Reading some of the replies - I have this to add.

Hot water lines are very often isolated from the tank to prevent
current leakage by dissimilar metals which can eat a tank up.

This isolation can be the issue - if the pipe doesn't dive under a
concrete slab or underground.

Remember many places have only air and brackets. Think non-1 family house.

Wire running a length along a pipe - in a race way - it being a single wire
e.g. one line of a 220 going to a heater... The other wire is there but
not as neat..

The wire induces current in the water pipe and the nominal voltage drop
drives a nominal current in the water stream.

Martin

Joseph Gwinn wrote:
I saw this article in The New England Journal of Medicine. Here is the
URL, unfortunately behind a paywall.

http://content.nejm.org/cgi/content/short/360/13/1363

The gist is that a danish doctor had a patient who was getting
inappropriate cardiac shocks from an ICD (implantable cardioverter-
defibrillator) while in the shower, which turned out to be due to bad
house wiring causing a 50 Hz leakage current from showerhead to floor
drain.

A current of 3.5 to 4.0 milliamps is not usually felt, but can spoof the
ICD, which looks for such signals lasting about 300 milliseconds.

This would be easy to check with a voltmeter. And to prevent by
running a ground wire from showerhead to drain pipe.

Joe Gwinn