Difference between Neutral and Ground

Discussion in 'General Electronics Chat' started by Barmaley, Nov 5, 2011.

  1. Barmaley

    Thread Starter New Member

    Nov 5, 2011
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    Hello everyone!

    I am an electrical engineer and I have a good knowledge of theory of circuits from theoretical point of view. I would like to learn more about safety and this is my question: why do not use neutral as a a ground wire? As far as I understand they are connected any way at a main panel. From theoretical point of view they have the same potential and if we disregard wire impedance they essentially are the same wire. I am sure that there is a reason to put so much extra copper in wiring, but what is the reason?

    Thank you in advance,

    Barmaley
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Neutral is the return path for current. All the current from the live must return through the neutral. Ground is a safety ground. No current should pass through the ground wire. If current is detected in the ground wire, it indicates there is a fault. If you remove the ground you remove your safety feature.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC

    U know that Neutral and ground are common at the power station. Right ?

    This way a fault like live to ground ( body of the item, also known as earth) would create a fault current through ground, not in the neutral. The Station ground completes the circuit which is basically considered as an Earth leakage.

    This is a safety feature to the consumer
     
  4. Barmaley

    Thread Starter New Member

    Nov 5, 2011
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    Mr. Chips,

    Thank you for your respond. I believe it was very accurate description of status quo. Indeed, there is nothing wrong in the currently accepted wire schedule, where ground is used for protection and neutral for currying of current. What prevents us from combining these functions in on wire?

    Correct.

    Unless we use neutral as ground. In this case the ground should carry the load current only and serve as ground in case of fault.

    Yes, it is possible to use ground as indicator of fault, however there are other methods of testing fault (like measuring resistance of a circuit). It would be cost prohibitive to use extra wire just to test faults.

    Unless you use neutral as ground.

    I understand that THERE IS A REASON WHY they use ground as a separate wire, the question is: what is the reason.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    It also gives the GFD (ground fault detector) something to work with. A GFD, usually found in bathrooms, switches off the outlet the moment it senses current through the ground wire. This usually in a millisecond or so.

    The other thing is, you ground the chassis of equipment. If a wire chaffs and makes contact then the house mains breaker will blow. With a two prong connector there is a very good chance of connecting it backwards, adding a third pin dramatically reduces the odds of this kind of problem.
     
  6. #12

    Expert

    Nov 30, 2010
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    Here's another aspect. To wire a remote location, the ground and neutral must not be connected together at the remote location because the return current flowing through that length of ground/neutral wire will develop enough voltage for humans to feel it when standing on damp concrete. The cases of the tools and recepticles are grounded to a non-current carrying ground so that the voltage on the case is actually zero.
     
  7. Barmaley

    Thread Starter New Member

    Nov 5, 2011
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    It sounds interesting. From this post follows that the main reason for a separate ground wire is to make sure that the a wire (ground) does not curry current and therefor has the same potential as Earth. Because of the impedance of neutral wire under a load it produces difference of potentials which can be harmful to humans.

    However as we know the resistance of 100 feet 14 AWG is 0.25 Ohms. At 10 Amp it gives us 2.5 volts which is not harmful. Any explanations?
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Except of course that under line-neutral fault conditions the neutral drop could be much higher, at least until fuses blew or breakers tripped out. It has to take a very large current for this to happen, so that the load exceeds its maximum expected value.

    For a short time, the neutral voltage might even approach half that of the supply. The case of any equipment returned to it would then be "hot" Bad enough on a 120V supply, worse with 230V or 440V.

    You may object that the earth voltage also rises in the case of a line-earth fault, but where a protective earth is used, it is possible to shut down the supply quickly when a relatively small leakage current is detected.
     
  9. Barmaley

    Thread Starter New Member

    Nov 5, 2011
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    I guess I will object it. From what you are saying I may construe that they use ground as means of detection of fault and not for another reason. However, I did not see any measures in residential - small commercial wiring to detect earth leakage to shut down supply. (I may not be aware about it since I am not a practicing electrician and it was just my guess).

    Anyway, can we conclude that the ground wire is eccentrically introduced for detection reason rather than for a reason of protection?
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Protection came first, as earth leakage circuit breakers (ELCBs) / and residual current devices (RCDs) came later.

    Their use for certain domestic applications e.g. in bathrooms is now required in my country (UK), but this is relatively recent. I do not know what the regulations may say e.g. in the USA. What is your location?

    While you may have made a breakthrough in concluding that earth conductors are an unnecessary luxury, consider the fact that this system is widely used throughout the world. Is it likely that all those electrical engineers and supply authorities have been mistaken for all these years, without anyone noticing?
     
    Last edited: Nov 6, 2011
  11. Wendy

    Moderator

    Mar 24, 2008
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    Repeating what I said earlier, you ground the chassis of large equipment (stoves, dryers, lathes, hand drills) as a major safety feature. I've gotten shocks as a kid from a hand drill in a shed/garage with a dirt floor and puddles, it's chassis was not grounded and it used a two wire plug.

    This could not have happened with a three wire setup.

    It is a lot more common than people realize for the chassis on two wire setups to go hot.
     
  12. someonesdad

    Senior Member

    Jul 7, 2009
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    Another thing to remember is that AC appliances 50 years ago were commonly built with metal enclosures and, as Bill mentioned, it wasn't unusual for a hot wire to contact the case through an insulation problem. I too remember getting shocked from a drill with a die-case metal case in the 60's -- and it was a drill that someone had "thoughtfully" cut off the grounding plug because then it would fit in the then-common two wire outlets. Today, it's less common to find hand-held power tools with metal enclosures and "double-insulated" is a more common type of construction. That third grounding wire was a very important safety invention -- in fact, in my humble opinion, it was the most important safety advance ever made for AC-operated appliances. Just ask all those people who DIDN'T get electrocuted from touching an appliance that developed a hot-to-case short -- the appliance blew the breaker instead. Thus, we just have a situation where the average person thinks "Gee, something must be wrong with this thing" when it blows the breaker and sends the appliance out to get fixed instead of attending their own funeral. That's a powerful argument in my mind for the cost of running that third wire.
     
  13. AcousticBruce

    Active Member

    Nov 17, 2008
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    That was a good explanation. :)


    I have always thought the neutral and earth were connected at the fuse box. If this is true then grounding a chassis to the earth would be connected to Neutral anyway. As Barmaley said about 14AWG @ 100 ft is .25 ohms is this what is significant or am I wrong about the ground and earth at the fusebox?
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    I guess you are having some serious problems understanding ground.
    NEUTRAL and GROUND are NOT the same thing!

    NEUTRAL is the return path for the supply current.
    GROUND is a safety feature and should conduct zero current.
     
  15. roadey_carl

    Active Member

    Jun 5, 2009
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    No, neutral and earth are not connected at the fuse board. They are connected and the transformer end or at the incoming head that the electricity board supply into your home.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Also, in an ideal world, it is earthed (as in connected to the dirt outside).
     
  17. roadey_carl

    Active Member

    Jun 5, 2009
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    Exactly bill. If the neutral conductor came disconnected from ground at the distribution end, you would have an a.c current arriving at the consumer unit and due to there being no live conductor at ground potential( a neutral), no earth fault protection would work as the earthing conductor would have nothing to discriminate between. That's why, like you said bill, a TT\Earth stake is an ideal option!
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Actually, it would all still work if it was connected at the pole/transformer, and not earthed. Connecting to earth increases the safety though, if hot touches earth ground you could still have a fuse blow. I had an experience similar to this (no fuse though) when I was a teen working on a metal chicken coop. The wires I was running for lights got away from me, and someone had left a switch turned on. I was clued by the sparks with the hot wire touched the metal shed. This was a two wire system in the 70's.
     
  19. roadey_carl

    Active Member

    Jun 5, 2009
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    That's a good point. That's caused by accidental earthing, for example a wet pylon, it would replicate a earth stake system but at the transformer end. If you take a isolation transformer and touch 1 ac output wire you won't get a shock. if you were to touch both the wires at the same time you would get a shock,( obviously because your the load ) however,put 1 wire into the ground ( creating a neutral ) then touch the other ac output ( now the live )you would get a shock. If you were to place and rcd/earth protection, and touch now what is the live wire, you would still get a shock but in the uk it would trip at 30mA within .3sec still enough to kill as we all know, but its a darn site better then being a lump of carbon. Hence the reason earth is good. I like earth.

    Don't touch wires people!
     
  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The original question, which has not yet been properly addressed, was about combining the protective earth and the neutral conductors in an AC power system. The discussion seems to have drifted to one about earths.

    1)
    Remember that wiring prectice is fundamentally different in North America and Europe. The OP seems to be asking about US practice.

    In Europe it is illegal to connect the neutral and the (edit: consumers local) earth.

    In the USA it is mandatory.

    In some installations in Canada they have no neutral nor supply earth.

    2)
    So why keep the protective earth and the neutral separate?

    Well remember that most of the earthed conductive material (metal) in a building is not part of the electricity supply.

    It comprises pipes, sinks, boiler cases, balustrades, etc etc.

    Would you want any of this connected to the neutral?
     
    Last edited: Nov 9, 2011
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