Air circuit breaker

Monday, 15 January 2007

An air circuit breaker is an electric protecting apparatus which is installed between an electric source and load units in order to protect a load unit and a load line from an abnormal current generated on an electric circuit and to perform distribution function for changing the electric power line to another line. The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through overcurrent protection devices to designated branch circuits supplying one or more loads. Electrical power distribution systems and their components need protection from numerous types of malfunctions, including overcurrent conditions, overvoltage conditions, undervoltage conditions, reverse current flow, and unbalanced phase voltages. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire. Electrical distribution systems are often provided with circuit breakers, which open to break the circuit a prescribed time after detection of a malfunction condition. Circuit breakers are widely used in residential and industrial applications for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or by ground faults. Initially used as a direct replacement for fuses, circuit breakers have been gradually called upon to provide more sophisticated types of protection other than merely interrupting the circuit when the current flow exceeds a certain level. More elaborate time-current trip characteristics have been developed such that a circuit breaker can rapidly open upon very high current with the time delay being roughly inversely proportional to the degree of overload. Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Circuit breakers can interrupt an electric circuit due to a trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current.

Circuit breakers are mechanical switching devices capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal conditions such as those described above. Basically, a circuit breaker comprises a pair of separable contacts, an operating mechanism for effecting separation of the contacts, and a tripping mechanism which automatically releases the operating mechanism upon the occurrence of an overload or fault condition. The medium in which circuit interruption is performed may be designated by a suitable prefix, for example, air-blast circuit breaker, gas circuit breaker, oil circuit breaker, or vacuum circuit breaker. In operation, a circuit breaker is opened by energizing a trip coil associated with the circuit breaker. Once the trip coil is energized, the main contacts are opened and then auxiliary contacts are opened to prevent damage to the trip coil. Circuit breakers may include an electronic trip unit that senses electrical current to the protected equipment. If the sensed electrical current indicates an overcurrent situation, the electronic trip unit provides a trip signal to a trip actuator. In response to the trip signal, the trip actuator actuates a mechanical operating mechanism. Actuation of the mechanical operating mechanism by the trip actuator causes the mechanical operating mechanism to separate the electrical contacts, stopping the flow of current to the protected equipment. Circuit breakers are rated by voltage, insulation level, current interrupting capabilities, transient recovery voltage, interruption time, and trip delay. Circuit breakers may be designed for high quiescent currents and high withstand currents. To maintain a high withstand current rating, the contacts must be clamped closed at the current withstand rating. In addition to sensing current to the protected equipment, the electronic trip unit may also sense the voltage across the protected equipment. The sensed current and voltage can be used for the purpose of metering quantities including voltage, current, power, energy, and power factor. Circuit breakers for low voltage generally find application in the current range of upto 6400A. These circuit breakers are available in fixed and drawout versions. Drawout version facilitates easy maintenance and installation.

Air circuit breakers are commonly used in electrical distribution systems. Air circuit breakers include operating mechanisms that are mainly exposed to the environment. Since the air circuit breakers are rated to carry several thousand amperes of current continuously, the exposure to convection cooling air assists in keeping the operating components within reasonable temperature limits. A typical air circuit breaker comprises a component for connecting an electrical power source to electrical power consumer called a load. The component is referred to as a main contact assembly. A main contact is typically either opened, interrupting a path for power to travel from the source to the load, or closed, providing a path for power to travel from the source to the load. In many air circuit breakers, the force necessary to open or close the main contact assembly is provided by an arrangement of compression springs. In many air circuit breakers, the mechanism for controlling the compression springs comprises a configuration of mechanical linkages between a latching shaft and an actuation device. The actuation device may be manually or electrically operated. In a common construction of a low voltage air circuit breaker, the movable contact is mounted on a contact arm that is pivoted to open the contacts by a spring powered operating mechanism triggered by a trip unit responsive to an overcurrent condition in the protected circuit. Various accessory devices are used with such air circuit breakers to provide auxiliary function along with overcurrent protection. One such accessory is the bell alarm accessory that provides local and remote indication as to the occurrence of circuit interruption. The current interruption capability of air circuit breakers is dependent in part upon their ability to extinguish the arc that is generated when the breaker main contacts open. An air circuit breaker is divided into a fixed type fixedly installed between power source and a load and a drawer type in which a breaker is movable so as to be separated from power source and a load in order to facilitate maintenance and secure stability.

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