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Miniature circuit breaker
Thursday, 01 February 2007
Circuit breakers are electrical switching devices for protecting and controlling the electricity supply to respective electrical circuits. Circuit breakers protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The electrical power is then delivered from the panelboard to designated branch circuits supplying one or more loads. Typically, various types of circuit interrupters are connected to the branch circuits to reduce the risk of injury, damage or fires. Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. In the event an overcurrent condition occurs, electrical contacts within the circuit breaker will open, stopping the flow of electrical current through the circuit breaker to the equipment. Circuit breakers have an operating mechanism and trip means, such as a thermal trip assembly and/or magnetic trip assembly, which are automatically releasable to effect tripping operations and manually resettable following tripping operations. Overload protection is provided by a thermal element which, when heated by the increased current, will cause the circuit breaker to trip and interrupt the power. Use of circuit breakers is widespread in modern-day residential, commercial and industrial electric systems, and they constitute an indispensable component of such systems toward providing protection against over-current conditions. Various circuit breaker mechanisms have evolved and have been perfected over time on the basis of application-specific factors such as current capacity, response time, and the type of reset (manual or remote) function desired of the breaker.

Circuit breakers used in residential and light commercial installations are referred to as miniature circuit breakers (MCBs). Miniature circuit breakers are commonly used in the electrical consumer units of domestic dwellings and small industrial premises to protect and control the electrical supply to respective electrical circuits of the building. Miniature circuit breakers have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker, which can be easily and economically manufactured on a large scale. Miniature circuit breakers often are single pole breakers and are configured to be installed in a cabinet that houses a plurality of such miniature circuit breakers. Miniature circuit breakers typically include an electrical contact mounted on a movable contact carrier which rotates away from a stationary contact in order to interrupt the current path. Such circuit breakers have molded insulative housings of standard dimensions sized to interchangeably plug into or bolt onto the hot stabs in a load center or panel board. Typically, there is a fixed contact attached to the housing and a movable contact coupled to an operating mechanism. The operating mechanism includes a movable handle that extends outside of the housing. The handle has essentially three stable positions: on, off, and tripped. These three positions tell the operator what condition the contacts are in when the handle is viewed. The operating mechanism may be actuated to move the contacts into a second, open position. The trip mechanism is automatically releasable to effect tripping operations and manually resettable following tripping operations. The trip mechanism may include a thermal trip capability, which responds to persistent low level overcurrents, and/or a magnetic trip capability, which responds instantaneously to higher overload currents. The thermal-magnetic device includes a bimetal which heats up and bends in response to persistent overcurrent conditions to unlatch a spring powered operating mechanism which opens the contacts which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. For short circuit protection, an armature, which is attracted by the sizable magnetic forces generated in a magnetic core by a short circuit, unlatches, or trips, the operating mechanism.

Miniature circuit breakers may be utilized in conjunction with arc fault and/or ground fault trip mechanisms. Some miniature circuit breakers include a ground fault detection system that is operatively connected with the operating mechanism to open the contacts in the event of a line-to-ground fault or a neutral-to-ground fault. Such circuit breakers typically are referred to as ground fault circuit breakers. A common ground fault circuit used in such applications includes a toroidal coil (current transformer) for differential current sensing. The ground fault detection system of the ground fault circuit breaker typically includes a pair of spaced current transformers and a circuit board. The primaries of the current transformers are conductors that pass the current traveling through the circuit breaker through the central bores of the toroidal current transformers forming the secondary windings. The secondary windings of the current transformers are connected with the circuit board. A ground fault causes an imbalance which generates a signal in the winding of the toroidal current transformer. This signal is processed, typically by circuitry implemented on a printed circuit board, and energizes a solenoid which actuates the trip mechanism. The purpose of the ground fault detector in residential circuit breakers is to protect persons from electrical shocks and the accepted level for tripping is 5 ma of ground fault current. Molded case circuit breakers for light to medium industrial and commercial applications have a case molded from an electrically insulative resin. Such circuit breakers can be single pole or multi-pole. One pole circuit breakers of this type are typically three quarters to an inch thick. Two pole circuit breakers of this type have side-by-side compartments for the poles making them proportionally thicker, but still relatively thin. For applications where ground fault protection is desired, the ground fault circuitry is housed in one compartment and the mechanical pole in an adjacent compartment. A load terminal is provided in such a miniature molded case circuit breaker by a rigid conductor which extends from the separable contacts within the housing to a terminal recess molded in the outer surface of the housing.