Rob: I believe that use of the water piping for grounding should be banned worldwide but that isn't my point. See Nitins last post. If a repair using plastic pipe isolates part of the plumbing in a formerly all metal pluming system from earth and a fault such as described happens within the isolated section then line voltage would be present on all interconnected exposed metal parts. There would probably be insufficient current to trip even a ground fault breaker. Now if you touch a sink or tap in that live section with one wet hand and a properly bonded metal object with the other you get a shock the may well be fatal.

I prefer plasitic pipe and if used everywhere in a building it is the safest possible plumbing. For new installations is is the very best. It is just repairs that concern me. Terry