On Feb 24, 9:24*pm, Tom Horne wrote:
On Feb 24, 8:52*pm, Metspitzer wrote:





On Fri, 24 Feb 2012 17:36:07 -0800 (PST), Tom Horne

wrote:
I think the return path for the other side of the primary
transformer is the earth. *Earth is used as a return in
many distribution systems and while it saves money,
it leads to some problems. *I see what the OP and you
are talking about all over here in rural NJ. *You have 3
wires on top of the poles, which are the 3 phases.
If you look at where you have houses, a single wire
leads from one of those to the transformer. *I believe
the other side is connected to earth ground. *That
arragement then serves a small group of houses. With a
larger commercial/industiral user you see 3 transfomers,
one connected to each of the 3 phases.

In most of North America we use a Multi Grounded Neutral (MGM). *That
means that there is a neutral conductor that goes back to the source
transformer's neutral point that is grounded at multiple points along
it's route. *Yes there are exceptions but they are not in common use..
The reason that system is used is because it saves a lot of money
while still providing reliable service. *The neutral in most utility
distribution systems is common to both windings of the transformer.
It connects to the uninsulated stud on the transformers case which is
internally bonded to one end of the primary winding and the center of
the secondary winding. *There is only one insulated connection to the
primary winding and that is the other end from the neutral
connection. *The grounded stud on the case is connected to the neutral
and to ground. *Since the current will flow in all pathways available
to it in proportion to the total impedance of the pathway some current
will flow via the earth on the order of a few amperes per grounding
electrode.

Here is where it gets confusing. *The reason that all of those
currents do *not add up to some phenomenal current flow through the
earth is that the flows from the three phases cancel each other out in
all of the common connections to the degree that the current being
drawn from the system is equal. *If you ran impossibly long leads from
a three phase power analyzer what you would see across any three
consecutive transformers grounds would approach zero current. *Since
no system is perfectly balanced across all three phases the current in
the neutral and the earth is never zero but if you check the current
flow in the source transformers neutral and it's grounding electrode
conductor back at the power substation you would find that it is
rather low.

So while it is true that in an MGM distribution system some of the
current is flowing through the earth the actual amperage doing that is

Wouldn't the situation where the primary is feed by a single conductor
have the same amperage returning through the earth as supplied by the
phase conductor? * *I wouldn't consider that "rather small"

rather small. *The earth carrying current seldom causes any problems
in systems that are maintained to the National Electrical Safety Code
standard. *The biggest exception is in the animal husbandry industries
were the four footed critters that spend much of their day standing or
lying in their own rather conductive waste do often suffer ill effects
from event the small stray currents that are flowing across the
ground. *When some defect in the distribution neutral raises that
current a little higher the animals suffer greatly and even die from
the effects. *In dairy cows for instance it will cause a drastic
reduction in production and radical changes in the cows behavior and
temperament because the animals are in nearly constant pain.
Utilities in areas with large dairying industry. *Have developed
transformers with high impedance connections to ground in order to
limit the stray current to levels that are imperceptible to the
livestock. *One of the utilities in Wisconsin painted these special
transformers to look like the coloration of *the locally dominant type
of dairy cow.

I hope that is helpful.

No. *Because the primary is supplied from two conductors. * In a Multi
Grounded Neutral (MGM) distribution system there is a continuous
neutral conductor. *It is the same neutral conductor that is part of
the low lines; that is the lines below the transformer; that supply
the individual premise wiring systems. *That one conductor is serving
as the neutral for both the distribution system and the premises
wiring systems served by the transformers that are connected in
succession to each phase of the distribution. *That uninsulated
conductor is grounded at intervals along it's route from the
substation transformer were the distribution current originated. *In
normal operation it carries comparatively little current because of
the cancellation that occurs in the common connections. *The only
current flowing on the neutral is the total difference between the
current flowing on the three phases. *Not the sum mind you just the
difference. *So if phase A is carrying one hundred, phase B one
hundred five, and phase C one hundred three amperes the maximum
current flowing in the neutral conductor at any given instant is five
amperes. *The actual purpose that the neutral current serves is to
provide a larger number of common neutral connections in which the
currents from the three phases can cancel each other out. *In any
portion of a three phase distribution system were all three phases are
not present the neutral will carry the same current as the highest
current on the one or two phases that are still present. *It is only
in the three phase portion of the network that the current will cancel
out to a very small value. *Once that portion of the system ties back
into the three phase portion of the network then the current averaged
from all such single phased stubs will again cancel out and the
neutral current will again be a rather small value.

--
Tom Horne- Hide quoted text -

- Show quoted text -

I'm with you on the essence of what you are saying regarding
how the power is distributed. Where
I still have questions is back to the original observation. Out
on country roads, you have 3 high voltage wires, one for
each phase. Then you'll have a group of houses. There
is a transformer for those houses and it's connected to
one phase. Since there are only 3 primary high voltage
wires, are you saying the other side of the transformer is
connected to the same neutral as the 240V services of
the houses? I can see that working, as then you have
multiple connections to earth ground for return on the
primary side. But then there is
also clearly substantial current flowing through earth
ground back to the substation or more likely I guess
to other earth ground points with other nearby pole
transformers that are on different phases.

If it's not done that way, then I don't understand the
return path from that pole transformer. All I see are
the 3 high voltage wires and then below it the 3
240V service wires, 2 hots, one neutral,
going down the road. In other
words, there is no neutral return path that I can
see other than the 240V, secondary one. So, is it shared?
I think that is what Bud said he thought might be
going on too. And that seems to be the essence
of the OP's question that has him stumped.