This question, concerning a Completion Certificate, and which box to tick if there are two supply systems, has been answered by the NICEIC (Theme - Regulation & Legislation):
Question: Concerning the 'Completion' Certificate, where it states what the system type is in the property, what box would you tick if there is a TN-C-S supply entering the property, but not being used as the earthing point?
Bear in mind that the consumer's main earth is via an earth rod.
Would the system type be classed as a TN-C-S or TT?
Answer: This arrangement would be classed as a TT system as the means of earthing used by the consumer is that of the consumer's earth rod electrode. It would, however, be good practice to record the presence of the TN-C-S system entering the building in the 'Comments' section of the certificate.
The installation earth electrode should be identified in the 'Means of Earthing' section and its details recorded, including type, location and resistance to earth in the 'Details of Installation Earth Electrode' section.
It is important to note that the prospective fault current for a TT system should be recorded between live conductors, live-live or live-neutral as this will be the highest value.
It is also important to ensure that the installation meets all the particular requirements of BS 7671:2008 (2011) for a TT system arrangement.
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