Circuit breakers are mechanical devices designed to close or open the contacts during fault condition and during switching conditions. Automatic circuit breakers which are normally employed for the protection of electrical circuits are equipped with a trip coil connected to a relay or other means, designed to open the breaker automatically under abnormal conditions such as over current.
Duties of Circuit Breaker:
The circuit breaker performs the following duties:
- It carries the full load current continuously without over-heating or damage
- It opens and closes the circuit on no load
- It makes and breaks the normal operating current
- It makes and breaks the short circuit currents of magnitude upto which it is designed for
The circuit breaker performs the first three duties satisfactorily, but in performing fourth duty (make and break the short circuit currents) it is subjected to mechanical and thermal stresses. The circuit breakers are rated for maximum voltage, number of poles, maximum continuous current capacity, maximum interrupting capacity and maximum short circuit current making capacity. The interrupting capacity or rupture capacity of the circuit breaker is the maximum value of the current which can be interrupted by the breaker without any damage.
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Operating principle of Circuit Breaker:
A Circuit breaker is a switching and current interrupting device. Circuit breaker consists of essentially of fixed and moving contacts which are touching each other and carrying the current under normal condition. When the circuit breaker is closed, the current carrying contacts (electrodes) engage each other under the pressure of spring.
During normal operating conditions the circuit breaker can be opened or closed by operating personnel for switching and maintenance purpose. Whenever a fault occurs on any part of the power system, the trip coils of the breaker gets energized and moving contacts are pulled away by some mechanism thus opening the circuit. The speration of the current carrying contacts produces an arc. The current is thus able to continue until the discharge ceases. The generation of the arc not only delays the interruption process but also produces enormous amount of heat which can damage the breaker. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that the heat generated may not reach a dangerous value.
The basic construction of a circuit breaker requires the separation of contacts in an insulating medium which serves tow functions:
- Extinguishes the arc drawn between the contacts when the circuit breaker opens
- Provides insulation between the contacts and from each contacts to earth
Some of the insuating media commonly employed in the circuit breakers are:
- Air at atmospheric pressure
- Compressed air
- Oil producing hydrogen for arc extinction
- Ultra high vacuum
- Sulphur hexa-fluoride (SF6 gas)
The insulating media used in the circuit breakers should have the properties of high dielectric strength, non-inflammability, high thermal stability, arc extinguishing capability, chemical stability and should be available commercially.
Air is the cheapest insulating medium and is widely used for low voltage applications. Hydrogen has better arc extinguishing capability but it has lower dielectric strength as compared to air. Also hydrogen if contaminated with air forms an explosive mixture. Nitrogen has similar properties that of the air. CO2 has almost same dielectric strength as air but it is better arc extinguishing medium at moderate currents. Oxygen has good extinguishing capability but is chemically highly active. SF6 has outstanding arc quenching properties and good dielectric strength. Of all the gases SF6 and air is used in the commercially gas blast circuit breakers