For cables there are many reasons for water penetration and growing of water trees inside the insulation. For example, it could be:

1️⃣ Production:
👉 the raw materials used were not dried;
👉 on the extrusion line (application of insulation or polymer sheath), when starting a new order or completing it, the water level in the cooling bath was not lowered, and the unprotected end of the cable passed under water;
👉 the cable was stored without sealed ends (without caps).

2️⃣ Installation:
👉 when pulling the cable, the cable grip (cable sock) damaged the cap at the end of the cable;
👉 damage to the cable sheath due to insufficient preparation of the route, the presence of stones or garbage;
👉 damage to the cable sheath due to incorrect selection of the number and location of the rollers;
👉 consecutive pulling of several cables in the same pipe/duct (this is prohibited because the cord which pulls a new cable may damage the sheath of those cables that are already in the pipe);
👉 damage to the cable sheath due to the laying at low temperature without cable preheating;
👉 lack of step-by-step monitoring of the cable outer sheath condition (DC 10 kV test);
👉 lack of timely detection of damage and delay in starting repairs;
👉 careless installation of joints and terminations;
👉 bonding cable problems.

3️⃣ Operation:
👉 natural absorption of water by the cable sheath;
👉 sheath damage that was not detected because the cable sheath was not tested (DC 10 kV) with proper frequency during operation (for new lines – once in 1-3 years);
👉 sheath damage that was not detected during tests (DC 10 kV) because the cable is surrounded by dry soil or laid in dielectric pipes/ducts (PE, PVC) which prevented current to flow from cable screen through damaged sheath to surrounding soil;
👉 a short circuit with a through-hole of the sheath (or just a mechanical break of the cable), which was not promptly repaired.

Constructive solutions can be proposed to increase the protection of the cable from water penetration and its consequences:
✅ the use of cores with longitudinal sealing by fibres, yarns etc;
✅ the use of water-blocking tapes (on top of the core, screen, etc.);
✅ increasing the thickness of the polymer sheath of the cable;
✅ the use of alum polymer tape (under the outer polymer sheath);
✅ the use of cables with a solid metal sheath (lead or aluminium);
✅ the use of cables with a corrugated aluminium sheath (as in the photo);
✅ use of armour for better protection from mechanical damages;
✅ the use of insulation with increased resistance to the growth of water trees (tree-retardant cross-linked polyethylene XLPE-TR or polypropylene PP).
✅ the use of semi-conductive layer over the sheath to improve control on sheath condition by DC test.

Do not let water into the cable. And the cable will work for a century!

Dry means alive!