Electricity: magnetically operated switches – magnets – and electr – Electromagnetically actuated switches – Automatic circuit-interrupting devices
Reexamination Certificate
1999-12-23
2001-04-10
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Electromagnetically actuated switches
Automatic circuit-interrupting devices
C335S172000
Reexamination Certificate
active
06215379
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the subject of shunts for indirectly heated bimetallic strips. While especially suitable for use in circuit breakers, the shunt of this invention is useful for heating any bimetallic strip.
Circuit breakers employing indirectly heated bimetallic strips are well known. A shunt, or heater strap, is attached to one end of a bimetallic strip via brazing, rivets, or screws. Electrical current from a distribution circuit passes through the shunt. When an overcurrent condition occurs, the shunt generates heat, which is transferred to the bimetallic strip across the junction of the shunt and the bimetallic strip. The bimetallic strip is formed of two metals having different coefficients of expansion such that a free end of the bimetallic strip bends or deflects when the temperature of the bimetallic strip exceeds a predetermined temperature. If the temperature of the bimetallic strip exceeds the predetermined value, the free end of the bimetallic strip deflects to actuate a linkage interconnected to a pair of separable contacts within the circuit breaker. The linkage then opens the pair of contacts to interrupt the current and, thereby, protect a load from the overcurrent condition.
Circuit breakers employing such indirectly heated bimetallic strips are well known. However, it is desirable to reduce the response time in obtaining the desired temperature distribution through the shunt and bimetallic strip and, thereby, reduce the amount of time to trip the breaker on an overcurrent condition. It is also desirable to reduce or eliminate the temperature hot spots at the extreme ends of the shunt. Attempts have been made in the prior art to address these deficiencies, such as by creating circular, rectangular or slotted openings in the shunt. While effective to some degree, these prior art approaches still leave room for improvement.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention, a shunt for a bimetallic strip is formed from a length of electrical and heat conductive material having a thickness of “t” throughout most of its length. A section of reduced thickness in the length of electrical and heat conductive material has a thickness ranging from 20% to 80% of the thickness “t”. This reduced thickness section produces a localized hot area, which decreases the time required to reach a predetermined temperature in both the shunt, at this localized hot spot, and in the bimetallic strip, and reduces the trip time of the rated circuit. The localized hot spot in the shunt results in increased temperatures along the bimetallic strip. This, in turn, increases the deflection of the bimetallic strip, for greater actuating force or greater range of movement. As a result of the greater range of movement, the gap between the bimetallic strip and the circuit breaker trip bar can be increased to reduce nuisance tripping.
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Arvidson Alan
Fulton James
O'Keefe Thomas
Cantor & Colburn LLP
Donovan Lincoln
General Electric Company
Horton Carl B.
Nguyen Tuyen T.
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