What is Circuit Breaker?
Circuit breaker is a mechanical switching device capable of making, carrying and
breaking currents under normal circuit conditions and also making, carrying for a specified
time, and automatically breaking currents under specified abnormal circuit conditions such
as those of short-circuits (faults). The insulating medium in which circuit interruption is
Performed is designated by suitable prefix, such as oil circuit breaker, air-break circuit
breaker, air blast circuit breaker, Sulphur hexafluoride (SF6) circuit breaker, vacuum circuit
breaker, etc.
The function of a circuit breaker is to isolate the faulty part of the power system in case
of abnormal conditions such as faults. A protective relay detects abnormal conditions and
sends a tripping signal to the circuit breaker.
CLASSIFICATION OF CIRCUIT BREAKERS
Circuit breakers can be classified using the different criteria such as, intended voltage application, location of installation, their external design characteristics, insulating medium used for arc quenching, etc.
Classification Based on Voltage
Low Voltage Circuit Breaker (less than 1 kV)
Medium Voltage Circuit Breaker (1 kV to 52 kV)
High Voltage Circuit Breakers (66 kV to 220 kV)
Extra High Voltage (EHV) Circuit Breaker (300 kV to 765 kV)
Ultra High Voltage (UHV) Circuit Breaker (above 765 kV)
2. Classification Based on Location
Indoor type
Outdoor type
3. Classification Based on External Design
Circuit breakers can be classified into following categories depending on their external design.
Dead tank type.
Live-tank type.
4. Types of circuit breaker
MCB (miniature circuit breaker)
MCCB(molded case circuit breaker)
RCCB ( Residual current circuit breaker)
SF6 (Sulphur hexafluoride)
VCB (Vaccum circuit breaker)
𝐌𝐂𝐁 (𝐦𝐢𝐧𝐢𝐚𝐭𝐮𝐫𝐞 𝐜𝐢𝐫𝐜𝐮𝐢𝐭 𝐛𝐫𝐞𝐚𝐤𝐞𝐫)
A miniature circuit breaker (MCB) automatically switches off electrical circuit during an abnormal condition of the network means in overload condition as well as faulty condition. Nowadays we use an MCB in low voltage electrical network instead of a fuse. The fuse may not sense it but the miniature circuit breaker does it in a more reliable way. MCB is much more sensitive to over-current than fuse. Handling an MCB is electrically safer than a fuse. Quick restoration of supply is possible in case of a fuse as because fuses must be re-wirable or replaced for restoring the supply. Restoration is easily possible by just switching it ON.
𝗠𝗖𝗖𝗕(𝗺𝗼𝗹𝗱𝗲𝗱 𝗰𝗮𝘀𝗲 𝗰𝗶𝗿𝗰𝘂𝗶𝘁 𝗯𝗿𝗲𝗮𝗸𝗲𝗿)
Molded case circuit breakers are a type of electrical protection device that is commonly used when load currents exceed the capabilities of miniature circuit
breakers. They are also used in applications of any current rating that require adjustable trip settings, which are not available in plug-in circuit breakers and MCBs.
The traditional melded-case circuit breaker uses electromechanical (thermal
magnetic) trip units that may be fixed or interchangeable. An MCCB provides
protection by combining a temperature sensitive device with a current sensitive
electromagnetic device. Both these devices act mechanically on the trip mechanism.
𝗥𝗖𝗖𝗕 ( 𝗥𝗲𝘀𝗶𝗱𝘂𝗮𝗹 𝗰𝘂𝗿𝗿𝗲𝗻𝘁 𝗰𝗶𝗿𝗰𝘂𝗶𝘁 𝗯𝗿𝗲𝗮𝗸𝗲𝗿)
The Residual Current Circuit breaker RCCBs are the safest device to detect and trip against electrical leakage currents, thus ensuring protection against electric shock
caused by indirect contacts. These devices must be used in series with an MCB or fuse
which protects them from the potentially damaging thermal and dynamic stress of against overload is designed with a time delay to allow short duration over-current, which is a normal part of operation for many devices. However, any over-current conditions that last more than what is normally expected represent an overload, and the MCCB is tripped to protect the equipment and personnel.
On the other hand, fault protection is accomplished with electromagnetic
induction, and the response is instant. Fault currents should be interrupted immediately,
no matter if their duration is short or long. Whenever a fault occurs, the extremely high
current induces a magnetic field in a solenoid coil located inside the breaker – this
magnetic induction trips a contact and current is interrupted. As a complement to the
magnetic protection mechanism, MCCBs have internal arc dissipation measures to
facilitate interruption.
𝗦𝗙𝟲 (𝗦𝘂𝗹𝗽𝗵𝘂𝗿 𝗵𝗲𝘅𝗮𝗳𝗹𝘂𝗼𝗿𝗶𝗱e)
SF6, as hashas excellent insulating strength because of its affinity for electrons (electronegativity) i.e., whenever a free electron collides with the neutral gas molecule to form negative ion, the electron is absorbed by the neutral gas molecule. The attachment of the electron with the neutral gas molecule may occur in two ways:
The negative ions formed are relatively heavier as compared to free electrons and,
therefore, under a given electric field the ions do not attain sufficient energy to lead
cumulative ionization in the gas. Thus, these processes represent an effective way of
removing electrons from the space which otherwise would have contributed to form
electron avalanche. This property, therefore, gives rise to very high dielectric strength
for SF6.The gas not only possesses a good dielectric strength, but it has the unique
property of fast recombination after the source energizing the spark is removed. This
property of SF6 makes it very effective in quenching arcs.
SF6 is approximately 100 times as effective as air in quenching arcs. SF6 has excellent
heat transfer properties because its high molecular weight together with its low gaseous
viscosity enable it to transfer heat by convection more effectively than the common
gases. The arc is thus better interrupted by slowing down the electrons by cooling in
case of SF6 as the arc quenching medium. The thermal time constant of SF6 is low and
as a result the pressures At which it should be stored and used are relatively smaller as
compared to air. Also for the same limiting voltage the natural frequency of mains may
be greater (almost 100 times) in case of SF6 as compared to air because of lower time
constant of SF6. This means that SF6 breakers can withstand severe RRRV and thus
are most suitable for short line faults without switching resistors and can interrupt
capacitive currents with restriking.
𝗩𝗖𝗕 (𝗩𝗮𝗰𝗰𝘂𝗺 𝗰𝗶𝗿𝗰𝘂𝗶𝘁 𝗯𝗿𝗲𝗮𝗸𝗲𝗿)
A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called a vacuum interrupter.
A vacuum that is used as the arc quenching medium in a circuit breaker is known as a vacuum circuit breaker because vacuum gives high insulating strength due to superior arc quenching properties. This is suitable for most standard voltage applications because, for higher voltage, vacuum technology was developed however not commercially feasible. The operation of current-carrying contacts & related arc interruption take place within a vacuum chamber of the breaker, which is known as a vacuum interrupter. This interrupter includes a steel arc chamber within the center of symmetrically placed ceramic insulators. The maintenance of vacuum pressure within a vacuum interrupter can be done at 10– 6 arc. The vacuum circuit breaker performance mainly depends on the material used for current carrying contacts like Cu/Cr.
The vacuum circuit breaker working principle is, once the circuit breaker contacts are opened within the vacuum, then an arc can be generated among the contacts through the metal vapors ionization in the contacts. But, the arc can be quenched easily as the electrons, ions & metallic vapors are generated throughout the arc quickly condense over the outsides of the CB contacts, so the dielectric strength can be quickly recovered.
The most important feature of a vacuum is that once the arc is generated within the vacuum, then it can be extinguished quickly because of the quick improvement rate in the dielectric strength of the vacuum.
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