>> ## If the business's don't get a neutral... then how do the
>> business's obtain 120 vac ??
Their supply is either 120/208 (e.g. wye or star) with neutral
being the center of the wye or they need to install their own
480 to 240/120 transformer on premesis.
73,
... Joe, W4TV
On 8/2/2010 10:54 AM, Jim Thomson wrote:
> Date: Mon, 2 Aug 2010 08:31:22 -0400
> From: "Paul Christensen"<w9ac@arrl.net>
> Subject: Re: [TowerTalk] Fwd: shack wiring
>> ## 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.
>
> ### Same thing here. BUT.... we have 3 phase running down the main drag.
> 1st side street gets phase A..... 2nd side street gets phase B.. etc, etc.
>
> Then the 30,000 resisdent's total load is divide up fairly evenly between
> all 3 x phases.
>
>
>
>
>
>
> 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.
>
> ## I don't think so. The most that could happen is the input
> fuse to the xfmr would blow open. The MGN is the return leg
> for the 7200/14.4 kv. Notice that the CT of the sec of the xmfr
> is also bonded to this same MGN . That's done...so each home
> that gets a neutral, also has the same neutral grnded to cold water
> pipe etc. So the MGN [HV return] gets grnded at each house as well.
> The MGN also gets grnded at each pole that has a xfmr.
>
> ## I call that an 'unbalanced HV primary. '... and a 'balanced 240 sec'..
> [with a grnded CT]
>
>
>
>
> 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 xfmr voltages would be all wrong! If you are going to
> use 2 x phases..then u really need 2 x xfmr's . Unless, what ur proposing,
> is run one xfmr between 2 x hv phases... which is doable. That requires
> 2 hot wires to each xfmr. They don't do it that way cuz they use an UN
> balanced
> distribution system.
>
> ## Notice right now.. that one side of the xfmr Primary is grnded. And that
> the CT
> of the sec is also grnded [ both to the same MGN].
>
> ## In the UK... they use no CT on their xfmrs. [230 vac 50 hz single phase],
> BUT, one side of the sec is bonded to the MGN..and also dirt grnded. One
> side of the HV primary is also grnded/bonded to the same MGN. What
> they have in effect is UN balanced sec power. What they call a neutral
> is just one side of the grnded 230 line. That way... you only need one
> spst switch for each circuit in a house...like a light switch. [you
> only have one hot leg].
>
>
>
>
> 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.
>
> ## The MGN also extends to each home...via the neutral [ CT]..where it's
> connected to a cold water pipe, etc... so the MGN gets grnded 9 x times
> for each xfmr..for 9 x homes. The fault current can't flow down the pole's
> safety grnd.. and zap anybody. As soon as it hit the MGN.. or anything else..
> HV fuse blows.
>
>
> 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.
>
> ## The reason they use the MGN set up is so if something happens
> like say the xfmr develops a pri to sec short.. ur safe. with a primary
> to sec short, since the CT [neutral] is bonded to the MGN.... all you end
> up doing, is shorting out the primary side ! .... and the HV fuse to the
> input of the xfmr blows open.
>
> ## If u did it ur proposed way..with one xfmr pri across 2 x phases...
> and say you had a pri to sec short in the xfmr.... then u would end up
> stuffing
> 7200/14.4 kv right into your living room!
>
> ## it's actually a well thought out system. Now, how 9 x homes..each with
> '200A' service can all hang off the same 50 kva xfmr..is beyond me..esp at
> dinner time.
> The xfmr is only rated for 210 A.
>
> Jim VE7RF
>
>
>
>
> Paul, W9AC
>
>
>
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