6491X as a separate earth conductor, when cables are laid direct underground

BS 7671 (which of course is for LV and below only, not HV) permits bare protective conductors underground provided they're at least 25mm² copper or 50mm² steel - which reduces to 16mm² if it has a sheath to protect it against corrosion (which I believe the "insulation" of normal singles will count as in circumstances like this where the insulation properties aren't required). Reg 543.1.1 & Table 54.1. So as long as you're G/Y meets that I think you're OK on that count.

I understand your unease though - it feels odd not to have protection on a safety critical conductor. Wire armour though, in general, provides rather limited protection against mechanical damage - even a simple garden fork can easily part the wire strands and penetrate to a core (I've done exactly that with SWA for an experiment) - the real protection comes from earthing the armour and using ADS. That of course won't work with a separate earthing conductor. There is of course the more general requirement that the conductor needs to be suitably protected against "mechanical and chemical deterioration" - but that's something that would have to be evaluated in the particular circumstances - how likely would it be that someone would start digging in that area without sensible precautions?

The side question of course is whether the extra G/Y is really needed - if the armour alone isn't sufficient you may have a problem with ADS for the cables themselves.

  - Andy.

Agree - its all about being 'suitable for the environment' - above ground in the sun the plastic will crack and flake off, underground buried direct with no sensitivity to sharp stones and so on, the jacket may be damaged and in either case at some point,  water gets in and starts to rot things.
Equally, carefully buried in soft sand or similar, and protected above ground, it will probably outlive the building and equipment it connects to.

In terms of mechanical damage, its all about how the ground is used - under a field regularly ploughed, or even a village green that sometimes hosts circus marquees with long pegs, maybe a problem. Under paving slabs, or even a raised flower bed, or anywhere that works are controlled, not so likely.

Mike.

Agree - its all about being 'suitable for the environment' - above ground in the sun the plastic will crack and flake off, underground buried direct with no sensitivity to sharp stones and so on, the jacket may be damaged and in either case at some point,  water gets in and starts to rot things.
Equally, carefully buried in soft sand or similar, and protected above ground, it will probably outlive the building and equipment it connects to.

In terms of mechanical damage, its all about how the ground is used - under a field regularly ploughed, or even a village green that sometimes hosts circus marquees with long pegs, maybe a problem. Under paving slabs, or even a raised flower bed, or anywhere that works are controlled, not so likely.

Mike.