25-01-2013, 04:38 PM
Underground Cables-
[attachment=48529]
Introduction
Since the loads having the trends towards growing density. This requires the better appearance, rugged construction, greater service reliability and increased safety.
An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.
The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service.
The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.
Advantages
The underground cables have several advantages such as,
Better general appearance
Less liable to damage through storms or lighting
Low maintenance cost
Less chances of faults
Small voltage drops
Disadvantage
The major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with equivalent overhead system.
Construction of Cables
Core or Conductor
A cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminium or copper and is stranded in order to provide flexibility to the cable.
Insulation
The core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.
The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.
Metallic Sheath
A metallic sheath of lead or aluminium is provided over the insulation to protect the cable from moisture, gases or othes damaging liquids
Properties of Insulating Material
The insulating materials used in cables should have the following properties
High resistivity.
High dielectric strength.
Low thermal co-efficient.
Low water absorption.
Low permittivity.
Non – inflammable.
Chemical stability.
High mechanical strength.
High viscosity at impregnation temperature.
Capability to with stand high rupturing voltage.
High tensile strength and plasticity.
[attachment=48529]
Introduction
Since the loads having the trends towards growing density. This requires the better appearance, rugged construction, greater service reliability and increased safety.
An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.
The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service.
The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.
Advantages
The underground cables have several advantages such as,
Better general appearance
Less liable to damage through storms or lighting
Low maintenance cost
Less chances of faults
Small voltage drops
Disadvantage
The major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with equivalent overhead system.
Construction of Cables
Core or Conductor
A cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminium or copper and is stranded in order to provide flexibility to the cable.
Insulation
The core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.
The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.
Metallic Sheath
A metallic sheath of lead or aluminium is provided over the insulation to protect the cable from moisture, gases or othes damaging liquids
Properties of Insulating Material
The insulating materials used in cables should have the following properties
High resistivity.
High dielectric strength.
Low thermal co-efficient.
Low water absorption.
Low permittivity.
Non – inflammable.
Chemical stability.
High mechanical strength.
High viscosity at impregnation temperature.
Capability to with stand high rupturing voltage.
High tensile strength and plasticity.