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| CIRCUIT BREAKER |
A circuit
breaker is an automatically operated electrical switch designed to protect an
electrical circuit from damage caused by excess current from an overload or
short circuit. Its basic function is to interrupt current flow after a fault is
detected.
OR
A circuit
breaker is a switching device that interrupts the abnormal or fault current. It
is a mechanical device that disturbs the flow of high magnitude current and
performs as a switch.
Circuit
breakers are mainly used for closing and opening of an electrical circuit and
thus protects the circuit from damage.
A small history of circuit breaker
An early form of circuit breaker was described by
Thomas Edison in an 1879 patent application, although his commercial power
distribution system used fuses. Its purpose was to protect lighting circuit wiring
from accidental short circuits and overloads. A modern miniature circuit
breaker similar to the ones now in use was patented by Brown, Boveri & Cie
in 1924. Hugo Stotz, an engineer who had sold his company to BBC, was credited
as the inventor on DRP (Deutsches Reichspatent) 458392. Stotz's invention was
the forerunner of the modern thermal-magnetic breaker commonly used in
household load centers to this day.
Interconnection
of multiple generator sources into an electrical grid required the development
of circuit breakers with increasing voltage ratings and increased ability to
safely interrupt the increasing short-circuit currents produced by networks.
Simple air-break manual switches produced hazardous arcs when interrupting high
voltages; these gave way to oil-enclosed contacts, and various forms using the
directed flow of pressurized air, or of pressurized oil, to cool and interrupt
the arc. By 1935, the specially constructed circuit breakers used at the
Boulder Dam project use eight series breaks and pressurized oil flow to
interrupt faults of up to 2,500 MVA, in three cycles of the AC power frequency.
Block diagram of a circuit breaker
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Operating principle or working principle
The circuit breaker must first detect a fault condition. In small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformers, protective relays, and an internal control power source.
Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; this is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts. Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Small circuit breakers typically have a manual control lever to switch off the load or reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load
current without excessive heating, and must also withstand the heat of the arc
produced when interrupting (opening) the circuit. Contacts are made of copper
or copper alloys, silver alloys and other highly conductive materials. Service
life of the contacts is limited by the erosion of contact material due to
arcing while interrupting the current. Miniature and molded-case circuit
breakers are usually discarded when the contacts have worn, but power circuit
breakers and high-voltage circuit breakers have replaceable contacts.
When a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity (or heat) is proportional to the current. This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit.
In short the working principle of circuit breaker is circuit
breaker essentially consists of fixed and moving contacts. These contacts are
touching each other and carrying the current under normal conditions when the
circuit is closed. When the circuit breaker is closed, the current carrying contacts,
called the electrodes, engaged each other under the pressure of a spring.
During the
normal operating condition, the arms of the circuit breaker can be opened or
closed for a switching and maintenance of the system. To open the circuit
breaker, only a pressure is required to be applied to a trigger.
Whenever a
fault occurs on any part of the system, the trip coil of the breaker gets
energized and the moving contacts are getting apart from each other by some
mechanism, thus opening the circuit.
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