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What earthing arrangement is this?

Former Community Member
Former Community Member
The supply is from a private transformer in a four core cable 3ph + n. The cable armour is earthed and connected to the MET. However there is also a green and yellow cable connected to the neutral terminal at the main isolator going back to a the transformer casing. The transformer is only 5 or 6 metres away. I think this must have been intended to make it a tncs supply but seems to me to just create parallel neutral conductors. Or is it tn-s-c-s?  I have only been able to go off visual inspection because I could not disconnect the supply..
  • 0
    its TNC-S


    standard setup with privately owned transformers. 


  • 0
    One NE bond, just for you, is strictly a PNB.


    PNB can be seen as being either TNS like or TNC-S like.


    Assuming only one NE bond and  one point of earthing for the LV side, it can be treated as TNS, just with the NE link outside the transformer tank, all the failure modes and fault currents are the same,

    If multiple links multiple transformers multiple earthing then treat more like PME.

    If in doubt about the NE links, then treat like TNC-s
    Like these chaps do


    Is there a separate HV earth ?

    If not you must assume HV and  LV earthing is shared and never interrupt the common CPC while the HV side is connected, even if the LV is off, or the transformer core and all the metalwork attached to it, will be driven to some nasty high voltage by the capacitance between the HV windings and the core and tank.

  • 0
    Mildly amusing, im in charge of our facility, but not an electrician, and know our setup was initially described as PNB. It has eight earth rods, and earth tape in a trench for the HV side, and the LV side, cant remember which way around, or if they are connected together, but the HV people added their own earth arrangement (I think theirs was the un-insulated tape and rods)


    Anyway, when I describe it as PNB to any electrician, or 'consultant', I get a sort of sympathetic dry smile and a pause, then I'm usually ignored. 


    Also I note that the EIC forms don't have a tick box for PNB


    On occasion, ill find an electrical sat on the floor of the powerhouse, studying the busbars and the N-E link 


    At a guess, I'd say most electricians don't get to work on or near private transformers much, hence the unfamiliarity, it is frustrating when they assume I'm just being thick though.
  • Former Community Member
    0 Former Community Member

    Johno12345:

    its TNC-S


    standard setup with privately owned transformers. 


     



    TN-S surely - it's a single consumer from the TX LV


    All that's happened is that the single N-E bond has been expressed to the offloading switchboard (usually to keep the N-E offset voltage low for the consumer)  - rather than as draw in in the textbooks as being made at the transformer spill box


    Regards


    OMS


     
     

  • 0
    i see your point, but everyone calls it TNC-S that I have come across. which probably means PNB is a better term for it. 


    Usually, the multiple earth rods and tape is used to explain why its TNC-S 


    its not combined though, Ill have to check but I think its got a link between N and the transformer frame in the spreader box too
  • 0
    I’ll agree with TNS as there is only a single neutral-earth bond. The bond is usually made at the incoming circuit breaker on the board, as this lowers the neutral to earth voltage. If it was TNC-S, there would be a conductor that fulfils both the neutral and earth function. In the setup you have described with only one neutral-earth bond, there isn’t one. 


    Regards,


    Alan.
  • 0
    would you go with PNB or TN-S though?
  • Former Community Member
    0 Former Community Member

    Johno12345:

    i see your point, but everyone calls it TNC-S that I have come across. which probably means PNB is a better term for it. 

    But it is actually TN-S - all that's happened is that the N-E Bond has been expressed out to the first consumer switchgear position


    Usually, the multiple earth rods and tape is used to explain why its TNC-S 

    It's just an electrode - the fact that there are multiple electrodes in the array does not make it a multiple earthed neutral - the complete electrode has a single connection to the system neutral


    its not combined though, Ill have to check but I think its got a link between N and the transformer frame in the spreader box too

    There will be a HV earth (or so called steelwork earth) and the LV earth, which is the mechanism that ensures the neutral is solidly and reliably at or around earth potential  - it should usually have a single N-E bond.  The connection or otherwise of the HV and LV earthing together is a function of the effective resistance  - calculated normally to limit the rise of earth potential under a HV fault (usually interwinding) and limt the rise of voltage on the neutral to 2kV  - historically this would have been a limit of 1 Ohm, but today we calculate the actual value of ROEP depending on the speed of protection. (eg 430V for "normal" and 630V for so called high speed disconnection. Earth connections to the transformer casing are usually bonding rather than earthing - depending on the physical make up of the receiving switchgear, that connection is from the MET - but may physically be at the neutral conductor depending on how the link is arranged on the incoming side of the de facto main switch (ie even when the switch is open, the N-E bond and equipotential bond of the transformer frame should remain in place




     

    Regards


    OMS
  • Former Community Member
    0 Former Community Member

    Johno12345:

    would you go with PNB or TN-S though?




     

    Depends on which capacity I was acting in - for the consumer, I would describe the system as PNB (assuming the DNO own the TX) and insist you achieve bonding as if the system were defined as PME


    If the consumer owned and operated the transformer, I would describe it as TN-S


    It's just semantics - you have a system with a single N-E bond - you don't have a system with a multiple earthed neutral (that would be illegal in the UK, unless you were operating under ESQCR)


    Regards


    OMS
  • 0

    Johno12345:

    would you go with PNB or TN-S though?




    The 'systematic' T[TN]etc naming came to the UK via CENELEC/IEC from other countries (French origin?).  PNB/PME are names of UK origin and almost purely UK use.  So they're not exclusive: a system can be both PNB and TN-S,  PME and TN-C-S,  non-PME but TN-C-S (in some countries, anyway), etc. 

    I'd go for TN-S (if it fits the description others have already given for having a single N-E conection), as it seems the EIC forms are aligned to the IEC-style naming.  Isn't it possible to append '(PNB)' to that (if handwriting), if you're really wanting to make sure you've declared every detail?

     

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