Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a private lighting column installation on a commercial scheme, is answered by SELECT:
Please could you clarify whether an earth electrode should be installed on a private external lighting column installation on a commercial scheme, having a TN-C-S supply?
The query has been raised by another party however we have been unable to find any clear guidance on this, in particular with regard to whether the circuit supplying the external lighting columns is classed as 'exporting' the TN-C-S (PME) supply from the building.
Our understanding is that the particular section 26 within BS 7430, for street lighting, is applicable to public lighting systems, and not commercial scheme.
Answer: Regulation 559.10 in BS 7671:2008 provides requirements for outdoor lighting installations, highway power supplies and street furniture and does not contain a specific requirement for the installation of an earth electrode and TN -C-S earthing is permitted as the supply earth on such installations.
Clause 26.3 of BS 7430 does however recommend that where TN-C-S earthing systems are used and if supplying more than one lighting column that an earth electrode should be installed at the last unit and the resistance of this should not be less than 20 ohms - where values greater than this are obtained, then other electrodes may need to be installed. This can be omitted completely, however, if a more reliable neutral connection can be made by connecting the neutral at the last column (unit) to the neutral of a different distribution or supply system.
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