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Re: [TowerTalk] Fwd: shack wiring

To: "John - K4WJ" <>, <>
Subject: Re: [TowerTalk] Fwd: shack wiring
From: "Jim Thomson" <>
Date: Mon, 2 Aug 2010 16:51:29 -0700
List-post: <">>

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!
> 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. 
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 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:
>> 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 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 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 beyond me..esp 
>> at dinner time.
>> The  xfmr  is only rated for 210 A.
>> Jim  VE7RF
>> Paul, W9AC
>> _______________________________________________
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>> TowerTalk mailing list


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