sparky sounds like you have a good handle on things. I have build a bunch of equipment and a kill switch on the lid of the furnace is nice. If you build it right the switch will operate correctly. I didn't have one on my furnace at first and i thought i was fine until i touched the wrong thing while working and got zapped. Then I put in a double pole mercury switch. After you get zapped once you take the time to engineer it right!