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Want to know how alternating current really works

  1. Nov 15, 2013 #1
    All I have read in the electricity class is that AC is any type of current which actually reverses its directions periodically. I have also read all sorts of mathematical expressions concerning sine-wave form of AC. I have also understood how AC generator works. These topics had never bothered me.

    But now when I think on how is AC really applied to household three pin plugs the story that follows really puzzles me. Well, I somehow understood that hot wire actually serves the purpose of having definite potential [which obviously changes]. The ground, as I have read in school text-books, "buries" the deadly current.
    But I could not understand as to what does the neutral sitting there do. I wish somebody from PF gives me the answer.
    Thanks in advance.
  2. jcsd
  3. Nov 15, 2013 #2


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    The "neutral" wire does absolutely nothing as long as there are no problems with the circuit or electric device it is attached to. But if there is a short circuit or other problem so that the current contacts the body of the device (which might electrocute the user) the neutral line, attached to the body of the device, gives that electricity a low resistance path to ground.
  4. Nov 15, 2013 #3
    I guess it may be wording differently in different parts of world but isn't the neutral wire the return for the AC and the ground wire the wire which actually " does nothing" as long as there is no fault in the device as it is connected to the chassis to offer a low resistance path to ground?
  5. Nov 15, 2013 #4
    But this definition is looking equivalent to the ground. If you are correct, then what's the difference between ground and neutral???
  6. Nov 15, 2013 #5
    To where???
  7. Nov 15, 2013 #6


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    Neutral wire is the wire which completes the circuit,so current can flow.
  8. Nov 15, 2013 #7
    The neutral wire loops back to the generator (transformer, really). It carries normal operating current, same as the hot wire. The ground/earth wire does not carry any current under normal operating conditions. If it ever does, that means somewhere either the cable or an appliance connected to it are dangerously broken and must be disconnected.
  9. Nov 15, 2013 #8
    The ground is there for safety. The neutral wire is there to complete the circuit. The ground wire connects, for example, to the metal housing of your power tool. That way, in case a connection comes loose and the metal housing gets charged with high voltage the current has a path through the metal housing to the earth through the ground wire (rather than going to the earth through your body). For this reason the ground connection actually connects to the earth somewhere near you. The neutral wire connects back to the power supply station and is used as the reference for your 120V current.
  10. Nov 15, 2013 #9
    exactly , that's what I thought
  11. Nov 15, 2013 #10


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    If it helps, understand that both the neutral and hot wire are effectively the same thing. Both deliver current and voltage from the generator to your appliances. If you look at a very simple circuit, a single wire running away from and then back to the power source, there is no distinction between hot and neutral. In a household circuit one leg is grounded and we call this leg the neutral leg.
  12. Nov 15, 2013 #11


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    Just to repeat and emphasize what CrazyMechanic said:

    All of the above only relates to the US wiring system..

    In other countries (e.g. the UK) several of the statements are wrong, or even dangerously wrong.
  13. Nov 15, 2013 #12
    and to add to what AlephZero just said for example in most parts of Europe and Russia you have only two plugs one hot one neutral or the return, normally under normal balanced loads the hot lights up using a tester screwdriver while the neutral does not.
    Now when people switch on several different loads there are many situations that the neutral starts to show voltage , maybe not the full voltage but half or some of the full phase voltage.
    In such a situation if that neutral would be connected to the device chassis or ground you would be in severe trouble.

    Actually I was once a victim of irresponsible wiring.Somebody had used the neutral wire here in europe and thought it is a ground wire also, so he earthed a water heater chassis with a NEUTRAL.!!!
    Now beware don't do anything like that, because when I, not knowing of this setup, wanted to wash my hands and the heater was working I felt this tickling sensation on my hands and understood that I was getting about 100 volts via the water from the pipes that were attached to the neutral.
    Wasn't much fun.:D

    That's why many people pay the local authorities for them to set up a special earth wire with beams put into ground and the wire connected to the necessary equipment.
    Back in the day there were only a few machines that worked on electricity and needed such grounding so I guess nobody gave much care about grounding.
    The situation is much different now en you have electric dish washers , microwaves , water heaters , bla bla bla.
  14. Nov 15, 2013 #13


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    It depends what country you are in but not many use Earth as you describe. In some countries like the UK you have..

    Live = 230VAC w.r.t Neutral
    Neutral = nominally 0V w.r.t Earth (although do not assume that it is safe to touch!)

    In this case it is normally the Neutral that "buries the deadly current" by returning it to the power station. The earth is provided for safety and other reasons. Modern Consumer Units contain Ground-fault circuit interrupter (GFCI) aka Residual Current Detectors (RCD) that check the live and neutral currents are the same. If they differ by more than 30mA then that suggests some must be leaking away somewhere (eg to Ground/Earth). That implies a fault and the RCD will open.

    In some countries you have

    Hot 1/Live 1 = 120VAC w.r.t Neutral (240V w.r.t Hot 2)
    Hot 2/Live 2 =120VAC w.r.t Neutral (240 w.r.t Hot 1)
    Neutral = nominally 0V w.r.t Earth (although do not assume that it is safe to touch!)

    In many systems the Neutral is connected to earth somewhere in the system but that does not mean its safe to touch. You can get quite high voltages on the Neutral if the connection between Earth and neutral is a long way away. You can even get dangerous voltages on the earth wire under fault conditions so never mess with mains wiring.
  15. Nov 15, 2013 #14
    In the US residential wiring the neutral and ground wires are usually connected to the same bus bar in the main circuit breaker box, which is then connected to a rod driven into the ground outside the house. A 120 volt outlet has 1 live terminal which has 120 volts of AC, neutral carries the current to complete the circuit and the ground is there for safety reasons and should carry no current if everything is working correctly.

    The main breaker box will have 2 or 3 different AC lines coming in from the transformer, each with 120 volts on it but they are out of phase with each other by 120 degrees. Appliances which require more power use a 220 volt outlet which is either a 3 or 4 terminal outlet with 2 hot terminals, 1 safety ground, and (in the 4 terminal case) a neutral wire. Since the 2 hot terminals are out of phase you get a push/pull action whereby the current is moved by the sum of the voltages on the 2 wires. This sum adds to 220 instead of 240 because the cycles are 120 degrees out of phase instead of 180 degrees out. The neutral terminal of the 4 terminal plug allows for appliances to use 120 volts for some functions and 220 volts for others.
  16. Nov 15, 2013 #15
    Neutral and ground are definitely NOT the same thing. THis type of advice is treacherous. Both do NOT deliver current and voltage from the generator to houses. Where do you get this stuff?

    Ground prong, aka "3rd prong", and its associated wire, conduct only when a fault is present. If a live wire shorts to a casing which is connected to 3rd wire, a very low impedance path to ground results in large current tripping the breaker and removing the danger. Without the 3rd prong, a live wire shorted to a metal case of a tool or appliance would not trip a breaker, and a person touching the case is subjected to severe shock hazard, maybe lethal.

    In a household 2 legs are grounded, not just 1, but only 1 of these wires carries actual load current, that being the neutral. Ground wire/prong conducts only when a fault takes place. The 2 should not be equated, they serve different functions. Both are important but they are not what you say they are.

    Last edited: Nov 15, 2013
  17. Nov 15, 2013 #16


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    I think you misread what was written - Drakkith wrote that neutral and hot are effectively the same thing, and both deliver current from the generator (effectively). You're right that ground should be extremely low (ideally no) current, but that doesn't contradict Drakkith's statement at all.
  18. Nov 15, 2013 #17


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    Of course. I think you misread. I said the hot and the neutral. (Though whether the neutral carries voltage, I'm not sure. I think it depends on the setup of the wiring. There is definitely a difference in potential between the hot and neutral legs though.)
  19. Nov 15, 2013 #18
    Well I did some readings on transformers, particularly single-phase transformer, and found that there exists central tapping that lets us access two different voltages. The central tapping, as understood by me, is what we call neutral (the current return path-as we call it here).

    Then I thought that if the two ends of the secondary transformer is provided load of unequal amounts and these loads are such that they are grounded (or connected to the earth) for the circuit to complete then I guess we would have a current flowing. But then because of unequal loads the charge distribution on the secondary transformer will change leading to something non-desirable (as I believe). And hence the purpose of neutral in this case is ascertained.

    What do you people say??? Is my justification correct???
  20. Nov 15, 2013 #19


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  21. Nov 15, 2013 #20


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