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

Declan A

Hi,

Apologies if i have posted this in the wrong discussion forum.

I am looking for advice regarding earthing when working with underground installations.

The job that has made me raise this query is HV but i believe it is still applicable to LV installations to.

We are installing 2 new HV feeders using BS6622 cable laid direct for around 200m, we are also running a separate earth conductor which at present is a 6491X cable, is this acceptable given there is no mechanical protection on the earth conductor.

As part of the scheme we also need to reconfigure the earth network diverting existing earth cables from within an integral sub to a external enclosure, to do this we are jointing on to the old cables using 6491X running them above ground on tray and underground laid direct, again is this proposal acceptable or should they be armoured?

  • 0

    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.

  • 0 in reply to AJJewsbury

    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.

  • 0

    Firstly, where does the requirement for the separate conductor come from?

    If the conductor forms part of the electrode then you need bare conductor to be in contact with the soil. Normally this is specified as hard drawn copper BS7884 for mechanical and chemical protection. Small sections might be sheathed for various reasons, but bare in mind that a) It will need joining and b) It will be derated compared with bare copper.

    Even if not, hard drawn is also pretty strong and tough, so I tend to find that's been used as the go-to for HV earthing, outside the substation, unless you need it to be in a sheath for some reason. 50mm² is the minimum size available commercially, but if you aren't able to rely on the armour you must have a chunky fault current anyway. Tape would be even more robust, but that's not cheap.

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