CIRCUIT BREAKERSElectrical Distribution SystemsA cutaway view of the molded case circuit breaker is shown in Figure 5.Figure 5 Cutaway View of Molded Case Circuit BreakerA circuit can be connected or disconnected using a circuit breaker by manually moving theoperating handle to the ON or OFF position. All breakers, with the exception of very small ones,have a linkage between the operating handle and contacts that allows a quick make (quick breakcontact action) regardless of how fast the operating handle is moved. The handle is also designedso that it cannot be held shut on a short circuit or overload condition. If the circuit breaker opensunder one of these conditions, the handle will go to the trip-free position. The trip-free positionis midway between the ON and OFF positions and cannot be re-shut until the handle is pushedto the OFF position and reset.A circuit breaker will automatically trip when the current through it exceeds a pre-determinedvalue. In lower current ratings, automatic tripping of the circuit breaker is accomplished by useof thermal tripping devices. Thermal trip elements consist of a bimetallic element that can becalibrated so that the heat from normal current through it does not cause it to deflect. Anabnormally high current, which could be caused by a short circuit or overload condition, willcause the element to deflect and trip the linkage that holds the circuit breaker shut. The circuitbreaker will then be opened by spring action. This bimetallic element, which is responsive tothe heat produced by current flowing through it, has an inverse-time characteristic. If anextremely high current is developed, the circuit breaker will be tripped very rapidly.ES-15 Page 8 Rev. 0
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