When we do this we consider the case when the 10mm is the only CPC to be an acceptable case as well.

The thinking goes as you chase back towards the origin, while you have meshes and multiple earthing at the low current load end. at some point there is a single CPC at a point where there is a lot of leakage, - even if it is only where it joins the suppliers cables, but by that point the connection  is so chunky and so  important already that accidental disconnection perhaps from a minor damage, is most unlikely.

This allows circuits that really need a 300mA RCDm by dint of leaking a lot more than 30,  to have one, and just be done in suitably chunky cable - even f at the load end we have racks with 3 core cables and additional braids and fly-leads.

We tend to go a bit beyond the regs and have additional warning labels for kit /installations where high CPC currents are expected.

Mike

Is this additional to the CPC?  

Nope, it's a single conductor. Duplicate c.p.c.s come under (iii).

The requirement is just 10mm² rather than 10mm² or 10mm² copper equivalent, so it's likely the steel armour of your cable alone would be sufficient. Use a core in parallel by all means, especially if there's any doubt of the reliability of the glanding or connections to the gland (e.g. via a painted gland plate), or you wish to minimise R2 (always a good thing), but 543.7 doesn't demand it.

   - Andy.