Underground cables

Transmission line

An underground cable essentially consists of one or more conductors covered with insulating material and surrounded by a protective cover.

 A cable should cover following requirements:

1. The conductor used in cables should be tinned stranded copper or aluminium of high conductivity. Stranding is done so that conductor may become flexible and carry more current.
2. The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.
3. The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.
4. The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
5. The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

Construction of Cables

1. Core or Conductors
2. Insulators
3. Metallic sheath
4. Bedding
5. Armouring
6. Serving

Insulating Material for Cables

Their Properties:

• High insulation resistance to avoid leakage current.
• High dielectric strength to avoid electrical breakdown of the cable.
• High mechanical strength to withstand the mechanical handling of cables.
•  Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic,it must be enclosed in a waterproof covering like lead sheath.
• Non-inflammable.
• Low cost so as to make the underground system a viable proposition.
• Unaffected by acids and alkalies to avoid any chemical action.

Material used for insulation

1. Rubber
2. Vulcanized Indian Rubber
3. Impregnated paper
4. Varnished Cambric
5. PVC

Classification of Cables

1. LOW TENSION CABLES upto 1000v
2. HIGH TENSION CABLES upto 11kv
3. SUPER TENSION CABLES from 22 to 33 kv
4. EXTRA HIGH TENSION CABLES from 33 to 66kv
5. EXTRA SUPER VOLTAGE CABLES beyond 132kv

Cables for 3-Phase Service

1. Belted cables — upto 11 kV
2. Screened cables — from 22 kV to 66 kV
3. Pressure cables — beyond 66 kV.

Laying of Underground Cables

1. Direct Laying
2. Draw in system
3. Solid system

Types of Cable Faults

Cables are generally laid directly in the ground or in ducts in the underground distribution system. For this reason, there are little chances of faults in underground cables. However, if a fault does occur, it is difficult to locate and repair the fault because conductors are not visible. Nevertheless, the following are the faults most likely to occur in underground cables :

1. Open-circuit fault
2. Short-circuit fault
3. Earth fault.

 

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