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