When John says the system supplying his computer has a ground wire along
the top, and this is for lightning suppression rather then current
return, it makes me think we are talking past each other and/or talking
about different parts of the system.
The transmission lines where the ground runs along the top, and is for
lightning, is the very high voltage three phase power towers. There is
neither an earth return nor a separate wire return in this case. In the
three phase system the current path is from one of the phases to the
other.
Power is not used from these until after the voltage is stepped down at
one or more substations. The step-down is done in stages. Perhaps it
goes from hundreds of kilovolts to tens of kilovolts, then continues
cross-country as a three phase transmission, then is stepped down to
about 7200 volts (still three phase).
The step-downs described so far can be done with a delta-configured
transformer. There is no ground wire and there is no ground current.
The primary current is from one phase to another. Likewise the
secondary current is from one phase to another. It is possible to use a
wye-configured transformer, with the center grounded, but this would
serve no purpose other than providing a ground reference... there would
be no ground current.
In the local neighborhood (village, small town, suburb, borough) the
distribution is 7200-volt three phase. There is no ground return
(neither by wire nor by the earth).
At some point, usually at the final destination building, or cluster of
homes, this is reduced from 7200 volts between phases to 208 volts
between phases. This is done with a wye-transformer in which the
primaries run current only to each other (there is no separate return;
just the three phases themselves). The secondary side of the wye
transformer is grounded at the center of the wye. Each secondary
winding yields 120 volts with respect to ground and 208 volts with
respect to each other.
Four wires go into the building... phase-1, phase-2, phase-3, and
ground.
Three-phase devices in the building (three-phase motors) run from the
three phases. There is no "return" for these devices other than the
3-power wires themselves. The ground wire going to these devices is
protective ground, goes to the metallic chassis or case, and carries no
current.
High-voltage two-phase devices in the building (like a stove or water
heater) use any two of the three phases and get 208 volts. The ground
wire going to these devices is protective ground, goes to the metallic
chassis or case, and carries no current.
Single-phase low voltage devices in the building get one of the three
phases, plus the ground wire twice. One ground wire is the neutral
(return) and the other is protective ground. If this goes to a typical
outlet, the "hot phase" goes to the shorter slot, one ground wire
(neutral) goes to the longer slot, the other ground wire (protective
ground) goes to the round hole.
Inside the building, the single-phase 120 volt return current flows
through the neutral wire and no current flows in the protective ground.
The neutral and protective ground join at the circuit breaker box which
also connects to the ground rod for the building. Outside there is a
ground rod at the building and another ground rod at the transformer,
and these are connected by a ground wire. The return current to the
transformer goes on this ground wire... not through the earth. If this
ground wire breaks such that the return current for the 120-volt devices
has to go to the transformer through the earth, the voltages can get
pretty messed up inside your building because the earth is too
resistive, and this would be a defective situation.
In the neighborhood, if the 7200-volt wiring is above ground, the three
phases run along the tops of the "telephone poles." Further down the
pole is a ground wire running from pole to pole, and connected to earth
at each pole. If the distribution is a true three-phase system this
ground wire carries no 7200-volt return current, but it might be
carrying 120-volt return current to a transformer depending on where
that pole-to-pole wire is situated with respect to the building and the
transformer.
Some neighborhoods only get one of the three 7200-volt phase. This
system starts out the same but differs slightly once the voltage is
reduced to 7200 volts. I'll describe that in a separate mailing
tomorrow.
At no place in the system described above is any current carried through
the earth when the system is working correctly.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu