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Circuit breaker

From Academic Kids

For the character named Circuit Breaker in the Marvel Transformers comics, see Neo-Knights.

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. Circuit breakers for larger currents are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.

Under short-circuit conditions a current of many times greater than normal can flow (see maximum prospective short circuit current). 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. Circuit breakers incorporate features to divide and extinquish the arc. In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and blowout coils deflect the arc into the arc chute. 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. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a higher prospective short-circuit current may result in failure of the breaker to safely interrupt a fault.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. Power circuit breakers are built into switchgear cabinets. High-voltage breakers may be free-standing outdoor equipment or a component of a gas-insulated switchgear line-up.

Contents

Internal details of a European breaker

Photograph of the inside of a 10 ampere 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.
Photo of inside of a circuit breaker
  1. 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.
  2. Actuator mechanism - forces the contacts together or apart.
  3. Contacts - Allow current to flow when touching and break the flow of current when moved apart.
  4. Terminals
  5. Bimetallic strip
  6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
  7. Solenoid
  8. Arc divider / extinguisher

Common trip breakers

Missing image
Breaker3phase2a_proc.jpg
Three pole common trip breaker for supplying a three-phase device. This breaker has a 2  A rating. Common trip breakers are usually purchased already-assembled into groups of two, three, 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 120/240 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

Missing image
LargeCircuitBreaker.jpg
Front panel of a 1250 A air circuit breaker. The breaker can be withdrawn from its housing for servicing. Trip characteristics are configurable via DIP switches on the front panel.
There are many different technologies used in circuit breakers and they do not always fall into distinct categories. The following types are common in domestic, commercial and light industrial applications for low voltage (less than 1000 V) use.
  • MCB (Miniature Circuit Breaker)—rated current not more than 100 A. 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 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable.
  • Air Circuit Breaker—Rated current up to 10,000 A. 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

Missing image
Circuit_Breaker_115_kV.jpg
A 1200 A 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 separate articles.

  • Breakers for protections against earth faults too small to trip an overcurrent device:
    • RCD—Residual Current Device (formerly known as a Residual Current Circuit Breaker) - detects current imbalance. Does NOT provide overcurrent protection.
    • RCDO—Residual Current Device with Overcurrent protection - combines the functions of an RCD and an MCB in one package.
      (In the United States, RCDs and RCBOs 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 devices of the style that looks like a circuit breaker and mount into panelboards (distribution boards) also provides overcurrent protection while devices of the style that looks like a wall outlet does not.)
    • ELCB—Earth leakage cuircuit breaker. This detected earth current directly rather than detecting imbalance. They are no longer seen in new insallations for various reasons.
  • Autorecloser A type of circuit breaker which closes again after a delay. These are used on overhead power distribution systems, to prevent short duration faults from causing sustained outages.

External links

  • [1] (http://www.cutler-hammer.eaton.com/unsecure/html/101basics/Module05/Output/)Fundamentals of Circuit Breakers

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