Top 4 Types of Cables | Conductors | Electrical Engineering

Different types of insulated wires and cables used for house wiring and service connections are:- 1. Flexible Cord and Flexible Cable 2. Paper Insulated Cable 3. Tropodur Cables 4. Mineral Insulated Cable or M.I. Cable.

Type # 1. Flexible Cord and Flexible Cable:

The stranded wire having cross-sectional area of each strand less than 4.28 mm² is called flexible cord. Stranded wire with higher cross-sectional area of each strand is called flexible cable.

Flexible cord usually consists of two separately insulated flexible stranded conductors twisted together. Such a cord is called twin flexible. These wires are extensively used for portable appliances like heaters, electric irons, refrigerators, hand lamps, table fans etc. With twin flexible also supply is given to pendant lamps, ceiling fans, electroliers, bracket lamps etc. The cord is available with two different types of insulation, namely high insulation and medium insulation. For pendant lamps, bracket lamps, ceiling fans etc. wires with medium insulation are usually used.

In place of twin flexible sometimes three separately insulated flexible stranded conductors are twisted together. Such a flexible cord is called triple flexible cord. This cord is usually used in a.c. circuits. There are various types of flexible wires.

They are briefly described as follows:

(i) Twin Rubber Insulated Flexible Cord:

This cord consists of two separately insulated stranded conductors, each conductor is at first covered with cotton insulation. Over the cotton insulation a fine layer of rubber is applied. Cotton covering prevents sticking of rubber to the conductor. The two conductors are then embedded in a solid mass of rubber insulation of sufficient thickness to build up to full size of finished cord.

No other insulation is further provided over it. The two conductors are so laid within the outer rubber insulation that there is a depression in the middle of them. This helps to separate the two conductors at the end where the cord is connected to the terminals of the appliance. The outer rubber insulation must be of much higher grade than that used for ordinary wire insulation. The rubber insulation immediately over each conductor has a particular colour. For a two-core cable these are usually red and black.

(ii) Twin Silk Flexible Cord:

This cord consists of two cores, each core having a number of fine copper conductors stranded together. Each core is at first covered with cotton insulation over which a layer of rubber insulation is provided. The cotton insulation between conductor and the layer of rubber prevents sticking of rubber to the copper conductors. Over the rubber insulation there is a loose braid of cotton insulation. Finally the two cores are laid side by side within a thick braid of silk insulation.

(iii) Twin Twisted and Cotton Braided Flexible Cord:

In this cord two separately insulated wires are twisted together as shown in fig. 72. Each wire consists of a number of fine copper conductors stranded together. The stranded conductor is covered with cotton insulation over which a layer of rubber insulation is applied. Cotton covering prevents sticking of rubber to the stranded conductor. Finally the insulation of a single wire is completed when a braid of cotton insulation covers the rubber insulation. The two wires are then twisted together.

(iv) Twin Tough Rubber Sheathed Flexible Cord or P.V.C. Cord:

This type of cord is resistant to moisture and can withstand much wear and tear. The cord consists of two or more separately insulated wires twisted together, each wire having a number of fine copper conductors stranded together. In order to form a round assembly the open space in between the twisted wires is filled up with jute or cotton threads. Over this assembly a loose braid of cotton insulation is provided. Finally a layer of high grade tough rubber insulation is applied all over the cotton insulation.

Now-a-days P.V.C. insulated flexible cord is widely used in place of rubber insulated cord. P.V.C. insulation may be directly applied over the conductor of each wire. It does not affect the copper or aluminium conductors of the wires.

Type # 2. Paper Insulated Cable:

The use of paper as insulating material has almost superseded the use of rubber insulation for the manufacture of power cables. The foremost reason of this is that, paper is cheaper than rubber and has lower capacitance and higher dielectric strength. Since paper is very much hygroscopic, it is soaked in insulating oil before use.

Further, a metallic sheath is provided over the insulation so as to prevent the entry of moisture into the insulating material. The two ends of the cable are always kept sealed with plumbing metal. The metallic sheath is usually made of lead or lead alloy or aluminium. Over the sheath comes a layer of bedding.

This bedding consists of jute tape of hessian tape or paper tape compounded with a fibrous material which protects the metallic sheath from mechanical injury. In case of armored cable armoring is provided to avoid mechanical injury to the cable. The armoring consists of one or two layers of galvanished steel tape or steel wires. Finally, over and above armoring a serving of fibrous material like Jute tape impregnated with bitumen compound is provided.

Paper insulated cables are in most cases placed underground. If the cable has two layers of steel tape armoring, it is called P.I.L.C.D.S.T.A cable (paper insulated lead covered double steel tape armored cable). With two layers of steel wires armoring, the cable is called P.I.L.C.D.S.W.A. (paper insulated lead covered double steel wires armored) cable.

Type # 3. Tropodur Cables:

This type of cables may have either copper or aluminium conductors, but in most cases aluminium conductors are usually used. Over the conductor specially treated P.V.C. insulation, called tropodur, is applied. This insulation is covered all around by a layer of regenerated rubber or P.V.C. tape. Tropodur cable may be either armored or unarmored like paper insulated cable.

The armoring consists of one or two layers of galvanised steel tape or steel wires. Finally, over and above armoring, a layer of very hard tropodur is provided. In case of unarmored cable this layer of hard tropodur is directly applied over regenerated rubber insulation or tropodur tape.

The voltage grade of tropodur cables is 650/1100 volts. At present this cable is manufactured in large scale in India. It is widely used for service connection and for transmission and distribution of electric power in place of paper insulated cable. Tropodur cable may be single- cored, twin-cored, three-cored and four-cored.

Type # 4. Mineral Insulated Cable or M.I. Cable:

In this cable copper conductor is placed within a copper sheath and the space between the conductor and the sheath is filled up with magnesium oxide under high pressure. This cable may be used anywhere with the help of copper clips and copper saddles. The cable may run through the concrete walls or it may be placed underground.

The specialty of this wire is that, it is totally fire proof and is not damaged easily by mechanical injury. It can be safely used in kerosene or petrol go-down, in factories where gunpowder or other explosive mixture is manufac­tured, in damp or moist place and in places where the surrounding atmosphere is filled with fumes produced from acid and other chemicals” The only difficulty with this wire is that, it requires highly skilled labour as some special type’ of tools are to be used while cutting and sealing the conductor.