Classification and installation of power cables

A power cable is an insulated conductor (or group of conductors) with a protective covering, used to carry power underground or through places where overhead lines are not possible.

Classification of power cables

By voltage:

By construction and insulation:

• Belted cables (up to 11 kV): Cheap and simple. Above 11 kV, tangential stress and voids cause breakdown.
• Screened cables, H-type and S.L.-type (up to 66 kV): Each core is screened with metallised paper, so the stress is purely radial and there are no voids or sheath losses.
• Pressure cables, oil-filled and gas-pressure (above 66 kV): Pressure prevents the formation of voids and ionisation at high stress.
• Modern solid-dielectric cables, PVC and XLPE: Light, dry-type and easy to joint. XLPE is now used up to EHV levels.

Reasons for using cables

• Lines in cities, substations, airports and river crossings where overhead lines are unsafe or not allowed
• Less exposure to lightning, storms and accidents, so fewer faults
• No effect on appearance, and better public safety
• Lower inductive reactance, so less voltage drop
• Main drawback: they cost 5–10 times more than overhead lines, and fault location and repair are difficult.

Methods of laying cables

1. Direct laying: An armoured cable is laid in a trench about 1.5 m deep on a sand bed and covered with bricks and soil. It is cheap and has good heat dissipation, but extension and repair are difficult.
2. Draw-in system: Ducts or pipes are laid with manholes at intervals, and cables are pulled through them. Repairs and additions are easy without digging, but the cost is high and cooling is poorer. This system is used in cities.
3. Solid system: The cable is laid in troughs (of wood or earthenware) and filled with bitumen. It gives good mechanical protection, but cooling is poor and the cost is high. It is now rarely used.