Maximum permissible

Discussion in 'General Electronics Chat' started by bwd111, Aug 5, 2013.

  1. bwd111

    Thread Starter Member

    Jul 24, 2013
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    What is the Maximum permissible voltage between equiment ground and the netural conductor


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  2. wayneh

    Expert

    Sep 9, 2010
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    Permissible by who/what? In what context?
     
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  3. paulktreg

    Distinguished Member

    Jun 2, 2008
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    What country are you in bwd111?

    In the UK I think the Neutral conductor is connected to Earth at the nearest sub-station which on the whole is never too far away from most locations. What your local rules are is hard to say unless we know where you are?
     
  4. Rbeckett

    Member

    Sep 3, 2010
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    You should not have any voltage across the neutral to ground. That is how a GFI circuit breaker senses a fault and shuts down the power. If you have voltage on the common leg or on the neutral/ground you have a circuit fault that needs to be addressed. Most GFI's sende in the range of Millivolts so the max leakage should be zero or nearly zero at all times.

    Bob
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Grounding is an often misunderstood practice.
    There is a minimum to ground resistance for any ground rod or pipe etc which is mostly universal, that being 25ohms.
    There is also a universal 3 point method of earth resistance testing using ground rods with current and voltage being measured at various points.
    Actually the GFI detects an inbalance in current between neutral and live conductor, indicating that current could be flowing from live conductor to earth, therefore a fault.
    Max.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Perhaps this relates to the integrity of the equipment earth wire/bond. Under fault conditions such as active to frame short, sufficient current must flow to ensure the protective breaker or fuse operates. This necessitates having a maximum allowable earth bond resistance.
    In my country the MEN system is used.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    I'm pretty sure that a GFI senses an imbalance between the line current and the neutral current.

    If the system is operating correctly, there will be no current in the ground at all but there will be a current in the neutral. Thus there will be a voltage drop between the neutral and the point where it connects to the ground.

    If fact, the neutral and the ground will be at the same voltage only if all of the line current is going out of the socket and through some other path, such as you to the bathtub to the plumbing, and none of it is going through either the neutral wire OR the ground wire.
     
  8. MaxHeadRoom

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    Jul 18, 2013
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    It also senses if the current is going through the ground wire or ground, period, from the live, or any other way to ground, therefore upsetting the balance between neutral and live conductor.
    All the GFI does is detect an unbalance between neutral and live.
    The current should be equal in normal circumstances.
    Max.
     
  9. WBahn

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    Mar 31, 2012
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    Agreed. My point was that requiring that the neutral be no more than a few millivolts different than the ground conductor is pointless since this will be the normal case when things are working fine and would actually be more indicative of there being a problem.

    14 guage wire has a resistivity of about 2.5Ω per 1000ft. So if the outlet is 100ft away from the panel (as measured by the wire run) that would be about 0.25Ω. If it is a 15A circuit the peak current could be over 20A resulting in a 5V drop along the neutral.

    The only thing that makes sense is to look for a mismatch between the line current and the neutral current, though you could also look for there being any current in the ground prong.

    The thing the GFI won't detect is a current that goes out the line prong, through you, and back into the neutral prong, because there is no ground fault in that case. But this situation is rare and there will usually be enough leakage between the fault path and something other than the neutral path to trip the GFI.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Err - not exactly.

    Most "earth leakage" breakers compare the current flow in line & neutral - if they're not equal; some current must be going somewhere it shouldn't

    There is frequently a small voltage difference between earth & neutral leads - the neutral might be carrying current (volt drop) and the earth wire shouldn't be (no volt drop).
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    A gfci doesn't need a ground conductor at all to operate properly.
    Its all about line vs neutral current.. What goes in must come out.. if not "pop"
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    To look at it from the other side, at my job any equipement we produce at work is tested to withstand 500Vac between the PE and any other connection.
     
  13. mcgyvr

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    2 x RMS voltage + 1000 for me at work.
     
  14. MaxHeadRoom

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    Jul 18, 2013
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    A bit misleading, if you don't have a ground referenced supply (grounded neutral) it won't work?
    Max.
     
  15. wayneh

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    Sep 9, 2010
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    Sure it will, it literally compares current out versus current returning and trips if they are not very, very close to equal. Ground, per se, has nothing to do with it.
     
  16. WBahn

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    Mar 31, 2012
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    Then why is one of the normal techniques to provide protection in an old home that only has line-neutral wiring to replace the first recepticle in each chain with a GFI?

    Why do you need a ground-referenced supply? The GFI uses a differential transformer that has the line and neutral leads passing through it in opposite directions and a control circuit that is powered by line-neutral.
     
  17. mcgyvr

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    Not misleading at all.. 100% correct. NO ground is needed at all for a GFCI to operate properly.
     
  18. MaxHeadRoom

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    Jul 18, 2013
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    Where is the return path?
    if no grounded neutral?
    The current has to go some where people!
    Max.
     
  19. LDC3

    Active Member

    Apr 27, 2013
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    It goes back to the transformer on the pole. Since it is now outside the home, you no longer care about it.
     
  20. MaxHeadRoom

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    Jul 18, 2013
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    But you still need a ground return, that is my point.
    OK I referred back to the 'Bible' Soares 'Book on Grounding' used by NEC and International Association of Electrical Inspectors.

    Quote:
    The GFCI sensing system continuously monitors the current balance in the ungrounded 'Hot' conductor and neutral conductor and the current in the neutral conductor.
    If the current in the neutral conductor becomes less than the current in the live conductor, a ground fault could exist.
    A portion of the current returns to the supply by some other path other than neutral wire, e.g. a ground path back to the neutral source.
    The GFCI does not limit the magnitude of the ground fault.
    It limits the time that a current of a given magnitude shall flow.
    Unquote:
    IOW the neutral has to be grounded at some point for this GFI to work.
    An ungrounded neutral offers no return path if a live conductor contacts earth ground.
    Max.
     
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