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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..
  • Former Community Member
    0 Former Community Member

    John Peckham:


    If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.

     




    That would be TN-S, John - all that's happened is that the physical star point has been expressed out to the Tier 1 switchboard in order to physically make the N-E bond and reduce the N-E offset for the installation.


    If it's a DNO transformer serving a single consumer they would describe it as PNB - but would also tell you it's TN-S, despite telling you to bond it as if it were PME


    Regards


    OMS

  • 0
    Evening OMS


    Not according to BS 7430, GN8 and the ECA Guide to the Wiring Regulations.
  • 0
    When I first read the OP, I assumed that the green/yellow cable was a protective conductor which was augmenting the armour of the 4-core cable, which I assume was insufficient on its own.


    Now what puzzles me ('cos I can just about cope with things on a domestic scale) is why this cable is marked as a protective conductor (514.4.2) when it is connected to a neutral terminal.


    Would somebody kindly explain please?
  • 0
    Chris


    Apart some special circumstances you are not allowed to combine the neutral and earth in the consumers installation.  The "S" in TN- C- S means separate. So we colour our protective conductors green/yellow even if they are connected to the supply neutral.


    I am a bit sceptical about the arrangement in the OP. Having a neutral coloured green/yellow run outside a SWA armour with one inside is a bit worrying! Running a separate earth with SWA or AWA cables is common practice but they are connected to the earth bar not the neutral.
  • 0

    Chris Pearson:

    When I first read the OP, I assumed that the green/yellow cable was a protective conductor which was augmenting the armour of the 4-core cable, which I assume was insufficient on its own.




    There are two earths to a transformer. The windings are both earth free.


    There will be a bare earth run from the transformer tank to the switchgear and too the HV earth rods. There is probably also a loop of copper round the substation, also connected to the HV earth in an attempt to give the operator an equipotential zone to stand in (as far as possible). 


    There will be an insulated earth conductor run from the LV earth rods (sufficiently far enough away from the HV rods to be out of their influence), which will be connected to the star point (neutral) of the transformer. This would normally be connected at the LV fuse board in the substation, but with a single customer off the transformer may be connected at the incoming circuit breaker on the customer’s LV switchboard. This is TNS as there is no conductor that carries both neutral current and earth fault current. 


    Depending on the rise of earth potential for an HV fault, the HV and LV earths may be linked, but this is still TNS. 


    Regards,


    Alan. 

  • 0

    mapj1:

    . . . 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. . . 




    With an ESI 35-1 standard transformer, the neutral - earth bond is always outside the tank. 

     


    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.


    It is actually worse than that. To reduce losses, the HV winding is wound over the top of the LV winding (I might have that the wrong way round), which gives the opportunity for a flashover between the HV and LV windings. For this reason, the neutral to earth bond must never be removed with the transformer in service. 


    Regards,


    Alan. . 

  • Former Community Member
    0 Former Community Member

    John Peckham:

    Evening OMS


    Not according to BS 7430, GN8 and the ECA Guide to the Wiring Regulations.




     

    Well, with all due respect to the publications author, they are wrong - and there is a certain amount of copying in class going on anyway on this issue


    Your argument is based on nothing more than the length of a conductor connecting to the means of earthing  - ie, if you physically don't make a connection at the TX spill box, and make it at the switchboard then it must be TNC-S - which is complete nonsense.


    If I give you the example of a switchboard with the transformer forming part of the board, your logic says TN-S (because you cannot see the short length of copper from the winding ends to the switchboard neutral)


    If I sperate the switchboard and transformer by a short distance and use a bit of cable to make that link, then your logic says "Ahh - but it's TN-C-S"


    Only if the neutral conductor is connected to earth at the transformer spill box and again at the consumer switchgear (or anywhere along that run) does it become TN-C-S. In most circumstances that would be unusual, because you cannot determine where Earth Fault current is going via the protection CT's - eg, you couldn't operate a Restricted earth fault scheme operating from the star point, including the LV windings and up to the primary circuit breaker


    I suggest you apply the same logic to a generator as you do to the transformer and then tell me it's TN-C-S


    Regards


    OMS


  • 0

    there is a certain amount of copying in class going on





    I agree - I have been told by folk who should know better that there is no such thing as TNS PNB , if it is PNB it must always be  TNCS.


    To which  I say, unless there is a PEN, and two points where N and E link it cannot be.  The point about earth fault detection is a good one, and chimes in as the big current version of the error folk make with  an RCD on a small genset with floating windings.


    By all means insist, as many DNOs do, that it should be bonded as if TNCs, as one day there may be another building added on the same transformer with a CNE supply, but that is not the same at all as the private case when it is all under one control.


    But then the same technical authorities are unable to separate heating due to magnetic hysteresis and that from eddy currents, and have a very wrong, but well copied diagram showing current in loops in the metal around a wire penetration.
  • 0
    In the original post it's said that there is a cable between the LV neutral and the transformer case surely this is wrong as the LV. Neutral will be raised to some high voltage in the event of an a HV flashover  as unlikely as that is  is it allowed to have that link?  I thought the two sides should be kept seperate
  • Former Community Member
    0 Former Community Member
    It depends on the actual resistance of the HV and LV electrode, Kelly


    Traditionally, if it was under 1 ohm, we combined them and called the site "cold" - usually the HV side was in underground cabling with a metallic sheath


    If it wasn't we separated them by some distance so the LV electrode didn't pick up the HV rise of earth potential - we called that a "hot" site - usually supplied by an overhead HV system with no over running earth conductor


    We now calculate the actual ROEP and base what's tolerable depending on expected speed of disconnection (keep in mind we often have intentional delay on the HV side to allow for discrimination) - unless you happen to be wandering around the transformer when wet and naked, it can easily get to somewhere near 2000V under your boots and still be considered "safe". Historically, most electrical equipment would be able to withstand a 2kV voltage being impressed on it - not so sure that equipment today is built to that kind of standard. Telephones are a particular problem, however - not that many still grip a handset of a traditional phone.

    In practice, with high-speed protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 650v or more. With slower protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 430v or more. If neither is true, the site is cold


    I agree that the cable connecting the system neutral bar to the case of the transformer sounds really suspect, however


    Regards


    OMS

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