Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Earth as return path for power transmission line?

  1. Sep 16, 2010 #1
    This is from How Stuff Works:

    The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. "Ground" in the power distribution grid is literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of the earth.

    Is this true? If yes, can you do something similar with a DC circuit (connect - of source to the earth, connect + of source to a resistor and then connect the other side of the resistor to the earth)? It seems that no current would flow, since the resistance of the earth is extremely large...
  2. jcsd
  3. Sep 16, 2010 #2


    User Avatar
    Gold Member

    A "pretty good conductor" does not have an "extremely large resistance".

    But yes, it is true.

    And you can do it with DC circuits.

    I've heard (but have never seen nor tried) that you can make worms come out of their holes by electrifying the ground in this way.

    For fishing of course.
  4. Sep 16, 2010 #3
    Actually yes it does if it is 1000 km long...
  5. Sep 16, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    You can (and do) use the earth itself as a "ground". But because of the very large resistance of the earth compared to copper wire, it would be pointless to try and make a circuit using the earth as a return.
  6. Sep 16, 2010 #5
    NEC standard is no more than 25 ohms resistance to ground. I realize this isn't the same as "resistance through the earth", which changes depending on moisture and mineral content along the path, but it at least gives you a starting point.

    Like OmCheeto said, yes, you can do it with DC -- in fact they actually DO. Look up "High Voltage DC single wire earth return". They key in both AC and DC "SWER" systems is the high voltage, which minimizes your losses to resistance.
  7. Sep 16, 2010 #6
    Having very poor conductivity, the earth is BY NO MEANS an ideal circuit link.
    It's just convenient and can handle large amperages.
  8. Sep 16, 2010 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Just this summer while cleaning out my father-in-laws garage I found his worm spike, it consisted of a 2ft long hollow wooden rod with a 120v AC cord running through it. One wire of the cord was connected to a 6in long spike. My wife recalls him plugging this into a wall socket then driving the spike into the ground, the worms would crawl out of their holes waiting to be picked up, yes, for fishing. Not sure how big of an area it would effect.

    The device gave me the creeps, it just looked dangerous! I disassmebled it and trashed the pieces.
  9. Sep 17, 2010 #8
    An electrician friend told me that when electricity was first used in the U.K.the earth was used as one of the wires.You had a possitive as one and the ground as another.
    It soon became obviouse that there was a problem with this system as many users were being killed by electrocution.
    So an extra wire was introduced to try to prevent this.
    Never done any research on this but it does sound plausable.
    Last edited: Sep 17, 2010
  10. Sep 17, 2010 #9
    No, they don´t. In a three phase system there is no return wire. And if they have 3+N (230/400V household systems in European countries) N is an extra wire.
    Railways use the rails as return wire, but that´s iron.
  11. Sep 17, 2010 #10
    Ummm... Yes they do, in the case of Single Wire Earth Return transmissions systems, which was the topic of this thread.
  12. Sep 17, 2010 #11
  13. Sep 17, 2010 #12


    Staff: Mentor

    This is a little deceptive IMO. "The power company" does not do this. It is certainly possible to do as other posters have pointed out, but this would be hugely expensive in terms of loss of power, and a power company that did this would be out of business very soon. The return path for electrons in a typical three-phase power distribution grid is the other lines. The dirt only carries current in the event of some failure or load imbalance, and often not even then since there is usually another wire for that purpose.
  14. Sep 18, 2010 #13
    One-wire power transmission lines have been used extensively in a number of countries. I do not know how many are using them currently.

    http://tdworld.com/mag/power_one_wire_enough/" [Broken]

    Early telegraph and telephone systems used one-wire systems with ground as the return.
    Last edited by a moderator: May 4, 2017
  15. Sep 23, 2010 #14
    One of my email communicants, an EE professor, related a communication he had with the Bonneville Power engineers. When Bonneville does maintenance on their +500,000 V and -500,000 V DC lines from Bonneville, OR to Los Angeles, they shut down the line they intend to do maintenance on and use earth ground as the return for the one in service. The Bonneville engineers stated they have less resistance using the ground return than when they have used the second line (without the opposite DC potential) as the return.

    All the mechanism(s) the contribute to earth conductivity are not fully understood. With a little depth, the increased pressure and temperature change the rules.
  16. Sep 24, 2010 #15
    Form what I remember there is no return in 3phase system only becouse at the both end of the line the Y star center is at the same potential 0 and is grounded, but only if the loading of the line is symmetric which is not always true. If you don't have a symmetric loading of the 3 phase, there is a return current through the neutral point. More than this, when you have a short circuit to ground of 1 phase or between 2 phases, the current path is through the earth.
  17. Sep 24, 2010 #16


    User Avatar
    Homework Helper
    Gold Member

    It has poor conductivity but there is so much of it, so does it have poor conductance?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook