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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