Types of Circuit Breaker
A circuit breaker is an automatically switching device which is designed to protect an electrical circuit from damage caused by overload or short circuit. It can be operated manually. Its major function is to detect a fault condition and interrupt current flow during the operation of circuit breaker.
Therefore we can define it as a mechanical switching device which is capable of switching, breaking and carrying current under normal conditions and under some specific time period.
Power system deals with huge power network and huge numbers of associated electrical equipment. At the time of short circuit fault or any other types of electrical fault the power network may release a high stress of fault current in the equipment which may damage both the equipment and networks permanently. Only solution for saving the equipments and the power networks is that the fault current should be cleared as soon as possible from the system. Once the fault is cleared, the system will recover its normal working condition and gets ready for supplying reliable quality power to the receiving ends.
There are different switching operations needed for proper controlling of power systems. For the protection and control of power system network some special type of switching devices are introduced which can be operated safely under huge current carrying conditions During the flow of huge current, there may be large arcing in between switching contacts, so it should be taken care to quench these arcs in circuit breaker in safe manner.
Basic Design of Circuit Breaker
Major components of a circuitbreaker are
- Frame ( cases made of metal or electrical insulation types)
- Contacts (Electrical)
- Arc (Arc extinguishing assembly)
- Operating mechanism
- Trip unit (having thermal element or magnetic)
It provides an insulated housing to mount the circuit components. Insulation materials are like thermal plastic or glass polymer. Quality of material will determine the interruption rating. Frame can be rated by ampere and voltage.
The flow of current in a circuit is mainly controlled by circuit breaks followed by the contacts. By separating its contacts the breaker interrupts the flow of current while it is tripped.
Arc (Arc extinguishing assembly)
At the time when the circuit breaker is open, current will continue to a period of time by arcing across the air space between the contacts. When the arc extinguishes the flow of current will stop.
Using an operating handle, circuit breaker provides energizing and dc- energizing of the circuit which is capable of reset. It is connected to a movable contact arm which is stated as operating mechanism.
Trip unit (having thermal element or magnetic)
When an over current is sensed the device will automatically open its contacts. At the time of opening, trip unit will be the part which determines the opening. Trip unit is able to sense the heat while it is over loaded.
Major Functions of Circuit Breaker
- It is able to sense the flow of current
- It is able to measure the flow of current
- It is possible to compare the current measured level to its pre-set trip point.
They have two purposes. It is able to insulate between the phases and between the phases and the ground. They are mainly designed for 11KV- 765KV. There are different divisions for oil circuit breaker such as:
Bulk Oil circuit Breaker
Here as shown in the figure contacts are immersed in to the oil bath. Using arc, they provide cooling effect with the hydrogen present in the oil.
Minimum Oil Circuit Breaker
Here they consist of mainly two parts a) supporting chamber b) circuit breaking chamber. Main advantage of this is they requre minimum quantity of oil, lesser space, low risk for fire and low maintainence.
It is a kind where the arc quenching takes place at vacuum. They have more insulating strength. Mainly this technology works on medium voltage application. The variance of pressure starts from 10-7 to10-5. This breaker has a long term service time than the others. They are much more compactable and robust. There are no chances of causing fire. They are much more user friendly. Generation of gas is impossible after the operation. They are soundless. They require less power for control operation. Because of the creation of vacuum it initiates high cost. Metal vapors are depleted over here.
Air blast circuit breaker
Air blast circuit breaker operates due to high blast velocity which automatically quenches the arc. Main components are blast valve, blast tube and contacts. Here blast valve is filled with the high pressure. And tube will carry the air which is in high pressure. And contact helps to open the attached spring. Surface is completely free from carbon. The main advantage is that they operate faster as compared to other circuit breakers. Able to switch frequently. Maintenance is very low. But there is a high risk of fire hazards due to high voltage.
Main component used for this breaker is Sulphur Hexa fluoride gas in the quenching medium. When it is closed the contact will remain with the SF6 gas with the pressure of 2.8 kb/cm2. This is a kind of electro negative gas. As a result they will absorb more electrons. It results in high power and voltage services. They are cheaper. Easily constructed. Gas can be reused in the same circuit. It has less arcing time. Free from fire. Maintenance cost is less.
Circuit breaker can be divided according to its basic services such as
They have rated voltage of 36 kV. And the breaking capacity 25kA.They maintain free poles. They are easily suitable for self enclosing cycles. The covers of poles are made with anodized aluminum.
Instrument transforms are allocated to indoor. It transforms usually 5A or 1A. For rating the high altitudes a correction factor have been introduced.
According to the operating mechanism circuit breaker can be divided into:
Spring operated circuit breaker
This is the most reliable mechanical drive architecture. They are powered with the compressed gas. They mainly include two sets of spring. The mains functions are for opening and closing the contacts.
Pneumatic circuit breaker
It is mainly helpful to reduce the air consumption. Due to the required fully loaded condition they are subjected to leakages over time. Theses breakers will include a driving piston within an operating cylinder which is helpful for effective movement.
Hydraulic circuit breaker
They are the most important apparel in the power system. They release a high voltage power grid. They are characterized into high speed, high power and high hydraulic pressure. They are long term waiting.
According to the voltage level of installation, types of circuit breakers are referred as
High Voltage Circuit breaker
They vary voltage up to 800kV and current upto the level of 63 kA. The main application is high power transmission and generation. The breaker with combined function will improve the reliability. An example is the high voltage circuit breaker having disconnected function.
Medium Voltage circuit breaker
They vary voltage up to 1-72kV and vary current up to 10kA. The main application is the power transmission and generation. They are mainly for indoor. The characteristics of medium voltage are given by the international standards IEC 62271. They use separate current sensors.
Low voltage circuit Breakers
They vary voltage up to 1kV. Current ratings from 0.1kA. Main applications include domestic consumer unit.
Residual Current Devices (RCD)
They provide a protecting and the property from electrical hazards in low voltage systems. They are mainly associated with incorporation in the breaking device.
This is mainly uses for preventing shock. It is helpful for detecting the small stray voltage. Main advantage of ELCB is to prevent human beings and animals from shock. Two types of ELCB are available.
- Voltage operated
- Current operated
They are basically a thermistor. They are a typical standard of protection from electrical hazards. Poly fuse is a major element in the circuit which helps circuit from low resistance to high resistance when it is over current condition. It is a mixture of semi-crystalline polymer and conductive particles.
It helps to close the breaker when it is opened due to fault. It helps for detecting the momentary faults. They are able to connect automatically after a brief interval. Within a period of time these will restore its services.
Therefore we can conclude that circuit breaker has a vital role in electrical networks. The main usage of this is to protect the electrical devices from damage by detecting the fault area within a short period of time.