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Power is supplied to the site through a two cored cable (active and neutral here). At the metering box all neutral leads connect to a common neutral link, i.e. leads coming back from various power and light circuits etc. Also connected to this neutral link is the earth wire leading to a bulk connection of all other earth wires. A further earth wire leads off to the water service and, in the case of more modern homes now, a separate earth stake driven into the ground.
Taking the case where no earth stake exists, if the main neutral conductor coming into the house is interrupted (and often can be for a variety of reasons) then the only return path for electrical continuity is the earthing system, down through the water service, out into the street and back up through EVERYBODY'S water service / earth system.
Modern installations have (a) a separate earth stake in the ground AND (b) an insulating bush in the water service where it connects to the water main. Other areas / countries might well be different. Certainly millions of older installations world-wide are as described above.
How do I know a water service is live? Standard procedure (for those who follow rules and procedures - idiots don't) is to first connect, by way of proper earth clamp, an insulated earth wire of standard 4 mm size to the water service as it emerges from the water main. This cable is then run out to the meter box for connection to the neutral link.
Many, many times I have scraped this earth wire down the side of the meter box to be greeted by a shower of sparks. At this point both the law and common sense compel me to cease all work and contact the electrical authority who will arrive within half an hour. Often the cause is simply a broken neutral at the point of attachment to the house (if the active broke you would have no power). Other times it has taken teams from the electricity supply several days to locate the source.
Strangely, I have been abused by consumers for "creating trouble", especially after the electricity supply have put up a ladder and cut the active line for "immediate" safety reasons.
Sorry it was long winded, but it is a VERY REAL hazard. Many a plumber has died by cutting through old galvanized pipes. One was charged with manslaughter (rightly so) after leaving an inexperienced apprentice alone on site with instructions to "start removing the old gal pipe - I'll get the materials".
Just be aware!
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