Underground cable laying systems

Underground cables in an industry are laid so that they are protected from mechanical damage, can dissipate heat and can be inspected or replaced. The three classical systems are direct laying, draw-in (duct) system and solid system. Inside plants, cable trenches and trays are also used.

1. Direct laying

 ground ════════════════════
        warning tape / route marker
        backfill soil
        ▭▭▭▭▭ protective bricks/tiles
        ░ sand ░  ●  ●  ●  ░ cables
        ░░░░ sand bed (~8 cm) ░░░
   trench about 0.75–1.2 m deep, 0.45 m wide

• A trench about 0.75 m (LV) to about 1 m or more (HV) deep is dug.
• A layer of sand is laid at the bottom, the armoured cable is laid on it and covered with more sand.
• Bricks or concrete tiles are placed on top for protection, then the trench is back-filled; a warning tape and route markers are placed.
• Advantages: simple, cheap, good heat dissipation, so higher current rating.
• Disadvantages: changes or extra cables need fresh digging; fault location and repair are costly; cables can be damaged by later excavation.

2. Draw-in (duct) system

   manhole ──── ducts (PVC/RCC/HDPE) ──── manhole
   [  ]═════════════════════════════════[  ]
        cables pulled through ducts

• Pipes or ducts of PVC, HDPE, RCC or stoneware are laid underground with manholes (pull pits) at intervals.
• Cables are pulled into the ducts after laying.
• Advantages: cables can be added, removed or repaired without digging; good mechanical protection; neat. Ideal under roads and plant areas with traffic.
• Disadvantages: high initial cost; poor heat dissipation, so cables are derated; difficult on bends.

3. Solid system

• Cables are laid in troughs of cast iron, stoneware or wood, which are then filled with bitumen or asphalt.
• Advantages: good mechanical protection.
• Disadvantages: costly, poor heat dissipation, hard to repair; now rarely used.

4. Cable trenches and trays (inside the plant)

• Concrete trenches with covers, with cables on brackets or trays along the walls, run between substation, panels and machines.
• Advantages: easy inspection, addition and maintenance; good ventilation.
• Disadvantages: must be kept free of water, oil and rodents; need fire barriers.

Comparison