrics.

Given the Battery sits in the rear footwell area, running two lengths
of cable to the front was just adding weight were I have been trying to
loose it.

So I have earthed the Battery where it is, taken the live, in 11mm cable
from the battery to the scuttle panel, where the kill switch is located
externally.
I have then gone off the other terminal of the kill switch to the
starter 11mm cable, and live feeds for fuse board 9mm cable etc.
Return from Starter / Alternator to this terminal as well. 6mm cable.

So now when the Kill switch is terminated, all the live feeds cease,
and so is the power to the ignition coil, so that the engine can not run on.
This can all be terminated externally.

Internally, I have an ignition Kill switch on the dash, and will fit a battery
pull / kill switch here to complete.

Thanks for your offer which I will get back to you on as I may require
a couple of purpose built cables.

Regards,

The Gorilla.

[LuckyP]

Sounds like a nice set-up. My only concern is that you've used pretty thin cable - it will be fine if you engine starts on the button, but if you need to crank it over a lot, the main power cables will get very hot. 11mm cable is only rated at about 80 amps continuous. Most starter motors can pull up to 200 amps continuous. See http://www.picotech.com/auto/waveforms.html for some graphs of cranking amps (fourth bottom of the list). I have no association with these guys, I just use their data occasionally. Some other useful stuff in there too.

My cables may be a bit heavy for you - the thinnest I make use 16mm2 cable, with 25 and 35mm2 being the most popular sizes. These are used for high compression car starters / boat starters / winches on the front of Land Rovers / domestic wind turbine set-ups etc.
They are all hand made though, so if I can get the materials I should be able to knock up something.

Off to watch the Grand Prix now - come on Lewis!