As we know wires and cables, are the main electrical accessories for the transfer of electrical energy, from one point to another. The wires are used for transfer,very small amount of electrical energy, whereas the cables are used to transfer large quantum of electrical energy. The wires are generally used for domestic as well as commercial wiring, where as the cables are used for transmission and distribution purpose. Depending upon the number of conductors, the wires and cables may be classified as single or solid conductor and stranded conductor. The solid conductors are rigid and liable to break if frequent bending or handling is required whereas the stranded conductors are more flexible and show more breaking strength on bending.
The numbers of strands are fixed and it may be 3, 7, 19, 37 and 61. As the number of strands goes on increasing, the flexibility will goes on increasing. The size of wire may be measured in “Standard Wire Gauge” (SWG) number and number of strands. For example if a wire is 7/26, it means there are 7 number of strands and SWG number of wire is 26. As the SWG number goes on increasing, the diameter of the wire goes on decreasing and vice-versa. But now a day, we are using cross-sectional area of the conductor instead of SWG number. So, now the size of wire is being expressed as number of strands/cross-sectional area of conductor. The use of wire is limited up to 650 volts whereas as the use cable is restricted to 132,000 volts, due to insulation problems. The wire and cables are explained separately.
i. Wires
Now we will study the different types of wires and their suitability, depending upon the environmental conditions. The wires may be insulated wire and bared wires.The wires are available in different voltage grading such as 250 volts and 650 voltsonly. The insulated wires being used in electrical wiring system are detailed
a) V.I.R. (Valcanized India Rubber): The conductor is valcanized by Indian rubber. Cotton tapes sheathe is provided over the valcanized Indian rubber. A very fine layer of any moisture proof insulating material, just as bitumen, wax or which can prevent ingress of moisture content to conductor. These wires are totally obsolete nowadays, as these wires absorb moisture quickly.
b) C.T.S./T.R.S. (Cab Type Sheathed/Tough Rubber Sheathed): These wires are almost moisture proof types. These are available in single core, and twin core. The conductor is covered with a layer of pure rubber, to protect from ingress of moisture. Over the layer of pure rubber, vulcanized Indian rubber layer is provided. Finally, a sheathing of tough rubber is provided to give mechanical strength to the wire. These wires are also obsolete now.
c) Lead Sheath Wires: These types of wires are similar in construction with T.R.S., as for as core section is concerned but having outer sheath of lead or lead alloy. The lead sheathing provides full moisture proof environment to the conductor. So, these wires can be used where the humidity is comparatively more or we may use even in snowfall area too.
d) P.V.C. (Poly Vinyl Chloride) Wires: In these types of wires, the conductor is insulated with P.V.C. insulation. There is no need for providing cotton tape,as the P.V.C. insulation is tougher than rubber, to protect the wire from moisture and mechanical failure. Nowadays, these wires are used for mostly batten,cleat, casing capping and conduit wiring. These wires should not be used for connecting heating appliances and pendent lights, etc.
e) Weather Proof Wires: These wires are used for outdoor service lines. The conductors are insulated first with rubber, then braiding of cotton thread. This braiding material is dipped in waterproof compound so that ingress of moisture could be avoided completely.
f) Flexible wires: The wires are used to connect the pendent lights and portable appliances. The insulation used is pure rubber or valcanized rubber on which single or double cotton or valcanized rubber layer is provided. Finally, single or double cotton or artificial silk braiding is provided or only P.V.C. layer is provided.
The following are the types of the flexible wires available in the market.
ii. Cables
The power from generating station can be transmitted by cables also, if the transmission distance is less. As seen already in comparison, the underground system is costlier and its maintenance is difficult. Still, the system of transmitting the power by cables is preferred in thickly populated areas and cities. The cables are usually classified according to the voltage for which they are manufactured.
According to the voltage they can be classified:
1. L.T. (Low Tension Cables) up to 1000 volts.
2. H.T. (High Tension Cables) up to 11,000 volts.
3. S.T. (Super Tension Cables) from 22,000 volts to 33,000 volts.
4. Extra High Tension Cables from 33,000 volts to 66,000 volts.
5. Oil Filled and Gas Pressure Cables from 66,000 volts to 132,000 volts.
iii. General Construction of Cable
The general construction of a cable is given below along with figure, which illustrates the various parts of the cable.
a) Core: All cables have one central conductor and a number of stranded conductors of copper or aluminium, wrapped around the central conductor. Generally, there are one, two, three or four cores (three and a half). This half core is used as neutral core.
b) Insulation: The different insulating materials are used to insulate conductors or cores such as impregnated paper, varnished cambric and vulcanized bitumen for low voltage. Impregnated paper is an excellent insulating material and widely used. When varnished cambric is used as an insulating material for low voltage cables, petroleum jelly is applied between the layers of the cambric type. It prevents the insulation damage by friction when the cables are handled.
c) Metallic Sheath: A metallic sheath is provided over insulation so as to prevent the entry of the moisture into the insulating material. The metallic sheath is usually of lead or lead alloy.
d) Bedding: Over the metallic sheaths, a layer of bedding is provided and it consists of paper tape compounded with a fibrous material. Also, sometimes,jute strands or hessian tape (strong coarse cloth of hemp or jute) also used for bedding. The purpose of providing the bedding is to protect the metallic sheath from mechanical injury due the armouring.
e) Armouring: Armouring is provided to protect from mechanical injury to the cable and it consists of one or two layers of galvanized steel wires or two layers of steel tape.
f) Serving: Over and above armouring, fibrous material is again provided which is similar to that of bedding but is called as serving.
The numbers of strands are fixed and it may be 3, 7, 19, 37 and 61. As the number of strands goes on increasing, the flexibility will goes on increasing. The size of wire may be measured in “Standard Wire Gauge” (SWG) number and number of strands. For example if a wire is 7/26, it means there are 7 number of strands and SWG number of wire is 26. As the SWG number goes on increasing, the diameter of the wire goes on decreasing and vice-versa. But now a day, we are using cross-sectional area of the conductor instead of SWG number. So, now the size of wire is being expressed as number of strands/cross-sectional area of conductor. The use of wire is limited up to 650 volts whereas as the use cable is restricted to 132,000 volts, due to insulation problems. The wire and cables are explained separately.
i. Wires
Now we will study the different types of wires and their suitability, depending upon the environmental conditions. The wires may be insulated wire and bared wires.The wires are available in different voltage grading such as 250 volts and 650 voltsonly. The insulated wires being used in electrical wiring system are detailed
a) V.I.R. (Valcanized India Rubber): The conductor is valcanized by Indian rubber. Cotton tapes sheathe is provided over the valcanized Indian rubber. A very fine layer of any moisture proof insulating material, just as bitumen, wax or which can prevent ingress of moisture content to conductor. These wires are totally obsolete nowadays, as these wires absorb moisture quickly.
b) C.T.S./T.R.S. (Cab Type Sheathed/Tough Rubber Sheathed): These wires are almost moisture proof types. These are available in single core, and twin core. The conductor is covered with a layer of pure rubber, to protect from ingress of moisture. Over the layer of pure rubber, vulcanized Indian rubber layer is provided. Finally, a sheathing of tough rubber is provided to give mechanical strength to the wire. These wires are also obsolete now.
c) Lead Sheath Wires: These types of wires are similar in construction with T.R.S., as for as core section is concerned but having outer sheath of lead or lead alloy. The lead sheathing provides full moisture proof environment to the conductor. So, these wires can be used where the humidity is comparatively more or we may use even in snowfall area too.
d) P.V.C. (Poly Vinyl Chloride) Wires: In these types of wires, the conductor is insulated with P.V.C. insulation. There is no need for providing cotton tape,as the P.V.C. insulation is tougher than rubber, to protect the wire from moisture and mechanical failure. Nowadays, these wires are used for mostly batten,cleat, casing capping and conduit wiring. These wires should not be used for connecting heating appliances and pendent lights, etc.
e) Weather Proof Wires: These wires are used for outdoor service lines. The conductors are insulated first with rubber, then braiding of cotton thread. This braiding material is dipped in waterproof compound so that ingress of moisture could be avoided completely.
f) Flexible wires: The wires are used to connect the pendent lights and portable appliances. The insulation used is pure rubber or valcanized rubber on which single or double cotton or valcanized rubber layer is provided. Finally, single or double cotton or artificial silk braiding is provided or only P.V.C. layer is provided.
The following are the types of the flexible wires available in the market.
ii. Cables
The power from generating station can be transmitted by cables also, if the transmission distance is less. As seen already in comparison, the underground system is costlier and its maintenance is difficult. Still, the system of transmitting the power by cables is preferred in thickly populated areas and cities. The cables are usually classified according to the voltage for which they are manufactured.
According to the voltage they can be classified:
1. L.T. (Low Tension Cables) up to 1000 volts.
2. H.T. (High Tension Cables) up to 11,000 volts.
3. S.T. (Super Tension Cables) from 22,000 volts to 33,000 volts.
4. Extra High Tension Cables from 33,000 volts to 66,000 volts.
5. Oil Filled and Gas Pressure Cables from 66,000 volts to 132,000 volts.
iii. General Construction of Cable
The general construction of a cable is given below along with figure, which illustrates the various parts of the cable.
a) Core: All cables have one central conductor and a number of stranded conductors of copper or aluminium, wrapped around the central conductor. Generally, there are one, two, three or four cores (three and a half). This half core is used as neutral core.
b) Insulation: The different insulating materials are used to insulate conductors or cores such as impregnated paper, varnished cambric and vulcanized bitumen for low voltage. Impregnated paper is an excellent insulating material and widely used. When varnished cambric is used as an insulating material for low voltage cables, petroleum jelly is applied between the layers of the cambric type. It prevents the insulation damage by friction when the cables are handled.
c) Metallic Sheath: A metallic sheath is provided over insulation so as to prevent the entry of the moisture into the insulating material. The metallic sheath is usually of lead or lead alloy.
d) Bedding: Over the metallic sheaths, a layer of bedding is provided and it consists of paper tape compounded with a fibrous material. Also, sometimes,jute strands or hessian tape (strong coarse cloth of hemp or jute) also used for bedding. The purpose of providing the bedding is to protect the metallic sheath from mechanical injury due the armouring.
e) Armouring: Armouring is provided to protect from mechanical injury to the cable and it consists of one or two layers of galvanized steel wires or two layers of steel tape.
f) Serving: Over and above armouring, fibrous material is again provided which is similar to that of bedding but is called as serving.
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