Under Ground Cables

An Under Ground Cables essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover. Under Ground Cables are used, where it is impractical to use overhead line such as, thickly populated area, Railway crossings, Plants and substations, Connection to equipment from switch board.

Advantages of Under Ground Cables:

• Less effect due to external atmospheric conditions such as lightening.
• Less chance of faults.
• Low maintenance cost.
• Small voltage drops
• Better appearance & occupies less space.
• Better Public Safety.
• Greater life span.

Disadvantages of Under Ground Cables:

• Greater Installation cost.
• More charging current.
• Difficulty to fault location.

Necessary Requirements of Under Ground Cables.

• High resistivity.
• High mechanical strength.
• High tensile strength & Low cost.
• Conductor should have high conductivity and flexible. Stranding is done so that conductor may become flexible and carry more current.
• The conductor should carry required current without overheating.
• High dielectric strength; Proper thickness of insulation to give high degree of safety.
• The cable must be provided with suitable mechanical protection, so that cable will not damage during laying it.
• Not affected by acid or alkalis: The material used should be chemically inactive.
• Non- inflammable.

Properties of Insulating Materials used in Cables

• High insulation Resistance.
• High dielectric strength.
• High mechanical strength.
• Non hygroscopic.
• Non inflammable.
• Low cost.
• Non corrosive material.
• The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.

Classification of Cables according to their voltage rating:

• Low-tension cables — the operating voltage is up to 1000 V ( 1KV)
• High-tension cables — the operating voltage of high tension cables is up to 11000 V
• Super-tension cables — the operating voltage of super tension cable is from 22 kV to 33 kV
• Extra high-tension cables — from 33 kV to 66 kV
• Extra super voltage cables — beyond 132 kV

Classification of Cables according to their Service:

• Single core
• two core
• three core
• Four core

For a 3 phase service, either 3 single core cables or three core cable can be used depending upon their operating voltage and load demand.