We had some plumbing work done today, which required the plumber to disconnect the water meter. As the plumber was re-connecting the meter, he noticed some sparking between the house-side pipe connection and the meter. What could cause this? Is it dangerous or will it cause problems such as pipe...

The previous (pretty old) thread on this subject didn't really answer the question, so: I just disconnected a ground wire that runs between my main breaker box and the cold water (copper) line. The disconnect was at the water line, and significant sparks fly every time I touch the ground wire...