AC/DC questions

Discussion in 'General Electronics Chat' started by electronis whiz, Jun 24, 2013.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    me and a friend were talking about electricity and I've always wondered and I guess he has too. how AC can be called that when it' don't rally alternate. if it did then you touched a white wire in wiring then you'd be shocked just as you would if you touched a black wire. the only way I could see AC being accurate is between 2 120V phases making 220V because there always opposite. but 120 seems almost like DC.
    can someone explain this. it makes no since to us.

    we both agreed liked DC better because a lot less confusing. and we both work on computers so used to working with a lot more than AC.
     
  2. LDC3

    Active Member

    Apr 27, 2013
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    Actually, the white wire is connected to ground, so there is no potential between it and the ground, even when you take hold of a bare wire. Usually only 2 of the 3 phases are brought inside a residential building. Each phase is 120º from the others. The voltage between any phase and ground is about 115V. The potential between any 2 phases is about 230V. When all 3 phases are available, it is 550V.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You could think of AC like two spinning jump ropes (double-Dutch style) with one black and one white. Then you would be right that either could give you a good jolt. However, in the US, the white wire (common) is connected to earth and is the center tap of a 220/240 line.

    That grounding makes the white rope appear stationary and the black rope appear to have double the amplitude (voltage).

    If you would mount a camera on the white rope and continue spinning double Dutch or hold the white rope tight and double the amplitude of the black rope, the view would be the same. Assuming the ropes do not tangle and the camera does not show the white rope.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The black wire voltage does go above and below ground (white wire) at the AC freuency rate. Thus for 120Vrms it goes to equal positive and negative peaks of 170V.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    There is no such thing as a voltage on a wire: a voltage needs TWO wires.

    One of these wires (the white one) is connected (literally) to the Earth to ground it.

    The other wire (the black one) alternates plus and minus with respect to the white one, and therefor the entire world.

    Touch the white one and you're fine. Don't touch the black one.
     
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  6. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    ok. this brigs up another question then. if neutral is connected to earth ground then why have bare wire and a 3rd prong wouldn't that be the same as a polarized plug?
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    Not really. The ground wire is usually connected to the metal conduit or metal spike driven into the ground for about 10 feet (if it has been properly installed) near the building. The neutral wire is fed back with the other wires to the transformer. It may go some distance before it is tied to ground.
    Also, the third terminal is a recent add-on. There were only 2 wires used in older homes and sometimes the person who wired the house didn't take the proper care to ensure the wiring was correct. So what you think is the neutral wire is really the live wire.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The ground wire carries no current and is strictly for safety use. It can be connected to the case of the electrical device to guard against a short from the hot wire to the chassis. You would not want the neutral, which carries current, to be connected to the case since, if the neutral wire developed an open, the hot voltage would now appear on the case when the power was turned on, thus electrocuting anyone touching the case. :eek:
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Google "belt and suspenders" for one reason.

    Two safety methods just make things safer. Suppose you have a 3-wire connection to a metal case appliance (dish washer or such) and one wire comes loose. Of course the ground is always safe to touch so it is connected to the metal body. Now if anything comes loose the ground keeps the frame safe (and hopefully blows the fuse/breaker so you are aware there is a problem).

    Also neutral is not quite the same as ground: since it carries current there may be several volts on it. True, that is a small voltage but it is different from ground.
     
  10. Stuntman

    Active Member

    Mar 28, 2011
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    As someone else posted, the white wire very well may have a slight voltage to it. Keep in mind, a branch circuit may have 20A running through some 12AWG, and a feeder may have close to 100+ amps in #1AWG (AL). Yes, copper has some resistance, so yes, neutral readily has a measurable potential between it and ground.

    Moreover, one bad connection downstream of this "neutral" wire, and you having a good ground (ie, standing on wet concrete) could dish that current through you. Hence why new standards have the (previously discussed) green ground wire, solely for safety. And why it is NOT the same to wire the ground prong of your outlet to the white wire. Point, do NOT touch the neutral wire of an energized circuit.

    This is not particularly part of the OP's question, so I won't dwell, but this is largely incorrect.
     
  11. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    ok. that's kind of what I was thinking, but somewhere I once saw a breaker panel where neutrals and GND shared the same bus bars. some reason I thought that was correct, and have seen panels that can join neutral to the frame ground.
    come to think of it I don't recall seeing a ground rod either. maybe doing it the old or easy way and just letting pole grounds do it.
     
  12. Stuntman

    Active Member

    Mar 28, 2011
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    Yes, neutrals and earth ground DO share the same bus bar, but only at the entrance to the house. Often times, you'll see your meter have a bare copper wire that comes down to your grounding rod. Sometimes you'll see this in the primary service panel as well. But past that, the ground an neutral should never meet again.
     
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