SEMINAR ON CIRCUIT

BREAKER

By:-Abhishek Agarwal

III year V semRoll NO.– eight(08)

CONTENTS

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.

WORKING OF THE 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.

PRINCIPLE OF CIRCUIT BREAKER

As shown in fig. there are two contacts –

i. Fixed Contactii. 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.

ARC INTERRUPTION

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.

CLASSIFICATION OF CIRCUIT BREAKER

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.

SF6 CIRCUIT BREAKER

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.

OPERATING MECHANISM For opening and closing the circuit breaker, one of

the following mechanism is employed.

1. Spring2. Solenoid3. 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

END

THANK YOU