I think it depends whether the cable is buried or not.

Regulation 543.6.1 says:

543.6.1  Where overcurrent protective devices are used for fault protection, the protective conductor shall be incorporated in the same wiring system as the live conductors or in their immediate proximity.

So provided the external cpc is in its immediate vicinity, it could be OK to provide a reinforcing cpc.

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc ... and that would also potentially imply an internal "additional cpc" is also out. Depending on how you view Regulation. 522.8.10 may also be interpreted that a suitable duct (for example, high impact resistance"twinwall" or similar) is able to provide mechanical protection for buried cables, so if using that you can ignore the fact the armour has to be a cpc.

BUT THEN AGAIN, nothing to say any cpc has to be able to take all the fault current, and can't be re-inforced by additional parallel conductors.

If you are using the reinforcing cpc method, the earth fault loop impedance of the combined line conductor and cpc are calculated in accordance with PD IEC/TR 50480. This is referenced from Appendix 4 to BS 7671, and further guidance can be found in the IET Electrical Installation Design Guide.

gkenyon said:

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc

The interesting phrase is, "suitable for use as a protective conductor". That might be read as excluding any cable whose armour is insufficient as a CPC, in which case a conduit or duct would be required and of course the extra CPC would be within the same duct.

I don't think that 543.6.1 is particularly relevant because "wiring system" as defined includes, "parts which secure and, if necessary, enclose the cable" so fastening the extra CPC under the same cleats or cable ties, etc. would include it in the same wiring system.

gkenyon said:

BUT Regulation 522.8.10 could be said to imply that if the cable is buried directly in the ground, the armour needs to be able to act as a cpc ... and that would also potentially imply an internal "additional cpc" is also out. Depending on how you view Regulation. 522.8.10 may also be interpreted that a suitable duct (for example, high impact resistance"twinwall" or similar) is able to provide mechanical protection for buried cables, so if using that you can ignore the fact the armour has to be a cpc.

mmmm..... i hesitate to put my interpretation of this reg to the guy who I had assumed wrote it? 

I've always interpreted this reg to say something along the lines of,

If the cable is buried directly in the ground without the use of proper duct (my interpretation here - Twin walled smooth bore duct)  then the cable shall be armoured and that armour shall be earthed. I've always assumed that the use of the words, "earthed armour or metal sheath or both" allowed for the use of cabling other than our standard SWA or AWA, Lead sheath maybe? Copper sheath of a Pyro (m.i.m.s) cable I suppose. Has to be........

I tend to use SWA as standard in buried (ducted) supplies, but I've always assumed that this regulation did not preclude the use of other cables types as long as it was in proper duct (or conduit - but who buries conduit? and does it need to be earthed too?) 

And as for 543.6.1, I've always understood the use of those words in the last part of the reg, " .......or in their immediate proximity" to allow for that parallel earth specifically used commonly with SWA supplies. Its why I thought that was there for?

Its My take on it anyway........

Hi Graham,

So if we are installing a SWA direct in the ground supplying a piece of equipment with a protective conductor current higher than 10mA, would we still be ok to run a separate cpc of not less than 10mm alongside the SWA to satisfy the requirements of 543.7.1.203 or would this need to be internal to the SWA or would this need to be 16mm (Table 54.1) as it is not provided with mechanical protection or could we just use the armour if this is sized adequately as a cpc and over 10mm.

Regards

Mark

So if we are installing a SWA direct in the ground supplying a piece of equipment with a protective conductor current higher than 10mA, would we still be ok to run a separate cpc of not less than 10mm alongside the SWA to satisfy the requirements of 543.7.1.203 or would this need to be internal to the SWA or would this need to be 16mm (Table 54.1) as it is not provided with mechanical protection or could we just use the armour if this is sized adequately as a cpc and over 10mm.

I would have thought it unlikely you'd need a duplicate c.p.c. for SWA to satisfy 543.7.1.203 - as the armour will almost certainly be >>10mm² already - the requirement is absolute c.s.a., not copper equivalent. If you did have a protective conductor buried in the ground, then yes it would need to satisfy all relevant requirements including table 54.1 (as per the last part of 543.1.1) - possibly larger if it also connects to extraneous-conductive-parts outside the building (i.e. has to serve as a main bonding conductor too).

   - Andy.

So if we are installing a SWA direct in the ground supplying a piece of equipment with a protective conductor current higher than 10mA, would we still be ok to run a separate cpc of not less than 10mm alongside the SWA to satisfy the requirements of 543.7.1.203 or would this need to be internal to the SWA or would this need to be 16mm (Table 54.1) as it is not provided with mechanical protection or could we just use the armour if this is sized adequately as a cpc and over 10mm.

I would have thought it unlikely you'd need a duplicate c.p.c. for SWA to satisfy 543.7.1.203 - as the armour will almost certainly be >>10mm² already - the requirement is absolute c.s.a., not copper equivalent. If you did have a protective conductor buried in the ground, then yes it would need to satisfy all relevant requirements including table 54.1 (as per the last part of 543.1.1) - possibly larger if it also connects to extraneous-conductive-parts outside the building (i.e. has to serve as a main bonding conductor too).

   - Andy.