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Probably a stupid question...

tonydon

Right...

We are on a site with 2 onsite substations providing our supplys.

Upon inspection the supply comes from the substation transformer room, into our switch room, as a TP&N feed to our section boards.

We cant see any earth other than the armour of the supply cable which is obviously attached to the section board.

A colleague and I have been debating whether this is a TNS or TNCS supply.???

Given that the transformer is just the other side of the wall in our switch room, and that the earth and neutral must be bonded at the star point of the transformer secondary.

Are we right in thinking the supply is TNS??? 

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    tonydon said:

    Given that the transformer is just the other side of the wall in our switch room, and that the earth and neutral must be bonded at the star point of the transformer secondary.

    Are we right in thinking the supply is TNS??? 

    It may well be an "effective TN-C-S" system, as technically N and PE will be coupled at both transformers, and earth nest at one transformer, or the main switchboard.

    However, in this case, PME conditions don't apply, and effectively from a BS 7671 perspective it's treated as a TN system that isn't PME ("TN-S" if you like, but perhaps not fully "TN-S").

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    tonydon said:

    Given that the transformer is just the other side of the wall in our switch room, and that the earth and neutral must be bonded at the star point of the transformer secondary.

    Are we right in thinking the supply is TNS??? 

    It may well be an "effective TN-C-S" system, as technically N and PE will be coupled at both transformers, and earth nest at one transformer, or the main switchboard.

    However, in this case, PME conditions don't apply, and effectively from a BS 7671 perspective it's treated as a TN system that isn't PME ("TN-S" if you like, but perhaps not fully "TN-S").

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