Underground Cables and Cable Jointing
Domain 8 - Underground Cables and Cable Jointing. The exam checks cable construction, laying practice, jointing and testing, where workmanship outweighs theory.
Construction and laying
- Layers from inside out: stranded conductor, semiconducting screen, insulation (PVC, XLPE or PILC), metallic screen or sheath, bedding, armour, outer serving.
- Grades: 1.1 kV LT, 11 kV HT and 33 kV; HT cables are screened and need stress control at joints and terminations.
- Laying depth is about 0.75–1.0 m with 75–100 mm sand bedding, protective tiles or bricks and a warning tape; route markers go at joints, bends and road crossings.
- Respect the bending radius, often 12–15 times the cable diameter; pull through ducts under roads rather than opening the surface.
- Joints and terminations use heat-shrink or cold-shrink kits in a clean, dry tent. Test after jointing with insulation resistance (500 V megger on LT, 5 kV on 11 kV), continuity, and a DC high-potential or very-low-frequency test.
Worked example
A cable rated 100 A in air is laid at 1 m depth in soil with derating factors 0.8 for soil thermal resistivity and 0.9 for depth. Continuous rating = 100 × 0.8 × 0.9 = 72 A. The connected load must be compared with 72 A, not 100 A, or the cable runs hot and the weakest joint fails first.
Traps
- Bending a cable tighter than its minimum radius and cracking the XLPE.
- Jointing in rain or dust without a shelter.
- Using a 500 V megger on an 11 kV cable and calling the result a pass.
- Skipping route markers, so the next excavation party cuts the cable.