On 2007-06-11, BobK207 wrote:

Sounds like he's got a grounding problem but desn't he also have a
problem in the furnace itself?

There's definitely a problem in the furnace or the circuit feeding it.

now you've got some juice in the (inadequate?) ground system that is
trying to find it's way back to earth.......

Slight clarification: the juice is trying to find its way back to the
secondary winding of the transformer powering the service. Because of
a fault, the normal return path using the neutral conductors is now
paralleled by a path involving the shower plumbing and probably the
earth. Because of some problem with the EGC (equipment grounding
conductor) system, the fault did not hit the desired "low-impedance
return path" to the service which would trip the breaker. Return
paths using the earth will almost never be low enough impedance to
trip a breaker.

Cheers, Wayne