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
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.