Question about the grounded neutral

Discussion in 'General Electronics Chat' started by RickinFlorida, Dec 28, 2008.

  1. RickinFlorida

    Thread Starter New Member

    Dec 28, 2008
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    In a typical split-phase AC circuit, one leg is grounded. I understand why it is. What I don't understand is why grounding one leg of an AC circuit doesn't short circuit the entire circuit. In other words, why don't all available electrons in the circuit, including from the hot leg, rush to the nearest ground electrode? Why would they continue to flow through the circuit, through the house and back to the ground tap on the distribution transformer? There are other grounding opportunities in the house for electrons to travel to. These grounding opportunities offer a low impedance to the flow of electrons.

    Second, what would happen in your basic household AC circuit if both legs were grounded instead of just one? How would that affect the operation of appliances? Since we ground one for safety, why not ground both?

    Thanks!

    Rick
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    It would be a short lived show. BTW, I am not sure by leg whether you meant L1 or L2; neither of them is grounded.

    Back to your original question. Consider typical DC wiring in airplanes, cars, etc. There is only one hot wire. The chassis is ground. Making the hot wire AC doesn't change that situation.

    In homes, one could in theory run only a black wire, and ground all appliances at the point of use. That of course is not code and would be inconvienent at best to get a good ground at each appliance, so we run a white (grounded wire).

    John
     
  3. Wendy

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    Mar 24, 2008
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    There is also the issue that without the neutral wire any other path is high resistance (enough to kill you, but not enough to run an appliance, Murphy's Law in action). Generally house wireing has three wires, Hot, Neutral (source and return), and Ground (for safety).

    The 220VAC going into homes is a 2 phase system (the L1 and L2 referred to earlier), each leg by itself is actually a 120VAC system. Ground is usually created where the power company introduces power into your home, and is connected to neutral at this point.
     
  4. SgtWookie

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    Jul 17, 2007
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    Actually, I think you're talking about home wiring.
    From the power company's transformer, you get three wires; L1, L2, and Neutral, the latter of which is the center tap of the transformer's secondary, while L1 and L2 are opposite ends of the secondary winding. Between L1 or L2 and Neutral, you'll measure 120VAC, but between L1 and L2 you'll measure 240V. That is because L1 and L2 are equal in voltage, but opposite in phase (180° out of phase).

    Neutral is connected to earth ground at the breaker/distribution panel for safety. This is to ensure that the voltages don't float at some arbitrary high level. The breaker/distribution panel is the ONLY place where ground and Neutral should be connected.

    If you happened to place a short circuit in one of your outlets, the power would go from the hot lead through to neutral, and rapidly trip the circuit breaker due to excess current.
    You mean light bulbs, appliances, etc? Well, they limit the current flow through themselves according to their power rating.
     
  5. PRS

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    Aug 24, 2008
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    I think both L1 and L2 are in phase with one another. Tha way, adding them doubles the voltage. If they were 180 degrees out of phase with each other they would add to zero volts.
     
  6. blocco a spirale

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    Jun 18, 2008
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    If L1 and L2 were in-phase with each other there would be no potential difference between them.
     
  7. jpanhalt

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    It is called split-phase --a term that has caused endless discussion and confusion. I have heard knowledgeable people call it "essentially" two phase, which to me makes it easier to grasp.

    [​IMG]


    John
     
  8. kdcouture2000

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    Dec 6, 2008
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    In House wireing there are normally three holes in a socket, and appliances usually have three prongs. The two prongs at the top are for the power to an appliance. The third prong at the bottom is to a G.F.I (Ground Fault Interuptor). Basicly if there is a short in an appliance where the case of a power supply for instance is energized, electricity will flow to the (GFI) and trip a breaker to that plug so you don't die. :eek:
     
  9. PRS

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    Aug 24, 2008
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    I was taught that sinusoids of the same frequency add algebraically only when they are in phase, if out of phase you use vector analysis. If 180 degrees out of phase their maginitudes subtract. For example if one source was 120 volts and it was added to a source of 120 degrees and both are in phase, then you get 240 volts, and if out of phase you get zero volts.
     
  10. PRS

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    Your analysis is gimicky? put plus signs at the top of each source, negative at the bottom. Without splitting them yet, we have 240 volts between the positive terminal of the upper source and the negative terminal of the lower source. This is how we get 240 volts in the home. To get 120 volts we split them between the sources with a ground. This gives us two legs of 120 volts each.
     
  11. Wendy

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    Take a look at the potential differences to see why...

    [​IMG]
     
  12. studiot

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    PRS, perhaps you are thinking of the Power equation

    P = IV cosθ, where θ is the phase angle between current and voltage.
     
  13. PRS

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    No, I'm not a guru, but I think I'm right in this particular case. By the way, are you the same studiot as at CARM? I'm Paul Solberg there.
     
  14. PRS

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    Aug 24, 2008
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    Did you draw those sinusoids? Something tells me we are all at odds about definitions. I would draw the first example as correct.

    Do you know how to add functions graphically? If so, just look at the second diagram. It adds to zero everywhere. On the other hand the first one doubles the voltage everywhere if both voltages are equal in magnitude.
     
  15. Wendy

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    The potential between T1 on the first peak is the same, 0V difference. You can't get work from that. Two sine waves side by side, depending on phase, can have no difference between them, or be 180 out of phase. This would create either 0V potential, or double the RMS value. Remember, these aren't two signals on one wire, but two wires each with it's own signal.

    Yes, I drew them.

    In an ungrounded system, if you were to hold the top example you wouldn't feel a thing.

    Using the second example you would have the 240VAC we've been talking about, but each leg compared to neutral is 120VAC.

    This is off topic, but you can do the same with solid state electronics to get an apparent increase if power out of two H bridges if they are out of phase.
     
    Last edited: Dec 29, 2008
  16. studiot

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    What's CARM ?
     
  17. studiot

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    PRS,

    Bill is quite correct. You will need to get a hold of these concepts to progress in electricity.

    Yes your teacher was also correct in your quote above.

    But

    Your teacher was talking about a different situation from Bill or the original poster.

    The addition you refer to happens when the voltages are at a single point.

    In this case we are talking about the voltage difference between two points.

    What Bill is saying is that if two points (not one ) are at the same voltage there is no voltage difference between them.

    What your teacher was saying was if two voltage generators are connected to the same point the resultant voltage, at that point, is the algebraic (or vector) sum of the generator voltages.

    Incidentally look again at John Panhalt's explanation
    It is good. Very good. Very very good.

    He is referring to split phase domestic mains supplies. This is a very efficient distribution method used in some parts of the USA. Other parts are more like the UK, where we do not use this system.
     
  18. Wendy

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    I think I see where some of the confusion is, looking at the batteries in the drawing they are facing the same way, but compared to ground they are reversed. If the same polarity was connected to ground then there would not be any voltage out.

    [​IMG]
     
  19. floomdoggle

    Senior Member

    Sep 1, 2008
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    KDcouture,
    The bottom prong on an outlet is to ground. Only the hot is connected to a GFI, or GFCI. Or breaker, or fuse. And switches.
    Dan
     
  20. leftyretro

    Active Member

    Nov 25, 2008
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    Off thread note:

    Well actually a GFI has to be wired and see both hot and neutral conductors (but your right, not the ground terminal) to work.

    It works by sensing if there is a unbalance between hot and neutral wire currents, and if unbalanced it implies that some hot current must be finding a alternate route through any grounded path.

    On Thread note: L1 and L2 most certainly have a 180 degrees phase difference, else there would be no 240 volt to run my oven ;)

    Lefty
     
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