Q & A of the Day - Can the client insist on a TN-S or TN-C-S supply?

Question: In a multi-tenancy retail unit under the same metal roof, we have been supplied with a TT supply by the electricity utility company. These units are 15,000 square feet each in area and although the electricity supplier has a TNCS supply into the first unit, it will not give a earth connection to subsequent retail units that are looped from the first unit.

The supplier has insisted that we supply our own earth via rods, for which we have a Ze of 20 ohms on the rods only. When connected up to steel etc, we are getting a reading of 0.06 ohms. Because we have a TT system, we have installed RCD of 300mA time-delayed on the mains as per client design. However, we are getting problems of intermittent tripping, which could be caused by heating or refrigeration, or simply by the lighting control gear.

The question is, can the client insist on a TN-S or TN-C-S supply, or is the client stuck with what supplier has given them?

If so, we can only remove the RCD from the mains, covering individual circuits or groups of circuits with RCDs - plus all the associated expense.

Answer: Is the structural steel in your unit bonded to your new TT earthing system? Also, is it the same structural steel frame as the TN-C-S supply to the first unit? It should be.

The 300mA RCD should not be tripping with healthy circuits and loads connected, unless there is some problem with the supply authority's neutral supply to the building, which could lead to normal load currents for the whole building returning to the transformer through the structural steel of the building creating a potential difference between your supply neutral and earth.

You cannot insist on a TN-S or TN-C-S supply, and you cannot simply remove the 300ma RCD, because you are not being provided with a sufficiently low Ze by the supplier.

I suggest that you speak to the supplier again, inform the people there that you are duty bound to connect your structural steel to your TT earthing terminal and are, therefore, connecting to their neutral earth in unit one anyway.

If they provide a TN-C-S connection within your unit, the supply neutral and earth will be connected at your origin and should prevent a potential difference occurring. Or you could try a 500mA time-delayed RCD, but that may not resolve the tripping problem.

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