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How does mains electricity work?

  1. Aug 10, 2011 #1
    Hello. I've been reading numerous texts about mains electricity but I'm not getting some of the things.

    I'll start with some questions, considering Live, Neutral, and Ground wires.
    1) Does the neutral wire return/come from the supply company? In that case, where does the company connect the neutral wire?

    I ask this because I've heard/read that "ground closes the circuit", asi if, the could have a grand neutral grounded wire, and an "independant" live wire. I have serious doubts about that.

    2) What does it mean that neutral wire is at zero potencial? OK, voltage is about differences. So when the wire is not connected to an appliance, say a bulb, it has a zero potential (conductor in equilibrium), but when it is conected, I asume it oscillates, and the voltage drop will be such that RMS is zero. Is that correct?

    Sorry for not writing the questions clearly, I think I have a big mess up about this, so any comments, or a diagram with WHERE DO WIRES COME FROM AND GO TO, including the supply company will be of great help.

    Thanks in advance to everyone.
  2. jcsd
  3. Aug 10, 2011 #2


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    Staff: Mentor

    (I moved your thread from General Physics to the EE forum for now)

    The wikipedia article on electrical power distribution is pretty good:


  4. Aug 10, 2011 #3
    As berkema mentioned, the Wikipedia page on http://en.wikipedia.org/wiki/Electric_power_distribution" [Broken] is a good general source. The first section gives a pretty good overview of why this is so complicated:
    The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customer's meter socket. A variety of methods, materials, and equipment are used among the various utility companies, but the end result is similar.

    In the U.S., household power is generally http://en.wikipedia.org/wiki/Split_phase" [Broken], with the neutral at ground potential -- literally. There is a long copper rod stuck in the earth outside my house. If I place a volt meter between a "hot" conductor (either of the two wires coming from the power company) and the ground (earth), it will measure 120 volts AC. I get 220 VAC by connecting to both of the "hot" conductors (each line is 120 VAC, at opposing phase angles.)

    In the simplest view, for (U.S.) household split-phase power, yes, you can consider the neutral and ground wires to be connected directly to the Earth, with the power company also connected to the Earth. The "live" conductor can be thought of as being physically connected to the power company.

    I don't understand the bolded portion. To me, "voltage drop" is the measurable reduction in voltage due to parasitic losses, such as the resistance in a long run of copper wire.
    The voltages talked about for electrical power distribution is expressed in VRMS, even if it's not written as such.

    The http://en.wikipedia.org/wiki/Electric_power_distribution" [Broken] mentioned above has a simple diagram that may help (although, it doesn't show the neutral or earth connections.)
    Last edited by a moderator: May 5, 2017
  5. Aug 10, 2011 #4
    Could someone confirm if what I'm saying about this picture is correct:



    There are three thick insulated black wires which are the split phase secondary winding, and the center wire should also be grounded (since it's the neutral wire and neutral is grounded), but you can't really see that in the picture. One of the leads of the primary winding is entering the bucket through the top, and is one of the three phases. But there needs to be another lead for the primary winding, and there seem to be two other wires, on the right side of the bucket, at the top and bottom. The Wikipedia article suggests that one of the three phases and a ground wire are the primary leads to create split phase, so I'm assuming that the two wires on the right side of the bucket are ground wires. But why are there two of them?
    Last edited by a moderator: Apr 26, 2017
  6. Aug 11, 2011 #5
    Looking at the image in http://upload.wikimedia.org/wikipedia/en/1/1c/Polemount-singlephase-closeup.jpg" [Broken], I can see the second primary lead connected to neutral/ground (the uninsulated cable), along with the secondary center conductor, and the steel case of the transformer (the lower connection seen toward the ride side of the image). The neutral is connected to the house as a bare steel wire, which doubles as a structural support/strain relief for the two conductors.

    The neutral-primary and case-to-ground connections on the transformer are separated in that manner as a type of redundancy. The reasoning, as I understand it, is to insure a solid connection to ground (earth), should there be an interruption of the neutral wire. I don't know enough of the electrical theory to understand how it would be any different than simply connecting the neutral and earth-ground to the same terminal.
    Last edited by a moderator: May 5, 2017
  7. Aug 11, 2011 #6
    I don't see how the secondary center conductor (the middle black wire) is connected to neutral/ground. That middle black wire seems to end at a dielectric spacer that separates a ground wire (the wire where you can see the braids) from other wires.
    Last edited by a moderator: May 5, 2017
  8. Aug 11, 2011 #7
    That connector (circled below) is metal, not an insulator. The white three-armed piece to the right is an insulating standoff. Farther right is another metal connector that attaches the household neutral.

    Attached Files:

  9. Aug 12, 2011 #8

    I find it interesting that the reason for separating the electric industry into power plants, transmission lines, substations, primary and secondary distribution, is to prevent a monopoly of the entire electric industry. But I find that odd, as I'd imagine that power plants have a monopoly because it doesn't seem profitable to have two power plants competing against each other to serve one area. Also, how many different transmission lines can you have coming out of a single power plant? It seems like only a few companies can own transmission lines. It just seems like the power plant people will charge the owner of the transmission lines for electricity, and the owners of the transmission line will mark it up and pass it to the substation owners, and the substation owners will mark it up and pass it to the primary, etc. At any one of those links, they could mark it up however much they want, and the end secondary user will blame it on the secondary distribution company because that's the only bill they see and the markups from all the previous stages are hidden.
  10. Aug 12, 2011 #9
    Where did you learn this? Power plants output very high voltage for reasons of long distance transmission efficiency. Substations are necessary to reduce the very high voltages. I'm not aware of legislation or agreements that prohibits power generation companies from owning and/or operating substations, etc.
  11. Aug 12, 2011 #10
    I got it from the link that was given by berekeman in this thread. The very last paragraph says:

    I can sort of understand how internet operates: some companies build a network of cables that cross the country, and charge ISPs to connect to this backbone. They sell usage of their lines to ISPs, and ISPs collect the money from their customers. I just can't see it scaling down to the local area that a powerplant serves.
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