Earth, dirt as a conductor

Discussion in 'General Electronics Chat' started by antennaboy, Nov 12, 2009.

  1. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hello Forum,

    for current to flow in a circuit and loads to operate the circuit needs to be closed with two wires, one for the current to go and the other as return wire.

    I have seen application (telegraph) where only one conductor is used. The other conductor is represented by earth.... Is earth a good conductor?
    If wet, maybe. I guess there is also a lot of resistance at the connection between the load and earth......

    If earth can behave as a low resistance conductor, why is it not used in power lines?

    thanks,
    antennaboy
     
  2. studiot

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  3. Duane P Wetick

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    Apr 23, 2009
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    It is used in power transmission. I worked at facility that generated AC power (from coal burning), rectified it to DC +300kV and -300kV and this two wire transmission crossed the state where it was transformed back to AC. The reason for this is that AC towers were too expensive to build, whereas the simpler DC 2-wire towers could be spaced much further apart, thus saving considerable construction cost.
     
  4. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hi Duane,

    thanks for the reply. I am interested in understanding better the application you worked on.

    So AC voltage was generated and converted to DC. What do you mean +300kV and -300kV?

    Also, do you mean that two wires, one for the +DC and other for the -DC were used instead of 4?

    Last, in what sense were AC towers more expensive? Could they have not used just one wire to send the AC and earth as return?

    thanks
    antennaboy
     
  5. thatoneguy

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    Feb 19, 2009
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    He's referring to +300,000 Volts, and -300,000 Volts, with respect to Earth/Ground.
     
  6. KL7AJ

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    Nov 4, 2008
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    Keep in mind that soil conductivity is a COMPLEX number...containing both a dielectric constant and a resistive component. This doesn't mean a whole lot for transmission line frequencies, but it's HUGELY important when you're working with R.F.! Not only that, but this complex number can change DRASTICALLY with depth (especially up HERE!)


    Eric
     
  7. Bernard

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    Aug 7, 2008
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    A small boy was electrocuted from ground current due to insulation failure on stadium lighting; if he had lifted one foot, might have survived. Ground not a good conductor for high power from a safety consideration. I did operate a two party , 1/4 mile, one wire & ground, telephone system, 1942.
     
  8. studiot

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    Eric is correct in mentioning that we normally talk about conductance rather than resistance in relation to the earth.

    You may be interested in the following information.

    The conductivity of the earth increases with depth, being of the order of 1/1000 Siemen/metres near the surface and increasing to 1/100 S/m in the middle of the crust and 1 S/m at the junction of the crust and mantle. It reaches 10 S/m in the lower mantle by the mantle/core interface. Values in the core are not known with any reliability.

    (after Condie 1976 -1997)
     
    Last edited: Nov 13, 2009
  9. ELECTRONERD

    Senior Member

    May 26, 2009
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    Tesla used the earth as the negative conductor in one of his projects. As most of you know, he was trying to put AC through the air without wires. He succeeded to some extent, but not very far. What he would do is take a light bulb and simply let the negative ternimal touch earth and it would light up. The positive AC was flowing through the air and he used the Earth as a negative conducter. You can find a great video on him if you google "The Missing Secrets of Nikola Tesla", it's well worth the time to watch.

    Austin
     
  10. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hi Austin,

    that seems to be a magic result. Before I go watch the video, let me make sure I get what you are saying.

    Tesla connected one terminal of a light bulb to ground and left the other free in the air.

    An AC battery somewhere had one of its terminals connected to earth as well, the other free. It behaves like an antenna.

    The light bulb behaves like a receiving antenna and lights up....
    It that what happens?

    thanks
    antennaboy
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you have enough RF energy around, it's easy to light discharge lamps. NE2 neons and fluorescent tubes as an example.

    One test for leakage on the USS Enterprise, back when the SPS-33 billboard radar was operating, was to open one of the several driver drawers and hold a fluorescent tube over it. If it did not light up, everything was operating properly.
     
  12. davebee

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    Oct 22, 2008
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  13. Darren Holdstock

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    Feb 10, 2009
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    This artist used a bunch of neon tubes stuck into the ground under HT pylons to light up a field near Bristol, UK.

    I've done work designing instrumentation to measure earth resistance, mostly for Safety Earth validation rather than power distribution or comms. Soil resistivity is a complex business, but has to be quantified as there are legal requirements to meet in terms of Earth resistance and current-carrying capacity. Google "Earth Resistivity" for more.

    During the course of this work, I found out that all these years I've been saying "homogenous" when I should have been saying "homogeneous". I blame milk.
     
  14. studiot

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    Nov 9, 2007
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    Please let's get this straight.

    The earth (planet) is not a return conductor, nor can it be so used.
    Period, point, full stop.


    'A conductor' begs the questions where from and where to?

    The whole point is that the earth absorbs the charge (current) without appreciably changing its own potential.

    It does not conduct it to anywhere.
     
  15. thatoneguy

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    Feb 19, 2009
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    That is a cool picture.

    Are there any more pitchers of it, or just that one? I'd like to see it in daylight, and how deep did he "plant" the other end of the flourescent tubes?
     
  16. davebee

    Well-Known Member

    Oct 22, 2008
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    Interesting point, studiot.

    If the earth were split into two hemispheres separating the generator from the consumer, but with the single supply wire still connecting the two, would you claim that the system would continue to operate exactly as before?
     
  17. studiot

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    Not quite sure what you mean here?

    If it is a supply wire how does it connect the earthed hemispheres?
    By definition anything connected to an earth is at earth potential.

    If you mean that there is one long supply wire, spanning but not connected to the hemispheres, with currents drawn through loads to each hemisphere, then yes in principle this would work.
    However in time the potentials of each hemisphere would drift apart.

    You can have more than one earth in a circuit, on this principle.

    We actually do this in circuit diagrams, although we normally mean the same one by the symbol.

    If you want more, look in an eletrical wiring textbook about PME (protective multiple earths). Connections to the planet may often be considered as if they were on separated hemispheres.
     
  18. peranders

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    May 21, 2007
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  19. Darren Holdstock

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    Feb 10, 2009
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    @thatoneguy, here you go, BBC link. Not up to the BBC's usual journalistic standards, but enough to be googling from. What would have been even neater, I reckon, would have been to turn each tube into a relaxation oscillator by strapping a 300 V cap across it, then they'd all be merrily flashing away.

    Good info from studiot there. Back in the days when hobby electronics magazines were worth buying/available/in existence (delete as applicable) there was a great Earth Charge Recorder project in one of them. Plus some (very old) info on ground current signalling. Not everything is on the Internet yet...
     
  20. davebee

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    Oct 22, 2008
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    I'm still having a hard time accepting that the earth is not a return conductor.

    Near the earth connections of a single-wire power system, I'd imagine there would be currents spreading out into the earth like roots of a tree.

    So if the two connections were close enough, I don't see why there wouldn't be a measureable current through earth from one to the other, in the sense that you could measure a voltage between two testing rods hammered into the earth between the power station generator and the consumer.

    As distances increase, the measureable current in any section of earth may become so low as to be unmeasureable, but it seems to me to be a stretch to go from that case to the statement that return currents simply do not exist.
     
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