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Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
Er. rahul sharma circuit breaker
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Er. rahul sharma circuit breaker

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Switchgear & Protection - Circuit Breaker (Ratings, Classifications)

Switchgear & Protection - Circuit Breaker (Ratings, Classifications)

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  • 1. PRESENTATION ON CIRCUIT BREAKER Er. By: Er. Rahul Sharma EE DEPARMENT SEMINAR April 23, 2012
  • 2. :Circuit Breaker Classification :1. AC Circuit Breaker 2. DC Circuit Breaker On basis of type of Current: On basis of type of Rated Voltage: 1. Low Voltage Circuit Breaker (<1KV) 2. High Voltage Circuit Breaker (>1KV) On basis of type of Medium Of Arc Extinction : Air Break CB MCB Oil CB Minimum Oil CB Vacuum CB SF6 CB Air Blast CB Plain Break Plain Break Axial Blast Magnetic Blow Out Impulse Radial Blast Arc Chute Self Blast Plain Explosion pot Cross Blast X-jet explosion Pot Self Compensated explosion pot
  • 3. Plain Break Oil Circuit Breaker •It consists of fixed and moving contacts enclosed in a strong weather-tight earthed tank containing oil up to a certain level and an air cushion above the oil level. •There is no special arc control system other than increasing length caused by separation of contacts. •The air cushion collects arc gases without generation of unsafe pressure in the dome of circuit breaker. • It also absorbs mechanical shock of the upward oil movement. •When fault occurs, the moving contacts are pulled down by protective system and an arc is struck which vaporizes the oil mainly into hydrogen gas. Oil CB Types
  • 4. Plain Break Oil Circuit Breaker Contd. The following processes facilitate the arc extinction: The hydrogen bubble generated around the arc cools the arc and aids deionistion of the medium between the contacts. The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc path. As the arc lengthens due to the separating contacts, the dielectric strength of the medium is increased. Disadvantages: There is no special control over the arc other than increase in length by, separating the moving contacts. Thus capacity installations for low voltages not exceeding 11 kV If some arc control is incorporated and the breakers are then called arc control circuit breakers. There are two types of such breakers: 1. Self-blast oil circuit breakers – in which arc control is provided by internal means i.e. arc itself facilitates its own extinction efficiently. 2. Forced blast oil circuit breakers – in which arc control is provided by mechanical means external to the circuit breaker. Oil CB Types
  • 5. SelfSelf-Blast Oil Circuit Breakers In this type of breakers, the gases produced during arching are confined to a small volume by the use of an insulating rigid pressure chamber or explosion pot surrounding the contacts. The space available for the arc gases is restricted by the chamber so a very high pressure is developed to force the oil and gas through or around the arc to extinguish it. The magnitude of the pressure depends upon the value of fault current to be interrupted. The arc itself generates the pressure so such breakers are also called self-generated pressure oil circuit breakers. The pressure chamber is relatively cheap and gives reduced final arc extinction gap length and arcing time as against the plain oil breaker. Oil CB Types
  • 6. Plain Explosion Pot Self-Blast Oil Circuit Breakers Contd. •It is a rigid cylinder of insulating material and encloses the fixed and moving contacts. •The moving contact is a cylindrical rod passing through a restricted opening called throat at the bottom. •When fault occurs the contacts get separated and an arc is struck between them. •The heat of the arc decomposes oil into a gas at very high pressure in the pot. •This high pressure forces the oil and gas through and around the arc to extinguish it. •Thus emergence of moving contact will be followed by violent rush of gas and oil through the throat producing rapid extinction. Limitation of this type of pot is that it cannot be used for very low or very high fault currents. •With low fault currents, the pressure developed is small, thereby increasing the arcing time. •And with high fault currents, the gas is produced so rapidly that the plot may burst due to high pressure. So this pot is used on moderate short circuit currents only where rate of gas evolution is moderate. Oil CB Types
  • 7. Cross Jet Explosion Pot Self-Blast Oil Circuit Breakers Contd. •The cross jet pot which is made of insulating material and has channels on one side that acts as arc splitters. •The arc splitters help in increasing the arc length, thus facilitating arc extinction. •When fault occurs, the moving contacts of the circuit breaker begins to separate and arc is struck in the top of the pot. •The gas generated by the arc exerts pressure on the oil in the back passage. •When the moving contact uncovers the arc splitter ducts, fresh oil is forced across the arc path. •The arc is therefore driven sideways into the arc splitters, which increase the arc length, causing arc extinction. •The cross jet explosion pot is used for interrupting heavy fault currents. •For low fault currents the gas pressure is small and consequently the pot does not give a satisfactory operation. Oil CB Types
  • 8. Self-Blast Oil Circuit Breakers Contd. Self Compensated Explosion Pot • This pot is a combination of plain explosion pot and cross jet explosion pot. •So it can interrupt low as well as heavy short circuit currents. Forced Blast Oil Circuit Breaker •In this type of circuit breaker there is a piston attached to a moving contact. •When fault occurs the moving contact moves and hence the piston associated with it also moves producing pressure inside the oil chamber. •So the oil gets movement or turbulates and quenches the arc. Oil CB Types
  • 9. Minimum Oil Circuit Breaker Minimum Oil CB
  • 10. Minimum Oil Circuit Breaker Contd. Construction •There are two chambers. •The oil in each chamber is separated from each other. •The main advantage of this is that low oil is required and oil in second chamber wont get polluted. •Upper chamber is called the circuit breaker chamber and lower one is called the supporting chamber. •Circuit breaking chamber consists of moving contact and fixed contact. •Moving contact is connected with a piston its just for the movement of the contact and no pressure build due to its motion. •There are two vents on fixed contact they are axial vent for small current produced in oil due to heating of arc and radial vents for large currents. •The whole device is covered using Bakelite paper and porcelain for protection. •Vents are placed in a turbulator. Minimum Oil CB
  • 11. Minimum Oil Circuit Breaker Contd. Operation •Under normal operating conditions, the moving contacts remain engaged with the upper fixed contact. •When a fault occurs, the moving contact is pulled down by the tripping springs and an arc is struck. •The arc vaporizes oil and produces gases under high pressure. •This action constrains the oil to pass through a central hole in the moving contact and results in forcing series of oil through the respective passages of the turbulator. •The process of turbulation is orderly one, in which the sections of arc are successively quenched by the effect of separate streams of oil ,moving across each section in turn and bearing away its gases. Minimum Oil CB
  • 12. Minimum Oil Circuit Breaker Contd. Advantages:Advantages:A low oil circuit breaker has following advantages compared to bulk oil circuit breaker •It requires lesser quantity of oil •It requires smaller space •There is reduced risk of fire •Maintenance problems are reduced Disadvantages:Disadvantages:A low oil circuit breaker has following disadvantages compared to bulk oil circuit breaker •Due to smaller quantity of oil, the degree of carbonization is increased •There is a difficulty of removing the gases from the contact space in time •The dielectric strength of oil deteriorates rapidly due to high degree of carbonization. Minimum Oil CB
  • 13. Vacuum Circuit Breaker Vacuum Circuit Breaker
  • 14. Vacuum Circuit Breaker Contd. •Vacuum is being used as the arc quenching medium. •Vacuum offers highest insulating strength. •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 that obtained with other circuit breakers. Vacuum Circuit Breaker
  • 15. Principle: Vacuum Circuit Breaker Contd. •When the contacts of the breaker are opened in vacuum (10 -7 to 10 -5 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. •The arc is quickly extinguished because the metallic vapours, electrons, and ions produced during arc condense quickly on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. •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: • It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. •The movable member 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 Breaker
  • 16. Vacuum Circuit Breaker Contd. Working: •When the breaker operates the moving contacts separates from the fixed contacts and an arc is struck between the contacts. •The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. •The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in 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. Applications: •For outdoor applications ranging from 22 kV to 66 kV. Suitable for majority of applications in rural area. Vacuum Circuit Breaker
  • 17. Vacuum Circuit Breaker Contd. Advantages: Advantages: a. They are compact, reliable and have longer life. b. There are no fire hazards c. There is no generation of gas during and after operation d. They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position. e. They require little maintenance and are quiet in operation f. Can withstand lightning surges g. Low arc energy h. Low inertia and hence require smaller power for control mechanism. Vacuum Circuit Breaker
  • 18. Air Blast Circuit Breaker •Fast operations •Suitability for repeated operation •Unit type multi break constructions •Auto Reclosure •simple assembly •Air blast circuit breakers is used for interconnected lines and important lines where rapid operation is desired. •High pressure air at a pressure between 20 to 30 kg/ cm2 stored in the air reservoir. Air is taken from the compressed air system. •Three hollow insulator columns are mounted on the reservoir with valves at their basis. •The double arc extinguished chambers are mounted on the top of the hollow insulator chambers. •The current carrying parts connect the three arc extinction chambers to each other in series and the pole to the neighboring equipment. •Since there exists a very high voltage between the conductor and the air reservoir, the entire arc extinction chambers assembly is mounted on insulators. Air blast circuit breaker
  • 19. Air blast circuit breaker Contd. Air blast circuit breaker
  • 20. Air blast circuit breaker Contd. •There are three double arc extinction poles in series, there are six breaks per pole. •Each arc extinction chamber consists of one twin fixed contact. •There are two moving contacts. The moving can move axially so as to open or close. •Its position open or close depends on air pressure and spring pressure. •The operating mechanism operates the rod when it gets a pneumatic or electrical signal. The valves open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the double arc extinction chamber. •As the air enters into the arc extinction the pressure on the moving contacts becomes more than spring pressure and contacts open. •The contacts travel through a short distance against the spring pressure. •At the end of the contact travel the port for outgoing air is closed by the moving and the entire arc extinction chamber is filled with high pressure air as the air is not allowed to go out. •However, during the arcing period the air goes out through the openings and take away the ionized air of the arc. Air blast circuit breaker
  • 21. Air blast circuit breaker Contd. •While closing, the valve is turned so as to close connection between the hollow of the insulator the reservoir. •The valve lets the air from the hollow insulator to the atmosphere. •As a result of the pressure of air in the arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts close over the fixed contacts by virtue of the spring pressure. •The opening is fast because the air takes a negligible time to travel from the reservoir to the moving contact. •The arc is extinguished within a cycle. •Therefore, air blast circuit breaker is very fast in breaking the current. •Closing is also fast because the pressure in the arc extinction chamber drops immediately as the valve operates and the contacts close by virtue of the spring pressure. Air blast circuit breaker
  • 22. Air blast circuit breaker Contd. Principle of Arc quenching in Air Blast Circuit Breaker (ABCB) •It needs an auxiliary compressed air system which supplies air to the air receiver of the breaker. •For opening operation, the air is admitted in the arc extinction chamber. It pushes away the moving contacts. •In doing so, the contacts are separated and the air blast takes away the ionized gases along with it and assists in extinction. •After a few cycles the arc is extinguished by the air blast and the arc extinction chamber is filled with high pressure air (30 kg/cm2). •The high pressure air has higher dielectric strength than that of atmospheric pressure. •Hence a small contact gap of few centimeters is enough. Air blast circuit breaker
  • 23. Air blast circuit breaker Contd. Axial Blast type air flow •The air flows from high pressure reservoir to the atmospheric through a convergent divergent nozzle. •The difference is pressure and the design of the nozzle is such that the air expands into the low pressure zone, it attains almost supersonic velocity. Air blast circuit breaker
  • 24. Air blast circuit breaker Contd. Axial Blast Contd. •The mass flow of air through the nozzle is governed by the parameters like pressure ratio, area of throat, nozzle throat diameter and is influenced by the diameter of the arc itself. •The air flowing at a high speed axially along the arc causes the removal of heat from the periphery of the arc and the diameter of the arc reduces to a low value at current zero. •At this instant the arc is interrupted and the contact space is flushed with fresh air flowing through the nozzle. •The flow of fresh air through the contact space ensures removal of hot gases and rapid building up of dielectric strength. Air blast circuit breaker
  • 25. Air blast circuit breaker Contd. Cross Blast flow •The air flows around the arc and the diameter of arc is likely to remain stable for higher values of current. •During the period of arc extinction, the air continues to flow through the nozzle to the atmosphere. •The mass flow rate can be increased by increasing the pressure of high pressure system. •The increase in the mass flow results in the increased breaking capacity. •The energy supplied for arc extinction is obtained from high pressure air and is independent of current to be interrupted. Air blast circuit breaker
  • 26. Air blast circuit breaker Contd. Sequence of Operation In ABCB After the brief duration of air flow, the interrupter is filled with high pressure air. The dielectric strength of air increases with pressure. Hence the fresh high pressure air in the contact space is capable of withstanding the transient recovery voltage. Air blast circuit breaker
  • 27. Air blast circuit breaker Contd. Resistance Switching •The post zero resistance of contact space is high in air blast circuit breakers. •This is because the contact clearance space is filled with high pressure air after final current zero and high pressure air has high dielectric strength. •The high restriking voltage appears across the contacts does not damp out through the gap because of the high post zero resistance. Air blast circuit breaker
  • 28. Air blast circuit breaker Contd. Air blast circuit breaker
  • 29. Air blast circuit breaker Contd. Opening operation •Air is admitted in the arc extinguishing chamber. It separates the main contacts and pushes the auxiliary contacts. •The auxiliary contacts close, thereby the resistors are connected across the arc for a short time of arcing. •The auxiliary contacts are located in the inclined V shaped insulators while the resistors are located in the vertical insulators. •Immediately after the arc extinction the pressure on either side of the piston of auxiliary contacts gets so adjusted that the auxiliary contacts open and resistor circuit is interrupted. •Ceramic resistances of non linear characteristics, similar to those used in the lightning arresters are used for resistance switching. •These consist of silicone carbide, bound by inorganic binders subjected to heat treatment. During high current, non liner resistor offers low resistance. •Thus the main arc current is partly diverted through resistor unit. As current reduces, the resistance offered by non linear resistors increases causing a greater drop across the resistor units. •Thereby the voltage available for arc between auxiliary contacts is no more sufficient and arc between auxiliary contacts is automatically extinguished. Air blast circuit breaker
  • 30. Air blast circuit breaker Contd. Depending upon the direction of air blast in relation to the arc;air blast circuit breakers are classified into: (i)Axial-blast type )Axial- Cross(ii) Cross-blast type Radial(iii) Radial-blast type in which air-blast is directed along the arc path in which air blast is directed at right angles to the arc path in which the air blast is directed radially Air blast circuit breaker
  • 31. Air blast circuit breaker Contd. (i) Axial-blast air circuit breaker Axial•The fixed and moving contacts are held in closed position by spring pressure under normal conditions. •The air reservoir is connected to the arcing chamber through an air valve. •This valve remains closed under normal conditions but opens automatically by tripping impulse when a fault occurs on the system. Air blast circuit breaker
  • 32. Air blast circuit breaker Contd. Axial-blast Contd. •The high pressure air entering the arcing chamber pushes away the moving contact against spring pressure. •The moving contact is separated and an arc is struck. •At the same time,high pressure air blast flows along the arc and takes away the ionised gases along with it. •Consequently,the arc is extinguished and current flow is interrupted. ,the contact separation required for interruption is generally small about 1.75 cm. •Such a small gap may constitute inadequate clearance for the normal service voltage. •Therefore,an isolating switch is incorporated as part of this type of circuit breaker. •This switch opens immediately after fault interruption to provide necessary clearance for insulation. Air blast circuit breaker
  • 33. Air blast circuit breaker Contd. (ii) Cross Blast air breaker •The cross-blast lengthens and forces the arc into a suitable chute for arc extinction. •When the moving contact is withdrawn, an arc is struck between the fixed and moving contacts. •The high pressure cross-blast forces into a chute consisting of an arc splitters and baffles. •The splitters serve to increase the length of the arc and baffles give improved cooling. •The result is that arc is extinguished and flow of current is interrupted. •Since the blast pressure is same for all currents, the inefficiency at low currents is eliminated. Air blast circuit breaker
  • 34. SF6 Circuit Breaker Working: •In the closed position of the breaker the contacts remained surrounded by SF6 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 synchronized with the opening of a valve which permits SF6 gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. •The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge a carriers. •The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. •After the breaker operation the valve is closed by the action of a set of springs. SF6 Circuit Breaker
  • 35. SF6 Circuit Breaker Contd. •When the contacts separate, an arc is established. •If the current is not very high, it is extinguished at its first zero crossing by deionizing effects of SF6, and by the pushing the SF6 through the arc by the piston. •The contact distance at this point is small and the pressure of the gas that goes through the arc is low. •This feature is important because it prevents current chopping when interrupting small inductive currents. •In the case of small capacitive currents, the maximum recovery voltage appears ½ cycle after the arc extinction. •This give the contacts sufficient time to reach a separation that will be able to withstand the voltage. •If the short circuit current is high, the arc extinction may not occur at the first zero crossing, but the gas pressure will increase sufficiently to blow the arc out. SF6 Circuit Breaker
  • 36. SF6 Circuit Breaker Contd. SF6 Circuit Breaker
  • 37. SF6 Circuit Breaker Contd. SF6 Circuit Breaker
  • 38. SF6 Circuit Breaker Contd. SF6 Circuit Breaker
  • 39. SF6 Circuit Breaker Contd. SF6 Circuit Breaker
  • 40. Advantages over oil and air circuit breakers: SF6 Circuit Breaker Contd. a. Due to superior arc quenching property of SF6 , such breakers have very short arcing time b. Dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. c. Gives noiseless operation due to its closed gas circuit d. Closed gas enclosure keeps the interior dry so that there is no moisture problem e. There is no risk of fire as SF6 is non inflammable f. There are no carbon deposits g. Low maintenance cost, light foundation requirements and minimum auxiliary equipment SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazard exists Disadvantages: A. SF6 breakers are costly due to high cost of SF6 B. SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose Applications: SF6 breakers have been used for voltages 115kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles. SF6 Circuit Breaker
  • 41. SF6 Circuit Breaker Contd. •SF6 is a colorless nontoxic gas, with good thermal conductivity and density approximately five times that of air. •SF6 is chemically inert up to temperature of 150 C and will not react with metals, plastics, and other materials commonly used in the construction of high voltage circuit breakers. •The principle of operation is similar to the air blast breakers, except that the SF6 gas is not discharged into the atmosphere. •The equipment consists of a compressor, a storage container, a blast valve that admits gas to the interrupting chamber, and a filter through which the exhaust gas is returned to the compressor. This is called the double pressure breaker design. SF6 Circuit Breaker
  • 42. Air Break Circuit Breaker •These circuit breakers employ high resistance interruption principle. •The arc is rapidly lengthened by means of the arc runners and arc chutes and the resistance of the arc is increased by cooling, lengthening and spilitting the arc. •The arc resistance increases to such an extent that the voltage drop across the arc becomes more than the supply voltage and the arc extinguished. •Air breaker circuit breakers are used in d.c circuits and a.c circuits upto 12 kV. •Magnetic field is utilized for lengthening the arc in high voltage air break circuit breaker. •The arc resistance is increased to such an extent that the system voltage cannot maintain the arc and the arc gets extinguished. Air Break Circuit Breaker
  • 43. Air Break Circuit Breaker Contd. Air Break Circuit Breaker
  • 44. Air Break Circuit Breaker Contd. •Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. •The arching contacts (2) are hard, heat resistance and usually made of copper alloy. •While opening the contact, the main contacts dislodge first. •The current is shifted to the arching contacts. •The arching contacts dislodge later and arc is drawn between them (3). •This arc is forced upwards by the electromagnetic force and thermal action. •The arc ends travel along the Arc Runner (Arcing horns). •The arc moves upwards and is split by arc splitter plates (5). •The arc is extinguished by lengthening, cooling, splitting etc. •In some breakers the arc is drawn in the direction of the splitter by magnetic field. Air Break Circuit Breaker
  • 45. Air Break Circuit Breaker Contd. Operating Mechanisms for Air Break Circuit Breakers The operating mechanisms are generally operating spring. The closing force is obtained from the following means: a. Solenoid b. Spring charged manually or by motor c. Pneumatic •The solenoid mechanisms drive power from battery supply or rectifiers. •The solenoid energized by the direct current gives the necessary force for the closing of the circuit breaker. •The springs used for closing operation can be charged either manually or by motor driven gears. •At the time of closing operation the energy stored in the spring is released by unlatching of the spring and is utilized in closing of the circuit breaker. Air Break Circuit Breaker
  • 46. Air Break Circuit Breaker Contd. Air Break Circuit Breaker
  • 47. Air Break Circuit Breaker Contd. Air Break Circuit Breaker
  • 48. ARC VOLTAGE The voltage that appears across the contacts of circuit breaker during the arcing period is known as arc voltage. It tends to maintain the current flow in the form of arc. RESTRIKING VOLTAGE It’s the transient voltage that appear across the contacts at or near current zero during arcing Period. RECOVERY VOLTAGE It’s the normal frequency voltage (rms) that appear across the contacts of circuit breaker after final arc extinction. It is approximately equal to the system voltage. When contacts are opened current drops to zero at every half cycle. At current zero dielectric strength of the medium can be increased and thus prevent the break down by restriking voltage. Consequently the final arc extinction takes place and circuit current is interrupted. After this current interruption the voltage appearing across the contacts is known as recovery voltage
  • 49. CURRENT CHOPPING •It is the phenomenon of circuit interruption before the natural current zero is reached. •It mainly occurs in air blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of current to be interrupted. • The powerful deionising effect of air blast causes the current to fall abruptly to zero well before the natural current zero is reached. •This phenomenon is known as current chopping and results in high voltage transient across the contacts of the circuit breaker. •The arc current i is chopped down to zero value as shown by point

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