An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.
Although several types of underground cables are available, the type of cable to be used will depend upon the working voltage and service requirements.
In general, a cable must fulfill the following necessary requirements :
- The conductor used in cables should be tinned stranded copper or aluminium of high conductivity. Stranding is done so that the conductor may become flexible and carry more current.
- The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.
- The cable must have a proper thickness of insulation in order to give a high degree of safety and reliability at the voltage for which it is designed.
- The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
- The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.
Construction of Cables
The figure shows the general construction of a three conductor cable. The various parts are:
- Cores or Conductors
- Metallic sheath
Parts of an Underground Cable
Cores or Conductors
A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended. For instance, the 3-conductor cable shown in the figure is used for 3-phase service.
The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable.
The aluminium or copper conductor carries the electrical current.
The conductor behavior is characterized by two particularly noteworthy phenomena:
The skin effect is the concentration of electric current flow around the periphery of the conductors. It increases in proportion to the cross-section of the conductor used.
The short distance separating the phases in the same circuit generates the proximity effect.
In practice, the proximity effect is weaker than the skin effect and rapidly diminishes when the cables are moved away from each other.
The proximity effect is negligible when the distance between two cables in the same circuit or in two adjacent circuits is at least 8 times the outside diameter of the cable conductor.
Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable.
The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.
Read more on different types of insulation materials used in power cables.
In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in Figure.
Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape.
The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armoring.
Over the bedding, armoring is provided which consist of one or two layers of galvanized steel wire or steel tape.
Its purpose is to protect the cable from mechanical injuries while laying it or handling it. Armoring may not be done in the case of some cables.
In order to protect armoring from atmospheric conditions, a layer of fibrous material like jute similar to bedding is provided over the armoring. This is known as serving.