James Frysinger <[EMAIL PROTECTED]> writes: > Because the ground (earth) connections at homes may or may not be very > effective (they get worse in times of drought), power companies often > run a neutral wire to each house to ensure that the secondary > transformer ground potential is the same as the house ground potential. > This practice was first started when automatic milking machines came > into wide use. Some farmers complained that their cows would not give > milk at times when the machines were used and sometimes the cows even > shied away from them. The "power" companies investigated and found that > there could typically be a quarter volt difference (sometimes as much as > a volt) in potential between the ground rod at the barn and the one at > the pole on which the secondary transformer was mounted. In effect, a > ground loop current was created in which the cows' teats were part of > the circuit. No wonder the cows shied away!
At least in the US, this is a problem only in rural areas which do not have central water supplies. Standard practice (required by national code, IIRC) is to connect a ground wire from each house's circuit-breaker box to its cold water supply pipe. Jumpers are also installed around each water meter, to ensure that all those grounds wind up connected to each other. The "floating neutral" can still happen if the ground wire breaks or corrodes, but that is quite rare. I don't know how this picture will change now that plastic pipes are becoming common in new construction. I wouldn't use the damned things.
