I have no idea why Rule 13.2 specifies the main positive power cable. It seems wrong to me since current flow is always from excess to deficiency despite automotive traditions.
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Racing is real competition and everything else is just a game.
My guess is because sometimes we find cars that have the frame negative grounded as students (and adults) are used to working on cars and even though 13.13 says not to who reads? I have had teachers argue that not using the frame as a ground it stupid and they wanted to do it even though it says not to. I guess if more of us worked on some of the old Lucas wiring that my old Triumphs had with positive ground we would have to use the negative wiring for our shut off.
With most modern automobiles running a 12V electrical system for accessories, lights, etc, grounding to the vehicle frame eliminates the need for extra wiring. The problem is that many Electrathon vehicles are made of non-conducting materials, or assemblies of different sections that are electrically isolated from each other. Isolating the positive power cable ensures consistency in the event of an emergency or accident. It is also the standard for DIY Electric Vehicle Conversions in the US.
I believe rule 13.13 is a safety measure. Many of our cars have at least some parts of the electrical system exposed and within reach of the driver. If the (metal) frame is grounded or acts as one side of the electrical system and the driver touches some exposed part of the opposite half of the system while sitting or lying in the car, he/she could suffer serious burns or even a heart attack. A typical 24 volt system like most of us use can weld steel together; imagine what it can do to human flesh that may be damp from sweating during a race... An isolated system is simply safer.
As for rule 13.2, I don't see what difference it makes whether the master switch is on the positive or negative side as long as it kills the power.
