one-wire pole transformers

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.

Reply to
dennisgauge
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Think about it. Only one (of the two) wires feeding the primary of a typical single phase distribution transformer needs to be insulated.

Reply to
George

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.

Reply to
bud--

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.

Reply to
trader4

The problem is, there is only ONE wire.

Reply to
dennisgauge

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

Reply to
Evan

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.

Reply to
dennisgauge

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.

Reply to
trader4

Google "Single-wire earth return" (SWER) for a diagram and explanation - as others have said, it uses earth grounding for the return path.

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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.

Reply to
SRN

There is no grounded conductor on the pole? That is unusual but not unheard of. They may be using earth return. That is the formula for "stray voltage" problems.

Get some pictures

Reply to
gfretwell

The norm is one to three phase conductors at the top of the pole or on the cross arm at the top of the pole, with the neutral for those on the pole a few feet down, and the low voltage secondaries if present a few feet below that. The neutral is grounded at least every few poles with a small uninsulated wire down the side of the pole that connects to a plate at the bottom of the pole underground. The SWER or delta (two phase conductors, no neutral) configurations are obsolete and only found in areas with old infrastructure.

Reply to
Pete C.

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I used 8000 for the primary from someone's earlier post. No clue what it really is.

Reply to
Metspitzer

For purposes of this discussion, you are correct. These IS another wire somewhere. It may be running down the pole to the ground (using the earth as most of the "wire") and may look like a steel support cable...

Reply to
Larry Fishel

In the US in a residential area, indeed.

In some (very) rural areas of Sask, CA, I saw one-line w/ earth return as recently as roughly 20-yr ago yet...

--

Reply to
dpb

I was looking outside one snowy day, and I saw a tree branch fall in my front yard with the end on fire. The branch had hit the single top wire connecting ground.

Greg

Reply to
gregz

I believe 7,200V is about the lowest you will find anywhere and most are

13,200V or more.
Reply to
Pete C.

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Reply to
HeyBub

I was lying in bed one day watching guys adding a new transformer outside the window. When trying to clip the wire onto the hv wire I saw a pretty good arc. Probably at least 3 inches. A transformer feeding the house burnt out, so they replaced that, and added another transformer in addition to original. Result our house had less voltage fluctuations. When young, I used to use the shortwave radio, and ever so often, maybe twice a ay, a horrendous arching- buzzing sound would build up and quickly stop. Lasting

3-4 seconds. I never found the source of that. Didn't sound like anything that would be consumer generated.

Greg

Reply to
gregz

It could have been a static discharge from your antenna if you had an external antenna. Or it could have been a static electricity discharge from another source, even atmospheric. Another source may have been power company or an industrial site switching high voltage power at certain times every day. I can remember listening to distant stations on an AM radio in different bands and hearing a "zip..zip..zip" sound at regular intervals.

TDD

Reply to
The Daring Dufas

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

Reply to
Tom Horne

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