General construction of cables
Cables consist of one or more conductors covered with suitable insulation and surrounded by a protecting cover. Although several types of cables are available, the type of cable to be used will depend on the working voltage and service requirements, the following figure shows the general construction of multi-core cables.
construction of cable
In general cables construction can estimate by the follows:-
(1) Conductor martial
(2) Insulation material
(3) Metallic sheath
(4) Bedding
(5) Armouring
(6) Serving
(1) Conductor material
A cable may have one or more than one conductor (core) depending upon the type of Service for which it is intended. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable. Copper has lower Resistivity and thus higher conductivity than aluminium.This means that copper Conductors have smaller cross sectional area take up less space than aluminium for the same current capacity, for the flexibility to the cables we stranded it.
Aluminium cable
Copper cable
(2) Insulating material
Each core or conductor is provided with a suitable thickness of insulation. The thickness of the layer depends on the voltage to be withstood by the cable, thus the insulation must have a very high resistance. The commonly used materials for insulation will be discussed in this chapter.The following figure shows a good insulating material and bad insulating material.
Insulation materials
(i) Type of insulating materials
-Rubber
-Vulcanized India Rubber (VIR)
-Impregnated Paper
-Polyvinyl chloride
-Polyethylene
-Cross-Linked Polyethylene
-Varnished Cambric
-Gutta_Percha
-Silk and Cotton
-Enamel Insulation
Polyvinyl Chloride (PVC):
Advantage:-
(1) Elasticity
(2) Recovery from stretching are not good as rubber
(3) Lower insulation resistance
Disadvantages:-
High immunity to acids & alkalis
Usages:-
Generally used for low & medium domestic lights and power
Cross-Linked Polyethylene (XLPE):
Advantage:-
Can operate at high temp.
Usages:-
Suitable for medium voltage application
(ii) The main properties of insulating material
(1) High dielectric strength.
(2) High insulation resistance.
(3) Good mechanical properties.
(4) Immunity to chemical attacks occur arrange of temperature of about -18 °C to 94 °C.
(5) Easily to manufacturing and not too costly.
(6) Non-hygroscopic i.e. it should not absorb moisture from air or soil.
(7) Non-inflammable.
(8) Low coefficient of thermal expansion.
(9) Low permittivity.
(10) Capability of withstanding high rupturing voltages.
(3) Metallic sheath
In order to protect the cable from moisture, gasses or other damaging liquids (acids or alkalis) in the soil and atmosphere, metallic sheath of lead or aluminium is provided over the insulation.(4) Aluminium Sheath
Aluminium has been used as a sheath to decrease the weight of the cable and it has the following advantages:-(1) The weight is decreased
(2) Increases tensile strength
(3) Improves the mechanical properties
(5) Bedding
To protect the metallic sheath against corrosion and from mechanical injury due to armouring(6) Armouring
Is used for the mechanical protection of the cable(A) Steel _ type armouring not flexible
(B) Wire armouring and it’s the better mechanical protection
(7) Serving (outer sheath)
Its consist of PVC compound of rated working temperatures up to 70c or PVC type for working temperature 90c.
It used to protect armouring from atmospheric conditions
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