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A circuit breaker is a piece of equipment which is designed to protect an electrical apparatus from damage caused by overload or short circuit. Unlike a fuse which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Circuit breakers are often implemented with a solenoid (electromagnet) whose strength increases as the current increases and eventually trips the circuit breaker. Alternatively a bimetallic strip may be used which heats and bends with increased current. Some circuit breakers incorporate both techniques. This allows the properties of the circuit breaker to be tailored to suit the application, with the electromagnet generally responding to short, large surges in current (short circuit) and the bimetallic strip responding to smaller but longer-term (overload) overcurrent conditions.
Under short-circuit conditions a current of several thousand amps can flow (see maximum prospective short circuit current) and when a circuit breaker tries to interrupt this current an arc may form allowing the flow of current to continue even though the contacts of the circuit breaker are open. Some smaller circuit breakers incorporate a device which divides and extinguishes this arc. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulfur hexafluoride or have contacts immersed in oil to suppress the arc.
Circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.
Internal details of a European breaker
Photograph of the inside of a 10 amp European DIN-rail mounted thermal-magnetic miniature circuit breaker. Circuit breakers such as this are the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. Unfortunately while the size and shape of the opening in the front and its elevation from the rail are standardised the arrangements for busbar connections are not so you should take care that the breaker you select fits the busbar in your board and preferably is the same make and range.
- Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the on position. This is sometimes referred to as "free trip" or "positive trip" operation.
- Actuator mechanism - forces the contacts together or apart.
- Contacts - Allow current to flow when touching and break the flow of current when moved apart.
- Terminals
- Bimetallic strip
- Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
- Solenoid
- Arc divider / extinguisher
Common trip breakers
 Three pole common trip breaker for supplying a three-phase device. This breaker has a 2 Amp rating. Common trip breakers are usually purchased already-assembled into groups of 2, 3, or the like. When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two out-of-phase live wires. Three pole common trip breakers are typically used to supply three phase power to large motors or further distribution boards.
Types of circuit breaker
Front panel of a 1250A air circuit breaker. The breaker can be withdrawn from its housing for servicing. Trip characteristics are configurable via DIP switches on the front panel. - MCB - Miniature Circuit Breaker - rated current not more than 100A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
- MCCB - Moulded Case Circuit Breaker - rated current up to 1000A. Thermal or thermal-magnetic operation. Trip current may be adjustable.
- Air Circuit Breaker - Rated current up to 10,000A. Trip characteristics often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled - some models are microprocessor controlled. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
- Vacuum Circuit Breaker - With rated current up to 3000 A, these breakers interrupt the arc in a vacuum bottle. These can also be applied at up to 35,000 V. Vacuum breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
High Voltage Circuit Breakers
A 1200 Amp 3 Pole 115,000 V breaker at a generating station in Manitoba, Canada.
Electrical power transmission networks are protected and controlled by high voltage breakers. The definition of "high voltage" varies but in power transmission work is usually thought to be 35,000 V or higher. High voltage breakers are nearly always solenoid operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault.
High voltage breakers are broadly classified by the medium used to extinguish the arc.
- Oil-filled (dead tank and live tank)
- Oil-filled, minimum oil volume
- Air blast
- Sulfur hexafluoride
High voltage breakers are routinely available up to 765 kV AC.
Other Breakers
The following types are described in a separate article.
- RCCB - Residual Current Circuit Breaker - detects current leakage. Does NOT provide overcurrent protection.
- RCBO - Residual Current Breaker with Overcurrent protection - combines the functions of an RCCB and MCB in one package.
(In the United States, these are both referred to as "GFCIs" (Ground Fault Circuit Interrupters). The U.S. description leaves it unclear whether overcurrent protection is provided, but the standard U.S. practice is that the style that looks like a circuit breaker and mount into panelboards (distribution boards) also provides overcurrent protection while the style that looks like a wall outlet does not.)
See also
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