Master Kill Swith

[Jon Davis]

Chaps,

I am working on a car that has the master kill switch on the negative side of the battery . . . in escence it puts a break in the earth. My own car has it on the positive side of the battery which I thought was the requirement in the blue book. I can't find any reference to it in the blue book and I can't really think of a good reason why having it on the negative other than my noddy theory that if the ngative cable came loose it could still earth on the car and allow the engine to keep running whereas if the live came loose and touched down on the chassis you would 'just' get a spectacular sparking light show.

Any comments.

I hate electricity, you can't see it leaking!

Cheers

[GORDON STREETER]

I have run the switch on the earth side for at least 20 years and never had a problem.

[graham bahr]

you can run in it either side of the circuit it doesnt matter which

[SAMD]

I would suggest it is safer to switch the negative for the following reasons. If the negative is connected directly to the chassis (lets call it earth) it only requires a single fault for any of the live connections to 'short' to earth and produce a spark as the whole of the metal parts of the car are connected to the negative of the battery all the time. Turning the isolator off still leaves all the metalwork connected to negative ensuring that any fault of the 'live' side will cause a spark. Switching the negative requires two faults for a spark to occur. The whole of the metalwork of the car is disconnected from the battery when the isolator is operated so any short to 'earth' on the live side has no impact. It would require a short to earth on the live side and a short to earth on the negative side to cause a spark with the isolator switched off.

[vjay48]

I know of a friend who took a car to be scrutinered,his switch was on the floor open to view ie earthed to the floor, the scrutineer suggested it was not a good practice for the reasons above.

[Jon Davis]

I've spent way too long thinking about this now . . . and have the following comments (which fly in the face of SAMD). I could of course be COMPLETELY WRONG and I am happy to have the pi$$ taken if this is the case.

Here goes

If the positive is on the kill switch I agree that the negative is always earthed to the chassis. However if the kill switch is in the positive loop then once the switch is thrown it is only the run from the battery to the kill switch that remains 'live' and so therefore you can't get a spectacularly sparky short circuit after the kill switch . . . the short circuit could only come from the run between the battery and the kill switch and this would not result in the engine continuing to run (for example), unless the short circuit managed to be across the terminals of the kill switch.

Conversly, if the negative is on the kill switch and the switch is thrown then the electrical circuit is still directly linked to the positive terminal . . . this is not a problem unless there is a short in the run from the negative to the kill switch, in this case the short would create an earth and this would allow the car to continue to run would it not?

Therefore is it not better for the kill switch to be on the positive as the liklihood is that if the switch is thrown the engine will shut down, unlike a short circuit on the negative.

[vjay48]

The cars i have the switch on the positive,since the instructions told you to do it that way, the little diode thinging that protects the alternator is also wired in. Is there a good reason to wire it on the negative? Can i gain extra power??? Arghh i,m tired bye chaps

[Jon Davis]

For me the advantage of wiring to the + is that I need less cabling and so carry less weight. Plus it feels like the right way round . . . re my thouhgts before.

[JimW]

Fire service policy for RTA's is to disconnect/cut the negative lead for the reasons stated by SAMD. Also helps reduce the consequences of an earthed metal rod penetrating the battery (and, yes, I have had to deal with that).

Don't see any convincing argument to depart from that for race cars.

Regards

Jim

[SAMD]

Originally Posted by Jon Davis

Conversly, if the negative is on the kill switch and the switch is thrown then the electrical circuit is still directly linked to the positive terminal . . . this is not a problem unless there is a short in the run from the negative to the kill switch, in this case the short would create an earth and this would allow the car to continue to run would it not?

Therefore is it not better for the kill switch to be on the positive as the liklihood is that if the switch is thrown the engine will shut down, unlike a short circuit on the negative.

This is not an exact science because of the variables in the type of fault. You are quite correct in your theorising. I actually had the same idea, in order to reduce the amount of heavy cable being used, especially as I moved the battery into the nose. All I can say is that at the end of the day, I just did it the same as everyone else. Not very creative I know. But I figured that everything like that had been tried on a FF1600 by now, so if the general concensus was, the negative side, it was for a reason and I would go with that too.

[Jon Davis]

Thaks to thise who have commented . . . the overwhelming selection seems to be on the negative side of the battery (I have also canvassed opinion elsewhere).

Negative side it is then.

Cheers,

Jon

[Triple J Motorsport]

Originally Posted by JimW

Fire service policy for RTA's is to disconnect/cut the negative lead for the reasons stated by SAMD. Also helps reduce the consequences of an earthed metal rod penetrating the battery (and, yes, I have had to deal with that).

Don't see any convincing argument to depart from that for race cars.

Regards

Jim

Surely that's the danger you get another earth created?

All my 12 cars have had well insulated well protected short + to master cut off fitted as standard.

[StephenRae]

On balance I think isolating the negative side is best but only if the battery is close to the switch. It is annoying when the jumper battery is wired to it's own earth and turning off the master switch has no effect until it is disconnected. I think they are wired this way so that the starter motor only takes power out of the slave battery.

[Jon Davis]

So we are still unble to come to a conclusion . . . I still think +, backed up by Triple J Motorsport. I have the same issue with my starter battery where the anderson plug is earthed independantly to the rest of the system and so is not isolated when the kill switch is flicked. This is compounded by the anderson plug being on the opposite side of the car to the kill switch so if you do have a problem you have to run round like a mad idiot to flick the switch and pull the plug. I was going to wire the anderson plug through the kill switch as well to resolve this.