By:- Abhishek Bhardwaj IV year VIII sem Roll NO.– four(04)

1. Working of Circuit Breaker. 2. Principle of Circuit Breaker. 3. Arc Interruption. 4. Classification of Circuit Breaker. 5. Operating Mechanism. 6. Selection of Circuit Breaker.

A circuit breaker is an automatically operated electrically 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.

As shown in fig. there are two contacts – i. Fixed Contact ii. Moving Contact Under normal condition these two contacts remain in closed position. When the circuit breaker is required to isolate the faulty part, the moving contact moves to interrupt the circuit. On the separation of the contacts, the flow of current is interrupted, resulting in the formation of arc between the Contacts.

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

Methods of Arc Interruption:-- 1. High Resistance Interruption. 2. Current Zero Interruption.  In high resistance its resistance is increased so as to reduce the current to a value insufficient to maintain the arc.The arc resistance can be increased by cooling, lengthening, constraning and splitting the arc. This method is not suitable for a large-current interruption, This can be employed for low power ac and dc circuit breakers.

 Current Zero Interruption is applicable only in case of ac circuit breakers. The current is not interrupted at any pt. other than the zero current instant, otherwise a high transient voltage will occur across the contact gap.

There are mainly five type of Circuit Breakers:-- 1. Oil Circuit Breaker. 2. Air blast Circuit Breaker. 3. SF6 Circuit Breaker. 4. Air - Break Circuit Breaker. 5. Vacuum Circuit Breaker.

Advantages :-- 1. It has better insulating property then air 2. It has also good cooling property. 3. When arc is formed,it decomposes oil into gases. Disadvantages :-- 1. It is inflammable may pose a fire hazard. 2. The possibility of forming explosive mixture with air and the production of carbon particles in the oil due to heating which reduces its dielectric strength.

In air blast circuit breakers, compressed air at a pressure of 20–30 kg/cm^2 is employed as an arc quenching medium and these are suitable for operating voltages of 132KV and above. Advantages:-- 1. High speed operation. 2. Elimination of fire hazard. 3. Less maintenance. 4. Facility of high speed reclosure. 5. Cheapness and free availability of the interrupting medium, chemical stability and inertness of air. 6. Short and consistent arcing time and therefore, less burning of contacts. 7. Suitability for frequent operation.

Disadvantages :-- 1. An air compressor plant has to be installed and maintained. 2. Upon arc interruption the air blast circuit breaker produces a high-level noise when air is discharged to the open atmosphere. In residential areas, silencers need to be provided to reduce the noise level to acceptable level. 3. Problem of current chopping. 4. Problem of restriking voltage. There are types of Air blast circuit breakers :-- 1. Cross-blast Circuit breakers. 2. Axial –blast Circuit breakers.

 These are quite suitable for high current interruption at low voltages.  Air at atmospheric pressure is used as an arc extinguishing medium.  Here the principle of high resistance is employed for arc interruption. The arc resistance is increased by lengthening, splitting and cooling the arc.

 AC air –break circuit breakers are available in the voltage range 400 to 12KV. They are widely used in low and medium voltage system.  Air –break circuit breakers are also used in dc circuits up to 12KV.

SF6 circuit breaker are manufactured in the voltage range 3.6 kv to 765 kv. However they preferred for voltages 132kv and above. Advantages:-- 1. It has good dielectric strength and excellent arc quenching property. 2. It is an electronegative gas,i.e. it has high affinity for electrons. 3. The SF6 gas has also excellent heat transfer property.

4. Its thermal time constant is about 1000 times shorter than that of air. 5. It decomposes to SF4,SF2,S2,F2,S and F at temp. of the order of 1000 degree Celsius. After arc extinction, the products of decomposition recombine in short time, with in about 1 microsecond. Disadvantages :-- 1. It is condensate at low temperature. 2. The temperature at which SF6 changes to liquid depends on the pressure. 3. SF6 breakers are equipped with thermostatically controlled heaters wherever such low ambient temperatures are encountered.

Types of Circuit Breaker :-- 1. Double-pressure SF6 circuit breakers. 2. Puffer-type Sf6 Circuit Breakers.

 The dielectric strength and arc interrupting ability of high vacuum is superior to those of porcelain, oil, air and SF6 at atmospheric pressure.  SF6 at 7 atm. Pressure and air at 25 atm. Pressure have dielectric strengths higher than that of high vacuum.  The pressure of 10^-5mm of mercury and below is considered to be high vacuum and it is possible to achieve pressures as low as 10^-8torr.  Copper-bismuth, Silver-bismuth, Silver-lead and copper-lead are good materials for making contacts of the breaker.

 The rate of dielectric recovery is many times higher than that obtained in other types of Circuit Breakers. Its typical value may be as high as 20 KV/us. It has other advantages like :- i. Suitability for repeated operations. ii. Least maintenance. iii. Silent operation. iv. Long life. v. High speed of dielectric recovery. vi. Less weight of moving parts

For opening and closing the circuit breaker, one of the following mechanism is employed. 1. Spring 2. Solenoid 3. Compressed Air  The force for the opening of a circuit breakers may be applied by spring or compressed air.  When spring is used of opening, it is precharged, i.e. compressed The precharging may be done by hand or a motor or by the closing mechanism.  The closing mechanism may be a spring solenoid or compressed air

 For large oil C.B., Solenoid is used for closing and spring is used for opening. While for medium size breakers, spring is used for both opening and closing.  In EHV oil circuit breakers,compressed air is used for closing and spring is used for opening.  In Air blast circuit breakers, compressed air is used for both closing and opening.  In SF6 C.B., compressed air may be used for closing and a spring for opening or compressed air for both closing as well as opening.  In Vacuum C.B., a solenoid or spring mechanism is fixed to lower end to move metallic bellows up & down inside the chamber during closing or opening the contact. The closing mechanism must be trip free.

 Spring is very good for opening as their force is large in the beginning and gradually decreases as the distance of travel of the moving contact increases, but it is not suitable for closing.  Solenoids are very good for closing The force of attraction increases when the distance between the contacts decreases, these are not suitable for opening because they are slow in action.  Compressed air is suitable for both closing as well as opening as its force remains almost constant even when the distance between the contacts increases or decreases.

Table shows the choice of circuit breakers for various voltage ranges Rated VoltageChoice of Circuit BreakersRemarks Below 1 KVAir – break Circuit Breaker 3.3 KV – 33 KVVacuum Circuit Breaker, Sf6 Circuit Breaker, Minimum oil Circuit Breaker Vacuum preferred 132 KV – 220 KVSF6 Circuit Breaker, Air blast Circuit Breaker, Minimum oil Circuit Breaker SF6 preferred 400 KV- 760 KVSF6 Circuit Breaker, Air blast Circuit Breaker Sf6 is preferred

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