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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..
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    AJJewsbury:

    Thus the neutral (star point) of the source could be distinct from the earthed point of the source.




    So in my diagram:
    cb4f0ba7a2819e2221fe9e9b04dd0042-huge-tn-s-paths_zpsyci5uu0c.png


    it doesn't matter how long the conductor between the star point ("0") and the N-PE link is - it's still just a neutral conductor.


    Ah, but it isn't!


    A line to PE fault will travel back to the N-PE bond and then back along the N conductor to the star point. If it headed for the general mass of the Earth, there would have to be another connexion between the star point and the Earth otherwise there is no circuit. Even if it did have a pathway through the Earth, most of the current would be carried by the N conductor because of its lower impedance.


    In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.

  • 0

    Chris Pearson:

    . . . In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.




    No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one. 


    if you take a good transformer and measure between the LV winding and the tank, it will be open circuit. Between the HV winding and the tank, it will be open circuit. Between the HV winding and the LV winding it will be open circuit. 


    Regards,


    Alan. 

  • 0

    Alan Capon:




    Chris Pearson:

    . . . In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.




    No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one. 



    Yes, I do understand that the connexion need not be made inside the transformer itself and it is self evident that there could be no connexion to the general mass of the earth within the tank. Of necessity in any system there must be at least a short length of cable internally from the star point to the connexion box (and similar cables for the lines at the other ends of the windings) and that cable must serve both as a neutral and a fault path. We can ignore that.


    My point is that in a TN-S system the PE must have its own connexion with the star point - you can have as many junctions as you like; but if the PE sprouts off the N at some distant point (as Andy drew it in his first diagram) then there must be a section of cable which serves both as N and PE - it cannot be any other way.


    Moreover, if we take a TN-S system, there are two cables which are connected to the star point: the N and the PE. If they are joined anywhere downstream of the transformer, they now become two cables in parallel and therefore the system becomes TN-C-S.


    If I am mis-reading Figures 3.8 and 3.9, I apologise for being thick.

  • 0

    Ah, but it isn't!


    A line to PE fault will travel back to the N-PE bond and then back along the N conductor to the star point.



    Which is why I said thinking about it in term of the complete earth fault loop isn't helpful. If you follow that logic you conclude that an entirely TT system is also TN-C-S because the connection to the star point carries both N and earth fault currents.

       - Andy.
  • 0
    I wonder if the argument could be made should the source earth be made at a point further away than would be considered “direct” such as in the PNB example, that the criteria implicit in the first letter “T” in the codes used for system earthing is not met. In other words unless the connection of the neutral to earth is direct (using the OE dictionary definition of “direct”) then it isn’t a “T” anything earthing system and not recognised by 7671.
  • 0

    AJJewsbury:




    Ah, but it isn't!


    A line to PE fault will travel back to the N-PE bond and then back along the N conductor to the star point.



    Which is why I said thinking about it in term of the complete earth fault loop isn't helpful. If you follow that logic you conclude that an entirely TT system is also TN-C-S because the connection to the star point carries both N and earth fault currents.


    Not at all. We are all agreed that the transformer's internal wiring is not included.

  • 0
    No we are not - the wiring within the transformer is just as much at phase or neutral voltage as the wiring outside. Of course the voltage changes each time it links round the core flux, but it is still an integral part of the circuit.
  • 0

    We are all agreed that the transformer's internal wiring is not included.



    But as Alan says the earthing of the N at source isn't internal to the transformer. Indeed if you look carefully at some rural systems where both HV and LV are overhead you'll often see N earthed not even at the pole the transformer is mounted on, but at the next pole several tens of meters away (as that's a simple way of segregating the LV and HV earths).


       - Andy.

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