Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Neutral wire and earth

  1. Aug 12, 2011 #1
    why during the the distribution of electricity to houses neutral wire is connected to earth wire?
  2. jcsd
  3. Aug 12, 2011 #2
    Check again.

    That would be illegal in UK.

    In the US the power feed neutral is connected to a supply transformer primary, not to earth.
    The local house neutral from the transformer secondary is earthed at the distribution board because the US practise needs is split phase and needs a stable cente zero.
  4. Aug 12, 2011 #3
    In the US, the center tap (neutral) of the transformer (secondary) supplying your house is connected to a large conductor with a spike on the end that is driven into the ground. One of the two connections on the primary of this transformer (the "return") is also connected to this very same spike in the ground at the utility pole.

    This neutral from the center tapped secondary is also connected to your service entrance (distribution board) at your house. Here again, it is also connected to a spike that is driven into the ground next to your house.

    This is also the place where the "equipment grounding conductor" is connected with the neutral and the spike and driven into the ground.

    Terminology is important here. What I just called the "equipment grounding conductor" is the technical term for this bare copper (or sometimes green insulated) wire in the US. This is more commonly referred to as "ground" or "the ground wire". From what I understand, this is called the "earth wire" in the UK.

    These conversations always seem to get confused because of the different terminologies we use on opposite sides of the pond. :-)
  5. Aug 12, 2011 #4
    but we can have split phase by using just neutral why connect it with earth or ground and how does connecting neutral to earth make it stable
  6. Aug 12, 2011 #5
    thankyou for your thorough reply but i still donot understand why neutral is connected to ground or earth because if they donot connect neutral eith earth we will never get electric shock even if we hold live wire with our bare hands
  7. Aug 12, 2011 #6
    I will leave you and Evil Bunny to have a long discussion about US practice.

    I repeat:

    In the UK the neutral is not connected to ground.

    Do not try the very unsafe experiment of grabbing hold of the line (it is not called live; both are actually 'live').
    Whether you get a shock or not depends upon many factors, but not upon whether the neutral is earthed.

    go well
  8. Aug 12, 2011 #7
    Suppose you have an electrical system in which neither side is grounded as you suggest. With such a system there could be a failure of the insulation and one side of the line could contact ground. Since the other side is not grounded there would be no indication that anything is wrong. If sometime later another point on the same side of the line contacted ground, there still would be no indication of a problem. However if there were a fuse or circuit breaker between those two points, that circuit breaker would now be bypassed and the risk of an electrical fire is greatly increased.
  9. Aug 12, 2011 #8


    User Avatar
    Science Advisor
    Gold Member

    How refreshing to read someone using the correct term. I though I was the only person who still used 'line' - and then only in good company, to avoid confusion for the masses. School books all have it wrong. The term 'line' seems to be restricted to mid-level audio output signals these days.
  10. Aug 12, 2011 #9
    thank you for your reply so the basic reason of connecting the neutral wire to earth is to find the electrical fault
  11. Aug 12, 2011 #10


    User Avatar
    Science Advisor
    Gold Member

    That is, in a funny sort of way, correct. The actual reason for having a neutral conductor is that you need to establish some degree of 'absolute potential' for your system (whether two or three phase). Without tying the live conductors, in some way, to a low potential wrt Earth, the whole system potential could drift to a very high value and deliver shocks. There wouldn't be a lot of charge available for these shocks because the charge would be limited by the Capacitance of the system but they would be unpleasant and sparks could cause fires etc.

    With a totally floating ac system with two conductors it is possible to touch either conductor safely, because the other conductor would instantly assume the supply potential wrt Earth. A lot of equipment supplies are floating (connected to the mains via an isolating transformer) - and it is a very safe system -EXCEPT when one side becomes grounded and this is not detected and then someone else happens to touch the other leg (assuming that they are safe).

    Any large network will have resistive paths to earth and so you can never be sure that it totally floats - so you may as well tie one side to near-earth and then you know where you are.
    In a three phase system, this is more obvious, I think. If the loading of a three phase system is equal on all three legs then very little (zero) current will actually flow in the neutral wire.
  12. Aug 12, 2011 #11
    The basic reason of connecting the neutral wire to earth is to DETECT the electrical fault. There may be additional reasons.
  13. Aug 12, 2011 #12


    User Avatar
    Science Advisor
    Gold Member

    There are many circumstances where a fault will not be detected. Only when there is enough current to blow a fuse will the mere grounding of the neutral "detect" a fault. (And that would never reveal a neutral-earth connection fault) The only way to detect a fault reliably is to use a residual current circuit breaker and that would work with or without a grounded neutral and it will spot a fault on either leg. Problem is that RCCBs are active devices and not actually fail-safe.
  14. Aug 12, 2011 #13


    User Avatar
    Science Advisor
    Gold Member

    Concerning what I have put in bold, you cannot be serious. What is a RCCB? Here in the USA we have GFCIs in outlet form and circuit breaker form. They detect an imbalance of current between the hot and neutral. I assume this is what you mean. With a grounded neutral at the service panel, an accidental (partial or full) short from the neutral or hot to the ground in a portable power tool for instance plugged into a GFCI outlet WILL trip this outlet. In a system where the neutral is NOT grounded at the service panel the GFCI is less likely to trip since there is no return path back. The case of the tool as well as the user hanging onto it will be at whatever potential the case is shorted to. A GFCI should not require a grounded neutral at the service to detect the imbalance of current between hot and neutral, but where will the current path be if we don't take steps to make sure it is through the actual earth?
    Almost forgot, I don't think I have ever seen a GFCI fail to open during a fault. It is more likely that they trip unnecessarily.
  15. Aug 12, 2011 #14
    Let's get a few things straight.

    Single phase electrical feeds to a property are normally controlled by a single pole switch, which is in the line circuit.

    So the line can be 'live' or disconnnected.

    The neutral is permanantly 'live' with this system.

    Now suppose someone was working on the wiring and had switched off the switch ie disconnected the line.

    And further suppose that a line- neutral cross fault occurred upline.

    The neutral would aquire the line voltage, a potentially lethal situation to anyone working on the wiring.

    If, however the neutral was earthed a fault current would flow, causing disconnection and resulting in safety.

    A further caveat.

    Birds can sit on transmission lines because they can fly up there and are small enough not to touch anything else at the same time.

    It is pure folly for a human, standing on the ground, to attempt to emulate this.
  16. Aug 12, 2011 #15


    User Avatar
    Science Advisor
    Gold Member

    But this is the whole point of disagreement of which is better. When the ground we stand on is not connected to anything at all why would the live wire touching the neutral make it any more dangerous? What would is actually happening in your scenario is a short circuit up-line.
  17. Aug 12, 2011 #16

    What disagreement?

    What is better?
  18. Aug 13, 2011 #17


    User Avatar
    Science Advisor
    Gold Member

    RCCB is spelled out in the para you have quoted. I could ask what your acronym stands for, too.
    Someone mentioned single pole switching. This is always a source of danger. Also, in a balanced system, fusing and switching can be problematical. Do you use double or single leg switching? Only one fuse will blow but this does not ensure isolation in a balanced system. Neutral fusing in a one sided system is even more deadly.
  19. Aug 13, 2011 #18
    What do you suppose is the ideal way to measure this leakage current of lets say that power tool, for instance.
  20. Aug 13, 2011 #19


    User Avatar
    Science Advisor
    Gold Member

    The standard way is to use a transformer with three windings. Two are put in series with each of the supply legs, connected in antiphase so as to cancel. When the load is balanced, there is no flux in the transformer so no volts on the third winding. Any imbalance (difference) in the two currents will result in a voltage on this winding. This is used to operate a trip switch. These devices can easily detect 10mA of leakage to Earth, giving good biological protection.
  21. Aug 13, 2011 #20
    Without getting into anything about what is "better" and what isn't... Studiot's example didn't lay out anything dangerous as far as I can tell... the short circuit he describes would create tremendous current but not where the hypothetical person was working on the wires... this would be upstream somewhere.

    As far as I can tell anyway... perhaps I've misunderstood something (it certainly wouldn't be the first time). Maybe another one of your sketches is in order?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook