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.