I learned about this the hard way a few years ago. I was also troubleshooting a 3-way situation and was using a voltage sensor to figure out what was live and what wasn't. The sensor, however, had a sensitivity adjusting dial on it which must have been set a little too high. It was indicating live wires which had no way of being live. I know this because I had disconnected the wire at both ends of the pipe run and verified that it was the same wire at both ends.
Mystified at this point, I got out my DMM and took a "real" measurement which was around...30-some volts. It seemed to me that the voltage pretty much had to be a result of induction from the other conductors in the pipe.
This phenomenon can really leave a guy scratching his head for a while until he figures it out.
I agree with wiking, it really is great to have access to the kind of wisdom available here.
Shawn.
