What is underground cable system? types, advantages & disadvantages

Underground Cables 

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What is underground cables?

An underground cable essentially consists of one or more conductors covered with suitable insula tion 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 aluminium of high con ductivity . 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 

shows the general construction of a 3 - conductor cable . The various parts are : 

( i ) 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 . 11.1 is used for 3 - phase service . The conductors are made of tinned copper or alu minium and are usually stranded in order to provide flexibility to the cable .

 ( ii ) Insulatian . 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 .

 ( iii ) Metallic sheath . In order to pro tect the cable from moisture , gases or other damaging liquids ( acids or alkalies ) in the soil and atmosphere , a metallic sheath of lead or aluminium is provided over the insulation

 

 ( iv ) 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 .

 ( v ) Armouring . Over the bedding , armouring is provided which consists of one or two layers of galvanised 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 . 

( vi ) 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

 The satisfactory operation of a cable depends to a great extent upon the charac teristics 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 - hygroscopic i.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 .

 

 Classification of Cables

 Cables for underground service may be classified in two ways according to ( i ) the type of insulating material used in their manufacture ( ii ) the voltage for which they are manufactured . However , the latter method of classification is generally preferred , according to which cables can be divided into the following groups : 

( i ) Low - tension ( L.T. ) cables - upto 1000 V 

( ii ) High - tension ( H.T. ) cables -

 ( iii ) Super - tension ( S.T. ) cables

 ( iv ) Extra high - tension ( E.H.T. ) cables

 ( v ) Extra super voltage cables -beyond 132 kV

 A cable may have one or more than one core depending upon the type of service for which it is intended . It may be ( 1 ) single - core ( ii ) two - core ( iii ) three - core ( iv ) four - core etc. For a3 - phase service , either 3 - single - core cables or three - core cable can be used depending upon the operating voltage and load demand .

 

Laying of Underground Cables

The reliability of underground cable network depends to a considerable extent upon the proper lay and attachment of fittings i.e. , cable end boxes , joints , branch con nectors etc. There are three main methods of laying underground cables viz . , direct laying , draw - in system and the solid system . 

1. Direct laying . 

This method of laying underground cables is simple and cheap and is much favoured in modern practice . In this method , a trench of about 1-5 metres deep and 45 cm wide is dug . The trench is covered with a layer of fine sand ( of about 10 cm thickness ) and the cable is laid over this sand bed . The sand prevents the entry of moisture from the ground and thus protects the cable from decay . After the cable has been laid in the trench , is covered with another layer of sand of about 10 cm thickness . The trench is then covered with bricks and other materials in order to protect the cable from mechani cal injury . When more than one cable is to be laid in the same trench , a horizontal or vertical inter axial spacing of atleast 30 cm is provided in order to reduce the effect of mutual heating and also to ensure that a fault occurring on one cable does not damage the adjacent cable . Cables to be laid in this way must have serving of bituminised paper and hessian tape so as to provide protection against corrosion and electorlysis .

 Advantages

 ( i ) It is a simple and less costly method . 

( ii ) It gives the best conditions for dissipating the heat generated in the cables .

 ( iii ) It is a clean and safe method as the cable is invisible and free from external disturbances . 

Disadvantages 

( i ) The extension of load is possible only by a completely new excavation which may cost as much as the original work . 

( ii ) The alterations in the cable netwok cannot be made easily . 

( iii ) The maintenance cost is very high . 

( iv ) Localisation of fault is difficult .

 ( v ) It cannot be used in congested areas where excavation is expensive and inconvenient .

 

2. Draw - in system .

In this method , conduit or duct of glazed stone or cast iron or concrete are laid in the ground with manholes at suitable positions along the cable route . The cables are then pulled into position from manholes . Fig . 11.11 shows section through four - way underground duct line . Three of the ducts carry transmis sion cables and the fourth duct carries relay protection con nection , pilot wires . Care must be taken that where the duct line changes direction ; depths , dips and offsets be made with a very long radius or it will be difficult to pull a large cable between the manholes . The distance between the manholes should not be too long so as to simplify the pull ing in of the cables . The cables to be laid in this way need not be armored but must be provided with serving of hessian and jute in order to protect them when being pulled into the ducts . Advantages

Advantages 

(i)Repairs , alterations or additions to the cable network can be made without opening the ground . ( ii ) As the cables are not armoured , therefore , joints become simpler and maintenance cost is reduced considerably . ( iii ) There are very less chances of fault occurrence due to strong mechanical protection pro vided by the system . 

Disadvantages

 ( i ) The initial cost is very high . 

( ii ) The current carrying capacity of the cables is reduced due to the close grouping of cables and unfavourable conditions for dissipation of heat . This method of cable laying is suitable for congested areas where excavation is expensive and inconvenient , for once the conduits have been laid , repairs or alterations can be made without opening the ground . This method is generally used for short length cable routes such as in workshops road crossings where frequent digging is costlier or impossible . 

3. Solid system .

 In this method of laying , the cable is laid in open pipes or troughs dug out in earth along the cable route . The troughing is of cast iron , stoneware , asphalt or treated wood . After the cable is laid in position , the troughing is filled with a bituminous or asphaltic compound and covered over . Cables laid in this manner are usually plain lead covered because troughing affords good mechanical protection . Disadvantages ( i ) It is more expensive than direct laid system . ( ii ) It requires skilled labour and favourable weather conditions . iii ) Due to poor heat dissipation facilities , the current carrying capacity of the cable is reduced . In view of these disadvantages , this method of laying underground cables is rarely used now - a days .

 

 

what is fuses || Characteristics of Fuse Element|| Current rating||Types of Fuses

What is Fuses?

 Fuses A fuse is a short piece of metal , inserted in the circuit , which melts when excessive current flows through it and thus breaks the circuit . The fuse element is generally made of mate rials having low melting point , high conductivity and least deterioration due to oxidation e.g. , silver , copper etc.

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Advantages

 ( i ) It is the cheapest form of protection available . 

( ii ) It requires no maintenance .

 ( iii ) Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate equipment for automatic action . 

( iv ) It can break heavy short - circuit currents without noise or smoke . 

( v ) The smaller sizes of fuse element impose a current limiting effect under short - circuit condi tions . 

( vi ) The inverse time - current characteristic of a fuse makes it suitable for overcurrent protection . 

( vii ) The minimum time of operation can be made much shorter than with the circuit breakers 

 Disadvantages

 ( i ) Considerable time is lost in rewiring or replacing a fuse after operation . 

( ii ) On heavy short - circuits , * discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned . 

( iii ) The current - time characteristic of a fuse cannot always be co - related with that of the pro tected apparatus . 

Characteristics of Fuse Element 

The function of a fuse is to carry the normal current without overheating but when the current excee its normal value , it rapidly heats up to melting point and disconnects the circuit protected by it . order that it may perform this function satisfactorily , the fuse element should have the followin desirable characteristics : 48 ( i ) low melting point e.g. , tin , lead . ( ii ) high conductivity e.g. , silver , copper . ( iii ) free from deterioration due to oxidation e.g. , silver . ( iv ) low cost e.g. , lead , tin , copper . The above discussion reveals that no material possesses all the characteristics . For instance , lead has low melting point but it has high specific resistance and is liable to oxidation . Similarly , copper has high conductivity and low cost but oxidises rapidly . Therefore , a compromise is made in the selection of material for a fuse .

Types of Fuses 

Fuse is the simplest current interrupting device for protection against excessive currents . Since the invention of first fuse by Edison , several improvements have been made and now - a - days , a variety of fuses are available . Some fuses also incorporate means for extinguishing the arc that appears when the fuse element melts . In general , fuses may be classified into : ( i ) Low voltages fuses ( ii ) High voltage fuses It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety . If such means of isolation are not available , the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed .

 Important Terms The following terms are much used in the analysis of fuses :

 Current rating of fuse element: It is the current which the fuse element can normally carry without overheating or melting . It depends upon the temperature rise of the contacts of the fuse holder , fuse material and the surroundings of the fuse . ) Fusing current . It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it . Obviously , its value will be more than the current rating of the fuse element . For a round wire , the approximate relationship between fusing current I and diameter d of the wire is

Prospective Current : shows how a.c. current is cut off by a fuse . The fault current would normally have a very large first loop , but it actually generates sufficient en energy to melt the fuse able element well before the peak of this first loop is reached . The r.m.s. value of the first loop of fault current is known as prospective current . Therefore , prospective current can be defined as under : It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance .

 Cut - off current . It is the maximum value of fault current actually reached before the fuse melts . On the occurrence of a fault , the fault current has a very large first loop due to a fair degree of asymmetry . The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached The current corresponding to point ' a ' is the cut off current . The cut off value depends upon : ( a ) current rating of fuse ( b ) value of prospective current ( c ) asymmetry of short - circuit current It may be mentioned here that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak . This gives the fuse a great advantage over a circuit breaker since the most severe thermal and elector - magnetic effects of short - circuit currents ( which occur at the peak value of prospective current ) are not experienced with fuses . Therefore , the circuits pro tected by fuses can be designed to withstand maximum current equal to the cut - off value . This consideration together with the relative cheapness of fuses allows much saving in cost .

 Pre - arcing time :. It is the time between the commencement of fault and the instant when cut off occurs . When a fault occurs , the fault current rises rapidly and generates heat in the fuse element . As the fault current reaches the cut off value , the fuse element melts and an arc in initiated . The time from the start of the fault to the instant the arc is initiated is known as pre - arcing time . The pre - arcing time is generally small : a typical value being 0.001second ( vii ) Arcing time . This is the time between the end of pre - arcing time and the instant when the arc is extinguished . ( viii ) Total operating time . It is the sum of pre - arcing and arcing times . It may be noted that operating time of a fuse is generally quite low ( say 0-002 sec . ) as compared to a circuit breaker ( say 0-2 sec or so ) . This is an added advantage of a fuse over a circuit breaker . A fuse in series with a circuit breaker of low - breaking capacity is a useful and economical arrangement to provide adequate short - circuit protection . It is because the fuse will blow under fault conditions before the circuit breaker has the time to operate . ( ix ) Breaking capacity . It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage .

What is Electric Potential , Resistance, Conductance?

 What is Electric Potential , Restence, Conductance?

1.3. The Idea of Electric Potential

In Fig. 1.1, a simple voltaic cell is shown. It consists of copper plate (known as anode) and a zinc rod (i.e. cathode) immersed in dilute sulphuric acid (H2SO4) contained in a suitable vessel. The chemical action taking place within the cell causes the electrons to be removed from copper plate and to be deposited on the zinc rod at the same time. This transfer of electrons is accomplished through the agency of the diluted H2SO4 which is known as the electrolyte. The result is that zinc rod becomes negative due to the deposition of electrons on it and the copper plate becomes positive due to the removal of electrons from it. The large number of electrons collected on the zinc rod is being attracted by anode but is prevented from returning to it by the force set up by the chemical action within the cell.

But if the two electrodes are joined by a wire externally, then electrons rush to the anode therebyequalizing the charges of the two electrodes. However, due to the continuity of chemical action, a continuous difference in the number of electrons on the two electrodes is maintained which keeps up a continuous flow of current through the external circuit. The action of an electric cell is similar to that of a water pump which, while working, maintains a continuous flow of water i.e., water current through the pipe (Fig. 1.2).It should be particularly noted that the direction of electronic current is from zinc to copper in the external circuit. However, the direction of conventional current (which is given by the direction of flow of positive charge) is from copper to zinc. In the present case, there is no flow of positive charge as such from one electrode to another. But we can look upon the arrival of electrons on copper plate (with subsequent decrease in its positive charge) as equivalent to an actual departure of positive charge from it. When zinc is negatively charged, it is said to be at negative potential with respect to the electrolyte, whereas anode is said to be at positive potential relative to the electrolyte. Between themselves, copper plate is assumed to be at a higher potential than the zinc rod. The difference in potential is continuously maintained by the chemical action going on in the cell which supplies energy to establish this potential difference.

Resistance

It may be defined as the property of a substance due to which it opposes (or restricts) the flow of electricity (i.e., electrons) through it. Metals (as a class), acids and salts solutions are good conductors of electricity. Amongst pure metals, silver, copper and aluminium are very good conductors in the given order.* This, as discussed earlier, is due to the presence of a large number of free or loosely-attached electrons in their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat. Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.c. (polyvinyl chloride) and dry wood etc. Amongst good insulatorscan be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p.v.c. It is helpful to remember that electric friction is similar to friction in Mechanics.

1.5. The Unit of Resistance

The practical unit of resistance is ohm.** A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals. For insulators whose resistances are very high, a much bigger unit is used i.e., mega-ohm = 106 ohm (the prefix ‘mega’ or mego meaning a million) or kilo-ohm = 103 ohm (kilo means thousand). In the case of very small resistances, smaller units like milli-ohm = 10−3 ohm or micro- ohm = 10−6 ohm are used. The symbol for ohm is Ù.

 

Ohm’s Law

This law applies to electric to electric conduction through good conductors and may be stated as follows :

The ratio of potential difference (V) between any two points on a conductor to the current (I) flowing between them, is constant, provided the temperature of the conductor does not change.

In other words, V/I= constant or V/I= R

where R is the resistance of the conductor between the two points considered. Put in another way, it simply means that provided R is kept constant, current is directly proportional to the potential difference across the ends of a conductor. However, this linear relationship between V and I does not apply to all non-metallic conductors. For example, for silicon carbide, the relationship is given by V = KIm where K and m are constants and m is less than unity. It also does not apply to non-linear devices such as Zener diodes and voltage-regulator (VR) tubes.

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what is circuit breaker, and its application full details

 Circuit Breakers 

what is a circuit breaker,

A circuit breaker is a piece of equipment which can 

( i ) make or break a circuit either manually or by remote control under normal conditions 

( ii ) break a circuit automatically under fault conditions 

( iii ) make a circuit either manually or by remote control under fault conditions 

Thus a circuit breaker incorporates manual ( or remote control ) as well as automatic control for switching functions . The latter control employs relays and operates only under fault conditions . The mechanism of opening of the circuit breaker under fault conditions has already been briefed in chap ter 16 . Operating principle . A circuit breaker essentially consists of fixed and moving contacts , called electrodes . Under normal operating conditions , these contacts remain closed and will not open auto matically until and unless the system becomes faulty . Of course , the contacts can be opened manually or by remote control whenever desired . When a fault occurs on any part of the system , the trip coils of the circuit breaker get energised and the moving contacts are pulled apart by some mechanism , thus opening the circuit . When the contacts of a circuit breaker are separated under fault conditions , an arc is struck between them . The current is thus able to continue until the discharge ceases . The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself . Therefore , the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value .

 Classification of Circuit Breakers 

There are several ways of classifying the circuit breakers . However , the most general way of classi fication is on the basis of medium used for arc extinction . The medium used for arc extinction is usually oil , air , sulphur hexafluoride ( SF ) or vacuum .

 Accordingly , circuit breakers may be classi fied into : 

( i ) Oil circuit breakers which employ some insulating oil ( e.g. , transformer oil ) for arc extinction . 

(ii ) Air - blast circuit breakers in which high pressure air - blast is used for extinguishing the arc .

 ( iii ) Sulphur hexafluroide circuit breakers in which sulphur hexafluoride ( SF ) gas is used for arc extinction .

 ( iv ) Vacuum circuit breakers in which vacuum is used for arc extinction . Each type of circuit breaker has its own advantages and disadvantages . In the following sections , we shall discuss the construction and working of these circuit breakers with special emphasis on the way the arc extinction is facilitated .

Oil Circuit Breakers

 In such circuit breakers , some insulating oil ( e.g. , trans former oil ) is used as an arc quenching medium . The con tacts are opened under oil and an arc is struck between them . The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous * hy drogen at high pressure . The hydrogen gas occupies a volume about one thousand times that of the oil decom posed . The oil is , therefore , pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc re gion and adjacent portions of the contacts . The arc extinction is facilitated mainly by two processes . Firstly , the hydrogen gas has high heat conductivity and cools the arc , thus aiding the de - ionisation of the medium between the contacts . Secondly , the gas sets up turbulence in the oil and forces it into the space between contacts , thus eliminating the arcing products from the arc path . The result is that arc is extinguished and circuit current finterrupted .

Oil Circuit Breakers

 Advantages . 

The advantages of oil as an arc quenching medium are : 

( i ) It absorbs the arc energy to decompose the oil into gases which have excellent cooling properties .

( ii ) It acts as an insulator and permits smaller clearance between live conductors and earthed components .

 ( iii ) The surrounding oil presents cooling surface in close proximity to the arc . 

Disadvantages . 

The disadvantages of oil as an arc quenching medium are : 

( i ) It is inflammable and there is a risk of a fire . 

( ii ) It may form an explosive mixture with air 

( iii ) The arcing products ( e.g. , carbon ) remain in the oil and its quality deteriorates with successive operations . This necessitates periodic checking and replacement of oil .

Air - Blast Circuit Breakers 

These breakers employ a high pressure * air - blast as an arc quenching medium . The contacts are pened in a flow of air - blast established by the opening of blast valve . The air - blast cools the arc and sweeps away the arcing products to the atomsphere . This rapidly increases the dielectric strength of be medium between contacts and prevents from re - establishing the arc . Consequently , the arc is extinguished and flow of current is interrupted . 

Air - Blast Circuit Breakers 

Advantages . 

An air - blast circuit breaker has the following advantages over an oil circuit breaker : 

( i ) The risk of fire is eliminated . 

( ii ) The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations ; the expense of regular oil replacement is avoided . 

( iii ) The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small . This reduces the size of the device . 

( iv ) The arcing time is very small due to the rapid build up of dielectric strength between con tacts . Therefore , the arc energy is only a fraction of that in oil circuit breakers , thus resulting in less burning of contacts .

 ( v ) Due to lesser arc energy , air - blast circuit breakers are very suitable for conditions where frequent operation is required .

 ( vi ) The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted . Disadvantages . The use of air as the arc quenching medium offers the following 

Disadvantges : 

( i ) The air has relatively inferior arc extinguishing properties . 

( ii ) The air - blast circuit breakers are very sensitive to the variations in the rate of rise of restrik ing voltage . 

( iii ) Considerable maintenance is required for the compressor plant which supplies the air - blast . The air blast circuit breakers are finding wide applications in high voltage installations , Major ty of the circuit breakers for voltages beyond 110 kV are of this type . 

 Sulphur Hexaflouride ( SF ) Circuit Breakers 

In such circuit breakers , sulphur hexaflouride ( SF ) gas is used as the arc quenching medium . The SF , is an electro - negative gas and has a strong tendency to absorb free electrons . The contacts of the breaker are opened in a high pressure flow of SF , gas and an arc is struck between them . The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions . This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc . The SF circuit breakers have been found to be very effective for high power and high voltage service . Construction . shows the parts of a typical SF circuit breaker . It consists of fixed and moving contacts enclosed in a chamber ( called arc interruption chamber ) containing SF gas . This chamber is connected to SF gas reservior . When the contacts of breaker are opened , the valve mechanism permits a high pressure SF gas from the reservoir to flow towards the arc interruption chamber . The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn . The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF gas to let out through these holes after flowing along and across the arc . The tips of fixed contact , moving contact and arcing horn are coated with copper - tungsten arc resistant material . Since SF gas is costly , it is reconditioned and reclaimed by suitable auxiliary sytem after each operation of the breaker . 6 Working . In the closed position of the breaker , the contacts remain surrounded by SF gas at a pressure of about 2-8 kg / cm² . When the breaker operates , the moving contact is pulled apart and an arc is struck between the contacts . The movement of the moving contact is synchronised with the opening of a valve which permits SF , gas at 14 kg / cm² pressure from the reservoir to the arc interrup tion chamber . The high pressure flow of SF , rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers . The result is that the medium be tween the contacts quickly builds up high dielectric strength and causes the extinction of the arc . After the breaker operation ( i.e. , after arc extinction ) , the valve is closed by the action of a set of springs .

sulphur hexaflouride CB

Applications . 

A typical SF circuit breaker consists of interrupter units each capable of dealing with currents upto 60 kA and voltages in the range of 50-80 kV . A number of units are connected in series according to the system voltage . SF , circuit breakers have been developed for voltages 115 kV to 230 kV , power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles . the range highest insulating example , when with dielectric obtained

Vacuum Circuit Breakers ( VCB ) 

In such breakers ,vacuum ( degree of vacuum being in the range from 107 to 105 torr ) is used as the arc quenching medium . Since vacuum offers the highest insulating strength , it has far superior arc quenching properties than any other medium . For example , when contacts of a breaker are opened in vacuum , the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers . Principle . The production of arc in a vacuum circuit breaker and its extinction can be explained as follows : When the contacts of the breaker are opened in vacuum ( 107 to 105 torr ) , an arc is produced between the contacts by the ionisation of metal vapours of contacts * . However , the arc is quickly extinguished because the metallic vapours , electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts , resulting in quick recovery of dielectric strength . The reader may note the salient feature of vacuum as an arc quenching medium . As soon as the arc is produced in vacuum , it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum . Construction shows the parts of a typical vacuum circuit breaker . It consists of fixed contact , moving contact and arc shield mounted inside a vacuum chamber . The movable mem ber is connected to the control mechanism by stainless steel bellows . This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak . A glass vessel or ceramic vessel is used as the outer insulating body . The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover .

Vacuum Circuit Breakers ( VCB ) 

Working . 

When the breaker operates , the moving contact separates from the fixed contact and an arc is struck between the contacts . The production of arc is due to the ionisation of metal ions and depends very much upon the material of contacts . The arc is quickly extinguished because the metal lic vapours , electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields . Since vacuum has very fast rate of recovery of dielectric strength , the arc extinction in a vacuum breaker occurs with a short contact separation ( say 0-625 cm ) .

Applications . For a country like India , where distances are quite large and accessibility to remote areas difficult , the installation of such outdoor , maintenance free circuit breakers should prove a definite advantage . Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV . Even with limited rating of say 60 to 100 MVA , they are suitable for a majority of applications in rural areas .

Applications . 

For a country like India , where distances are quite large and accessibility to remote areas difficult , the installation of such outdoor , maintenance free circuit breakers should prove a definite advantage . Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV . Even with limited rating of say 60 to 100 MVA , they are suitable for a majority of applications in rural areas

types of switchgear,function of switchgear and different type of switch

 Chapter 1:Switchgear 

 

Overview 1

 General A great demand for electrical energy is a notable feature of modern civilisation . Most of this energy is needed for lighting , heating , domestic appliances , industrial electrical machinery and electric traction . The importance of electric supply in everyday life has reached such a stage that it is desirable to protect the power system from harm during fault conditions and to ensure maximum continuity of supply . For this purpose , means must be provided to switch on or off generators , transmission lines , distributors and other equipment under both normal and abnormal conditions . This is achieved by an apparatus called switchgear . A switchgear essentially consists of switching and protecting devices such as switches , fuses , circuit breakers , relays etc. During normal operation , switchgear permits to switch on or off generators , transmission lines , distributors and other electrical equipment . On the other hand , when a failure ( e.g. short - circuit ) occurs on any part of power system , a heavy current flows through the equipment , threatening damage to the equipment and interruption of service to the customers . However , the switchgear detects the fault and disconnects the unhealthy section from the system . In this way , switchgear protects the system from the damage and ensures continuity of supply . In this chapter , we shall present the elementary introduction to switchgear . 

 Switchgear The apparatus used for switching , controlling and protecting the electrical circuits and equipment is known as switchgear . The switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions . The tumbler switch with ordinary fuse is the simplest form of switchgear and is used to control and protect lights and other equipment in homes , offices etc. For circuits of higher rating , a high - rupturing capacity ( H.R.C. ) fuse in conjunction with a switch may serve the purpose of controlling and protecting the circuit . However , such a switchgear cannot be used profitably on high voltage system ( 3.3 kV ) for two reasons . Firstly , when a fuse blows , it takes some time to replace it and consequently there is interruption of service to the customers . Secondly , the fuse cannot successfully interrupt large fault currents that result from the faults on high voltage system .

 Switchgear Equipment Switchgear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions . It includes switches , fuses , circuit breakers , relays and other equipment . A brief account of these devices is given below . I. Switches . A switch is a device which is used to open or close an electrical circuit in a convenient way . It can be used under full - load or no - load conditions but it cannot interrupt the fault currents . When the contacts of a switch are opened , an arc is produced in the air between the contacts . This is particularly true for circuits of high voltage and large current capacity . The switches may be classified into ( i ) air switches ( ii ) oil switches . The contacts of the former are opened in air and that of the latter are opened in oil . 

( i ) Air - break switch . 

It is an air switch and is designed to open a circuit under load . In order to quench the arc that occurs on opening such a switch , special arcing horns are provided . Arcing horns are pieces of metals between which arc is formed during opening operation . As the switch opens , these horns are spread farther and farther apart . Conse quently , the arc is lengthened , cooled and interrupted . Air - break switches are generally used outdoor for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder .

( ii ) Isolator or disconnecting switch . 

It is essentially a knife switch and is designed to open a circuit under no - load . Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line . Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger . They should never be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed .

 ( iii ) Oil switches .

 As the name implies , the contacts of such switches are opened under oil , usually transformer oil . The effect of oil is to cool and quench the arc that tends to form when the circuit is opened . These switches are used for circuits of high voltage and large current carrying capacities .