Introduction to the circuit breaker
An electrical power system is very complex, in order to operate it in a controlled and effective manner some switch gears are needed. A circuit breaker is switchgear which can be used to control and protect electrical power system. A circuit breaker can be called as an automatically operated electrical switch. it can protect the electrical circuit from the severe damages the damages could be caused by the overload and short circuit. So whenever a fault occurs in a power system that part of the system can be isolated from the remaining part of the system by the help of a circuit breaker. A circuit breaker could be able to make or break circuit either manually or automatically under certain conditions such as no-load, full –load, short circuit so it can be used for the switching and protection of different parts of the power system. A circuit breaker has an inbuilt fixed electric current load capacity and when this load capacity is breached then it will automatically shut-down the circuit, it detects the fault conditions like a short circuit or overload in the circuit and when this fault occurs it will interrupt the continuity and stops the current flow. So with all these features of the circuit breaker, it is inevitable in a power system.
What are the functions of a circuit breaker
- Breaks a circuit automatically under fault condition
- Sense when an overcurrent occurs
- Measures the amount of overcurrent
- it makes or breaks both normal and abnormal currents
- It carries the full load current continuously without overheating or damage
- A circuit breaker can open and close the circuit on no load
- It makes and breaks short circuit current of magnitude up to which it is designed for
Circuit breakers provide manual means of energizing or de-energizing circuit, it does the automatic overcurrent protection of a circuit. A circuit breaker does the reactivation of the circuit immediately after clearing the short circuit or overload. After the circuit breaking a simple flip of the breakers operating handle restores the circuit.
What is the difference between fuse and a circuit breaker
Circuit breaker and fuse can be used for the circuit protection from overload but circuit breaker is different from a fuse, a fuse would only operate once and then it has to be replaced but in case of the circuit breaker, it can be reset either manually or automatically to resume normal operation. Circuit breakers can be of various size, from a small device which is used to protect the home appliances to a large device which can protect the entire city
Working principle of circuit breaker
Mechanical working – in a circuit breaker it has fixed contacts and moving contacts these two contacts are physically connected in normal conditions potential energy is stored in the operating mechanism of a circuit breaker potential energy is stored in a circuit breaker by means of compressed air or by hydraulic pressure and this potential energy is released during operation and this makes the sliding of moving contact quickly.
Electrical working – circuit breaker has to carry fault power and this would be large power and due to this and it could cause high arching between moving contacts and fixed contacts during the operation of circuit breaker
Types of circuit breaker
Oil circuit breaker (Types minimum oil & bulk oil)
In this type of circuit breaker, mineral oil is used as an arc quenching medium in this circuit breaker the contacts are separated inside steel filled with dielectric oil. Oil is used in this circuit breakers for two purposes it insulates between the phases and between phases and the ground it provides the medium for the extinguishing of the arc
Bulk oil circuit breaker – Bulk oil circuit breakers are enclosed in metal grounded waterproof tanks and it is referred to as dead tanks. These type of circuit breaker has 3 separate tanks for 72.5KV and above, single tanks are used for 36Kv and below
Working of oil circuit breaker – When the electric arc is drawn under oil the arc vaporizes and creates a large bubble that surrounds the arc and the gas inside the bubble is 80 percent hydrogen which impairs ionization and the decomposition of oil into gas requires energy and that comes from the heat generated by the arc and the oil surrounds the bubble conducts the heat away from the arc and thus deionization of the arc is done.
Minimum oil circuit breaker – These type of circuit breaker is also known as a small oil circuit breaker, this type of circuit breaker is developed to reduce the oil volume to the required rate for arc extinguishing oil is injected into the arc to improve the breaker performance. In MOCB the current interruption takes place inside the interrupter.
Advantages
- Oil provides insulation between the contact after the arc has been extinguished
- Oil provides insulation for the live exposed contacts from the earthed portion of the container
- Hydrogen gas is produced by arc energy and this is used to extinguish the arc
Disadvantages
- Oil is inflammable and can cause fire hazards and it can also cause an explosion
- The generated hydrogen could combine with air and may form an explosive mixture
- Maintenance is required frequently for oil changing
Vacuum circuit breaker
In this type the vacuum is used as an arc quenching medium it comprises of one or more interrupter units per pole. Moving contact in the interrupter is connected to the insulating operating rod. It takes advantage of non-sustainability of electric arc in the vacuum and the contact separation takes place in the vacuum where there is no ionization due to medium. The initial arc will be removed because of vacuum.
Advantages
- No emission of gases so pollution-free
- Non-explosive
- Compact in size Long life
- Less operating energy is needed
- There is no environmental effect
Disadvantages
- Vacuum interrupter is very expensive
- High technology is needed for the production of the vacuum interrupters
Sulphur-hexafluoride circuit breaker (SF6)
In this circuit breaker, sulphur hexafluoride gas is used as an arc quenching medium. SF6 is an electronegative gas and it has the tendency to absorb free electrons. Breaker contacts are opened in the high-pressure flow of SF6 gas and an arc is struck between them, free electrons in the arc are quickly captured by the gas so that it can form immobile negative ions and this loss of conducting electrons in the arc rapidly builds up enough insulation strength to extinguish an arc. These circuit breakers are very effective for high power applications. It can be used in the voltage range of 115KV-230KV.
Advantages
- Arc energy is low because of high conductivity
- Contact erosion is small due to the low energy
- Higher dielectric and arc quenching features for SF6 gas
- Because of its closed gas circuit, the operation is noiseless
Disadvantages
- Expensive because SF6 gas is costly
- SF6 gas needs to be reconditioned after every operation of the breaker
Airblast circuit breaker
This circuit breaker uses high-pressure air-blast as an arc quenching medium and the contacts are opened in a flow of air blast established by the opening of the blast valve. The air blast will cool the arc and sweeps away the arcing products of the atmosphere, and so the arc is extinguished and the flow of the current is interrupted. Airblast circuit breakers are used for high voltage applications for voltages of 245KV and 400KV and up to 765KV
Advantages
- There is no risk of fire
- Auto reclosure
- Modest maintenance
- Multi break construction
- Arcing time is very small so less burning of oil
Disadvantages
- They are very sensitive to vibrations
- Compressor plant supplies air-blast and that needs considerable maintenance.
Air circuit breaker
In this type of circuit breaker, the contact opens and closes in the air at atmospheric pressure, these type of circuit breakers are restricted to low voltage applications or high-security installations where the risk of oil fire or oil contamination of the environment is too high to be tolerated. This circuit breaker can be used for AC and DC types of circuit up to 12Kv. In normal conditions, the contact remains closed and whenever a fault occurs the tripping signal makes the circuit breaker contacts to open and the arc is drawn between the contacts
