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Choice of SWA cable for burial below the water table

Hydrothusiast

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?

  • 0 in reply to Grumpy

    You could try emailing me at randomcalculations.co.uk

  • 0 in reply to Hydrothusiast

    XLPE sheathed cable is best for this situation and available, although not easily at wholesalers because it is more expensive. I would expect that it would last a lifetime, it seems to on HV DNO cables etc.

  • 0 in reply to Hydrothusiast

    In the old days I could've PM'd you. However, I am an impecunious hydrothusiast, with a summer stream and winter small river running through my grounds so I am particularly keen to learn from your experiences. Are you able to share?

  • 0 in reply to broadgage

    Yes, I have been buying everything possible on Ebay but even so it's not a cheap way of getting electricity.  I can see why most people favour the National Grid.

    The cottage is off-grid so the alternatives are a petrol generator for electricity and an oil-burning Rayburn for heating.   None of the local LPG suppliers are willing to install a tank there and the low-sulphur regulations mean that oil stoves need far more frequent servicing than was the case 20 years ago.    It gives us a decent water supply too and will power a UV steriliser for the 95% of the year when I'm allowed to extract water from the stream.  NRW explain that the stream will get "stressed" in very dry conditions beyond Q95 - I asked whether it was the rock or the water that feels stressed (there are no fish in the depleted reach and as little as 5% of the flow would run the smalll turbine) but did not get an answer. 

    The feed-in tariff (8p/kWh, generation only) will help a little: I must get it commissioned in the next 11 weeks for this. A totally unnecessary deadline now that the scheme is closed to new applicants!  Ofgem extended the ROOFIT deadlines by 12 months, to allow for 13 months of lockdowns and furlough schemes, but are unwilling to extend it further than that.

  • 0 in reply to broadgage

    Well I get all that Broadgage. Hopefully Hydrothusiast might come back with costings of the alternatives.

  • 0 in reply to Grumpy

    9 kw used only intermittently with an average load of less than 1 kw is not worth that much.

    9 kw used continually for electric  heating is more valuable since it will fully heat a reasonably insulated home. In a remote location natural gas may not be available and LPG or oil may be expensive to deliver.

    Presumably this installation was planned BEFORE the recent increase in energy prices. It will now be even more attractive.

  • 0

    Really interesting post. Iwas just wondering how the economics stack up. It sounds as if the capital costs are significant for a 9kW load.

    Or am I missing something as usual?

  • 0 in reply to Hydrothusiast

    The inductance is low because we assume a balanced load and the magnetic fields of the flow and return currents more or less cancel,  if you split the cores and measured as the inductance of a single wire with no return current in the bundle it would be more serious.

    For single wire, I'd expect 1-2 nano-henries per mm per single core (1-2mH per metre = 1-2 henries  per km ) and better than 90% cancellation at 50Hz so yes, credible.

    Beware of skin depth at 400Hz the currents will not be uniform accross the full 10mm2

    That said at 50 Hz with 13 a per core and no neutral current to speak of it will be fine and at 400Hz you have  only 6 or 7A single phase  so the VD/ core will be similar

    I'll have a ponder and come back.

    Optic fibre works fine when wet - glass has a higher refractive index than water and does not dissolve or corrode so the light stays inside even when immersed..  There is 'SWA' fibre  example  which can be knocked about a bit more than the normal stuff.

    mike

  • 0 in reply to Richard Clifford-Smith

    That is very reassuring, thank you.

  • 0 in reply to mapj1

    Thanks - see previous reply about voltages.  Yes, the land is owned by the National Trust.  We (and they) are very keen to keep it unspoilt by poles and overhead wires.

    In principle I could replace sections of the SWA cable following water ingress (how would one know?) but I'm also laying an optical fibre so that control systems at the extraction point, cottage and turbine hut can talk to each other, and I don't think that can have sections replaced.  For the fibre I am tempted to pick corrugated steel tube reinforcing as I can have it over-sheathed with HDPE to make it properly waterproof (comparable in price to standard SWA fibre with a PVC sheath) - if successful it should never need replacing.

    One thing I have wondered is whether any obvious damage to the sheath during installation could be patched-up with amalgamating tape to water-proof small nicks.  I'm wary though of using anything that might cause chemical damage to the insulation.

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