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Choice of SWA cable for burial below the water table

I am building myself a small off-grid micro-hydro system.  The turbine hut will be connected to the house by 560m of buried cable (SWA, 10mm^2 chosen for allowable voltage drop) and there is a second 340m leg of thinner cable above the house taking power up to the stream level sensors.

The ground is very boggy moorland and the cable will be below the water table for at least 10 months of the year.  

Having read previous discussions it appears that neither PVC nor LSZH sheathing is really suitable for continuous immersion. (see also https://uk.prysmiangroup.com/media/news/underground

and https://www.molexces.com/webfoo/wp-content/uploads/Water-Ingress-In-Structured-Cabling-Systems-2013-1.pdf).

Which would be better, PVC or LSZH sheathing?  It seems very strange that there is no British Standard for installations like this.

Talking to companies in this field, they say "don't worry about it".  They just use standard SWA cable, directly buried because "that's all that is available" and "the armouring may rust but each core has XLPE insulation that will continue to insulate even when wet".  Sometimes they make a nick in the outer sheath before it enters the turbine hut so that if there is any sheath damage higher up, the water running down past the armouring can drip outdoors rather than inside the switchgear.

I don't want to spend a fortune on submarine-rated cable; equally, it has taken weeks with an 8-ton digger to carve the trench and I really do not want to be replacing the cable in 10 or 20 years' time when the rest of the groundworks should be good for a century or more.

In principle I could thread 50m sections of 63/50 twinwall ducting along the cable before dropping it in the trench.  This would give better protection against sharp stones when back-filling.  If the joints were taped and sealed the downhill sections could then avoid being water-logged but they would probably still be damp.   Part of the cable though runs down a hill, under a stream and back up again, like a giant u-bend, and this would inevitably fill with water.  Ducting would be expensive, both to purchase and install.

One supplier says they could add an outer MDPE or HDPE sheath to improve the water resistance; again, this is an expensive option.  It would terminate with the last 4m inside an outhouse, so it does not run into the house itself.  It seems a bit illogical that the BS standards (I think) prohibit domestic use of PE-sheathed cables on fire-safety grounds but allow the use of PE cable ducting.

Any suggestions?

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  • You must use a submersible specification of cable. 

    I looked at this many years ago in the water industry.  After talking with Pirelli cables experts no PVC XPLE cable will withstand submerging in water, even intermittently for short periods. 

    The sheath material is micro porous.  

    I have seen with my own eyes PVC sheathed XPLE cables in sand and gravel quarries, when cut back to be remade.  They are damp, with water under the outer sheath and into the core bedding.  This was due to sitting in intermittent water levels when direct buried.  Water will track into the internal bedding and reduces the cables insulation properties. 

    Carrying out periodic inspections of industrial installations had shown me.  Over time cables take on moisture through their sheathing, if they are not specified correctly for the environmental conditions.  

  • Thanks.  I cannot afford anything much more expensive than standard SWA cable.  I made preliminary enquiries about custom cables a couple of years ago but they were hugely expensive by comparison.   Can you recommend any specific cable, and somewhere that stocks it?

  • The key is specification of the cable.  

    When a cables insulation is tested, the test is carried out in relation to the voltage rating of the circuit. 

    You must use the correct cable for the environment.  Taking into account the worst case. 

    I worked in the water, for Scottish Water and infrastructure industry for Scottish Power.  Cables specification must be suitable.  

    See this link for some ideas 

    https://www.cef.co.uk/catalogue/products/2015173-10mm-3-core-ho7rnf-rubber-flexible-cable-cut-length-sold-by-the-mtr?gclid=Cj0KCQiAuP-OBhDqARIsAD4XHpfbsG1mE0NIvlC6Yi7SwFgq0dtghk2TXR5GqhvKhtps5Kh8KQJQPsgaAjU7EALw_wcB&gclsrc=aw.ds

    Tough Rubber Sheathed cables are used in the renewables sectors for many applications.  

    I'm not saying you must not use PVC but you will experience issues down the line, from my experience.  

    Don't just take the word of others, here's what the cable manufacturers say about it.  

    uk.prysmiangroup.com/.../underground

    When I was on infrastructure engineering we would specify lead sheathed for chemical/water ingress.  DNO to consumers we're always direct buried into free draining soil/trenches.  If there was issues with free draining the specification was increased for better IP protection. 

    Check your BS7671 for environmental protection standards.  

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