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Classification of an earthing arrangement where an interconnected earth electrode system provides the earth fault return path — TN-S or TT?

CH

Hi all. I am trying to determine if this system is considered as a TT or TNS based on the image above.

From BS7671

TN System is

A  system  having  one  or  more  points  of  the  source  of  energy  directly  earthed,  the  exposed conductive-parts of the installation being connected to that point by protective conductors.

TT System is

A  system  having  one  point  of  the  source  of  energy  directly  earthed,  the  exposed-conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source.

The design earth fault current return path is from the equipment earth bar to the equipment earth electrode to the buried bare copper ring back to the transformer earth electrode and back to the transformer earth bar then back to the transformer neutral.

I am trying to determine whether a fault current path that uses an earth electrode system can still be considered a protective conductor, and whether this arrangement would be classified as a TN-S system rather than a TT system, given that the installation earth electrodes are not electrically independent from the source earth electrodes.

Parents
  • 0

    Where do the live conductors go? Surely, they must be accompanied by a protective earth, which would join the various earth bars/terminals?

  • 0 in reply to Chris Pearson

    Hi Chris,

    Thanks for the reply. From my knowledge, the live conductors to the equipment are accompanied by a protective earth. The issue i have is with the earth fault return path that slightly deviate from the conventional TN system as the earth bar is connected via the buried earth ring and earth electrode instead of the conventional bus bar to bus bar connection above ground. 

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  • 0 in reply to Chris Pearson

    Hi Chris,

    Thanks for the reply. From my knowledge, the live conductors to the equipment are accompanied by a protective earth. The issue i have is with the earth fault return path that slightly deviate from the conventional TN system as the earth bar is connected via the buried earth ring and earth electrode instead of the conventional bus bar to bus bar connection above ground. 

Children
  • 0 in reply to CH

    It does not seem to fit the examples given in 312.2 or . I would find it easier to understand if the live conductors were included in the diagram.

    CH said:
    I have been thinking about this for abit and have a quick follow up question.If this is considered a TN system for fault current calculations, should the earth loop impedance include the parallel return path through the earth?

    I think that over-complicates matters. I suspect that the multiple earth electrodes in a PME system are not taken into consideration.

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