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## Re: [TowerTalk] Fwd: shack wiring

 To: "John - K4WJ" , Re: [TowerTalk] Fwd: shack wiring "Jim Thomson" Mon, 2 Aug 2010 16:51:29 -0700 mailto:towertalk@contesting.com>
 ``` -------------------------------------------------- From: "John - K4WJ" Sent: Monday, August 02, 2010 1:56 PM To: "Jim Thomson" ; Subject: Re: [TowerTalk] Fwd: shack wiring > Jim, > > You said, > "connected to a cold water pipe, etc... so the MGN gets grnded 9 x times" > > What does 9 x times mean? To me x represents the multiplier symbol so it > means "times". Are you saying 9 times times? Help! > > 73..de John/K4WJ ##### Here, we have 9 x homes hanging off one 50 kva xfmr. This part of town is all on poles. My drop wire is a mid span drop...between 2 x poles. Since that neutral goes to all 9 x homes.. and the neutral [ CT of xfmr sec] is also bonded to the MGN, via a short loop on the pole.... the MGN just got grnded 9 more times....one per home. Repeat for every xfmr on the same street.... so the 'return' wire for the HV xfmr gets gnded at every house on the street.. + every home that hangs off that hv phase. Jim VE7RF > > > ----- Original Message ----- > From: "Jim Thomson" > To: > Sent: Monday, August 02, 2010 10:54 AM > Subject: [TowerTalk] Fwd: shack wiring > > >> Date: Mon, 2 Aug 2010 08:31:22 -0400 >> From: "Paul Christensen" >> 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 >> >> >> >> _______________________________________________ >> >> >> >> _______________________________________________ >> TowerTalk mailing list >> TowerTalk@contesting.com >> http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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