Actually, I'd never thought of the metal bit hitting a live cable with one of these drills, although metal appliances have always required an earth (ground) wire in them here.
I thought about it, when I was, like, a second-year apprentice. That doesn't mean I never did anything stupid like what Harold mentioned! (Yeah, remember standing at the top of the extension ladder leaned up against a pole, resting on the truck bumper, tied off only with a piece of romex, stripping a wire I wasn't sure was dead, only because I thought my foreman would say something bad about me...
Don't ever do that!!)
No, on this job, my foreman gave me a Hole Hawg with a 3.5" hole saw and told me to cut a hole for a smoke detector in a lath-and-plaster ceiling. This house had "knob and tube". I put that in scare quotes because there were no actual knobs or tubes anywhere. The conductors were lead sheathed and stapled, and I'd seen enough of the house to know they would be right between the lath. Yes, both the cord and the drill had grounding continuity. Yes, I tested the GFCI, and it was functional. Still, you ever try to hold a Hole Hawg in such a way as not to touch metal? It may be plugged in to a GFCI, but I'd bet whatever circuit I might drill into isn't. A GFCI doesn't interrupt the EGC, nor does it do anything about other circuits.
Well, I cut the hole, and there wasn't a wire there. There are always risks. It's always good to think them through beforehand.
