Ground-Neutral Connections and the Current Return Path

Discussion in 'General Electronics Chat' started by zachl1220, May 20, 2016.

  1. zachl1220

    Thread Starter New Member

    May 20, 2016
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    Hi Everyone,

    I've been reading up on earthing/grounding practices, and this bit keeps confusing me. I will quickly run through my understanding of the return path for current in a domestic situation:

    In normal conditions, current flows in the live and neutral conductors, and the return path is generally provided back to the substation through these conductors as a low impedance connection (alternating flow and return of course in an AC circuit). In this situation, though, the neutral line is also connected to the separate earthing conductor at the main panel (if we consider this to be a TN-S system), which is then connected to the earthing rod at the house. According to wire regulations (at least where I am in the UK) the impedance of the ground path back to the substation through the ground rod to the substation's earth stake is required to be below a certain value (for 32A circuits, this may be <5 Ohm for example) in order for overcurrent devices to trip in the appropriate time frame. To me, that is quite low resistance indeed.

    Given this low impedance path back to the substation from the house's grounding rod, is this path not constantly used for flow and return of normal operating current? If the path is indeed low impedance, then why do we need a separate neutral conductor as a flow/return into our houses? I know that this does occur in certain situations and is labelled Single Wire Earth Return (SWER), but I am struggling to understand why we don't always do this if wiring regulations require us to maintain a low impedance path back to the sub-station through the Earth.

    In addition, if we consider an earth fault on a conducting surface of equipment, because the ground and neutral conductors are connected, fault current must be flowing through both the Earth loop path as well as the normal neutral return. Is this correct?
     
  2. BR-549

    Well-Known Member

    Sep 22, 2013
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    What is a TN-S system? My understanding of a "neutral" must be different from yours.
     
  3. zachl1220

    Thread Starter New Member

    May 20, 2016
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    I think you've caught on to a problem I have noticed in my understanding of TN and TT earthing systems. I believe my mistake is in thinking that most/all electrical systems have a grounding rod. You are right in suspecting my understanding of TN-S is wrong, because from what I have just read, it seems that a TN-S system is characterized by no grounding rod at all, and the ground conductor uses the neutral return path in a ground-fault. Perhaps this is the key to what was confusing me, but I'd still welcome some clarification from anyone. Another question that comes to mind is that if transformers typically have an earth stake, then how does current flow and return further upstream from the transformer? I guess in a three phase situation, there should be minimal current returning through the neutral (which may go through the Earth path?) unless loads across phases are out of balance. In a single phase situation, I am still at a bit of a loss.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    Show me a print of your distribution system. A picture will define your terms for understanding.
    What is TN and TT?
    I'm still trying to understand the question.

    Is English your first language? And where are you at?
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    :confused:
    His "English" is 100% fine..
    Is English your first language?
    The terms he is using is just over your level of knowledge.
    You can learn the terms here..
    https://en.wikipedia.org/wiki/Earthing_system
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In some early cases a transmission system was used and earth ground was used for the return path.
    Unless things have changed in the UK the service Co. does not provide a earth ground conductor, it is up to the end user to provide a suitable ground resistance path back to the grounded neutral at the distribution transformer.
    Using a metallic water pipe or ground rod, if the resistance does not meet spec, then a GFCI breaker should be used.
    Ground Practices
    Max.
     
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  7. BR-549

    Well-Known Member

    Sep 22, 2013
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    English was always the hardest foreign language. No doubt. Thanks for the clarification.

    Care you answer his question?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A separate conductor (neutral) is used as a return path to provide a stable and known path to supply the current demanded, this does not place any burden on the earth part of the system which is reserved particularly for safety reasons.
    For an in depth explanation on the theory behind Earth Grounding I suggest the book by Eustace Soares "Book on Grounding" published by the International Electrical Insp. Assoc.
    It is used world wide as a reference.
    Max.
     
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  9. Jahnlee

    Member

    Jul 2, 2015
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    I came across 2 videos explaining grounding systems in the UK and US.




    The TN-S has a separate earth wire. In a L-E fault, it does not use the neutral return path.
    The low impedance path you mentioned is the Ze, kept low for the purposes of triggering the protective device and not meant for normal current return path. Also, I think in the UK (I'm not from UK), the earth and neutral are not connected at the consumer unit unlike in the US where it is connected at the main panel as per the videos. So, under normal operating conditions, current does not return via the earth. If you have RCD (aka GFCI) it would trigger the RCD if current (above 30mA) flows via the earth.

     
    Last edited: May 20, 2016
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  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In the UK the Neutral and Earth ground are bonded at the utilitiy sub-station but not at the consumer unit. The consumer ground is (or at least used to be) a copper conductor clamped to the galvanised steel wire armouring of the incoming mains supply cable when such armouring is available.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    When I practiced there that was a no-no in residential installations.
    Max.
     
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