why battery cut off goes on negative terminal instead of positive?

[autobonrun]

Originally Posted by Algernon

A lot of these replies are not really accurate, or not truly helpful. The fact is, if either terminal is disconnected, the battery is no longer involved

You are absolutely correct. The direction of current flow from negative to positive is accurate when discussing electron flow, but it doesn't determine where to place a switch. Electron versus Conventional flow direction has been going on for years. Both are correct. If you're really bored, this article has one of the best descriptions I've seen of the two.

http://www.allaboutcircuits.com/textbook/direct-current/chpt-1/conventional-versus-electron-flow/

With respect to the cutoff switch location, the racing world says to cut the positive first. See NHRA rule for example:

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8.4 MASTER CUTOFF
Mandatory when battery is relocated, or as outlined in Class Requirements. An electrical power cutoff switch (one only) must be installed on the rearmost part of each vehicle and be easily accessible from outside the car body. This cutoff switch must be connected to the positive side of the electrical system and must stop all electrical functions including magneto ignition. The external control switch for this cutoff switch will be clearly indicated by a red flash inside a white-edged blue triangle with a base of at least 12 cm (see drawing 25). The positions must be clearly indicated with the word "OFF". If switch is "push/pull" type, push must be the action for shutting off the electrical system, pull to turn it on. Any rods or cables used to activate the switch must be minimum 1/8-inch (3.2 mm) diameter. Plastic or keyed switches prohibited.
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The difference between typical race cars and our cars is that for street vehicles there may be multiple small wires connected to the positive terminal to power everything from aftermarket radios to alarm systems and radar detectors. Breaking the main positive cable does nothing to stop power distribution from these "leech" circuits. So you still have power, and arcing potential in the car. However, there is typically one main cable connected from the negative of the battery to chassis ground. Breaking this single cable breaks the current flow to everything connected to the battery. For this reason, I would break the negative side of the battery.

With 120v AC in the home, always break the hot, not the neutral. My neighbor was shocked badly because some jack leg electrician wired the light switch to the neutral wire. He turned off the light to work on the fixture thinking he had shut power off to fixture. However, the hot was still connected to the light, the circuit was just interrupted; until he formed the new path to ground when he touched it.

[Marc-André]

So, for a true cutoff switch, is it the "B+" terminal from the alternator that should be shorted to ground with a resistor (as seen on various cutoff switches wiring schematics)?








Thanks!

[Marc-André]

Here's another diagram I've found out there:



Contact #2 (or Z): Isn't this redundant since the ignition switch takes it's source from the "main electrical feed" ??