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Download Definition of Circuit Breaker
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Circuit Breaker ; A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Origin : An early form of circuit breaker was described by Edison in an 1879 patent application, although his commercial power distribution system used fuses. Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads. Operating Principle The primary function of the circuit breakers mechanism is to provide the means for opening and closing the contacts. Initially, this seems to be a rather simple and straightforward requirement. However, when one considers the fact that most circuit breakers, once placed into service, will remain in the closed position for long periods of time, and yet on the few occasions when they are called upon to open or close, they must do so reliably, without any delay or sluggishness, then one realizes that the demands on the mechanisms are not as simple as was first thought. A circuit breaker essentially consists of fixed and moving contacts. These contacts are called electrodes. The need for carrying the continuous current and for withstanding a period of arcing makes it necessary to use two sets of contacts in parallel, one is the primary contact and the second is the arcing contact. The primary contact is always made of a high conductive material such as copper and the arcing contact is made of arc resistance material such as tungsten or molybdenum, which has a much lower conductivity than those used for primary contacts. When the circuit breaker opens to interrupt the current, the primary contacts open before the arcing contacts. Under the normal operating conditions, these contacts remain closed and are not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control when ever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. When the contacts of a circuit break 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 the 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 the circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. In single phase (1-phase) circuits (i.e., lighting circuits etc.), a switch is located in only one of the two conductors to the load. However in the power circuits, a circuit interrupting device (i.e., circuit breaker) is put in each phase or conductor. These are, sometimes, called 3-pole circuit breakers. Arc Phenomenon Arc in an ac circuit breaker occurs in two ways: 1. When the current-carrying contacts are being separated, arcing is possible even when the circuit e.m.f. is considerably below the minimum cold electrode breakdown voltage, because of the ions neutralizing the electronic space charge and thus allowing large currents to flow at relatively low voltage gradients. This way of occurrence of an arc is common to both dc and ac circuit breakers. 2. The other way of occurrence of an arc happens only in ac circuit breakers. In such case, the arc is extinguished every time the current passes through zero and can restrike only if the transient recovery voltage across the electrodes already separated and continuing to separate, reaches a sufficiently high value causing breakdown. The function of an ac circuit breaker is to prevent restriking of the arc, which depends upon the following important factors: The nature and pressure of the medium of arc. The external ionizing and de-ionizing agents present. The voltage across the electrodes and its variation with time. The material and configuration of the electrodes. The nature and configuration of the arcing chamber. Classification of Circuit Breakers: Based on voltage : Low voltage Medium voltage High / Extra voltage Ultra voltage Based on location : Indoor Outdoor Based on external design : Dead tank Live tank Based on interrupting media : Air Break Air Blast Oil SF6 Vacuum However, the most general way of classification is on the basis of medium used for the arc extinction , so we will talk about this type . Oil circuit breaker : In such circuit breakers, some insulating oil (i.e., transformer oil) is used as an arc quenching medium. The contacts 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 hydrogen gas at a high pressure. This large volume of the hydrogen gas pushes the oil away from the arc. In an oil circuit breaker, the arc quenching process is entirely dependent on arc energy generated. The arc drawn across the contacts is contained inside the interrupting pot, and thus the hydrogen gas formed by the vaporized oil (gas) is also contained inside the chamber. As the contacts continue to move, and the moving contact rod separates itself from the orifice of the chamber, an exit similar to a nozzle allows escape of the hydrogen gas trapped inside the interrupting chamber. The escaping high pressure hydrogen gas, having a high thermal conductivity, takes away the heat, thus making the contact gap cool and free from ionization, immediately after current zero. Advantages : 1. Oil absorbs the arc energy to produces hydrogen gas during arcing. The hydrogen has excellent cooling properties and helps extinguish the arc. (In addition to hydrogen gas, a small proportion of methane, ethylene, and acetylene are also generated in oil decomposition.) 2. The oil provides insulation for the live exposed contacts from the earthed portions of the container. 3. Oil provides insulation between the contacts after the arc has been extinguished. 4. The oil close to the arc region provides cooling surface. Disadvantages : 1. Oil is inflammable and may cause fire hazards. When a defective circuit breaker fails under pressure, it may cause an explosion. 2. The hydrogen generated during arcing, when combined with air, may form an explosive mixture. 3. During arcing, oil decomposes and becomes polluted by carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement. Sulphur Hexafluoride (SF6) Circuit Breakers : In these circuit breakers, Sulpher hexafluoride gas (SF6) is used as the arc quenching medium. The SF6is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6gas 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 SF6 circuit breakers have been found to be very effective for high power and high voltage services. The excellent insulating properties of SF6 gas make it possible to design circuit breakers with smaller overall dimensions, shorter contact gaps, which help in the construction of outdoor breakers with fewer interrupters. Advantages: 1. Because of the high conductivity of the arc in the SF6 gas, the arc energy is low. (Arc voltage is between 150 and 200V.) 2. Due to the low energy the contact erosion is small. 3. The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. 4. Due to the superior arc quenching property of the SF6 gas, such circuit breakers have very short arcing time. Furthermore, they can interrupt much larger current. 5. These breakers give noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air-blast circuit breaker. 6. The SF6 gas is not inflammable, so there is no risk of fire in SF6 breakers. 7. Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazards exist, i.e. in coal mines. Disadvantages: 1. These circuit breakers are expensive due to the high cost of SF6 gas. 2. Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose. 3. The SF6gas has been identified as a greenhouse gas, and safety regulations are being introduced in many countries in order to prevent its release into the atmosphere. Vacuum circuit breaker : In such circuit breakers, the vacuum is used as the arc quenching medium. The vacuum circuit breaker takes the advantage of nonsustainability of electric arc in vacuum, and employs the principle of contact separation under vacuum where there is no ionization due to medium. The initial arc caused by field and thermionic emissions during the contacts separation, will die away soon, as there is no further ionization because of vacuum. The degree of vacuum in these circuit breakers is in the range from 10-7 to 10-5 torr . Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. When the contacts in the vacuum circuit breakers are opened in vacuum, an arc is produced between the contacts by the ionization of metal vapors of contacts. However, the arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts. Advantages: 1.The vacuum circuit breakers are compact in size and have longer lives. 2. Operating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances. 3. Because of the very low voltage across the metal vapor arc, energy is very low. (Arc voltage is between 50 and 100V.) 4.Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapor recondenses on the contact, contact erosion is extremely small. 5.There is no generation of gases during and after the circuit breaker operation. 6. The outstanding feature of these breakers is that it can break any heavy fault current perfectly just before the contacts reach a definite open position . 7.They can successfully withstand lightning surges Air Circuit Breakers : Air circuit breaker is defined as a circuit breaker, in which the contacts open and close in air at atmospheric pressure. In general, the use of this type of circuit breakers is restricted to low voltage applications or high security installations where the risk of an oil fire or oil contamination of the environment is too high to be tolerated. Countries following the American practice used air circuit breakers almost exclusively for systems up to 15 kV until the advent of the new vacuum and SF6technologies. The principles of arc interruption used in an air circuit breaker are rather different from those in any other type of circuit breaker. However, the objective is the same for both categories of the circuit breakers, i.e. to prevent the resumption of arcing after current zero by creating a situation where in the contact gap will withstand the system recovery voltage; the air circuit breaker does this by creating an arc voltage in excess of the supply voltage. This can be achieved in three ways: 1. 2. 3. Intense cooling of the arc plasma, so that the voltage is very high . Lengthening the arc path to increase the arc voltage Splitting up the arc into a number of series arcs It is inflammable and there is a ris k of f