John Nichols wrote:
>
> Anecdotally, I have been told that the reason each home in the US has its
> own transformer is due to the problems of distribution. If the 120 volts
> was carried distances over power lines voltage drop would be an issue. By
> transforming the high voltage at each home this problem is minimised. My
> understanding why Australia adopted the 240/415 volt system on the
> south/east coast, and a 250/440 volt on the west coast, so we can use a
> single transformer to service a whole street of homes. While three phases
> pass each home, typically only one of these is brought into the home for a
> 240 volt 80 amp single phase service.
John, you seem to be mistaken about homes in the US.
Typically, several homes are supplied by a "secondary crib" which is
"split phase 235 V". By that is meant that there are two "hot" wires 235
V apart and centered about ground (earth). Each of those two phases to
ground, then, is 117.5 V. The secondary crib is fed by a transformer,
stepping down the potential from (typically) a primary which operates in
the 8 kV to 14 kV range.
Each house is connected to both phases of the secondary crib and this
enters the house's fuse or breaker box after first passing through that
house's energy meter (mistakenly refered to colloquially as a power
meter). A few loads that a house might have are wired across both of
these hot phases, taking advantage of a 235 V supply: clothes driers,
ranges ("cookers"), and perhaps heat pumps. Most other loads are wired
from one phase to ground, thus being supplied with 117.5 V. The wire
return to the ground bus in the fuse or breaker box from the load is
called the "neutral" or "cold" wire. Properly wired houses have about
equal loading of each "hot" phase.
An earth ground is established at the output (the winding's center tap)
of the transformer supplying the secondary crib and also at each house
receiving service. An extremely hazardous but fortunately rare occurance
can arise, called a "floating neutral". If the connection to earth is
lost, the house's "ground bus" actually floats between the two hot
phases. If one phase is more heavily loaded than the other phase, the
voltage on the heavily loaded phase drops while the voltage on the
lightly loaded phase rises. I have observed this twice, once in
Connecticut and once in South Carolina. In each case, I could typically
measure 70 V on one side and 165 V on the other! The two voltages I
mention observing still total to 235 V, but they differ because the
neutral has floated away from ground potential. When this happens, one's
attention is called to the matter because some of the lights in the
house shine more brilliantly than ever and others glow very dimly. Or,
the starting of a large load causes even more momentary dimming than
normal in the lights on that load's phase while the lights on the other
phase brighten momentarily. If it is not noticed, it can eventually lead
to damage of appliances or even a house fire.
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!
Jim
--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE
http://www.cofc.edu/~frysingj
[EMAIL PROTECTED]
[EMAIL PROTECTED]
Office:
Physics Lab Manager, Lecturer
Dept. of Physics and Astronomy
University/College of Charleston
66 George Street
Charleston, SC 29424
843.953.7644 (phone)
843.953.4824 (FAX)
Home:
10 Captiva Row
Charleston, SC 29407
843.225.0805
