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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?

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    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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    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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