Confused About Ground in AC Circuits

Discussion in 'General Electronics Chat' started by davertron, Oct 22, 2012.

  1. davertron

    Thread Starter New Member

    Oct 22, 2012
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    So after reading this page, I'm pretty confused. This seems to only make sense if the circuit is DC, right? If the circuit is AC, then the polarity is flipping constantly, so wouldn't it mean that the side of the circuit with the ground is only safe half of the time? I know that that doesn't make sense, but I don't understand what I'm not understanding.

    Also, my father is an electrician, and we were talking about electronics today, and he said "unless there's a fault in your circuit, you can NEVER get 'bit' off the neutral line because there's never power on the neutral line". This also doesn't make sense to me; wouldn't both sides of the AC circuit be "hot" half of the time? I think I must be missing something pretty fundamental here but I can't figure out what it is. :(
     
    Last edited: Oct 22, 2012
  2. davertron

    Thread Starter New Member

    Oct 22, 2012
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    So I hate to try and answer my own question, but perhaps the explanation is this:

    The "neutral" wire is only safe BECAUSE it is connected to ground. Since it's connected to ground, it's electrically common with ground, which basically means there is no difference in voltage between neutral and ground. Since there is no difference in voltage, it's basically "the same" wire. Therefore, that side of the circuit is always safe.

    However, if you were to remove the ground from that side of the circuit, there would effectively be no difference in either side of the circuit if it were AC, correct?
     
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  3. Stuntman

    Active Member

    Mar 28, 2011
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    This is a difficult concept to master, so let's make some statments:

    Remember that "ground' in this particular case means Earth Ground, hence why your home should have one (or two) 8 foot rods driven into the earth and then attached to your electricity meter or breaker box.

    Safe AC designs assume that you (the person) are either not strongly attached to any potential (IE, not holding a 50V DC line), or strongly grounded to earth (disregard the former if it doesn't make sense). This means that if you, the person, are connected to earth ground, and the neutral conductor of an AC system is connected to ground, there should never be a large potential between you and the neutral conductor, therefore, no shock if you were touch that conductor.

    The "hot" wire of an AC goes from positive potential (WRT to earth ground) to negative potential (WRT to earth ground) in a full wavelength. So, you as a person (connected to earth ground), will get shocked from the "hot" lead. I believe the reason you are having trouble comprehending this is due to the verbage you used "polarity is flipping". No, the neutral line never "changes" potential (with respect to you, or the earth), the hot lead just goes negative to positive. You, as a bystander, won't know the difference between negative voltage and a positive voltage, either will shock you, just he electrons will be flowing opposite directions.

    Finally, although your father is partially right, he is not completely right. Yes, if wires were perfectly conductive, you would never see the neutral line "float" above ground. And in practice, only in a poor electrical system would you see the neutral line voltage float enough to truly shock you. However, do you notice that third prong on many standard electrical outlets around your home? That is a third wire that ties directly to earth ground (not neutral). Why not just tie that third prong to neutral? You guessed it, because neutral is not always perfectly grounded.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    The drawing in that chapter was made for illustrative purposes only and happens to be a poor example of reality.

    No. To say that the circuit is safe half the time is incorrect. The situation remains the same for both DC and AC.

    The NEUTRAL and GROUND wires are NOT the same.
    The NEUTRAL wire is used to conduct the return current. If there is a fault in the NEUTRAL wire one can experience an electrical shock when in contact with the NEUTRAL wire.

    GROUND is a SAFETY GROUND. No current ought to flow through the GROUND wire.

    Your father is partly correct. The NEUTRAL wire is at earth (GROUND) potential.

    Edit: Stuntman explains why your father is only partly correct. There is current flowing in the NEUTRAL wire.
    If there is current flowing, there must be a potential difference between the NEUTRAL wire where the load is plugged in at the outlet and where the NEUTRAL wire enters your distribution panel.
     
    Last edited: Oct 22, 2012
  5. Stuntman

    Active Member

    Mar 28, 2011
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    You are getting closer. Yes, the neutral wire is safe because it is connected to ground. And yes, if you removed this wire from ground, there would be no difference between the two leads. This is the theory behind an isolation transformer.
     
  6. davertron

    Thread Starter New Member

    Oct 22, 2012
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    Well, he actually said the same thing as you: theoretically it should be safe, but in the "real world" it in fact could have the potential to shock you.

    The main thing we kept arguing over was that he kept saying "there is no power in the neutral wire, so you can't get shocked by it". I kept saying that I believe that was ONLY because neutral and ground were common; if you were to remove the ground, then there shouldn't really be a difference between neutral and hot in an AC circuit, since the potential voltage between the circuit and ground would presumably be different, and therefore you could receive a shock if you were to touch the circuit on either side.

    I think the main thing I was getting hung up on was that for electricity to flow, there HAS to be a potential difference in voltage. Since the neutral wire and ground wire are electrically common, there should be no potential difference, and therefore no reason for electricity to flow. I was thinking that there's current flowing through the circuit in both directions depending on the AC cycle, and that that current is what would get you on the neutral side even if there was ground on that side, but I can see now because of the lack of potential would stop the current from flowing from neutral to ground.
     
  7. davertron

    Thread Starter New Member

    Oct 22, 2012
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    Yes, I'm aware that neutral and ground are not the same. I think my issue was mainly that I was not thinking about voltage and the potential between neutral and ground.

    However, several times I told my father that I thought the only real difference between "hot" and "neutral" was the fact that the neutral side was connected to ground, and that if you were to remove ground from the neutral side there would be no difference to the two lines. He kept telling me I was wrong, and that there was only power flowing to the hot side, so it was not possible to get shocked from the neutral side.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I think it's time to cut dear old Dad some slack and stop using him as a source of theoretical information. You say your "father is an electrician, and we were talking about electronics today." Would you ask a boxer about wrestling? Electricians don't use theory to design devices, they apply standard solutions to problems to produce safe and effective power sources. It's very involved: try scanning the National Electric Code (NEC) to see how to do anything and see how far you get.

    As far as the power goes your Dad may be using the word a little loosely (as a synonym for current) but again he's not completely wrong. Unless he works for the power utility company he can't "remove ground from the neutral side" your question is nonsense.

    I believe you have the concept of neutral and ground correct. Yes you can take an ungrounded AC source and ground either side to create a neutral.
     
  9. davertron

    Thread Starter New Member

    Oct 22, 2012
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    I don't think my question is nonsense at all. I'm just trying to understand how grounding one side of an AC circuit works. I wasn't asking my father to remove the ground; I was discussing theoretically what would happen if the ground were not there.

    I'm not trying to prove my father wrong, that's certainly not the point. The point is for me to get a better understanding. Perhaps when I said we were arguing I made it sound more combative than it was. We were discussing it because it's obvious I don't have a fundamental understanding how this stuff works, that's all.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Imagine a center-tap transformer:

    [​IMG]
    Both sides of the secondary would be live. So obviously you cannot ground either of these.


    Now imagine a transformer with no center tap:
    [​IMG]
    You may connect either side to GROUND.
    In this case, one side is live while the grounded side is "neutral".
    Electrical power from your electricity supplier looks like the second example.
     
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  11. sdowney717

    Member

    Jul 18, 2012
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    Think of how a GFCI device works. It measures the difference in the current going in and the current coming back. If the difference exceeds a set amount of leakage current, then it trips off the circuit to help keep you from dying. With GFCI no ground is needed for it to function.

    If you think of utility power as a generator, look how they are usually wired. To make it safer, the gen frame as ground is tied to the neutral wire. Then the ground wire is run to an appliance in the wire bundle and the ground attached to a metal exterior part. If the hot wire touched the metal it will energize the device exterior until the current flow trips a breaker and that will likely happen very quickly. but perhaps not quick enough to keep your heart from stopping.

    Some generators are not bonded neutral, so the neutral 'floats'. So for 120v AC gen, they have voltage of 60 volts on neutral and 60 volts on hot = 120 volts total. Each wire is hot. But can you get shocked holding onto the neutral? NO, only if your body can make a return circuit back to the source of power.

    If you grab a 120 VAC hot wire, and your body is not able to conduct any current back to the gen source, your not shocked.

    On my boat this summer. The AC water pump which pumps seawater developed a wire fault which partially energized the exterior metal of the AC compressor and the water flow into the ocean. When I touched the flow, I got a small shock so I must have been able to complete part of the circuit. I found that a wire taped connector had gotten salty water and was very damp and laying on the damp surface inside the metal connection box of the cruisair heat pump. That was enough to create a small shock hazard but not enough to blow the breaker. Something like that issue would be great to use a GFCI.

    The water exiting the boat from the pump had voltage on it and the brackish water of the slip conducts current back to shore ground since the utility power is grounded to earth. Somehow my body conducted current from the water flow back to an earthed ground which could have been on the boat, something I was close to or in contact with. I found that the Cruisair for its ground uses the copper freon tubing. Even so the current leakage can still flow in multiple paths including me. The leakage did not all flow back threw the copper pipe. If it did I dont think I would have felt a shock. Maybe that ground has an issue where the supply wire from my breaker panel connects to the control unit.?
     
    Last edited: Oct 23, 2012
  12. davertron

    Thread Starter New Member

    Oct 22, 2012
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    Yeah, I think I get at it at this point, but correct me if I'm wrong: there has to exist a voltage difference (or potential) between two points in a circuit for current to travel through that point in the circuit. The main reason for putting ground on one side of a circuit is to try to eliminate any potential between that side of the circuit and ground, because humans are effectively always grounded. If there was no ground on the "neutral" side of the circuit, you would still be able to touch that side of the circuit and not get shocked as long as there was no path back to the power source through ground. But since it's hard to know FOR SURE that there is no accidental path back to the power source through ground, we intentionally ground one side of the circuit to make it "electrically common" with ground just in case.
     
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Fundamentally correct. We ground one side (and call it neutral) to insure what voltage is on both these lines: without grounding one side then either side can be any voltage with respect to ground. Floating voltage sources can be extremely dangerous.
     
  14. ramancini8

    Member

    Jul 18, 2012
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    You seem to forget that plus 120V kills as fast as minus 120V, so polarity doesn't enter into the discussion.
     
  15. JMW

    Member

    Nov 21, 2011
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    If you remove the ground from the neutral, you will have 1/2 voltage on each leg. Remember you must also disconnect the ground at the transformer nearest your home. "Neutral" is not a good term it should be called return. Where you will run into problems is with small portable generators. These will have 1/2 line voltage on each leg. Inexpensive inverters do the same thing. Neither device should be wired into a home circuit, for obvious reasons. Where are floating systems in use? The marine industry is primarily "floating" grounds. USN, USCG and US Army (more boats than Navy and Coast Guard combined) go to great lengths to make certain there are no "grounds" on the system. Even with all there efforts line to line, and line to hull capacitance are sufficient to be lethal.
    Any of the above can be verified using one of those polarity detectors.
     
  16. electron_prince

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    Sep 19, 2012
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    What will happen if his body has less resistance than resistance of the connecting wires?
     
  17. JMW

    Member

    Nov 21, 2011
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    It will follow Kirchhoff's Laws
     
  18. electron_prince

    Member

    Sep 19, 2012
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    I don't mean that. I want to say that most of current will then flow through his body to ground and he'll get shock. There must be some potential difference between ground and the "neutral". But I assume that the potential difference is too low to generate a reasonable amount to current through us (because human body resistance >> resistance of wire).
     
  19. JMW

    Member

    Nov 21, 2011
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    The "neutral" wire should have close to 0 volts. However, the current flow will equal that of the "hot" wire. Want to prove it. Make up a short length of extension cord, with black white and green wires and a male and female plug. Connect this to a toaster or some other moderate device. Using a suitable AC ammeter (clamp on is preferable), measure the current in the black wire, then the green wire and finally the white wire. The green should be 0 amps. The black and white should read the same. Now measure voltages placing the black meter lead on the green wire. Black should read 123 or so, and white close to 0. As you've demonstrated above, even though the white is near 0 VAC, it is still carrying substantial current.
     
  20. SPQR

    Member

    Nov 4, 2011
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    I think this is an absolutely fantastic question - thank you Davertron for asking it.


    The concepts of "Hot" (black) and "Neutral" (white) in my opinion, were constructed to make it easier for people to be consistent in their wiring of houses. "The 'hot' always goes here, and the 'neutral' always goes there.".


    To an oscilloscope, the "hot" and "neutral" wires are equivalent in the sense that in an AC circuit one line is positive for 1/120th of a second, and the other line is negative for the same time (not counting the sine wave crossing zero), then the wires change polarity - 60 times per second.


    The real difference between "hot" and "neutral" are beautifully stated in the transformer examples of MrChips (I've never seen that analogy, and it is superb!), and the only difference between the hot and neutral is the presence of the "ground" attachment to one of the lines.


    Years ago when I started seeing the wall sockets with the differences in slot height, I chuckled to myself, because the increase in height of the left slot corresponds to the larger left plug of the male component (when it exists). But that is another way to assure that electrical "stuff" is connected to the backbone in a consistent way.

    NICE article that explains the colors and the sizes of the slots of a plut.
    But even they aren't completely accurate - note the sentence:
    "In a house outlet, power flows from hot to neutral" - they are right for 1/60th of a second, then they are wrong for the other 1/60th of a second.

    Again, thanks for your question.
     
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