When I was teaching auto mechanics, I had a Sun distributor tester set up in my classroom with a General Motors HEI distributor, powered by a 12 volt battery charger. 8 spark plugs threaded into a piece of aluminum flat bar, plug wires, etc. The tester was on rubber caster wheels, which made pretty good insulators. I would disconnect one of the plug wires, attach it to a spare spark plug, and hold onto the plug with the machine running- - - - -and let my students watch the arc jump the plug gap. Then I'd challenge anyone in the class to do the same thing. Yes, it gave me a little jolt every time the plug fired, but as long as I didn't touch the machine and get a good ground, no problems. It was amusing to watch a big, tough football player invent dance steps that John Travolta or Patrick Swayze would envy when he grabbed onto that spark plug!
Jerry
