1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conflict in AC electricity

  1. Jan 28, 2013 #1
    helllo ALLLLLL

    my question
    in Ac single phase without earth

    only one phase live and one neutral

    why we say that live line would kill but neutral won't kill

    isn't neutral carry current for return "same live current"
  2. jcsd
  3. Jan 28, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Since the current oscillates directions at the frequency of your power supply, both wires carry current at all times. I believe the "neutral" wire is simply a distinction to label the wires differently. Someone correct me if I'm wrong.
  4. Jan 28, 2013 #3
    The neutral wire is tied to ground.
  5. Jan 28, 2013 #4


    User Avatar

    Staff: Mentor

    By code (at least in the US), the Neutral wire is Earth grounded at the breaker panel. So there is very little voltage developed between the Neutral wires and grounded metal surfaces. The Hot lead is carring the full AC Mains voltage, and both the Hot and Neutral wires carry the current to the load from the source distribution transformer.

    The usual risk from electric shock is from a "ground fault". That is, when you are touching some grounded piece of metal, and you come in contact with the Hot wire. That's bad. But if you are in contact with a grounded metal surface, and you come in contact with an exposed Neutral wire, you will only experience a few volts of AC voltage at most, which you will likely not even feel.
  6. Jan 29, 2013 #5

    It's possible you might be suffering from a couple of common misconceptions. The first is that that the absolute 'voltage' matters. It doesn't. All we care about is the difference in potential (the potential difference or 'voltage') between two wires, points etc.

    The second is that wires somehow 'carry' a fixed current. Current flows between two points where there there is a potential difference and sufficiently low resistance. That why we only care about the potential difference, not absolute voltages.

    In a single phase AC supply, the potential difference between the 'live' wire and 'neutral' oscillates between (for example) around +340 and -340 V in Europe. So if you connect them together via, say a motor, there is a potential difference and current flows (in this case an alternating current).

    The 'neutral' wire is earthed, so is at the same voltage (roughly) as you. So if you touch the neutral wire, nothing happens as there is no potential difference and hence no current flow.

    The live wire varies between +340 and -340 V relative to the neutral wire. So there is a large potential difference between you and the wire - if you touch it, bang!
  7. Jan 29, 2013 #6


    User Avatar
    Gold Member

    If Neutral gets tied to Earth what's the purpose of the third grounding pin / wire that's sometimes found?

  8. Jan 29, 2013 #7
    Hello mickybob, Berkeman is quite correct, the building neutral is earthed by code in the US.

    This is not the case in Europe, where it is illegal to earth the neutral.

    This is because the two distributions system work entirely differently.

    Hello rollingstein.

    The neutral in both systems carries the return current.

    The aditional earth in both systems should carry zero current in normal operation. It's sole purpose is for safety protection, in which case it carries a fault current, sufficient to blow the fuse or breaker. It also serves to prevent hazardous potentials developing on exposed metalwork.

    Hello ,klmnopq

    mickybob is on the right lines, the actual current flowing in the wires does not present the danger. It is the voltage difference between yourself and something else.
    The power company maintains the neutral at somewhere near earth potential (mine is often about 30 volts) and the phase at line voltage above that. It is this potential difference the delivers the electrical power to your appliances.

    You are also at approximately earth voltage so if you touch the neutral of the supply wire you are probably safe, but don't try it to see.

    If, however your supply is from a (portable) generator then you will get a belt off either terminal. Neither terminal of a portable generator is maintained near earth, without further intervention.

    Remember safety first, second and last.
    Last edited: Jan 29, 2013
  9. Jan 29, 2013 #8
    Yes, but it's earthed the other side of the circuit breaker/fuse box. Which is a crucial difference under fault conditions.
  10. Jan 29, 2013 #9


    User Avatar

    Staff: Mentor

    I'm not sure these answers really answer the question. The key, to me, is that the neutral wire comes after the load in the circuit, so the voltage is essentially zero, regardless of if/where it is tied to the ground.
  11. Jan 29, 2013 #10
    What do you mean by voltage? Voltage is a difference in potential between two points, if you'er saying the voltage is zero, do you mean the potential difference between that point and ground is zero. In which case it's the same as saying it's tied to ground.

    The key point is that the p.d. is zero because it is tied to ground. This is irrelevant of whether there is a load or not.
  12. Jan 29, 2013 #11
    No, this is illegal.
  13. Jan 29, 2013 #12
    If the neutral naturally rests at approximately zero in relation to earth as Russ suggested, or if it is tied to earth as mickybob suggested, then how come my neutral is usually about 30 volts under no fault conditions.

    Many folks will have seen a neon tester light when touched to a neutral. The striking voltage of a neon is about 70 volts.

    Finally why do you get a belt from the 'neutral' terminal of a portable generator if it naturally rests near earth?

    The OP is fully answered in my post#7 edit which was being composed whilst Russ was posting.
    Last edited by a moderator: Jan 29, 2013
  14. Jan 29, 2013 #13
    You'd expect a small voltage drop, but 30 V seems pretty dangerously highly. Is that under load?
    Last edited by a moderator: Jan 29, 2013
  15. Jan 29, 2013 #14
    sorry friends

    I got more worried

    we difference between

    1-single phase 2 wire
    not as USA and has only 2 wires for current & return??

    IN AC current alternate so, Isn't neutral must carrry current &therefore not safe

    2-single phase 3 wire

    as in USA there is 3 wires and there is the same problem
  16. Jan 29, 2013 #15
    is it mean eath carry current for return !!!!!!!!!!!!!!!!!!! in case of who say return is grounded

    how current return into supply!!!
  17. Jan 29, 2013 #16


    User Avatar

    Staff: Mentor

    I know it is tied to ground, but it doesn't have to be for this to be true.

    Neutral and ground are tied together to ensure exactly zero potential between them, but even if they weren't, there would still be close to zero because the load is in between the neutral and the voltage source.
    Last edited: Jan 29, 2013
  18. Jan 29, 2013 #17
    The debate has got a bit side-tracked, so lets go back to the 'physics' of your question which I'm sure we'll all agree on. Firstly - please re-read my first post - I think you have some basic misunderstandings.

    'Dangerous' means current flowing through you, not through a wire.

    You are standing on the ground, then to a reasonable approximation you are grounded, i.e. at what we usually call 0 V. This will be a better approximation if you stand in a pool of water.

    The live wire is at, for example, 240 V relative to ground.

    The neutral wire is at (give or take) 0 V relative to ground.

    If you touch the live wire, there will be a 240 V potential difference between your hand and your feet and hence ground. Your resistance is not that high, so a big current will flow and you'll probably die.

    If you touch the neutral wire, there will be (in principle) a 0 V potential difference between your hand and your feet - nothing will happen.

    There is a voltage drop of 240 V across the 'load' - this can be your TV or, if nothing is plugged in, air.

    If it's your TV, then there is a relatively low resistance, and so there is a current running through the live wire, across the load, through the neutral wire and back into the supply. If the 'load' is air (nothing is plugged in) then the resistance is huge and the current running along this path is essentially zero.

    Either way, it doesn't matter - only a current running through you is dangerous and that depends on the voltage drop across you (and your resistance).

    Having said all that, in practice things might be slightly more complicated, and I wouldn't suggest you still metal objects in the neutral plug.
  19. Jan 29, 2013 #18


    User Avatar
    Gold Member

    That's a good explanation. But why is there a separate ground and neutral wire then in some cases?
  20. Jan 29, 2013 #19
    0V is an arbitrary concept, its only has meaning if things are tied to ground.

    Otherwise I could say the voltage of the live is 10000 V and the voltage of the neutral is 99760 V, giving a p.d. of 240 V. That is possible if it was a floating power supply.
  21. Jan 29, 2013 #20
    The earth is often connected to the chassis of the device.

    If you did this with the neutral, it would - all else being equal - be okay.

    But now suppose there was a break in the neutral wire between the plug and the power supply.

    The neutral wire would jump up to 240 V and you've now got a live chassis.

    Not good!

    There are other reasons as well, to do with the way circuit breakers are set up to detect faults.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook