Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Suggested Answer
  • Locked Locked
  • Replies 26 replies
  • Helpful Answers 3 Helpful answers
  • Subscribers 44 subscribers
  • Views 6578 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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?

Parents
  • 0

    I suspect that modern plastic insulated cable will be fine in practice, though not actually recommended for such use.

    Consider the DNO network for example, what type of cables do they use for mains and for service cables in areas with a high water table ? Exactly the same as they use in less waterlogged areas, and such cables give very little trouble in practice, except when damaged during excavations.

    Also this proposed cable is sized for voltage drop and run well below its current carrying capacity and therefore wont run significantly over ambient temperature. Most types of chemical or physical degradation are temperature related and proceed more slowly at low temperatures. I would be SLIGHTLY more concerned about a cable heavily loaded, in a swamp, in the tropics.

    The fact that these cables are generally rated at 600/1000 volts gives a further safety margin as the operating voltage is a lot less than half that.

    As regards actually installing the cable, be aware that cables in ducts laid on significant gradients tend to very slowly "creep" downwards until the uppermost connections are strained or pulled out. Leave a large loop at the top to allow for this.  Also allow a similar loop immediately below any joints. Try to get the cable in as few lengths as possible to minimise joints, jointing kits cost money, and joints tend to be a weak spot.

Reply
  • 0

    I suspect that modern plastic insulated cable will be fine in practice, though not actually recommended for such use.

    Consider the DNO network for example, what type of cables do they use for mains and for service cables in areas with a high water table ? Exactly the same as they use in less waterlogged areas, and such cables give very little trouble in practice, except when damaged during excavations.

    Also this proposed cable is sized for voltage drop and run well below its current carrying capacity and therefore wont run significantly over ambient temperature. Most types of chemical or physical degradation are temperature related and proceed more slowly at low temperatures. I would be SLIGHTLY more concerned about a cable heavily loaded, in a swamp, in the tropics.

    The fact that these cables are generally rated at 600/1000 volts gives a further safety margin as the operating voltage is a lot less than half that.

    As regards actually installing the cable, be aware that cables in ducts laid on significant gradients tend to very slowly "creep" downwards until the uppermost connections are strained or pulled out. Leave a large loop at the top to allow for this.  Also allow a similar loop immediately below any joints. Try to get the cable in as few lengths as possible to minimise joints, jointing kits cost money, and joints tend to be a weak spot.

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines