Circuit breaker is a device that interrupts fault current or short circuit current under abnormal condition. In addition they perform the function of a switch. It operates practically instantaneously on short circuit and with definite time delay on overloads. Generally switching and protection of electric circuits above 3.3 kV is done by circuit breaker.
In power system network circuit breakers are employed along with isolators.
Duties of Circuit Breaker
Circuit breakers are mechanical devices which closes or opens the contacts. Thus opening or closing the circuit under abnormal or normal conditions. The duties of the circuit breaker are summarized in following points;
- Without damage or overheating it carries full-load current continuously.
- Under no load it opens and closes the circuit.
- It makes (closes) or breaks (opens) the normal operating currents.
- It makes and breaks the short circuit current up to the value of current it is designed for.
Circuit breakers are employed for protection of electrical circuits and are equipped with a trip coil. Trip coils are generally connected to the relay. Hence, breakers are designed to operate automatically under fault or short circuit.
When circuit breaker is subjected to make or break short circuit currents then it is put through mechanical and thermal stress.
Rating of Circuit Breaker
They are rated in terms of maximum voltage, frequency, number of poles, maximum continuous current carrying capacity, maximum interrupting capacity and maximum momentary and short circuit current carrying capacity.
Standard ratings of breakers are:
1A, 2A, 3A, 4A, 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 100A and even much more kA.
The maximum value of current that a breaker can interrupt without facing any damage is called interrupting or rupturing capacity of circuit breaker.
They are also rated in MVA. It is the product of interrupting current and rated voltage.
It has a moving contact and a fixed contact. Both the contacts are touching each other during normal condition i.e. when the breaker is closed. During closing of the breaker the moving contact is held under by the pressure of a spring.
Normally, it can be closed or opened manually by a person or an operator for switching and maintenance purpose. But when fault occurs and heavy short circuit current flows through the breaker. The trip coil of the breaker gets energized and moving contacts are pulled apart by some mechanism resulting in opening of the breaker. Arc is formed when the contacts separates. Until the discharge of arc ceases current continues to flow. Production of arc delays the current interruption process thereby generating enormous amount of heat which may damage the system or the breaker itself.
The main challenge in a circuit breaker is to extinguish the arc within the shortest time possible. For that various mechanisms are employed in various types of circuit breaker.
Basic construction includes separation of contacts in an insulating medium. Insulating medium extinguishes the arc produced between the contacts when the breaker opens and provides insulation between the contacts and from each contact to earth.
Insulating materials used are:
- Air at atmospheric pressure
- Compressed air
- Sulphur Hexafluoride (SF6)
These insulating material used have the following property:
- High dielectric strength
- No inflammability
- High thermal stability
- Arc extinguishing ability
- Chemical stability
- Commercial availability
- Available at moderate cost
Air circuit breaker is the cheapest and widely used. Also SF6 has excellent arc extinguishing property and is widely used.
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