Insulated Cables
INTRODUCTION
All the electric cables consist of three essential parts.
The conductor for transmitting electrical power.
General Construction of Cable The underground cable used for transmission of power at high voltage consists of one central core or a number of cores (two, three or four) of tinned stranded copper conductors (sometimes use of aluminium conductor is also made) insulated from each other by paper or varnished cambric or vulcanized bitumen or impregnated paper. A metallic sheath of lead or alloy or of aluminium is provided around the insulation to protect it against ingress of moisture. For the protection of metallic sheath against corrosion and from mechanical injury from the armouring a layer of bedding consisting of paper tape compounded with fibrous material is provided over the metallic sheath.
Requirements of cable
Conductor Material Used The materials employed for conductors generally are annealed copper and aluminium.
Copper The used of copper is limited to cables used for control circuits, signalling and communications.
Aluminium Power cables use standard or sector shaped aluminium conductors.
Insulation Main requirements of the insulating material :
Insulation Material
PARAMETERS OF CABLES
Insulation Resistance Figure shows a single core cable of conductor radius r and a sheath of radius R.
Where r is the resistivity of the insulating material in W-m and l is the length of cable in metres.
Capacitance
Capacitance
Where Îr is the relative permittivity (dielectric constant) of the cable insulation and Î0 = 8.85 × 10–12 F/m.
Capacitance between core and sheath
THREE CORE CABLES
These are used only upto 11 kB. In these cables a potential difference exists between any two pairs of conductors and also between each conductor and sheath.
Measurement
Assumption:
Capacitance between each conductor and sheath
The effective capacitance between each core and neutral (or capacitance per phase).
EXTRA HIGH VOLTAGE (EHV) CABLES
Electrostatic stress Let r be the radius of the conductor, R the inner radius of the sheath, Î the permittivity of the dielectric, q the charge per unit length.
GRADING OF CABLES
Methods of Grading Capacitance Grading
Thus Ex is constant throughout the thickness of insulations.
Intersheath Grading
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1. What are insulated cables used for in electrical engineering? |
2. What are the advantages of using insulated cables? |
3. How is the insulation of cables tested in electrical engineering? |
4. What are the different types of insulation materials used in cables? |
5. How can I select the right insulated cable for my electrical engineering project? |
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