The circuit breaker is a device that interrupts fault current or short circuit current under abnormal conditions. Besides, they perform the function of a switch. It operates practically instantaneously on a short circuit and with a definite time delay on overloads. Generally switching and protection of electric circuits above 3.3 kV is done by a circuit breaker.
In power system network circuit breakers are employed along with isolators.
Duties of Circuit Breaker
Circuit breakers are mechanical devices that close or open the contacts. Thus opening or closing the circuit under abnormal or normal conditions. The duties of the circuit breaker are summarized in the 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 the current it is designed for.
Circuit breakers are employed for the 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 the 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 the 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 conditions i.e. when the breaker is closed. During the closing of the breaker, the moving contact is held under the pressure of a spring.
Normally, it can be closed or opened manually by a person or an operator for switching and maintenance purposes. But when a 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 the opening of the breaker. Arc is formed when the contacts separate. Until the discharge of the arc ceases current continues to flow. Production of arc delays the current interruption process thereby generating an 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 the circuit breaker.
Basic construction includes the 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
The air circuit breaker is the cheapest and widely used. Also, SF6 has excellent arc extinguishing property and is widely used.
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