1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Electronics AC power - how does it work?

Discussion in 'Modding' started by MisterX, 26 Dec 2004.

  1. MisterX

    MisterX New Member

    Joined:
    24 Dec 2004
    Posts:
    12
    Likes Received:
    0
    Hi, everybody,
    Can anyone tell me how the AC current works? I know that it uses two wires, hot and neutral, and, sometimes, the third one, ground wire. I read that the current flows from hot to neutral, but after looking at the sinusoid shape of the AC graph I was pretty much confused. Doesn't it mean that the polarity of the wires changes 50/60 times per second? If it does, then how can we say that the current flows from one wire to another, if its direction currently reverses?
    Also, in the US one of the slots in the outlet is wider (neutral), than another one (hot), but many European countries use the outlets with two identical slots, or holes. Why is it made in this way?
     
  2. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    Live (or "hot") varies at 50-60 times per second between 240v to -240v
    the ground stays as close to 0v as possiable. it is for this reason that all fuses, switches etc switch the live wire only, as it is safe to touch ground (otherwise they would switch both). Many people think that the live and ground switch places 50 times per second, but if that were true, touching neutral would be just as bad as touching live!

    basically "hot" is a perfect sine wave while ground is a 0v referance. :)

    a little scope screen that is horriably not accurate, but shows the princable:

    [​IMG]
     
    Last edited: 26 Dec 2004
  3. myke

    myke New Member

    Joined:
    15 Dec 2002
    Posts:
    68
    Likes Received:
    0
    That's a good explination! I was one of the people who thought they switched polarity and always wondered why only live was switched :duh:
     
  4. cpemma

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    The average voltage (root-mean-square voltage as it's a sine wave) is about 230V in Europe, 120V or so in the US. The peaks are at ±325V over here, so rectifier diodes and capacitors for UK mains use are rated at 350V or 400V.

    Voltage is the potential difference between two points, so if you bury one connection in the ground (earth) you can give it a zero value and assume only the other connection (live, hot) changes in value.
     
    Last edited: 26 Dec 2004
  5. Smilodon

    Smilodon The Antagonist

    Joined:
    25 Mar 2003
    Posts:
    6,244
    Likes Received:
    102
    here in norway we have two "hot" wires.. and no neutral. only ground.

    so the polarity switches place. It doesn't matter what way you plug in something. also, both wires are fused but often only one of the wires is switched (in those wall switches). So if you measure in a lamp socket that is switched off, you will have around 80 volts on one pole, and 0 on the other. In relation to ground that is.

    Things that are places outdoors or in bathrooms are often switched on both wires.

    i'm not quite sure how the rules is now, though. i think we have got new rules on some of this, as all outlets should have ground now.
     
  6. myke

    myke New Member

    Joined:
    15 Dec 2002
    Posts:
    68
    Likes Received:
    0
    I get it so far... But what's the difference in earth and neutral then? (For uk spec)..
     
  7. SteveyG

    SteveyG Electromodder

    Joined:
    23 Nov 2002
    Posts:
    3,049
    Likes Received:
    8
    Nothing, until after the electricity meter for a PME system. After the meter the earth and neutral are kept separate, (it's actually illegal to join neutral and earth anywhere after the meter) and in some premeses it's quite possible for the voltage between earth and neutral to reach maybe 50 or 60V.

    Some areas that don't use the PME system use an earth rod instead. This earth is kept separate from the neutral at all times.

    Using the PME system guarantees that a fault to earth will provide enough current to blow/trip any fuse, provided you use the correct wire sizes.
     
  8. Lynx

    Lynx New Member

    Joined:
    24 Nov 2001
    Posts:
    864
    Likes Received:
    2
    earth is there to stop you electricuting yourself if there is a fault. Instead of going through the case to you to earth it will instead go throught eh case direct to earth and also trip the earth leakage breaker.
     
  9. Xiachunyi

    Xiachunyi New Member

    Joined:
    3 Nov 2003
    Posts:
    1,404
    Likes Received:
    0
    There are probably devices in your bathroom that have a ground-fault circuit interrupter in them, NEC 2002. They are utilized for protection of personnel if the current to ground exceeds an established value. I think for a class A device it is about 4-6mA.
     
  10. roll1

    roll1 New Member

    Joined:
    21 Sep 2004
    Posts:
    149
    Likes Received:
    0
    If you connected an electric motor to earth instead of neutral all the power would go to ground and the motor would not run ? Also does the power run in a circle so it must return to where it was generated to give a live and neutral ? Also what is PME ?
     
  11. SteveyG

    SteveyG Electromodder

    Joined:
    23 Nov 2002
    Posts:
    3,049
    Likes Received:
    8
    PME = Protective Multiple Earthing.
    At various points before the mains reaches your house, the neutral is grounded. In substations, a huge metal grid is often buried underground and tied to the neutral, that neutral will be joined to earth stakes/grids at various points until it reaches your meter, at which point the neutral and earth separate - earth is connected to pipes etc, and neutral straight to consumer unit.

    If you connected an item between live and earth, it would still run if there's no RCD anywhere in the house, but any sinks etc could end up giving you a shock.

    The circuit path must be complete, so in effect you'll have a circle to the other end of the transformer winding. One end of the winding in a two phase system is live, the other taken as neutral and connected to earth at the substation.
     
  12. roll1

    roll1 New Member

    Joined:
    21 Sep 2004
    Posts:
    149
    Likes Received:
    0
    Thanks for that SteveyG, just one point how do they manage to separate the earth and neutral outside premises.
     
  13. SteveyG

    SteveyG Electromodder

    Joined:
    23 Nov 2002
    Posts:
    3,049
    Likes Received:
    8
    There's just a connection block in the little unit where the electricity board's fuse is, connecting the return path to the substation, the earth, and the neutral to the consumer unit together.

    That is the last point there is a join between earth and neutral. Anywhere else inside the house and it's a very dangerous thing to do...
     
  14. myke

    myke New Member

    Joined:
    15 Dec 2002
    Posts:
    68
    Likes Received:
    0
    Thanks for the answers! Just had our electricity redone after a flood and the guy fitted an rcd and put a really meaty earth cable to a rod outside. Apparently we should have had one of these in the first place!
     
  15. roll1

    roll1 New Member

    Joined:
    21 Sep 2004
    Posts:
    149
    Likes Received:
    0
    That's cleared a few things up. I remember now seeing earthing rods being hammered into the ground outside newly built houses.Sorry to bore you but what happens in a jumbo jet.
     
  16. Xiachunyi

    Xiachunyi New Member

    Joined:
    3 Nov 2003
    Posts:
    1,404
    Likes Received:
    0
    Here you go
     
  17. roll1

    roll1 New Member

    Joined:
    21 Sep 2004
    Posts:
    149
    Likes Received:
    0
    Xiachunyi

    Interesting article but that begs the question ,do they have to earth the fuselage when it lands before the passengers disembark to protect them from this huge buildup. It's bad enough getting a jolt off an ordinary car door handle.
     
  18. Xiachunyi

    Xiachunyi New Member

    Joined:
    3 Nov 2003
    Posts:
    1,404
    Likes Received:
    0
    From what I looked up, I am not sure how to derive the exact answer but these two may be of some help.

    Here and here.

    It seems static electricity is automatically discharged when the plane touches the ground. I haven't been shocked when riding an airplane and touching a piece of metal on the ground so I can't provide the answer. I wonder if people do get shocked if they ride that slide during an emergency airplane landing.
     
  19. roll1

    roll1 New Member

    Joined:
    21 Sep 2004
    Posts:
    149
    Likes Received:
    0
  20. zz300

    zz300 New Member

    Joined:
    7 Mar 2002
    Posts:
    95
    Likes Received:
    0
    Aircraft are grounded via a port on the outside of the fuselage. It's rare to get shocked while connecting them. Next time you're at an airfield, have a close look at the pad they're parked on, there are grounding ponts formed into the concrete (& marked with a yellow date when they were last checked for continuity)
     

Share This Page