It has been found that most of the line faults on overhead transmission system are transient in nature. About 90% of faults are caused by lightning, birds, vines, tree branches etc. these conditions result in such arcing faults that if the fault energy is interrupted for a short period, the arc extinguishes and the line can be re-energised. This fact is employed as a basis for auto reclosure schemes. In such schemes, after the relays at both ends of the line have picked up, the circuit breakers are tripped as far as possible at the same time and reclosed after time has been allowed for deionization. The fault disappears if it is transient, and line is fully restored to service after the reclosure. If the fault is not cleared after the first reclosure, a double or triple attempt of read more

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In this breaker, vacuum is being used as the arc quenching medium. Vacuum offers highest insulating strength, it has far superior arc quenching properties than any other medium. 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. Principle: 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 read more

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In this circuit breaker, sulphur hexaflouride ( SF6 ) gas is used as the arc quenching medium. The sf6 gas 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 sf6 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 sf6 circuit breakers are very effective for high power and high voltage service. Construction: Fig 15 shows the parts of a typical sf6 circuit breaker. It consists of fixed and moving contacts enclosed in a chamber called arc interruption chamber containing sf6 gas. read more

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Depending upon the direction of air blast in relation to the arc;air blast circuit breakers are classified into: (i)Axial-blast type in which air-blast is directed along the arc path as shown in figure below. (ii) Cross-blast type in which air blast is directed at right angles to the arc path as shown in figure below (iii) Radial-blast type in which the air blast is directed radially as shown in figure below (i) Axial-blast air circuit breaker The figure below shows the essential components of a typical axial blast circuit breaker.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 read more

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These type of breakers employ ‘air blast’ as the quenching medium.The contacts are opened by air blast produced by the opening of blast valve.The air blast cools the arc and sweeps away the arcing products to the atmosphere.This rapidly increases the dielectric strength of the medium between contacts and prevents from re-establishing the arc.Consequently the arc is extinguished and the flow of current is interrupted. Advantages An air blast circuit breaker has the following advantages over an oil circuit breaker: The risk of fire is eliminated The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations;the expense of regular oil is replacement is avoided The growth of dielectric strength is so rapid that final contact gap read more

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Construction There are two chambers in a low oil circuit breaker,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. Operation Under normal read more

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In case of plain oil circuit breakers artificial control of arc is less therefore long arc length is necessary for arc quenching. Its necessary that final arc extinction should occur while the contact gap is still shorter. For this 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. Self-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 read more

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In this circuit breaker the contacts are separated under the whole of the oil in the tank. There is no special arc control system other than increasing length caused by separation of contacts. Arc extinction occurs when a critical gap is reached between the contacts. Double break plain oil circuit breaker is shown in fig 7 below 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. 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. Under normal operating conditions the fixed and moving contacts remain closed and the circuit breaker carries the normal circuit read more

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