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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

    HallsofIvy

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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

    adjacent

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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

    Drakkith

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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

    AlephZero

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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

    CWatters

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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!)
    Earth

    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!)
    Earth

    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.

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

    cjl

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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

    Drakkith

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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

    MacLaddy

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  21. Nov 15, 2013 #20

    Drakkith

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  22. Nov 16, 2013 #21
    @Justwild , you are correct to say that the center tapped middle of a secondary winding is the return path , but normally if the apparatus or device has no fault current goes back through that center wire.
    The earth wire under normal conditions is current free and better should be that way.
    Only when the isolation inside a device is broken and current can escape to the chassis or any other place where you can get in contact with only then the grounding or earth comes into play.
     
  23. Nov 16, 2013 #22
    That's a very well written explanation that I'm sure I will reference in the future any time this question pops up. Kudos to the author!
     
  24. Nov 25, 2013 #23

    jim hardy

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    Think of the humble flashlight - it teaches us a lot.
    Alternating current is, at any frozen instant of time, unidirectional like DC.

    Consider a two cell flashlight with metal case. The metal case is not connected to earth , is it, unless we set the flashlight on the ground? Yet it does not shock us when we pick it up. That's because as Kirchoff tells us, electric current "wants" to flow back to where it came from. Every unit of charge that comes out one end of the flashlight battery "wants" only to get back to that battery. It will ignore "ground" unless "ground " can take it back to the battery from whence it came.

    Same is true of current emanating from one end of a transformer winding.

    A two cell flashlight is like a center tapped transformer. The junction of the two cells , + of one and - of other is the centertap, though we don't bring it outside of the flashlight.
    But if we did, we could drive a 1.5 volt lamp from centertap to case in addition to the 3 volt lamp in the flashlight.

    NOW - we are completely free to connect "ground" to any part of the flashlight.
    So let's use the analogy of battery to transformer winding
    In US, we "ground" the centertap with a green wire and insulate both ends of the winding(batteries)
    and place a white wire from centertap to one side of our half-voltage loads for current return
    and place a green wire from centertap to outer case of all loads, both half and full voltage
    the green wire is there only to intercept current resulting from an accidental short circuit inside the load..

    So you have it right- neutral is return for current. Current through half voltage loads has its choice, return through neutral, or if available through another half voltage load to opposite end of transformer . If current gets into green wire something is wrong, and green wire gives a current return path that should be preferable to one that goes through somebody.

    Point of this tome is - simple minded thinking resolves a lot for us.
    When you can make the math and simple thinking agree you probably have both of them right.

    old jim
     
  25. Nov 25, 2013 #24

    sophiecentaur

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    If you have a split phase supply (as in the US), the centre tap is the neutral. The neutral is often held at or near Earth potential but it does not need to be for the equipment to all work - i.e the 110 or 220V equipment will work because the potentials are all relative between the conductors.
    The UK uses a three phase supply with a single phase plus neutral, connected to each consumer. In this case, instead of 180 degrees of phase between any two 'live' two conductors, there is 120 degrees of phase. Large loads are supplied directly with three phases. Nowhere in the UK uses a split phase domestic supply. All appliances use 240V (nominal).

    As for the current flowing in the neutral conductors, this is a bit more complicated. If a split phase supply has equal loads connected between each leg and neutral, there will be no return current via a common neutral return wire to the transformer centre tap. Likewise, if all phases of a three phase supply feed equal loads, there will be no net return neutral current. The neutral conductor, in both cases will just carry any unbalanced current (which will usually be lower than the current in any individual live conductors).

    The Earth conductor is merely there to ensure that all metal cased equipment is kept at Earth potential - which removes the possibility of a shock if there is any leakage to the case. In extreme cases, the leakage current will actually blow a fuse and disconnect the supply.
     
  26. Nov 25, 2013 #25
    Someone mentioned that electrical panels often have the neutral and ground wires connected on the same bus. Although this is often the case it is incorrect wiring. The reason being that neutral and ground must be isolated after the first means of disconnect. The goal is to have only one connection between neutral and ground for each electrical service. The ground is literally one or more conductive rods that are driven into the earth.

    If the service only has one panel then it's ok to connect them on the same bus but it's still bad practice because if you ever add a new panel upstream then you would have to separate neutral from ground in the original panel. Panels can now come equiped with a bonding screw and two busses for the neutrals and grounds. You can put the neutrals on right side bus and the grounds on the left side bus and control whether they are connected by simply adding or removing the screw.

    What many people like to do now is install a main switch after transformer and connect the neutral to ground right there. All the panels that follow have the grounds and neutrals isolated from each other.

    That applies to 3-phase and single-phase services in the US.

    Like I said, it's often the case that the neutrals and grounds are mixed in the panels because either it was grandfathered-in under older rules or an electrician did some shoddy work. This causes many problems because current returns to the transformer on the ground wires as well as the neutrals. If the panel in question is especially far from the main service connections then all the chasis connected to neutral become electrified because V=IR and real wires have resistance. It can be bad in industrial settings where currents are high and poor planning resulted in load imbalances. You would probably never notice in a house with a few incorrect panels.


    In theory the neutral wire should be at 0V. It's bonded to the earth ground which itself acts like a big current sink when their is any positive or negative potential. In single phase systems the center tap of the transformer is held firm at 0V by it's connection to ground and this forces the ends to settle on 120V potentials that are 180 degrees apart. In three phase systems, one of the three transformers can also be a center-tapped transformer. If you connect that tap to ground it will help stabilize the three phase-to-neutral outputs.
     
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