Richie Webster asks, in terms of inspection and testing, which certification figures to be used when using cable armour as the circuit protective conductor (CPC). His question has been answered by the NICEIC (Theme – Cables, Wiring & Wiring Accessories):
Question: When filling out the circuit schedules on certification, I am unclear as to which figures should be used when using the armour or sheath of a cable as the circuit protective conductor (CPC).
My understanding is that these figures should be the K values in the various tables within Guidance Note 1. However, it has been brought to my attention by a former collegue, that it should be the equivalent copper size that should be input in this box. Is that so?
Nevertheless, whichever way is correct, would it be acceptable to input the circuit protective conductor (CPC) size as sheath, armour etc., instead of the size stated in these tables, as long as the inspector confirms that the size of CPC complies with BS7671, either by these tables, or by the adiabatic equation, as this would save a lot of time?
Answer: The electrical certificates ask for the size of the earth. GN1 page 148 Table D.10A provides you with the actual size of the armour CSA from BS 5467 and BS 6724. You may wish to put an asterix and a note to say where these figures were derived from.
The adiabatic calculation provides you with the minimum size earth. That is not what is asked for on the form. If the size of the armouring is shown to be too small from the table, you should still note, but then show that by using the adiabatic equation, the earthing will be compliant and a note should be made in the summary of installation.
But first, one would need to verify that the steel wire armouring is suitable for use as a CPC, with reference to Table 54.7 of BS7671, page130. If this is proved to be sufficient, then you need to record on the certificate that the conductor material is steel and enter the actual cross section area of the steel armouring on the certificate.
Reference to the size of cross sectional area of armouring can be accessed from the manufacturers, or from Appendix D of IEE Guidance Note 1.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions