Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a Completion Certificate, and which box to tick if there are two supply systems, is answered by the NICEIC:
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 consumers 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 on existing installation' section of the installation certificate.
The installation earth electrode should be identified on the certificate by ticking the box 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 for a TT system arrangement, reference to Regulations 411.5 and 531.4 for example should be made.
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