Ground vs Neutral

  1. I understand that in an electrical outlet, "ground" is wired directly from the outlet to the ground, whereas neutral is wired from all the outlets to the breaker box or wherever in the house, then to the ground, and that neutral is intended to carry current.

    That being said, I am confused about the following line in my textbook: "This leads us to the major point of the discussion: inside the house or the laboratory, the neutral wire is NOT AT GROUND POTENTIAL... When the neutral wire is carrying current, it can't be at ground potential." I don't see why that is true. If I were to draw the circuit, there would be one wire at potential between -120V and +120V, a resistor, and another wire carrying the current to ground with potential equal to ground. What am I missing? Is my picture overly idealized?
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    The current that is flowing in the Neutral wire causes a voltage drop across the resistance of the wire. This can cause an AC voltage of several volts when you are drawing large currents down a long run of wire. Makes sense?
     
  4. Yes. That makes sense to me. So in the perfectly idealized situation (or if we used a superconducting neutral wire), there would be no voltage drop, correct? (But there obviously is, in the real world.)
     
  5. berkeman

    Staff: Mentor

    If there were no resistance, there would be no voltage drop along either the Hot or Neutral wire, so the Neutral wire would always be at ground, by virtue of the tie at the Breaker Panel.
     
  6. Ok. So in that situation, all of the voltage drop would occur over whatever appliance we have plugged in, and the neutral wire would be exactly the same as earth ground always. Correct?
     
  7. berkeman

    Staff: Mentor

    Correct-a-mundo. :smile:
     
  8. Drakkith

    Staff: Mentor

    Berkeman, if the neutral isn't at ground potential, what does it have potential relative to? It should have potential relative to the hot wire, but what about relative to ground? I was under the impression that it was at ground potential, otherwise current would flow through the neutral wire to ground, which would lead to a mismatch in current flowing through the hot wire and neutral wire.

    Is that right or wrong?
     
  9. berkeman

    Staff: Mentor

    It's easiest to explain with a diagram. Let me see if I can find something or scan a sketch...
     
  10. berkeman

    Staff: Mentor

    Neutral and Earth Ground are tied at the Breaker Panel. There is some resistance in the wires as they run out to the load. When current is flowing out to the load, you get some voltage drop across the wires (call it 1VRMS), and the rest of the source voltage drops across the load.

    The whole electrical distribution system of the house is floating because the output of the transformer is floating. The only reference happens at the panel where Neutral is tied to Earth. So you won't have any current flowing in the Ground wires -- the only time you get current in the ground lead is if you have a Ground Fault, which is described pretty well at the website below where I got the picture:

    http://www.thecircuitdetective.com/hot_neutral_ground_wires.gif
    [​IMG]
     
  11. As from my own experience in wiring houses, all three wires run from the breaker box to an outlet together, in fact, they generally come in a roll encased together. The grounded wire goes to/from the breaker box along with the hot and neutral. In many cases, a single room is on a single breaker. This is done by running the three wires from one outlet / light to another, and in these cases the ground does not directly go to the box/ground outside. It goes to all the other outlets between the outlet and the breaker box in the circuit, and then to the breaker box, and then to the earth. The grounds in a circuit are all wired together, so I suppose you could say it is a direct route to the earth, but this gets confusing.

    There is a physical pole buried in the ground in my yard which is connected to the ground in my breaker box. The neutral is intended to carry current, and the ground wire is basically an added safety. Many old homes did not have 3-prong AC outlets


    Just so we're clear, I am not a pro electrician. So I might be wrong on some points.
     
    Last edited: Jul 14, 2014
  12. berkeman

    Staff: Mentor

    Looks correct to me! :smile:
     
  13. Drakkith

    Staff: Mentor

    Berkeman, I'm not seeing how your explanation touches on the OP's question about the wire not being at ground potential when current flows through it. Have I misunderstood something?
     
  14. berkeman

    Staff: Mentor

    I think we got it answered and the OP agreed. Then he just went on to ask about "thought experiments" to make sure his understanding was correct. Am I missing another part to his questions? Or if the explanations so far don't make sense to you, let me know and I'll try to post more info. :smile:
     
  15. sophiecentaur

    sophiecentaur 13,541
    Science Advisor
    Gold Member

    There is no current flowing through the earth conductor. It is at earth potential.
    There can be many Amps flowing through the Neutral (return) so there will be a PD across the length of it due to its resistance. Hence, at the room socket, there will be a potential difference between the Neutral and earth pins.
    In the UK (totally different system from the US), the Neutral goes right back to the supply transformer, which can feed tens of homes off each of its phases. There can be a massive imbalance of current which will produce a significant PD on the neutral cable. Es ist verboten to connect neutral to earth in the home. The supply company do that back at the transformer.
     
  16. Drakkith

    Staff: Mentor

    Hold on... are we talking about a potential difference along the length of the neutral conductor, or about whether the neutral conductor is at ground potential? Or are these one and the same?
     
  17. sophiecentaur

    sophiecentaur 13,541
    Science Advisor
    Gold Member


    One end of the neutral is at ground potential. If there is current flowing through it, the other end will not be. V=IR
    0.05Ohms, 30A gives 1.5V
    The idea is that the Earth wire carries no current - except in a fault.
     
  18. Drakkith

    Staff: Mentor

    Oh, of course. This is just like grounding a DC circuit. The part of the circuit at the ground is at ground potential, but if we measure the voltage further away from the ground, it will have some finite voltage with respect to ground. In neither case will current flow through ground, but there's still a voltage between ground and any part of the circuit. Right?
     
  19. sophiecentaur

    sophiecentaur 13,541
    Science Advisor
    Gold Member

    It's just a potential divider chain with mains Volts across the whole thing.
     
    Last edited: Jul 15, 2014
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