Circuit Breakers & Isolater
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Circuit breakerFrom Wikipedia, the free encyclopedia
For other uses, seeCircuit breaker (disambiguation).
An air circuit breaker for low voltage (less than 1000 volts) power distribution switchgear
A 2 pole miniature circuit breaker
Four 1 pole circuit breakers
A circuit breakeris an automatically operatedelectricalswitchdesigned to protect anelectrical circuitfrom damage
caused byoverloadorshort circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse,
which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to
resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual
household appliance up to largeswitchgeardesigned to protect high voltage circuits feeding an entire city.
http://en.wikipedia.org/wiki/Circuit_breaker_(disambiguation)http://en.wikipedia.org/wiki/Circuit_breaker_(disambiguation)http://en.wikipedia.org/wiki/Circuit_breaker_(disambiguation)http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Overcurrenthttp://en.wikipedia.org/wiki/Overcurrenthttp://en.wikipedia.org/wiki/Overcurrenthttp://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Switchgearhttp://en.wikipedia.org/wiki/Switchgearhttp://en.wikipedia.org/wiki/Switchgearhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Four_1_pole_circuit_breakers_fitted_in_a_meter_box.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:Jtecul.jpghttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/File:AEG_Circuit_breaker_type_ME_800_(1).JPGhttp://en.wikipedia.org/wiki/Switchgearhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Overcurrenthttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Circuit_breaker_(disambiguation) -
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Operation
All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current
rating and type of the circuit breaker.
The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit
breakers for large currents or high voltages are usually arranged withpilot devicesto sense a fault current and to operate the trip
opening mechanism. The tripsolenoidthat releases the latch is usually energized by a separate battery, although some high-voltage
circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using
something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the
energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated, larger units
havesolenoidsto 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 high ly
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 current is interrupted, anarcis generated. 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. Different circuit breakers usevacuum, air,insulating gas,
oroilas the medium the arc forms in. Different techniques are used to extinguish the arc including:
Lengthening / deflection of the arc
Intensive cooling (in jet chambers)
Division into partial arcs
Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at
the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz and 120 times per
second for 60 Hz AC)
Connectingcapacitorsin parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.
Types of circuit breakers
Low voltage circuit breakers
Magnetic circuit breakers
Thermal magnetic circuit breakers
Common trip breakers
Medium-voltage circuit breakers
High-voltage circuit breakers
Sulfur hexafluoride (SF6) high-voltage circuit-breakers
Disconnecting circuit breaker (DCB)
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Disconnector (Isolater)
High voltage disconnector.
Inelectrical engineering, a disconnectororisolator switch ordisconnect switch[1]
is used to make sure that an
electrical circuit can be completely de-energised for service or maintenance. Such switches are often found inelectrical
distributionandindustrialapplications where machinery must have its source of driving power removed for adjustment or
repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit
breakersandtransformers, and transmission lines, for maintenance. Often the isolation switch is not intended for normal
control of the circuit and is used only for isolation; in such a case, it functions as a second, usually physically distant
master switch (wired inserieswith the primary one) that can independently disable the circuit even if the master switch
used in everyday operation is turned on.
Isolator switches have provisions for apadlockso that inadvertent operation is not possible (see:Lockout-Tagout). In
high voltage or complex systems, these padlocks may be part of a trapped-key interlock systemto ensure proper
sequence of operation. In some designs the isolator switch has the additional ability to earththe isolated circuit thereby
providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems
where both end of the circuit need to be isolated.
The major difference between an isolator and acircuit breakeris that an isolator is an off-loaddevice intended to be
opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent
any attempt to open the disconnector while it supplies a circuit.
Standards in some countries for safety may require either local motor isolators or lockable overloads (which can be
padlocked).
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