Circuit Bre Ker

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Power point presentation on circuit breaker

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  • Introduction A circuit breaker is a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions. It is also capable of making and carrying currents for a specified time and breaking currents under specified abnormal circuit conditions, such as those of a short circuit.

  • Arc Phenomenon

    An arc is struck when contacts are separated. The current is thus able to continue. Thus the main duty of a circuit breaker is to distinguish the arc within the shortest possible time.The arc provides the low resistance path to the current and the current in the circuit remains uninterrupted.

  • Fixed contactMoving contactARCFixed contactMoving contactARCARC IS QUENCHED BY MEDIUMIN A CIRCUIT BREAKEROPERATING PRINCIPLE OF BREAKER

  • Operating MechanismCircuit Breaker consists of two contacts: Fixed contact. Moving contact.Moving contact is used to make and break the circuit using stored energies in the form of spring or compressed air.Spring, pneumatic or oil damping is used to arrest the speed of mc while closing.FC contains a spring which holds the mc after closing.

  • Circuit breaker consists of two coils: Closing coil Used to close the circuit. Tripping coil-Used to trip the circuit.These coils activate the stored energy and directs the mc to open or close.DC batteries are used to energize these coils.Solenoids are used to close or trip it.CBs are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.

  • The Electric ArcDuring the separation of contacts, due to large fault current and high current density at the contact region the surrounding medium ionizes and thus a conducting medium is formed. This is called the ARC.

    Factors responsible for arc:-Potential difference between the contacts.Ionized particles between the contacts.

  • Arc quenching is achieved by:Greater dielectric strength than restriking voltage.Faster rate of heat removal than rate of heat generation.

    Arc extinction methods are: By lengthening the gap.Cooling the arc.Inserting medium of high dielectric strength.PRINCIPLES OF ARC EXTINCTION

  • Types Of Circuit Breakers

    Oil Circuit Breakers

    Vacuum Circuit Breakers

    Air Blast Circuit Breakers

    SF6 Circuit Breakers

  • OIL CIRCUIT BREAKERThese are of two typesBOCB (Bulk oil Circuit Breaker)MOCB (Minimum oil Circuit Breaker)

  • OIL CIRCUIT BREAKERIt is designed for 11kv-765kv.The contacts are immersed in oil bath.Oil provides cooling by hydrogen created by arc.It acts as a good dielectric medium and quenches the arc.

  • BULK OIL CB

  • MINIMUM OIL CIRCUIT BREAKERIt is designed for 11kv-765kv.The contacts are immersed in oil bath.Oil provides cooling by hydrogen created by arc.It acts as a good dielectric medium and quenches the arc.

  • Advantages:Oil has good dielectric strength. Low cost.Oil is easily available.It has wide range of breaking capability.Disadvantages:Slower operation , takes about 20 cycles for arc quenching.It is highly inflammable , so high risk of fire.High maintenance cost.

  • VACCUM CIRCUIT BREAKER

  • 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.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 ionisation of metal vapours of contacts.The salient feature of vacuum as an arc quenching medium is that 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

  • 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 metallic 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, thearc extinction in a vacuum breakeroccurs with a short contact separation (say 0.625 cm)

  • Advantages:Free from arc and fire hazards.Low cost for maintenance & simpler mechanism.Low arcing time & high contact life.Silent and less vibrational operation.Due to vacuum contacts remain free from corrosion.No byproducts formed.Disadvantages:High initial cost due to creation of vacuum.Surface of contacts are depleted due to metal vapours.High cost & size required for high voltage breakers.

  • AIR BLAST CIRCUIT BREAKERSThis operates using high velocity blast of air which quenches the arc.It consists of blast valve , blast tube & contacts.Blast valve contains air at high pressure.Blast tube carries the air at high pressure & opens the moving contact attached to spring.There is no carbonization of surface as in VCB.Air should be kept clean & dry to operate it properly.

  • Types of Air circuit breakerAxial Blast Type - air blast is directed along the arc path. Cross Blast Type - air blast is directed at right angles to the arc path.Radial Blast Type - air blast is directed radially.

  • Plain air circuit breakerorCross-Blast Air CB

  • In this type of breaker, an air-blast is directed at right angles to the arc. The cross blast lengthens and forces the arc into a suitable chute for arc extinction.The circuit breaker is fitted with a chamber surrounding the contact. The chamber is known as arc chute. The arc is made to drive in it. The arc chute will help in achieving cooling.The inner walls of arc chute are shaped in such a way that arc is not only forced into close proximity, but will drive into the serpentine channel projected on arc chute wall.Initial arc will split into a series of arcs this will make all arc voltages higher than system voltage.They are preferable choice in low voltage application.

  • Axial Blast Type circuit breaker

  • Under normal conditionThe fixed and moving contacts are held in closed position with the help of spring pressure.There is an airreservoirconnected to the arcing chamber through an air valve. The air valve controlthe flow of air into the arcing chamber. The valve is closed under normal conditions.Under Faulty Condition When a fault occurs a tripping impulse is produced which causes opening of the air valve. Since the air valve connects the airreservoirand the arcing chamber, a high pressure air enters to thearcing chamber.

  • This air pushes away the moving contactagainstthe 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 arc extinction is very small generally (1.75 or so ).This small gap may sometimes inadequate clearance for the normal voltage. Therefore an isolating switch is included as a part of this CB.This switch opens immediately after the fault interruption to provide necessary clearance for insulation.

  • Advantages:High speed operation as compared to OCB.Ability to withstand frequent switching.Facility for high speed reclosure.Less maintenance as compared to OCB.Disadvantages:Little moisture content prolongs arcing time.Pressure should be checked frequently for frequent operation.Risk of fire hazards due to over voltages.It cant be used for high voltage operation due to prolonged arc quenching.

  • SF6 CIRCUIT BREAKERSIt contains anarc interruption chamber containing SF6gas.In closed position the contacts remain surrounded bySF6gas at a pressure of 2.8 kg/cm2 .During opening high pressure SF6 gas at 14 kg/cm2 from its reservoir flows towards the chamber by valve mechanism.SF6rapidly absorbs the free electrons in the arc path to form immobile negative ions to build up high dielectric strength.It also cools the arc and extinguishes it.After operation the valve is closed by the action of a set of springs.Absorbent materials are used to absorb the byproducts and moisture.

  • Advantages:Very short arcing period due to superior arc quenching property ofSF6 .Can interrupt much larger currents as compared to other breakers.No risk of fire.Low maintenance, light foundation.No over voltage problem.There are no carbon deposits.

    SF6breakers are costly dueto high cost ofSF6.SF6gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

    Disadvantages:

  • CONCLUSION:Therefore, we conclude that circuit breaker is the most essential part of the electrical networks as it protects every device from damage. It helps us to detect the fault and area affected by it. Nowadays vacuum and SF6 circuit breakers are widely used due to their reliable and fast operations.

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