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

Why Single-wire earth return systems are only used in rural areas?

  1. Feb 21, 2012 #1
    Why Single-wire earth return systems are only used in rural areas? Why not also in urban areas where there is only demand for single phase power?
     
  2. jcsd
  3. Feb 21, 2012 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Primary reason I would assume would be cost... you only pay for one copper conductor NOT 2 same as they used to do for out of the way rural phone lines

    Dave
     
  4. Feb 21, 2012 #3
    Of course that would be the main reason for using it, that`s why the question is why not also in urban areas where there is only demand for single phase power?
     
  5. Feb 22, 2012 #4

    NascentOxygen

    User Avatar

    Staff: Mentor

    I'll venture a guess, to see whether anyone can shoot this down. :wink:

    I seem to recall seeing in rural areas that the single wire feeds a pole-mounted transformer as it passes each isolated farm. The mains return is via an earth at the power pole, and from the transformer the farmer gets both active and neutral wires. I would expect earth return to involve appreciable losses, compared with copper return.

    So, extending this to suburban areas, almost every power pole would have to incorporate a transformer and a buried earth return, if we were to avoid twin wires running along the streets.
     
  6. Feb 22, 2012 #5
    This was done for cost - and typically old farm houses used very little power. Really there are 2 conductors missing - a Neutral and a Ground - I do not know if they will allow this type of install today - but they used to then make a secure earth connection ( Grounding rod ) Really every installation has one, but in this single line type connection the ground rod is actually conducting current.
    The Grounding rod actually can have relatively low resistance - back to the pole, ~1 Ohm - but the real issue is that the grounding rod will deteriorate over time - high rate of corrosion due to the current - so this connection needs to be checked and maintained.

    In Urban or more densely populated areas the total load is greater and the utility will try to maintain balanced 3 phase loads in their system, but also one transformer will be feeding many customers - typically the transformers in an area will be matched and to do this well they need a neutral. Also - modern feed is 240/120 ( here int he USA) - split phase - this also requires the neutral.
     
  7. Feb 22, 2012 #6
    This Wiki article explains the system.
    http://en.wikipedia.org/wiki/Split_phase

    You see that the rural residant obtains 2 live wires and a third neutral line that go into his fuse panel. By selection of proper terminals, he/she can have 120 v for lighting and outlets, and 240 v for stoves, dryers just like an an urban residant.
    The neutral line could carry a current depending upon how the electrical balance was set up from the fuse panel for circuiitry in the house or other farm building. A ground wire for safety reasons could be set up from the fuse box to the earth ground.

    The main reason for this setup was cost of one wire versus 2 for a long transmission line. And with one resident per transformer, load balance was not that much of a problem. In urban areas, the reverse is true - it is less costly to set up more wires than have a transformer for each dwelling - imagine 10 transformer units on a pole for a 10-unit apartment building.
     
  8. Feb 22, 2012 #7
    Thank you for your answer!
     
  9. Apr 23, 2012 #8
    Single wire earth return pertains only to the primary (19 kv, etc.) circuit from the substation. Each customer still has a neutral wire (grounded/earthed conductor) in the respective secondary circuit from the pole transformer to the building fuse/breaker panel.

    In a SWER system one pole transformer can also server multiple customers.

    In rural areas the overall amount of current drawn by the primary circuit (at primary circuit voltages) is usually small enough that the voltage drop across the earth return is not too great. If the current draw got too large for the earth conductivity, as would happen in an urban or suburban setting, then the primary voltage and therefore the customers' secondary service voltage would drop noticeably. Because the current draw could vary greatly over the course of a day or even an hour, automatic voltage regulation would be needed.
     
    Last edited: Apr 23, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why Single-wire earth return systems are only used in rural areas?
Loading...