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Generating station return path

  1. Mar 21, 2013 #1
    They state that the earth ground cable connecting ground to the neutral wire inside the transformer is a feedback path for electrons. From that information I would conclude that without it the circuit would not be complete and current wouldn't flow.

    Then they follow by saying that the use of a ground cable is a safety measure. I have read that in other forums that no current normally flows through the earth ground cable unless there is an emergency situation such as a voltage spike.

    These are contradictory statements..

    I think this will answer my question. If we were to take out the earth ground wire, would all the circuitry would still operate? I am thinking yes, the circuits would still operate because there is the neutral and -120V lines that always act as return cables back to the transformer. It would just be more dangerous without the earth ground cable. Right??

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  2. jcsd
  3. Mar 21, 2013 #2

    jim hardy

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    1. The circuits will still work.
    In household wiring neutral wire and earth wire are not the same.
    In USA Earth is green, Neutral white.
    Europe uses different colors, even here I find brown and blue inside some power cords from thrift shops.

    2. That book is misleading but not totally wrong.

    Every time you go through a transformer you establish a new "ground" ( I call it EARTH for clarity).
    The neutral wire leaving the transformer is tied to earth.
    The neutral wire returns whatever 'unbalance' current may need to return to the transformer.
    If your transmission system is balanced you have no return current - look up "three phase power" .
    So the neutral wire where it's still three phase can be small.

    The power plant itself is connected through a transformer and is three phase so it needs no return path through earth.
    I would wager that book's author was in a hurry and did not think that paragraph through.

    Systems that use earth for return DO exist - years ago it was done in rural settings, but it tends to shock cows and is increasingly rare.

    I am told it is still done in High Voltage DC systems that operate at ~a megavolt. But I have never worked around one of those so cannot say for sure, perhaps somebody else has experience there..

    old jim
     
  4. Mar 21, 2013 #3
    I do not like how this section was written, it makes statements that are not needed to discuss the issue - so agreeing with Jim- it is misleading.

    The Ground - in almost all cases is a safety related addition to the system, it should carry no CURRENT in normal operation. (Personally I do not like referencing electrons in power, we do not measure them directly and it causes the reader to think about meaningless details for this level of discussion).

    In normal operation the Neutral is the return path for current.

    Also "to prevent "charging up of machinery" -- ugh, not it's primary focus ( but an important one) and the machinery is in the residence. This is discussing the ground of the step-down transformer. If it showed a ground rod for the residence, which there should be, that would be more applicable, but still not the primary purpose. The grounding of the transformer - and properly also at the residence - is to ensure there is no shock hazard when touching the electrical devices in the residence.

    As drawn - if a device in the residence malfunctions - for example one of the 120V lines comes in contact with the device's surface, think outside of the washing machine - there is no solid connection shown between the device ( washer) and Ground - so depending on the essentially random arrangement of the whole system the surface of the washer is now at 120 - and the floor or water pipe that goes into the earth is at the "ground potential" - a person could be shocked.

    With out proper grounding system there is no guarantee that the surface of the devices are safe to touch.
     
  5. Mar 21, 2013 #4

    sophiecentaur

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    Any book that talks of electrons 'going' anywhere in an AC circuit and uses the term "feedback path" is hardly an academic masterpiece. In that house wiring diagram, there is no mention of a safety Earth connection between appliances with a metal case and with pipework. Is that not a normal requirement in US installations?
     
  6. Mar 21, 2013 #5
    Hello again MotoP - not to be too harsh - but this is not a "Generating station return path" - question. Go with AC Source Ground Return - or something of that nature.
     
  7. Mar 21, 2013 #6
    The grounding stakes driven into the earth are only for safety, they carry no load current. Should a hot wire short to ground then fault current is carried by the wire and through the earth. Grounding the power system keeps all wires in the grid at a known potential wrt earth.

    Neutral wires in the 240 volt center tapped single phase household supply do carry current. The unbalance in current between the 2 outer legs of the 240 volts is carried by the neutral. In 3-phase ac power systems, the neutrals of Y-connected transformers, and the earth, and the overhead ground wires for lightning protection do not carry load current. They conduct current during a fault or lightning stroke.

    Even if the 3-phase power is severely unbalanced, the system is designed so that the neutral carries virtually no current. Each transformer has 1 or more windings connected in delta which keeps the line & phase voltages well balanced even when the load currents are very unbalanced. A small fraction of the unbalanced current does conduct in the neutral, but a high impedance path (neutral conductors with their large inductive reactance over miles of wire) in parallel with a low impedance path (local leakage path in delta winding) still conducts a small fraction of the total current.

    Does this help at all? BR.

    Claude
     
  8. Mar 21, 2013 #7

    jim hardy

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    Last edited by a moderator: May 6, 2017
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