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Neutral - Earth Link in Main Switchboard

Nick Parker
When is an earth neutral link required in an main switchboard? Is it particular to the earthing system?


Is it common to earth the source neutral at the first point in the LV system where the neutral is terminated - the main LV switchboard. 


If the Neutral terminal of the transformer is grounded near the transformer and if it is interconnected with the other earth rods/bars? Is still neural - earth link to be provided at the Main switchboard?
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    Glibly, when there is not already a link somewhere else - there has to be at least one in the system for the ADS to work. - in TN-S and TT it is at the transformer, for TNC-s (PME) at each point where the TNC part becomes TN-s, that is at least at each building origin, usually.


    Also needed  if the feed from the transformer is a combined neutral and LV earth, in the manner of PNB.


    Also, often, but not always done at the board when there are multiple sources, such as pairs of private transformers, or private generators  to be combined - though in such cases where there are more than one  link the part  of the earthing between links needs to be sized for carrying the neutral current too, depending which supply or supplies is active or may be active in combination, and at least in the UK, this is carefully regulated, needing DNO or BNO involvement -ESCQR applies.


    In summary, there are quite a few situations - not sure which one you have in front ot you.
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    When is an earth neutral link required in an main switchboard? Is it particular to the earthing system?


    Is it common to earth the source neutral at the first point in the LV system where the neutral is terminated - the main LV switchboard.



    In general there's exactly one N-PE link in an TN-S system - it'll only appear at the main swicthboard if there isn't one at the transformer. (The text books often seem to assume it's always at the transformer, but it's often convenient in practice to have it at the switchboard when using a private transformer, especially if the HV and LV earths are separated).


    The same basic principle applies with TN-C-S systems but the link is sort of "stretched" along the length of the CNE (or PEN) conductor - earthed at the source end and N/PE split at the consumer end(s).


    Likewise TT systems will have the N earthed at one point only (again possibly 'stretched' along a CNE if the system also supplies TN-C-S installations).


    In the UK we never "re-earth" a true N - as I understand is traditional in the US and similar (although of course a CNE can be earthed multiple times along its length).


       - Andy.
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    In the UK we never "re-earth" a true N




    That is certainly the official position,and should be the situation all the time. Although we do sometimes come very close to it with some generator switching and building sized UPS arrangements where there may be multiple N-E links as part of the local distribution networks,and to make sure that ADS works when runing on UPS. Data centres with dual 11kV transformers that can be switched to be paralelled or split and have an N-E link each can become particularly confusing.

      However  as the wiring between the links is rated for load current, arguably that is electrcially more like having redundant PENs. It does mean great care should be taken if it ever seems that an NE bond needs to be removed for any reason.


    I have also had the fun (as a one off special), of wiring the CPC of an American made UPS back though the current transformer of the earth fault relay, so it's case could be earthed, but  the internal NE-short it created  did not look like a fault, to make it work, the CPC of the final circuits did not go via the UPS or pass through the core, so the final circuits still had earth fault protection when the UPS was not operating.  Given the whole thing was running from a priveate genset, there was no real issue.

    Very much not  BS7671 compliant, but it is not actually an offence to link a CPC and a neutral, so long as the same length wire is not used for both functions for any non-distribution wiring. (The UK LAW is A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation )

    Does not quite say you cannot have two conductors and link them in more than one place, though BS7671 does not like that.

     


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    Surely, we need to link the neutral[N] conductor to the Earth[E] in the consumers board to make sure we can hold the bonded metalwork in the house down to a low potential. The use of initials like CPC confuse me as the live phase wire is [L] not [P] and earth[E] and not [T]..[C] is something else I guess?
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    Hi Clive S

    Which country  are you from, and how many wires come in from the supply company ?

    Here (UK ) a single phase 230V supply  is normally 2 wires  (Live and Neutral, no company provided earth) or 3 wires (Live Neutral and a CPC = Circuit Protective Conductor = A wire for the ground/ earth connection.)


    If the company does not provide an earth you must not connect your private earth to the company neutral.


    3 phase supplies, common in larger buildings, are 4 wires or 5.

    There is an alphabet soup of shorthand...


    Now to make sure fuses will blow,  and the ADS (Automatic Disconnection Supply ) occurs in an earth  fault condition the CPC is connected to neutral, but  this link is supposed to be in the supply company equipment, not the consumer's - in fact to share the two functions of CPC (earth) and Neutral by a single wire is not permitted on the consumer side of the puibic supply, by UK law.


    IF the supply company distribution has the NE bond at the transformer, and then separated N and E to each consumer that is TN-S , if it is combined,and the splitting of the neutral to be a neutral and  a CPC is done at or near each property, that is TN C  (French, Terra/neutral separe  and Terra/Neutral combine)

    TNC-s is a short hand for neutral  and earth combined for the big stuff outside, and separated at the consumer side, common in newer towns and where rewiring of street mains has occurred. PME - protective multiple earthing, is another name for the same system, where for distribution, earth and neutral are combined, with lots of earth electrodes along the path of the neutral (PEN) - Protective Earth and Neutral.

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    The use of initials like CPC confuse me as the live phase wire is [L] not [P] and earth[E]



    The regs are written in a "living language".. (i.e. it keeps changing). At the moment..


    Brown wires are "line" (hence L is OK again).

    Blue wires are neutral (N)

    Green/yellow ones are protective conductors - often either protective bonding conductors or protective earth (PE or earthing symbol). Protective conductors associated with a particular circuit are circuit protective conductors (and in some system - e.g. separated systems supplying more than one item - aren't actually earthed at all).


    Combined neutral and earth conductors are PEN (Protective Earth and Neutral) (or sometimes referred to as CNE - Combined Neutral Earth - especially by those who doubt the safety of that approach and so like to deliberately omit any reference to "protective" in that context!)


    The term "live" now means any Line or Neutral conductor - i.e. the normal current carrying conductors in normal service (but not a PEN, and ignoring protective conductor currents).


    Mike's already described the different system for describing how an earthed system is arranged (which doesn't apply to non-earthed systems, of course, even when they have green/yellow protective conductors)


       - Andy.

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