Stuart Paterson relates a story that happened to him a number of years ago. He’s from Scotland, so don’t let the high voltage cited alarm you:
“A friend had asked me to have a look at a light switch in her house. Every now and again they got an electric shock when switching on the light. Our mains is 230V so the shock was sometimes quite painful, and I’m sure it was pure luck that no one had been injured. As she rented the property, my friend had contacted her landlord numerous times. In fact, three different electricians had been out to have a look at the switch. It had been replaced twice.
I removed the switch and checked the wiring, which looked okay. I then checked for continuity between the earth point on the switch and a radiator on the other side of the room. It checked out fine, and as I could see no other fault in such a simple device, I was stumped.
Just then someone walked in the room and a big spark flashed on the radiator where I had terminated the test wire. The wire was lying on top of a metal strip holding the carpets down between the door frame. It was then that the fault dawned on me — the metal strip had been screwed down with screws that were far too long. When someone stepped on the metal strip their weight caused it to sag slightly, pushing the screw into what I can only presume was a live cable below. When a person touched one of the metal screws securing the light switch, they completed the circuit and got a shock!
The fix involved simply changing the screws for ones of the correct length.”