I can imagine different companies have chosen different layouts for the
wires on their poles. I have seen wooden telephone poles with ground
wires running along the top. However, around here we have ground as the
lowest wire on the pole, with the 7200-V wire(s) at the top, and the
208/220 wires (if present on the pole) between the ground and HV. We do
have ground wires for lightning protection running on the very top of
the metal HV towers.
The reason I said 208/220 above is because some residential areas have
3-phase running through the neighborhood. In the 3-phase case the rms
voltages after the final 3-phase wye-transformer are 208 between phases
and 120 volts to ground. Older neighborhoods have a single HV phase at
the top of the pole. After the leaving the final center-tapped
transformer this yields 220 volts between the two phases and 110 volts
from either phase to ground.
Therefore we have the interesting situation that the 3-phase customers
have a slightly lower high-voltage (208) and slightly higher low-voltage
(120). The 1-phase customers have higher high voltage and lower low
voltage (220/110). At least these are the typical voltage around here.
In my neighborhood the 1-phase customers are not getting a completely
different phase from the substation. They are getting just one-phase of
the 3-phase (plus ground). That is, the power company just didn't
bother to run all three phases into the neighborhood. I believe the
situation is that these older neighborhoods just never got upgraded
after 3-phase came to town.
Now that we seem to be in agreement that we are not using earth for
return current, but we are using earth as a reference point for some
wires, I think we are back at the original question.... Is a
ground-referenced system more safe or less safe than a system that is
not ground referenced? If less safe, why was a ground referenced system
chosen? I don't have a clear answers to these questions.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu