Gus Ferguson asks whether a statement in the NICEIC’s Pocket Guide 14 about conductor cross sectional areas is correct. His question has been answered by the NICEIC (Theme – Cables, Wiring & Wiring Accessories):
Question: In carrying out checks on the size of a main earthing conductor (TN-S earthing arrangement) as part of an Electrical Installation Condition Report (EICR), I noticed that the NICEIC Pocket Guide 14 indicates that 'Certain lower limits apply to the cross sectional area (CSA) of a protective conductor.........and not less than that determined by selection (see later)'.
Is this an error, as I understand that the conductor CSA can be less than the table selection value if the adiabatic equation allows?
I have attached a PDF document containing an extract from the guide, which I have marked up accordingly.
Please click on the link near the bottom of this web page to download the attachment (PDF doc - 652kB file size).Answer: You are correct in stating that when carrying out an EICR, you can use an earthing conductor with a cross sectional area less than the sizes quoted in Table 54.7 - if the adiabatic equation is used.
If, however, the size is calculated to be less than 2.5mm, and no mechanical protection is provided, then a minimum size of 4mm would have to be used.
Where mechanical protection is provided as shown, the minimum size of earthing conductor is 2.5mm - this would be usually be part of a TT earthing arrangement.
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