Positive or Negative?

[SAMD]

Hi there, The rebuild of my RF86 nears completion and it is currently at the 'battery' stage.
Why does it appear that the majority of cars break the Negative cable through the emergency cut out, and run the positive to the starter system?
Rather than come off the Negative battery terminal and straight onto an appropriate bolt on the chassis. Then run the Positive through the cut out to the starter system? Many thanks.

[Dave Brand]

Maintenance manuals always warn that when disconnecting a battery the negative terminal must be removed first. I believe this is because if the positive terminal is removed first there's a possibility of a spark being produced. When you're sitting in a crashed car with petrol vapour wafting around you don't want even the possibility of a spark!

[StephenRae]

Your car is wired so that the battery is isolated from the jumper battery when the master switch is off. I guess its so the jumper battery turns the starter on its' own. I prefer to wire the jumper battery so that it helps the on board battery, the bonus is you can plug in and charge them both together...and you don't panic when you can't switch the thing off when the jumper battery is plugged in!

[John Wilkinson]

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.

[SAMD]

Yes I see what you mean John, The entire chassis and its associated parts are at 'negative' potential on my system. Whereas the other way they are isolated. This must increase the risk of a fault hazard.
Thanks to all for their replies, especially John whose logic I appreciate. Tiny performance gain (less cable = less weight) versus small but someday could be significant! risk of 2 stage fault.
I think I will stick with the Isolator on the Negative.
SAMD

[redturner37]

Originally Posted by SAMD

Yes I see what you mean John, The entire chassis and its associated parts are at 'negative' potential on my system. Whereas the other way they are isolated. This must increase the risk of a fault hazard.
Thanks to all for their replies, especially John whose logic I appreciate. Tiny performance gain (less cable = less weight) versus small but someday could be significant! risk of 2 stage fault.
I think I will stick with the Isolator on the Negative.
SAMD

One of the safest ways to isolate the system, though not practiced on many racing cars is to have TWO solenoids, one in the neg and one in the pos. When the starter button is pressed, both solenoids engage allowing the power to go to the starter motor through the heavy cables and when it fires, both drop out leaving it double isolated. The ignition circuit only requires light cables which of course won't normally carry sufficient current to do real damage.