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Why can current flow from power line to earth ground?

  1. Aug 23, 2015 #1
    Take some high voltage power line. Place a conductor between the wire and earth ground. Current goes to ground.

    Why? I understand what is voltage, resistor, circuit etc. I do know that the wire is at a higher voltage than the ground. But isn't earth also a kind of huge resistor? Thus preventing current to flow.

    And when it flows into ground, where does it go? Does it go back to the power plant generator?

    And finally: If current flows, it means the circuit must be closed. What if the power plant generator is isolated from ground? How can there any closed circuit including the ground in such a case?

    This has probably a simple answer, but I can't find it. :)
     
  2. jcsd
  3. Aug 23, 2015 #2

    mfb

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    The resistance is not that huge, but the voltage is of the order of 100 kV. This leads to a significant current flow.
    Simple example: a resistance of 10 kOhm leads to a current of 10 A, which is a power flow of 1 MW, or 10 MW if you go to 300 kV. Enough to evaporate a lot of soil.

    The earth has a huge capacitance - even if you could perfectly isolate it from the whole power grid, you would get significant current flow with AC current.
     
  4. Aug 23, 2015 #3
    Thank you for answering!

    Ok I see for the resistance of earth. Indeed with such high voltages the current is high anyway! In addition I've just looked up resistance of earth (I could have done that earlier...) and it is indeed suprisingly small. I've read 5 ohms for instance.

    However I didn't get the part with the capacitance of earth affecting the current even with isolated generator. Can you explain this to me?

    Also, for the second part of my question, concerning closed circuit, I think I actually said something totally wrong : I wrote that if current flows, then the circuit must be closed. Actually, current can flow without that, it just needs voltage difference...Right? But then, a bird touching a wire doesn't get current, while if earth "touches" the wire, it does...I'm even more confused now. :D
     
  5. Aug 23, 2015 #4

    NascentOxygen

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  6. Aug 24, 2015 #5
    New enlightenment! I'll try to organize a bit this post to make things clearer since I had many questions.

    1. About a non grounded power plant

    This link (found here BTW) explains that, in theory, if the whole circuit (power plant included) is not grounded, then current would not flow to ground when, say, a person touches a wire of the circuit. But in practice, "ground faults" can appear on the circuit (for instance, a tree touching a wire), making unpredictable if the circuit is "safe" (you could touch it without closing the circuit, thus without being shocked) or not, and where it is safe and where it is not.

    The article explains that power plants are actually grounded, so we can always be sure what part of the circuit is safe, and what part is not. It also seems to suggests from the pictures that if current flows to ground, it does go back to the power plant.

    Ok, that would answer my first questions. Except maybe for the current going back to the power plant, still not 100% sure about that...

    2. About current flowing without closed circuit

    In my last post, I wasn't sure anymore that a circuit actually needs to be closed for the current to flow. Let's examine the confusion with two cases: a discharging capacitor, and a bird landing on a power line.

    Case A: a capacitor

    Take a charged parallel plate capacitor. There is a potential difference between each plate. Now connect the plates: current flows.

    Case B: a bird around a power line

    Take a bird at 0 V (with respect to ground) flying above a power line at 100 kV (still with respect to ground). The bird now lands on the wire. The bird is NOT shocked (see the article mentioned above as well). Current does not flow?

    The problem

    Well, what's the difference? In both cases, there is an initial potential difference, and in both cases there is no closed loop. Why is the outcome different in those two cases?

    I would suggest that in the case of the bird, current actually flows from the wire to the bird, BUT it is very brief. Not long enough for the bird to be shocked, just long enough to make the bird 100 kV.

    Seems to be a weird answer, can't be right. :D What is the truth?

    EDIT: If we model the bird has being a resistor in parallel on the circuit (each "foot" being one end of the resistor), then current actually flows continuously trough the bird, but only in a very small amount if the bird has a high resistance. Thus the bird is neither at 0 V or 100 kV. But there is a small potential difference between each of the bird "foot". Is this approach correct?
     
    Last edited: Aug 24, 2015
  7. Aug 24, 2015 #6

    CWatters

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    When the bird lands it's voltage (wrt ground) changes from 0 to 100kV. Some current will flow because the bird has capacitance wrt ground. Don't forget most high voltage lines are AC not DC. Fortunately for the bird the capacitance is small and at 50 of 60 Hz the impedance is quite high.

    Elsewhere on the internet I've seen a comment that the current can be enough to make birds feel uncomfortable so they are rarely seen on 132kV lines and never on 400KV lines. I don't know if that's true.
     
  8. Aug 24, 2015 #7

    CWatters

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    No that's wrong. No current flows because both feet would be at the same voltage1. The bird would only be in trouble if he stepped off the line one foot at a time onto a wet tree branch or metal roof (not that you should build a metal roof that close).

    Note 1: The voltage on an AC power line can be considered the same at every point. At least for distances as short as those between the feet of a bird.
     
  9. Aug 24, 2015 #8

    mfb

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    710 µF, charged with 100 kV and discharged again 100 times per second (120 in the US), gives several kA of current.
    Anyway, a perfect isolation from the power grid is unrealistic.
    I think that was DoobleD's point. The resistance of the wire between the bird's feet is tiny compared to the resistance of the bird, so DC-like current flow through the bird is tiny compared to the current flow in the wire.
     
  10. Aug 24, 2015 #9
    Thank you guys for your answers.

    I think I am missing something when you both talk about capacitance and AC here...All I know regarding AC current with capacitor in a circuit, is that a capacitive reactance is produced, delaying the current with respect to voltage. Does this has anything to do with what you discuss? What is this thing of capacitance with the earth (and the bird?) you are referring?

    It is still not clear to me what happens to the bird.
     
  11. Aug 24, 2015 #10
    absolutely true - the large voltage gradient around the conductor
     
  12. Aug 24, 2015 #11
    the resistance is often sufficiently low to allow significant currents to flow in the earth. Depends on the type of soil, moisture etc. Power systems protection schemes are used to detect this current and disconnect the circuit.

    It goes back to the point where the circuit is (deliberately) earthed. There is an earth point at every voltage level on the system.

    If the electrial system is completely isolated from the earth, a single earth-fault will not see any current flowing through earth because there is no circuit. A second earth fault will. This is one of the reasons power systems ARE earthed. You want ANY fault to be detected and the circuit disconnected. If it was not earthed, a fault would go undetected.

    Some circuits are isolated from earth for safety reasons - obvious example being SELV circuits. Other examples include circuits in explosive atmospheres (some mines).
     
    Last edited: Aug 24, 2015
  13. Aug 24, 2015 #12
    its a particular problem for birds with large wing spans, such as raptors - many get killed by power lines because their span is big enough to touch one conductor and earth. or two conductors.
     
  14. Aug 24, 2015 #13
    Thanks William, this confirms the answers to my initial questions! :) Any idea regarding what is happening to the bird?
     
  15. Aug 24, 2015 #14
    Simply (and one can over complicate things) current flows between two points when there is a potential difference between those two points.

    If the bird's feet are at the same potential, it does not get zapped.

    If the bird opens its wings, and touches the earthed-steelwork of the tower carrying the conductor, then there is a potential difference between its feet and its wings, and it gets zapped.


    They don't like EHV conductors because the air immediately around the conductor is charged. This gives a very steep voltage gradient around the conductor. As the bird gets closer, it starts to feel this (just like you get a tingle if you lick a 9V battery!) and flys away.
     
  16. Aug 24, 2015 #15
    So the bird cannot be modeled as a resistor in parallel when it has only the two foot on the same wire? I find it hard to believe. No current at all through it, even a tiny? Crappy drawing below. :D (Haven't draw transformers etc for simplicity)

    202619IMG20150824192040.jpg
     
  17. Aug 24, 2015 #16

    Nugatory

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    Sure it can - it's just that the voltage difference between the wire under the left foot and the wire under the right foot is fairly small, so the voltage across the bird/resistor is also small.
     
  18. Aug 24, 2015 #17
    So there is a tiny current? Even too small for the bird to actually notice.
     
  19. Aug 24, 2015 #18
    the bird is connected at two points...what is the potential difference between these two points?

    just say (for arguments sake) the volt drop along the conductor is 0.5mV per metre and the distance between the birds feet is 2cm

    the potential difference between the birds feet is then 10 uV.

    now say the birds resistance is 100 ohms (I dont know what it is, it could be thousands)

    so the current flowing is i = v/r = 100 nA. This is about as close to zero as you can get
     
  20. Aug 24, 2015 #19
    All right I think I get it. :)

    Also, even if the wire would be a perfect conductor, without any potential drop, there would still be a (very) small potential drop between each foot I think. Since the other parts of the bird body act as a (huge) resistor.
     
    Last edited: Aug 24, 2015
  21. Aug 24, 2015 #20
    for simplicity, along an electrical conductor, the potential difference between two nearby points is zero.


    There are regulations which minimise the volt drop. In the UK a 230V supply can vary from 253V to 216V : a drop of 37V along the entire conductor (for those of us that live close by to the transfomer, we blow a LOT of light bulbs!. My measured voltage is 250V!)
     
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