[TowerTalk] Fwd: shack wiring

John - K4WJ k4wjfoc1749 at wildblue.net
Mon Aug 2 13:56:04 PDT 2010


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

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <towertalk at contesting.com>
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" <w9ac at 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://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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