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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.

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  • 0

    Is the means of earthing distributed from the transformer via a conductor to the installation? If it is then it is a TN system.

    If the means of earthing is not distributed from the transformer by a conductor ,and the means of earthing for the installation utilises an earth electrode then it is TT. 

    JP

  • 0 in reply to John Peckham

    Hi John,

    Thanks for the reply. The means of earthing is distributed from the transformer by a conductor via the buried earth ring connected to the earth electrode. This means the earthing for the installation also utilizes an earth electrode which makes it a hybrid? Sorry, i am still having issue wrapping my head around this.

  • 0 in reply to CH

    So, if the underground electrode failed, ,would there be a total loss of earthing, or would the CPC coming in alongside the power conductors on its own be adequate ? - if the latter there is no doubt it is TN. It just happens to have a shared  electrode as well, which in reality a lot of TNs supplies into rows of metal frame buildings sort of do by accident. 
    Mike

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  • 0 in reply to CH

    So, if the underground electrode failed, ,would there be a total loss of earthing, or would the CPC coming in alongside the power conductors on its own be adequate ? - if the latter there is no doubt it is TN. It just happens to have a shared  electrode as well, which in reality a lot of TNs supplies into rows of metal frame buildings sort of do by accident. 
    Mike

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