GCR 17.27: "The master switch shall be installed directly in either battery cable and shall cut all electrical circuits..." (my emphasis) I was describing above with this requirement as an assumption.

You don't have a choice: the battery must be completely isolated from the rest of electrical system. If you did it as you describe, then the remainder of the car's electrical system is still getting juice from the battery.

....

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Greg,

Though I'd mention one exception which is electrically actuated fire systems must be hot at all times even when the BCS is off. If it's a pull cable like mine then all dead is good.

James