> "Paul, with 2 phases, wouldn't they be phased at 120 degrees apart instead of
> 180? Seems that would cause problems with 220/240vac loads. 73, Gerald K5GW"
The transformer's secondary winding creates the 180-degree phase relationship
on the 120/240 side -- not the transformer's primary.
Paul, W9AC
In a message dated 8/2/2010 7:36:13 A.M. Central Daylight Time, w9ac@arrl.net
writes:
> ## If the business's don't get a neutral... then how do the business's
> obtain 120 vac ??
The neutral for premise distribution is created at the utility transformer
secondary. Speaking of electrical distribution and use of neutrals, this
evolution of plant step-down architecture has always bothered me:
http://72.52.250.47/images/power-tap.gif
http://www.psc.state.fl.us/consumers/utilitypole/images/utilitypole5.jpg
In the U.S., the HV primary on a pole transformer feeding a home is tapped
between one phase of a three-phase system and a Multi-Ground Neutral (MGN).
The photo in the top link shows only one phase on the pole insulator.
Often, the other two phases are not passed in deeper residential
distribution.
At each pole, a grounding conductor (shown in the second link) is run from
the MGN to earth to keep the distribution's MGN ground line at earth
potential. That's a good thing because if a ground fault occurs, it's
possible that the pole grounding conductor (runs from the top of the pole
to
the ground stake) would elevate to the full 7200V delivered by the
distribution phase through the transformer primary. During a fault
condition, a person standing on the ground while touching the pole's
grounding conductor would be guaranteed electrocution.
Albeit more expensive for utility companies, it makes more sense to me that
for new neighborhood construction, two phases should be carried to the home
transformer's HV primary, and not between one phase and the MGN. This is
especially the case where 3-phase is available on the pole. Yet, in cases
where a residential transformer is mounted on a 3-phase pole, the utility
companies still use the MGN instead of a second phase. The safety of the
exiting distribution is wholly dependant on the bonding quality of all
components between the MGN, the pole grounding conductor, and the earth
grounding rod. Tapping two phases instead of one eliminates the deadly
ground fault condition. Then again, just how many accidents or deaths
result each year from such a ground fault? In areas of *well-maintained*
plant, probably not too many.
Paul, W9AC
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