From: "John - K4WJ" <firstname.lastname@example.org>
Sent: Monday, August 02, 2010 1:56 PM
To: "Jim Thomson" <email@example.com>; <firstname.lastname@example.org>
Subject: Re: [TowerTalk] Fwd: shack wiring
> 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
is all on poles. My drop wire is a mid span drop...between 2 x poles.
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.
> ----- Original Message -----
> From: "Jim Thomson" <email@example.com>
> To: <firstname.lastname@example.org>
> 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" <email@example.com>
>> 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
>> secondary. Speaking of electrical distribution and use of neutrals, this
>> evolution of plant step-down architecture has always bothered me:
>> In the U.S., the HV primary on a pole transformer feeding a home is
>> between one phase of a three-phase system and a Multi-Ground Neutral
>> 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
>> ### Same thing here. BUT.... we have 3 phase running down the main
>> 1st side street gets phase A..... 2nd side street gets phase B.. etc,
>> Then the 30,000 resisdent's total load is divide up fairly evenly
>> all 3 x phases.
>> At each pole, a grounding conductor (shown in the second link) is run
>> 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
>> 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
>> for new neighborhood construction, two phases should be carried to the
>> transformer's HV primary, and not between one phase and the MGN. This
>> especially the case where 3-phase is available on the pole. Yet, in
>> 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
>> is run one xfmr between 2 x hv phases... which is doable. That
>> 2 hot wires to each xfmr. They don't do it that way cuz they use an UN
>> 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
>> BUT, one side of the sec is bonded to the MGN..and also dirt grnded.
>> 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
>> 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
>> safety grnd.. and zap anybody. As soon as it hit the MGN.. or anything
>> 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
>> 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
>> 7200/14.4 kv right into your living room!
>> ## it's actually a well thought out system. Now, how 9 x homes..each
>> '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
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