Al,
The water pipe is always a hazard, not only when there is a fault, but always. It can even be a hazard when the main for the building is off. It is being used to carry part of the grounded conductor current and in some cases a majority of that current. Any time the pipe is "opened" and you touch both sides, you become part of the circuit. Look at what the code requires. It says we must bond the grounded conductor to the metal underground water piping system, and so does your neighbor. If you have a 100 amp service with a long service drop and next door has a larger service with a shorter service drop and both homes are on a city under ground water system, the path via the water pipe to next door and then to the transformer may have a much lower impedance then your service drop grounded conductor. The grounded conductor current will be split between these two paths. The amount on each path will be determined by the impedance of the two paths with the most current on the path with the least impedance.
As far as the touch voltage at the time of a fault, it exists on everything that is bonded to the electrical system grounding system. The most touch potential will appear on the enclosure where the fault occurs, but the voltage drop on the grounded conductor between the main bonding jumper and the utility transformer will appear on everything that is connected to the electrical grounding system. This could still be a high enough voltage to cause serious harm. Fortunately this voltage to ground only exists as long as the fault exists. In a properly designed, installed and maintained system the fault should only exist for a very short time.
Don
