Electricity: magnetically operated switches – magnets – and electr – Electromagnetically actuated switches – Automatic circuit-interrupting devices
Reexamination Certificate
2000-02-10
2001-05-29
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Electromagnetically actuated switches
Automatic circuit-interrupting devices
C335S023000, C335S172000
Reexamination Certificate
active
06239677
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to circuit breakers and more particularly to a circuit breaker employing a thermal-magnetic trip unit having an over centering mechanism for unlatching the circuit breaker operating mechanism and a trip flag system that discriminates between a short circuit trip and an overcurrent trip.
Circuit breakers typically provide protection against persistent overcurrent and against very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a thermal-magnetic trip unit having a thermal trip portion, which trips the circuit breaker on persistent overcurrent conditions, and a magnetic trip portion, which trips the circuit breaker on short-circuit conditions.
In order to trip the circuit breaker, the thermal magnetic trip unit must activate an operating mechanism. Once activated, the operating mechanism separates a pair of main contacts to stop the flow of current in the protected circuit. Conventional trip units act directly upon the operating mechanism to activate the operating mechanism. In current thermal-magnetic trip unit designs, the thermal trip portion includes a bimetallic strip (bimetal), which bends at a predetermined temperature. The magnetic trip portion includes an anvil disposed about a current carrying strap and a lever disposed near the anvil, which is drawn towards the anvil when high, short-circuit currents pass through the current carrying strap. The force created by the bimetal or lever, and the distance that they travel, may be insufficient to directly trip the operating mechanism. A conventional way to solve this problem is to use a latch system as a supplemental source of energy. However, the drawback of a latch system is the use of latching surfaces, which degenerate over repeated use.
Further, a circuit breaker having a thermal-magnetic trip unit can be tripped by three events, namely: overcurrent, short circuit and ground fault. It is important to know the cause due to which a breaker has tripped. Distinguishing the reasons for tripping allows the user to determine if the breaker can be reset immediately, as in the case of an overcurrent, or only after careful inspection of the circuitry, as in the case of a short circuit or ground fault.
Circuit breaker trip mechanisms of the prior art have solved this problem by the use of flags, which are visible through windows disposed in the case of the circuit breaker. In such trip mechanisms, a flag appears in one window upon the occurrence of an overcurrent condition, while another flag appears in another window upon the occurrence of a short-circuit condition. This solution works well for trip units having an inactive bimetal. That is, for trip units where the bimetal does not carry electrical current, but is attached to a current-carrying strap. However, this solution can provide indeterminate indications when it is used with a trip unit having an active bimetal. That is, when it is used with a trip unit where the bimetal carries electrical current. When such an active bimetal is used, it is possible during a short circuit event that, in addition to the magnetic trip portion, the bimetal also moves to expose the overcurrent flag, thereby leading to both the short-circuit and overcurrent flags being shown thus providing an indeterminate indication to the user.
SUMMARY OF INVENTION
In an exemplary embodiment of the present invention, a circuit breaker trip mechanism includes an over centering spring tripping linkage. The trip unit consists of a trip bar having a first leg and a second leg. The trip bar is rotatably mounted within the case about a first pivot where the first leg is adjacent to a bimetal mounted within the circuit breaker trip mechanism. A link, having a third leg and a fourth leg, is rotatably mounted within the case about a second pivot. The second leg is pivotally engaged to the third leg of the link by a moveable pin which slides in a slot in the trip bar. The fourth leg of the link is pivotally engaged to a slide by a moveable pin. A slide projection extending outward from the slide is disposed between the first end and the second end of the slide. Further, the link is biased in a first direction about second pivot when the trip unit is in a reset condition and biased in a second direction about pivot when the trip bar is rotated about first pivot thereby urging the slide to interact with the trip lever of the circuit breaker operating mechanism.
In a further exemplary embodiment of the present, an improved indication-of-trip system is employed comprising a two-piece trip bar mechanism. In this embodiment of the invention, visual confirmation of the cause of the trip is provided. This embodiment includes a second trip bar having a fifth and sixth leg. The second trip bar is rotatably mounted within the case about a third pivot. A second link, having a seventh leg and an eighth leg, is rotatably mounted within the case about a fourth pivot. The sixth leg is pivotally engaged to the seventh leg of the second link by a moveable pin. The eighth leg of the second link is pivotally engaged to a second slide by a moveable pin. A slide projection extending outward from the second slide is disposed between the third end and the fourth end of the second slide. Further, the second link is biased in a first direction about the fourth pivot when the trip unit is in a reset condition and biased in a second direction about the fourth pivot when the second trip bar is rotated about the third pivot thereby urging the second slide to interact with the trip lever of the circuit breaker operating mechanism.
The circuit breaker casein this embodiment of the invention includes a window disposed in the case in a location conducive to a user viewing a position indicator thus enabling the rapid determination of the type of trip that has occurred. To identify a trip caused by an overcurrent condition, an overcurrent indicator is employed with the first trip bar whereby the indicator senses the bimetallic force applied on the heat sensitive bimetal. To identify a trip caused by a short circuit condition, a short circuit indicator is employed with the second trip bar whereby the indicator senses the magnetic force applied to the improved indicator of trip bar system.
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Castonguay Roger
Ramakrishnan Bhaskar T.
Cantor & Colburn LLP
Donovan Lincoln
General Electric Company
Horton Carl B.
Nguyen Tuyen T.
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