Classification of Cables

Classification of Cables Points : classification of electric cables, v.i.r. vulcanized insulated rubber cables, .t.s or t.r.s (cabtyre sheathed) or (tough rubber sheathed) cables, poly vinyl chloride (p.v.c) insulated cables, lead sheathed cables, flexible cables and cord, weather - proof cables, properties of insulation, according to the number of cores, advantages of stranded conductors over solid conductors, The cable employed for internal wiring of buildings may be divided into different groups according to:
(i) Type of insulation used
(ii) Number of cores used
(iii) Voltage grading and
(iv) Conductor used (i) ACCORDING TO INSULATION The following cables are used for different types of wiring installation.
(I). V.I.R (Vulcanized Insulation Rubber) Cable
(ii) C.T.S (Cabtyre sheathed) Cable or T.R.S (Tough Rubber Sheathed) Cable.
(iii) P.V.C (Poly vinyl Chloride) Cable.
(iv) Lead sheathed cable.
(v) Flexible Cable.
(vi) Whether Proof Cables. (i) V.I.R. VULCANIZED INSULATED RUBBER CABLES V.I.R insulation is applied on wires and cables of low and medium voltage. Vulcanized rubber contains sulphur, which reacts readily with copper. A thin layer of pure rubber is first applied beneath the V.I.R insulation.

The copper conductor is tinned to provide protection against corrosion due to presence of traces of sulphur, zinc oxide and other mineral ingredients in the V.I.R. V.I.R. cables are available in 250/440v as well as in 650/1100 volts cables and are used for general electrical wiring in cleat, casing capping and conduit. (ii) C.T.S OR T.R.S (CABTYRE SHEATHED) OR (TOUGH RUBBER SHEATHED) CABLES The rubber layers serve as insulation covering of V.I.R cables. As rubber is easily damaged, a. protective layer of tarred jute braiding is applied over it. Conductors with insulation covering of V.I.R and protective covering of Tough rubber are called Tough Rubber sheathed (T.R.S) and Cab tyre sheathed (C.T.S) Cables.

OR

These cables are available in 250/440 volts grade and used in C.T.S or T.R.S wiring. T.R.S. cable in nothing but a vulcanized rubber insulated conductor with an outer protective covering of tough rubber. These cables are waterproof, hence can be used in wet, condition. These cables are cheaper in cost and lighter in weight than lead alloy sheathed cables. (iii) POLY VINYL CHLORIDE (P.V.C) INSULATED CABLES In this type of cable conductor is insulated with P.V.C insulation. It is a thermoplastic material, which is resistant to oil grease and water and is suitable for insulation. P.V.C is harder than rubber so it does not require covering over P.V.C for mechanical and moisture protection. These cables are available in 250/440 volts grades, and are used for concealed wiring. These cables are not used for heating appliances. (iv) LEAD SHEATHED CABLES The lead sheathed cable is a vulcanized rubber insulated conductor covered with a continuous sheath of lead. The lead sheath provides good protection against mechanical injury and so can be used without casing or conduit system. These cables are available in 250/440 volts grade and are used for internal wiring where climatic is not dry and has little bit moisture. (v) FLEXIBLE CABLES AND CORD The flexible cable consist of wires either silk or cotton or plastic covered. Plastic cover is more popular as it is available in various colours. Flexibility and strength is achieved by using the conductors having large number of strands. These wires or cables are used for household appliances such as radios, Fans, Lamps etc. (iv) WEATHER - PROOF CABLES These cables are used for out-door wiring and for power supply or industrial supply. These cables arc either P.V.C insulated or vulcanized rubber-insulated conductors being suitably taped (only incase (V.I.R) braided and then compounded with weather resistant material. These cables are not affected by heat or sun or rain. Properties of insulation The conductor is covered with insulating material so that it may prevent leakage of current from the conductor, i.e., the insulating material should be extremely high resistive to the flow of electrical current though them. The insulating material used in electrical cables must have the following properties.
(i) High resistivity
(ii) High flexibility
(iii) Non inflammability
(iv) High resistance against moisture, acids and alkalies.
(v) Capacity to withstand high temperature without much deterioration.
(vi) Capacity to withstand high rupturing voltages.
(vii) Should not be hygroscopic
The property of absorbing moisture is called hygroscopic property. VOLTAGE GRADING Cables and insulated wires are manufactured to work up to a particular voltage. The dielectric failure of cable insulation depends on voltage. The insulation may puncture if the voltage exceeds the limit.

Therefore, the classification of cables is done according to the “voltage” at which the cables are designed to work.

A 250 / 440 V grade cable is suitable for use on single phase or three-phase system, where the voltage between each conductor and earth does not exceed 250 V (i.e.. solidly earthed 400 / 440 V system).

A 650 / 1100 Volts grade cable is suitable for use on medium voltage 3-phase system, where the voltage between each conductor and earth does not exceed 650 V (i.e. solidly earthed 1100 V system). (ii) ACCORDING TO THE NUMBER OF CORES The cables of the cables may be divided into the classes known as single core cables, twin core cables; three core cables; two core with (E.C.C) Earth continuity conductor etc. CORE It is one or several twisted strands of wire used as a conductor of electric current. A multi-core conductor is a cable with several cores insulated from one another and enclosed in a common sheath. The cables are manufactured as single core, twin core, three core (flat or circular), and two cores with E.C.C. (iii) ACCORDING TO VOLTAGE GRADING The cables may be divided into two classes:
(i) 250/440 volts cables and
(ii) 650/1100 volts cables (IV) ACCORDING TO THE CONDUCTOR A number of single bare conductors twisted together from a multi-standed conductor. Advantages of Stranded Conductors Over Solid Conductors 1. Stranded conductors are more flexible. Chances of cracking of insulation at bend are less. They can be easily laid, are comparatively easier to handle.
2. Connections and joints of stranded conductors are stronger and of longer life.
3. Stranded conductors become stronger when reinforced and have better gripping of insulation.
4. Solid conductors between supports of overhead lines may break due to vibration. It is avoided with stranded conductors.
5. Space between strands permits flow of oil in the (UG) under ground cable to cool and insulate it. Size of Wire The size of the wires is given with the help of S.W.G. number. For example a wire may be indicated as 3/.029. The given size of wire 31.029 means that of wire numerator indicates the number of strands employed and denominator indicates the SWG number of each strand. Now-a-days the sizes of the wires in metric system is given by 19/2.24 means that there are nineteen strands, each strand being of diameter 2.24 mm. Similarly 7/1.70 size wire means that wire consisting of 7 strands, each strand of 1.7O mm diameter.