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Question about Earthing options for two 3 phase mains supplies

Paul Duffin

The site has two buildings (A&B) each with a 3 phase mains supply and its own earth. One is TN-C-S, one is TN-S.  I wish to run one of the phases from building A into building B.  The supplies come off the DNO road cable approx. 100m apart.  I am interested in what options there are for providing an earth for the phase from building A running into building B.  I can see two options but am looking for advice about whether either of these would be acceptable.  The options are:

1. Connect the two earth's together.

2.  Use earth A up to the building but earth B inside the building.

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  • 0

    I think that the CPC must accompany the live conductors.

    You must not have two simultaneously accessible different earthing systems. If one is TN-C-S and the other is genuinely TN-S, then I would say that they are different. However, if they are connected to the same transformer, and one is TN-C-S, the apparently TN-S one is also effectively TN-C-S.

    100 m apart is quite a long way. Are you certain that the two service cables are branches of the same street mains?

    How far apart are the buildings? If the supplies are both perpendicular to the road, that suggests 100 m, in which case voltage drop and EFLI might be considerations.

  • 0 in reply to Chris Pearson

    I think it's highly likely the supplies come from the same transformer but will check with the DNO. I agree with the points about voltage drop and EFLI.

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