one-wire pole transformers

Page 1 of 4  
I just noticed that all the residential transformers on the poles on my street only have one wire feeding them. There are three wires on top of the pole.
One wire in, and three wires out to the triplex feeding my house. There are no other visible connections on the transformer.
Heck, the next street over only has ONE wire at the top of the pole...
I thought the three wires on the pole corresponded to each leg on my breaker panel, and a ground wire.
How is the transformer creating two 110V legs and a ground from one feed wire? This just goes against everything I learned in school about electricity.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/24/2012 10:14 AM, snipped-for-privacy@gmail.com wrote:

Think about it. Only one (of the two) wires feeding the primary of a typical single phase distribution transformer needs to be insulated.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The 3 wires you see running down the road on poles are each one phase of a 3 phase distribution system. They are 120deg out of phase with each other. For larger industrial loads, like motors, they use all 3 phases and step it down. For light loads, residential, they use any one of the three phases and run it into a center tap transformer. The center tap becomes your neutral and each of the ends becomes one of the hots, giving you 240V between them, 120V between each of them and neutral.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The problem is, there is only ONE wire.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/24/2012 9:14 AM, snipped-for-privacy@gmail.com wrote:

In the alley behind my house the poles have one high voltage wire (8000V) and 3 low voltage wires (H-N-H). The low voltage H-H wires break between transformers but the neutral is continuous. The high voltage return must be on the secondary neutral.
--
bud--


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@optonline.net wrote:

pic
http://thereifixedit.files.wordpress.com/2012/02/white-trash-repairs-what-i-grounded-it-to-earth.jpg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I got to admit, there is clueless, and there is clueless. This was to the extreme.
Christopher A. Young Learn more about Jesus www.lds.org .
pic
http://thereifixedit.files.wordpress.com/2012/02/white-trash-repairs-what-i-grounded-it-to-earth.jpg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/24/2012 6:25 PM, HeyBub wrote:

http://thereifixedit.files.wordpress.com/2012/02/white-trash-repairs-what-i-grounded-it-to-earth.jpg
Ahhhh-haaaaa!
So that's how they ground the Space Station and other satellites!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 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.
-- Tom Horne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 24 Feb 2012 17:36:07 -0800 (PST), Tom Horne

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"

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Metspitzer wrote:

Do you know what the currents are at primary voltages? Your 240V 100A residential service is some 3.3A at the lowest 7,200V primary voltage and something like 680mA at 35kV distribution, and that's only if it's running at the full 100A load which it isn't ever supposed to do for more than a few seconds.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Tom is correct, we have had that sort of a system here in Illinois for at least 50 years.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/25/2012 9:44 AM, snipped-for-privacy@optonline.net wrote:

I have been on a lot of country roads and have never seen what you described. Typically there are the Y derived phase conductors on a crossarm or bracket with three insulators and an uninsulated neutral below. As density gets lower they will only carry one of the primary conductors and the neutral.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 02/25/2012 11:30 AM, George wrote:
[snip]

The one here appears to have the neutral on top. See the picture at
http://notstupid.us/pix/IMG_3427.JPG
[snip]
--
Mark Lloyd
http://notstupid.us
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mark Lloyd wrote:

The theory with that is the neutral up top is most likely to take a lightning strike. They do the same on the long haul transmission lines.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Feb 24, 10:14am, snipped-for-privacy@gmail.com wrote:

Read up more on transformers...
Residential power is typically single phase (although there are some exceptions to this) with a center tapped secondary winding on the transformer...
~~ Evan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The thing is, everything I know about electricity says that you need TWO wires to make a complete circuit. AC or DC, doesn't matter.
Most streets have two overhead wires, with two wires going to the transformer.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.