Published:
2 December 2014
Category:
Q&A
John asks this question, which concerns a distribution board that doesn’t comply with Table 54.7 from BS 7671: 2008, and the size of earth cable that is required. It has been answered by SELECT (Theme – Distribution Equipment including Cable Management):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I’m looking for advice regarding the earth size for an existing installation.
I have been checking an EICR certificate and there is a distribution board fed from 25mm thermoplastic singles with only a 10mm earthing conductor.
I have highlighted this as a new observation that needs to be added to the EICR certificate, as it does not comply with Table 54.7 from BS 7671: 2008. Table 54.7 states that a 16mm earth cable would be adequate, but I have carried out the adiabatic calculation as a cross check and the result of the calculation is a minimum of a 32.67mm cable.
The power factor correction (PFC) at the distribution board is very high (11.88kA) and the overcurrent protective device for the distribution board is BS88 fuses, rated at 80A.
Should I recommend a replacement 16mm or 35mm earth cable?
If a 35mm was recommended, I assume that the live and neutral conductor size would also need to be increased to 35mm for fault current reasons.
Answer: The cross-sectional area of a protective conductor, other than a bonding conductor, can either be calculated using an adiabatic equation or by applying Table 54.7 BS 7671.
Calculation would normally result in a smaller conductor size than that is required by the table?. Assuming that the 25mm live conductors are adequate, and from the information provided, it would appear that they are, then without calculation, a 16mm earthing conductor would suffice.
A BS 88 fuse would normally have a breaking capacity well in excess of your measured 11.88 kA fault current.
I hope this is of some help.
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