Applications of Heating Effect of Electric Current

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Applications of Heating Effect of Electric Current

Now, the generated heat can be viewed from many points of angles. Sometimes, it is only seen as a loss and is trying to be minimized. Various steps are taken to minimize heat dissipation from the conductor. But we can see many positive applications of heating effect of electric current in our daily life. Electric iron, the whole idea or working principle depends upon the heating effect of electric current. High resistant wire is used as the main coil in the electric iron when electric current flows through the coil, the coil gets heated and the iron works. But what about over heating of electric iron? This problem can be solved by using bimetallic conductors. Bimetallic plates made of two different metals are used in the circuit. As expansion co-efficient of heat is different for two metals, so due to heating effect one metal's expansion is different from the other metal; as a result the plate is bent and after reaching at a certain temperature the contact of the circuit is broken and electric current flowing through the coil is stopped and the electric iron too is not heated any more.

The same mechanism is used in electric heater, the only difference is that there is no bimetallic plate or circuit breaker involved.

Another application of heating effect of current is seen in electric bulbs. The wire which is used inside the bulb gets illuminated and emits light after reaching certain temperature. The metal used in the bulb is mainly made of tungsten.

Finally and perhaps the most important application of the heating effect of electric current is in electrical fuses, that are used in almost everywhere. From huge industries to domestic level, everywhere electrical fuse is a must. The fuse is made of such metals which has a certain melting point. They are okay for normal electric current but when over electric current flows through the circuit; the generated heat in the fuse wire is enough to melt the metal portion of the fuse wire and break the circuit. In this way the costly equipment is protected as huge electric current flow, can damage the equipment permanently.

Alternating Current

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Alternating Current

The electric current whose flow is not unidirectional moreover it alternates at a frequency, is called alternating current. In other words, the direction of the electric current continuously changes from forward to backward and then backward to forward in the circuit. The number of times, this direction changes from forward to backward or from backward to forward per second, is referred as frequency of the current. The electric current produced in an alternator is always an alternating current. The shape of the waveform of an alternating electric current is usually sinusoidal. But square, triangular and other types of waveform are also available for attending current.

Conventional Direction of Alternating Current

As direct current, alternating electric current is denoted with arrow. An AC has both forward and backward direction of flow. The arrow head always indicates the forward direction of the current. In different point of view, when the electric current has a positive valve, the direction of electric current is same as the reference arrow and when the electric current gets negative value; its direction is just opposite of the reference arrow.

Effects of Electric Current

There are mainly two effects of electric current, such as heating effect and magnetic effect. Each and every utilization of electricity, we see in our daily life, is either due to heating effect or due to magnetic effect of electric current. For examples, the light bulb glows in our house is due to heating effect of electric current and the fan rotates in our house is due to magnetic effect of electric current. There are thousands of other examples which can illustrate the effect of electric current, too.

Heating Effect of Electric Current

Whenever electric current passes through a conductor there would be a generation of heat due to ohmic loss in the conductor. This is commonly known as heating effect of electric current.

Since, we cannot use electric power directly, we need to convert it into another usable power, like heat, light, or mechanical power etc. When electric current flows through a conductor some loss occurs and this loss is almost inevitable, and more the resistance of the conductor, more the loss. This loss due to the electrical resistance of conductor is mainly responsible for the heating effect of electric current.

As some electric power is converted into heat energy, this phenomenon can be described by Joule's law, which states that,

Where H is the generated heat in calories, "i" is the electric current that is flowing through the wire and it is measured in amperes, "r" is the resistance of the conductor in ohm(Ω) and "t" is the duration of electric current flowing in seconds. If we know the time of electric current flowing, the resistance of wire, and amount of electric current flow, we can easily find out the generated heat of the circuit. This heat can be utilized in various ways. 

We saw that the more the electrical resistance of the wire the more the generated heat in the circuit, but to know more accurately about the heating effect of current, we should know about it from the atomic level. As the flow of electric current is nothing but the flow of electrons there will always be resistance from the fixed atoms of the conductor. The fixed atoms of the wire resist the flow of electrons and as a result there are collisions and as the kinetic energy converts into heat energy we see that the wire is getting hot.

Types of Current

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Types of Current

There are only two types of electrical current, direct electric current and alternating current.

We abbreviate them as DC and AC respectively. Concept of DC was developed before AC. But AC becomes most popular means of generating, transmitting and distributing of electric power. The direction of the flow of direct electric current is unidirectional, means this electric current does not alter its direction during flowing. Most common examples of DC in our daily life, are the electric current that we get from all kinds of battery system. But most popular form of electrical electric current is alternating electric current or AC. AC does have some advantages over DC for generating, transmitting and distributing and that is why the electric current we get from our electric supply companies, is normally alternating current.

Measurement of Current

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Measurement of Current

The most common method of measuring electric current is to connect an ammeter in series with the circuit that’s electric current to be measured. This is so because; the entire electric current flowing through the circuit must also flow through the ammeter also. The ideal internal resistance or impedance of an ammeter is zero. Hence, ideally there is no voltage drop across the ammeter connected in the circuit. A conventional analog ammeter consists of a electric current coil. Whenever electric current flows through this coil, it deflects from its position depending upon the amount of electric current flowing through it. A pointer is attached to the coil assembly; hence it points the electric current reading on the dial of the ammeter. For measuring alternating current, clip on meter or tong tester can also be used instead of conventional ammeter. In this ammeter a current transformer core is attached to the meter which can easily be clipped on the live electric current carrying conductor. Due to this arrangement, electric current in the circuit transforms to the secondary of the CT and this secondary electric current then measured on the dial of clip on meter without disturbing the continuity of the electric current unlike conventional ammeter.

In the early days, it was thought that the electric current is, flow of positive charge and hence electric current always comes out from the positive terminal of the battery, passing through the external circuit and enters in the negative terminal of the battery. This is called conventional flow of current. On the basis of this conception, all the theories of electricity, formulas, and symbols were developed. After the development of atomic nature of matter, we have come to know, that actual cause of electric current in a conductor is due to movement of free electrons and electrons have negative change. Due to negative charge, electrons move from the negative terminal to the positive terminal of the battery through the external circuit. So the conventional flow of current is always in the opposite direction of electrons flow. But it was impossible to change all the previously discovered subsequent rules, conventions, theories and formulas according to the direction of electrons flow in the conductor. Thus the concept of conventional electric current flow was adopted. The true electron flow is used only when it is necessary to explain certain effects (as in semiconductor devices such as diodes and transistors). Whenever we consider the basic electrical circuits and devices, we use conventional flow of electric current i.e. electric current flowing around the circuit from the positive terminal to the negative terminal.

Theory of Electricity

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Theory of Electricity

There is an equal number of electrons and protons in an atom. Hence, atom is in general electrically neutral. As the protons in the central nucleus are positive in charge and electrons orbiting the nucleus, are negative in charge, there will be an attraction force acts between the electrons and protons. In an atom various electrons arrange themselves in different orbiting shells situated at different distances from the nucleus.

The force is more active to the electrons nearer to the nucleus, than to the electrons situated at outer shell of the atom. One or more of these loosely bonded electrons may be detached from the atom. The atoms with lack of electrons are called ions. Due to lack of electrons, compared to number of protons, the said ion becomes positively charged. Hence, this ion is referred as positive ion and because of positive electrical charge; this ion can attract other electrons from outside. The electron, which was previously detached from any other atom, may occupy the outer most shell of this ion and hence this ion again becomes neutral atom. The electrons which move from atom to atom in random manner are called free elections. When a voltage is applied across a conductor, due to presence of electric field, the free electrons start drifting to a particular direction according the direction of voltage and electric field. This phenomenon causes electric current in the conductor. The movement of electrons, means movement of negative charge and rate of this charge transfer with respect to time is known as electric current.

The amount of negative electric charge in an electron is 1.602 X 10^-19 Coulomb. Hence, one coulomb negative electric charge consists of 1/1.602 X 10 ^-19 = 6.24 X 10 ¹⁸number of electrons. Hence, during drift of electron to a particular direction, if 6.24 X 10 ¹⁸ number of electrons cross a specific cross-section of the conductor, in one second, the electric current is said to be one ampere. Since, we have already seen the unit of electric current, ampere is coulomb/second.

What is Electric Current?

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What is Electric Current?

Electric current is nothing but the rate of flow of electric charge through a conductor with respect to time. It is caused by drift of free electrons through a conductor to a particular direction. As we all know, the measuring unit of electric change is Coulomb and the unit of time is second, the measuring unit of electric current is Coulombs per second and this logical unit of current has a specific name Ampere after the famous French scientist André-Marie Ampere.

If total Q Coulomb charge passes through a conductor by time t, then electric current I = Q / t coulomb par second or Ampere.

For better understanding, let give an example, suppose total 100 coulombs of charge is transferred through a conductor in 50 seconds. What is the electric current? 

As the electric current is nothing but the rate at which charge is transferred per unit of time, it would be ratio of total charge transferred to the required time for that. Hence, here electric current I = 100 coulombs / 50 second = 2 Amperes.

'Ampere' is Sl unit of current.
Definition of Electric Current

While a potential difference is applied across a conductor, electrical charge flows through it and electrical electric current is the measure of the quantity of the electrical charge flowing through the conductor per unit time. 

CONDUCTORS

CONDUCTORS

Commonly used conductor materials:

 The most commonly used conductor materials for over head lines are copper, aluminium, steel cored aluminium, galvanised steel and cadmium copper. The choice of a particular material will depend upon the cost, the required electrical and mechanical properties and the local conditions. All conductors used for overhead lines are preferably stranded in order to increase the flexibility. In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers, the total number of individual wires is 3n(n + 1) + 1. In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiralled in opposite directions so that layers are bound together.

 TYPES OF CONDUCTOR

 Copper

 Copper is an ideal material for overhead lines owing to its high electrical conductivity and greater tensile strength. It is always used in the hard drawn form as stranded conductor. Although hard drawing decreases the electrical conductivity slightly yet it increases the tensile strength considerably. Copper has high current density i.e., the current carrying capacity of copper per unit of Xsectional area is quite large. This leads to two advantages. Firstly, smaller X-sectional area of conductor is required and secondly, the area offered by the conductor to wind loads is reduced. Moreover, this metal is quite homogeneous, durable and has high scrap value. There is hardly any doubt that copper is an ideal material for transmission and distribution of electric power. However, due to its higher cost and non-availability, it is rarely used for these purposes. Now a days the trend is to use aluminium in place of copper.

  Aluminium

 Aluminium is cheap and light as compared to copper but it has much smaller conductivity and tensile strength. The relative comparison of the two materials is briefed below:

 (i) The conductivity of aluminium is 60% that of copper. The smaller conductivity of aluminium means that for any particular transmission efficiency, the X-sectional area of conductor must be larger in aluminium than in copper. For the same resistance, the diameter of aluminium conductor is about 1·26 times the diameter of copper conductor. The increased X-section of aluminium exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater transverse strength. This often requires the use of higher towers with consequence of greater sag.

 (ii) The specific gravity of aluminium (2·71 gm/cc) is lower than that of copper (8·9 gm/cc).Therefore, an aluminium conductor has almost one-half the weight of equivalent copper conductor. For this reason, the supporting structures for aluminium need not be made so strong as that of copper conductor.

 (iii) Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are required.

 (iv) Due to lower tensile strength and higher co-efficient of linear expansion of aluminium, the sag is greater in aluminium conductors. Considering the combined properties of cost, conductivity, tensile strength, weight etc., aluminium has an edge over copper. Therefore, it is being widely used as a conductor material. It is particularly profitable to use aluminium for heavy-current transmission where the conductor size is large and its cost forms a major proportion of the total cost of complete installation.

 

 Steel cored aluminium

 Due to low tensile strength, aluminium conductors produce greater sag. This prohibits their use for larger spans and makes them unsuitable for long distance transmission.In order to increase the tensile strength, the aluminium conductor is reinforced with a core of galvanised steel wires. The composite conductorthus obtained is known as steel cored aluminium and is abbreviated as A.C.S.R. (aluminium conductor steel reinforced).

Steel-cored aluminium conductor consists of central core of galvanized steel wires surrounded by a number of aluminium strands. Usually, diameter of both steel and aluminium wires is the same. The X-section of the two metals are generally in the ratio of 1 : 6 but can be modified to 1 : 4 in order to get more tensile strength for the conductor. Fig. shows steel cored aluminium conductor having one steel wire surrounded by six wires of aluminium. The result of this composite conductor is that steel core takes greater percentage of mechanical strength while aluminium strands carry the bulk of current. The steel cored aluminium conductors have the following

 

Advantages:

 (i) The reinforcement with steel increases the tensile strength but at the same time keeps the composite conductor light. Therefore, steel cored aluminium conductors will produce smaller sag and hence longer spans can be used.

(ii) Due to smaller sag with steel cored aluminium conductors, towers of smaller heights can be used.

 

Galvanized steel

 Steel has very high tensile strength. Therefore, galvanised steel conductors can be used for extremely long spans or for short line sections exposed to abnormally high stresses due to climatic conditions. They have been found very suitable in rural areas where cheapness is the main consideration. Due to poor conductivity and high resistance of steel, such conductors are not suitable for transmitting large power over a long distance. However, they can be used to advantage for transmitting a small power over a small distance where the size of the copper conductor desirable from economic considerations would be too small and thus unsuitable for use because of poor mechanical strength.

 

Cadmium copper

The conductor material now being employed in certain cases is copper alloyed with cadmium. An addition of 1% or 2% cadmium to copper increases the tensile strength by about 50% and the conductivity is only reduced by 15% below that of pure copper. Therefore, cadmium copper conductor can be useful for exceptionally long spans. However, due to high cost of cadmium, such conductors will be economical only for lines of small X-section i.e., where the cost of conductor material is comparatively small compared with the cost of supports.

 

INSULATOR TESTING TYPES

INSULATOR TESTING TYPES

 According to the British Standard, the electrical insulator must undergo the following tests

 1.     Flashover tests of insulator

2.     Performance tests 

3.     Routine tests

Let's have a discussion one by one

FLASHOVER TEST

There are mainly three types of flashover test performed on an insulator and these are-

Power Frequency Dry Flashover Test of Insulator

First the insulator to be tested is mounted in the manner in which it would be used practically.

Then terminals of variable power frequency voltage source are connected to the both electrodes of the insulator. 

Now the power frequency voltage is applied and gradually increased up to the specified value. This specified value is below the minimum flashover voltage. 

This voltage is maintained for one minute and observe that there should not be any flash-over or puncher occurred. 

The insulator must be capable of sustaining the specified minimum voltage for one minute without flash over.

 

Power Frequency Wet Flashover Test or Rain Test of Insulator

 In this test also the insulator to be tested is mounted in the manner in which it would be used practically.

Then terminals of variable power frequency voltage source are connected to the both electrodes of the insulator.

After that the insulator is sprayed with water at an angle of 45o in such a manner that its precipitation should not be more 5.08 mm per minute. The resistance of the water used for spraying must be between 9 kΩ 10 11 kΩ per cm3 at normal atmospheric pressure and temperature. In this way we create artificial raining condition.

Now the power frequency voltage is applied and gradually increased up to the specified value. This voltage is maintained for either one minute or 30 second as specified and observe that there should not be any flash-over or puncher occurred. The insulator must be capable of sustaining the specified minimum power frequency voltage for specified period without flash over in the said wet condition.

 Power Frequency Flashover Voltage test of Insulator

 The insulator is kept in similar manner of previous test. In this test the applied voltage is gradually increased in similar to that of previous tests. But in that case the voltage when the surroundings air breaks down, is noted.

 Impulse Frequency Flashover Voltage Test of Insulator

 The overhead outdoor insulator must be capable of sustaining high voltage surges caused by lightning etc. So this must be tested against the high voltage surges.

 The insulator is kept in similar manner of previous test.

Then several hundred thousands Hz very high impulse voltage generator is connected to the insulator 

Such a voltage is applied to the insulator and the spark over voltage is noted.

The ratio of this noted voltage to the voltage reading collected from power frequency flashover voltage test is known as impulse ratio of insulator.

This ratio should be approximately 1.4 for pin type insulator and 1.3 for suspension type insulators.

PERFORMANCE TEST OF INSULATOR

Now we will discuss performance test of insulator one by one 

Temperature Cycle Test of Insulator

The insulator is first heated in water at 70oC for one hour. 

Then this insulator immediately cooled in water at 7oC for another one hour. This cycle is repeated for three times.

After completion of these three temperature cycles, the insulator is dried and the glazing of insulator is thoroughly observed.

After this test there should not be any damaged or deterioration in the glaze of the insulator surface

Puncture Voltage Test of Insulator

The insulator is first suspended in an insulating oil.

Then voltage of 1.3 times of flash over voltage, is applied to the insulator. A good insulator should not puncture under this condition

Porosity Test of Insulator

The insulator is first broken into pieces.

Then These broken pieces of insulator are immersed in a 0.5 % alcohol solution of fuchsine dye under pressure of about 140.7 kg ⁄ cm2 for 24 hours.

After that the sample are removed and examine.

The presence of a slight porosity in the material is indicated by a deep penetration of the dye into it.

Mechanical Strength Test of Insulator

The insulator is applied by 2½ times the maximum working strength for about one minute.

The insulator must be capable of sustaining this much mechanical stress for one minute without any damage in it.

 

ROUTINE TEST OF INSULATOR

Each of the insulator must undergo the following routine test before they are recommended for using at site.

 

Proof Load Test of Insulator

In proof load test of insulator, a load of 20% in excess of specified maximum working load is applied for about one minute to each of the insulator.

Corrosion Test of Insulator

The insulator with its galvanized or steel fittings is suspended into a copper sulfate solution for one minute.

Then the insulator is removed from the solution and wiped, cleaned. Again it is suspended into the copper sulfate solution for one minute.

The process is repeated for four times.

Then it should be examined and there should not be any disposition of metal on it.

INSULATED CABLE - INTRODUCTION

INSULATED CABLE - INTRODUCTION

 Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages such as less liable to damage through storms or lightning, low maintenance cost, less chance of faults, smaller voltage drop and better general appearance. However, their major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with the equivalent overhead system. For this reason, underground cables are employed where it is impracticable to use overhead lines. Such locations may be thickly populated areas where municipal authorities prohibit overhead lines for reasons of safety, or around plants and substations or where maintenance conditions do not permit the use of overhead construction. The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively low or moderate voltages. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltages. This has made it possible to employ underground cables for transmission of electric power for short or moderate distances. In this chapter, we shall focus our attention on the various aspects of underground cables and their increasing use in power system.

  UNDERGROUND CABLES

 An underground cable essentially consists 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 upon the working voltage and service requirements. In general, a cable must fulfill the following necessary requirements:

 (i) The conductor used in cables should be tinned stranded copper or aluminum of high conductivity. Stranding is done so that conductor may become flexible and carry more current.

 (ii)  The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.

 (iii)  The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.

 (iv)  The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.

 (v) The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

CONSTRUCTION OF CABLES

Fig shows the general construction of a 3-conductor cable.

 

The various parts are

 

 

a)Cores or Conductors

 

A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended. For instance, the 3- conductor cable shown in Fig. is used for 3-phase service. The conductors are made of tinned copper or aluminum and are usually stranded in order to provide flexibility to the cable.

 

b) Insulation

 

Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.

 

c)Metallic sheath.

 

In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation as shown in Fig.

 

d) Bedding.

 

Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

 

e) Armouring.

 

Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables.

 

f) Serving.

 

In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring. This is known as serving.

 It may not be out of place to mention here that bedding, armouring and serving are only applied to the cables for the protection of conductor insulation and to protect the metallic sheath from Mechanical injury.

 

 

 

INSULATING MATERIALS FOR CABLES

INSULATING MATERIALS FOR CABLES

 The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. Therefore, the proper choice of insulating material for cables is of considerable importance. In general, the insulating materials used in cables should have the following

 Properties

(i) High insulation resistance to avoid leakage current.

(ii) High dielectric strength to avoid electrical breakdown of the cable.

(iii) High mechanical strength to withstand the mechanical handling of cables.

(iv)  Non-hygroscopici.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath.

(v) Non-inflammable.

(vi)Low cost so as to make the underground system a viable proposition.

(vii) Unaffected by acids and alkalies to avoid any chemical action. No one insulating material possesses all the above mentioned properties. Therefore, the type of insulating material to be used depends upon the purpose for which the cable is required and the quality of insulation to be aimed at. The principal insulating materials used in cables are rubber, vulcanized India rubber, impregnated paper, varnished cambric and polyvinyl chloride.

 

Rubber

Rubber may be obtained from milky sap of tropical trees or it may be produced from oil products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30 kV/mm and resistivity of insulation is 1017 cm. Although pure rubber has reasonably high insulating properties, it suffers form some major drawbacks viz., readily absorbs moisture, maximum safe temperature is low (about 38ºC), soft and liable to damage due to rough handling and ages when exposed to light. Therefore, pure rubber cannot be used as an insulating material.

 

Vulcanised India Rubber (V.I.R.)

It is prepared by mixing pure rubber with mineral matter such as zinc oxide, red lead etc., and 3 to 5% of sulphur. The compound so formed is rolled into thin sheets and cut into strips. The rubber compound is then applied to the conductor and is heated to a temperature of about 150ºC. The whole process is called vulcanisation and the product obtained is known as vulcanised India rubber. Vulcanised India rubber has greater mechanical strength, durability and wear resistant property than pure rubber. Its main drawback is that sulphur reacts very quickly with copper and for this reason, cables using VIR insulation have tinned copper conductor. The VIR insulation is generally used for low and moderate voltage cables.

 

Impregnated paper

It consists of chemically pulped paper made from wood chippings and impregnated with some compound such as paraffinic or naphthenic material. This type of insulation has almost superseded the rubber insulation. It is because it has the advantages of low cost, low capacitance, high dielectric strength and high insulation resistance. The only disadvantage is that paper is hygroscopic and even if it is impregnated with suitable compound, it absorbs moisture and thus lowers the insulation resistance of the cable. For this reason, paper insulated cables are always provided with some protective covering and are never left unsealed. If it is required to be left unused on the site during laying, its ends are temporarily covered with wax or tar. Since the paper insulated cables have the tendency to absorb moisture, they are used where the cable route has a few joints. For instance, they can be profitably used for distribution at low voltages in congested areas where the joints are generally provided only at the terminal apparatus. However, for smaller installations, where the lengths are small and joints are required at a number of places, VIR cables will be cheaper and durable than paper insulated cables.

 

Varnished cambric

It is a cotton cloth impregnated and coated with varnish. This type of insulation is also known as empire tape. The cambric is lapped on to the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. As the varnished cambric is hygroscopic, therefore, such cables are always provided with metallic sheath. Its dielectric strength is about 4 kV/mm and permittivity is 2.5 to 3.8.

 

Polyvinyl chloride (PVC)

This insulating material is a synthetic compound. It is obtained from the polymerization of acetylene and is in the form of white powder. For obtaining this material as a cable insulation, it is compounded with certain materials known as plasticizers which are liquids with high boiling point. The plasticizer forms a gell and renders the material plastic over the desired range of temperature. Polyvinyl chloride has high insulation resistance, good dielectric strength and mechanical toughness over a wide range of temperatures. It is inert to oxygen and almost inert to many alkalies and acids. Therefore, this type of insulation is preferred over VIR in extreme environmental conditions such as in cement factory or chemical factory. As the mechanical properties (i.e., elasticity etc.) of PVC are not so good as those of rubber, therefore, PVC insulated cables are generally used for low and medium domestic lights and power installations.