Basic question about home power distribution

Discussion in 'General Electronics Chat' started by epsilonjon, Feb 15, 2011.

  1. epsilonjon

    Thread Starter Member

    Feb 15, 2011
    65
    1
    Hi there.

    Firstly I apologise as this is probably a very basic question. I am okay with DC circuits but we haven't done very much in class on AC circuits yet, so please don't give too complicated answers :p

    We are learning a little bit about home power distribution but there is something I don't understand. I'll refer to the diagram below to make my question clearer. The hot line is kept at 240V and the neutral line is kept at 0V since it is grounded right after the meter? So when you plug something in a socket and complete the circuit, do all the electrons flow round through the neutral wire and into the ground? If so, why do you need to have the neutral wire going back into the meter and back to the power company? Could you not just have a wire going straight into the ground and connect the hot line to this instead?

    I have said "the hot line is kept at 240V" but I don't really understand how this happens, and what is going on in the meter and further back along the line. I suspect if I understood these things I would probably not ask the questions above ;)

    If someone could explain it to me I'd be really grateful!

    Many thanks,
    Jon.

    [​IMG]
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
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    Ah, common misconception.

    The "ground" connecting to the neutral is probably not grounded at the meter, but at the nearest substation. Which could be a few miles away, so the neutral is not necessarily at the same potential as the ground.

    The reason there is a second safety or protective ground is that the resistance of the neutral wire can put it at a dangerous potential relative to the actual earth (think V = IR.) Protective earth does not in an ideal situation carry any current.

    Some companies have experimented with transmitting power through the earth, and it can and does work, especially for rural areas. However, concerns have been raised about the potential for stray voltage in single-wire earth return systems. They are generally considered safe.

    It doesn't really matter if the neutral is 0V; this is all relative. Because the polarity switches 100 to 120 times per second, the hot and the neutral can be viewed as switching roles at this rate.
     
  3. alim

    Senior Member

    Dec 27, 2005
    113
    1
    The circuit is completed through LIVE, LOAD, NEUTRAL, the connection to EARTH (GROUND) IS FOR SAFETY PURPOSES. This is for in the event there is any unsafe condition of any leakage of electricity from the LIVE to the BODY or FRAME of the EQUIPMENT.The mass of earth is not used to carry the NEUTRAL current.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I can't speak for European standards, but in the US the ground is very close to the residence. This is part of the safety requirements for ground, if it is too far away it won't be of much use I suspect.

    From the OPs description it sounds European, 220AC isn't an American standard, though we do use 240VAC for driers and climate control.

    Ground is connected to neutral at the power pole (or transformer), but is kept as a separate wire from there on.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    In the US and Canada, your service neutral is grounded at the service entrance, and at it's supply, typically the local step down trans.

    240vac is derived between two hots, L1 and L2, with the neutral falling(phased) between for 120vac.

    Herein lies the misconception.

    Current is intended to return on the neutral, as ground flow will create a much higher potential.
     
  6. epsilonjon

    Thread Starter Member

    Feb 15, 2011
    65
    1
    Thanks for your replies but i'm still not sure i understand.

    I understand there is a protective earth which does not carry any current, and is connected to exposed metal parts so that current will flow to the ground and not through you.

    But I don't understand why the neutral wire is grounded, if it's not being kept at 0V?

    Thanks,
    Jon.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Well, in a sense it is at 0V, at the point (or points) where the earth connection is made. We could get into a discussion about what "0V" means anyway, but we had better leave that for now.

    The cable will inevitably drop some voltage though, so away from the ground connection there will be some potential on the neutral wire. Even if the neutral is connected to ground in more than one place, eg at the sub-station and at the consumer's premises, the neutral potential will not be identical everywhere.

    Similarly, the live wire is not perfectly "kept at 240V". The sub-station transformer supplies approximately this voltage, but some voltage is dropped in cable losses - the amount will depend on the load current and cable resistance. With a correctly designed system, the customer will receive a supply which is within some stated tolerance values.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    one reason might be is that the neutral is often used as a physical support for the live and insulated transmission wires, and often is without an insulating jacket. This environmental exposure can bring unwanted charges that need to be shunted to earth.
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,165
    From a electrical surge protection point of view it's grounded at the house so in case of a (near) lighting strike the potential of the wiring in the house stays at the same as the grounding point. This house grounding point might actually be at a raised voltage in reference to utility feed ground but because everything in your house is at the same potential there normally will not be current flow between wires inside your home due the GPR, reducing the chance of surge damage.
    http://lightning-protection-institute.com/lightning-protect.htm

    In the case of a direct strike or very near miss (EMP) all bets are off the table on non mil-spec electrical protection systems.
     
    Last edited: Feb 15, 2011
  10. Wendy

    Moderator

    Mar 24, 2008
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    Again, American standards, Neutral and Ground (even though they are connected way back at the power tap) are treated as different wires. Neutral carries current, Ground never does.
     
  11. epsilonjon

    Thread Starter Member

    Feb 15, 2011
    65
    1
    Okay thanks, I think I understand better now. After we learn more about AC circuits hopefully i'll have a more detailed understanding.

    Thanks for all the replies,
    Jon.
     
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