I suppose it's possible that not all electric companies in the USA do things
identically, but Jeffrey Schnick's description of transformer grounding does
not describe the situation where I live.
(1) The center-tap is connected to the transformer can, and that wire
continues to the ground rod at the base of the pole (I have above-ground
wiring). That is, the transformer can and the center-tap are not separately
grounded by American Electric Power.
The center-tap wire also runs to the house along with the two 115-volt
phases. At the entrance to the house, the center-tap-wire is grounded again
to a ground rod. Note, our voltage for single-phase systems is 230 volts
which splits as two phases of 115-volts each. Also note that I've always
felt that calling house wiring single phase is a misnomer. The house wiring
is really two phase. The single-phase name derives from the neighborhood
wiring, not the house wiring. The transformers in my neighborhood are
powered by a single high-voltage feeder of about 7500 volts. So I am in a
single-phase neighborhood. Some of the newer residential areas are three
phase and are powered by three 9000-volt feeders. A resident in the
three-phase neighborhoods can pay extra and bring three-phase to the house,
but most homes are fed by a standard center-tapped transformer fed by just
one of the three high-voltage phases. So in Bluffton we have some
single-phase neighborhoods and we have some three-phase neighborhoods. All
the single-phase-neighborhood houses are two-phase. In the three-phase
neighborhoods most houses are two phase, but some are three-phase. It can
pay to get three-phase if you are going heat and cool with a heat pump.
(2) If the center-tap wire gets cut between the transformer and the house,
the house neutral is still connected to ground at the house, and the
transformer's center-tap is still grounded at the pole. This means the
home's neutral wiring (and ground wiring) are still connected to the
transformer's center-tap via an earth path. However, this path is not
conductive enough to keep the the phases balanced at 115 volts each if a
significant number of appliances are operating and they are not balanced.
However, the earth connection may keep the voltages closer to 115 volts each
better than what happens in (3) below.
(3) If the center-tap wire is cut at the transformer such that the
center-tap is not connected to earth ground at the pole, this is worse than
a cut between the transformer and house because now there isn't even an
earth path between the house neutral/ground and the center-tap. This means
the two house phases are likely to be more different from each other in this
case than in case (2) above.
(4) Jeff mentioned grounding at the house to the water main. That is
illegal here. The house ground must go to a ground rod designed for such
purposes. There are three different problems. (A) In some neighborhoods
the water lines are plastic. Even if metal leaves the house and then
changes to plastic underground, connecting to the metal will not provide
sufficient grounding if there are only a few feet of metal before the
transition to plastic is made. (B) The grounding also can be insufficient
if connected to water mains because the mains are not deep enough.
Depending on the frost line, water lines might be anywhere from 2 feet deep
to 4 feet deep. (Ours are 4 feet deep.) The ground rod is supposed to go a
minimum of 8 feet deep. One reason is that in dry spells the earth can dry
out down to several feet deep. (C) If the water company has to work on your
meter, particularly if they need to remove it, then they break your home's
ground and also set themselves up for electrocution.
For these reasons, if AEP notices that your ground is connected to the water
line, they will make you change it to a proper ground rod, and if you don't
do that promptly, they will shut off your electricity.
(5) William Maddox claimed that an older home that does not have grounded
circuits would not be affected by the cut neutral. This is not true. Old
homes with fuses and two-prong outlets still use neutral. What they lack is
the independent second connection to the ground rod.
(6) William also suggested the old mechanical circuit breakers (and he
probably would include fuses) are reliable and we don't need the solid state
GFCI breakers. I strongly disagree. GFCI is a wonderful invention and has
no doubt saved many lives. The GFCI breakers are still mechanical as far as
over-current is concerned, and retain that old-fashioned reliability. The
more complicated circuitry is for detection of ground faults which is a very
different problem then over-current.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu