[TowerTalk] Fwd: shack wiring

[Paul Christensen]

> "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 at 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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