Patent Publication Number: US-6215379-B1

Title: Shunt for indirectly heated bimetallic strip

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings, wherein like elements are numbered alike in the several figures: 
     FIG. 1 is a side view of a circuit breaker including a shunt of the present invention; 
     FIG. 2 is a perspective view of the shunt of FIG. 1; 
     FIG. 3 is a cross-sectional elevation view of the shunt of FIG. 1; 
     FIG. 4 is an enlarged view of the reduced thickness area of the shunt of FIG. 3; 
     FIG. 5 is a side elevation view similar to FIG.  3  and showing a bimetallic strip attached to the shunt; and 
     FIG. 6 is a is a graph showing circuit breaker trip time as a function of rated current for comparison of the present invention with the prior art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an embodiment of a circuit breaker, generally shown at  10 , includes a thermal trip unit  12 . Circuit breaker  10  is electrically connected to an electrical distribution circuit (not shown) via line and load side connections  14 ,  16  to provide overcurrent protection to the distribution circuit. Circuit breaker  10  includes a pair of moveable contacts  18 ,  20 , disposed on opposite ends of rotating contact arm  22 . The moveable contacts  18 ,  20  are in opposing alignment to fixed contacts  24 ,  26  respectively. The rotating contact arm  22  is mounted pivotally to the circuit breaker frame at  28 . The rotating contact arm  22  engages a circuit breaker operating mechanism  30  at a pair of pivotal engagements  32 ,  34  that are interposed between the moveable contacts  18 ,  20 . 
     The thermal trip portion  12  includes a bimetallic strip  34  having one end attached to a shunt  36  by a rivet  38 . While a rivet  38  is shown for connecting bimetallic strip  34  to shunt  36 , bimetallic strip  34  may be connected to shunt (heater strap)  36  by brazing, screws, or by any other means known in the art. Shunt  36  is electrically connected to a contact strap  40  at one end of shunt  36 . The other end of shunt  36  forms load-side connection  16 , which is electrically connected to the electrical distribution circuit. 
     The operating mechanism  30  includes a series of linkages and levers for interconnecting the rotating contact arm  22  and the thermal trip unit  12 . Lever  42  cooperates with the thermal trip unit  12  to actuate a trip latch  44  of operating mechanism  30  and separate the movable contacts  18 ,  20  from the fixed contacts  24 ,  26 . 
     The bimetallic strip  34  provides the thermal trip for an overcurrent condition. Increased current generates heat in the shunt  36  which further heats-up the bimetallic strip  34 . When the temperature of the bimetallic strip  34  exceeds a predetermined set point, the free end of the bimetallic strip  34  deflects to engage lever  42 , which releases the trip latch  44  of operating mechanism  30 . Operating mechanism  30  then separates the movable contacts  18 ,  20  from the fixed contacts  24 ,  26  to interrupt the current and, thereby, protect the load side of the distribution circuit from the overcurrent condition. 
     FIG. 2 is a perspective view of shunt  36 . Shunt  36  is constructed of electrical and heat conducting material such as copper or aluminum and is formed in a desired shape depending on the circuit breaker in which it is to be used. Preferably, shunt  36  is constructed of a copper material with some copper derivative such as titanium, brass, tin, or chromium. As shown, shunt  36  has a generally vertical main body portion  50 , an upper generally horizontal section  52 , a lower generally horizontal section  54 , and load-side connection section  16 , which is generally horizontal. Upper section includes an aperture  56  formed on a tab  58  extending from upper section  52 , allowing connection between shunt  36  and contact strap  40  (FIG.  1 ). Main body section  50  includes elongated slots  60  and apertures  62  disposed in a central portion thereof. Aperture  62  allow for a rivet connection between shunt  36  and bimetallic strip  34  (FIG.  1 ). Elongated slots  60  help to increase the temperature of shunt  36  at a location between the elongated slots  60 . Lower section  54  includes an aperture  64  formed in a central portion thereof and slots  66  extending from side edges thereof. Aperture  64  and slots  66  allow for mounting of shunt  36  within the circuit breaker. An aperture  68  formed in load-side section  16  allows for connection with a phase of an electrical distribution circuit. The overall shape shown in the drawings is illustrative and is not required for the invention. Tab  58 , apertures  56 ,  62 ,  64 ,  68 , and slots  60 ,  66  are optional. Such tabs, apertures, slots and the like may be added or removed depending on the circuit breaker in which shunt  36  is to be used. 
     The thickness “t” of the material forming shunt  36  is essentially constant throughout the entire extent of shunt  36  except in the area  70  defined between lines A and B. Area  70  extends the entire width of heater strap  10 . As is best seen in the cross-sectional view of shunt  36  shown in FIG. 3, the thickness “r” of the shunt in area  70  is reduced to a thickness in the range of 20% to 80% of the thickness “t”. 
     FIG. 4 is an enlarged view of the reduced thickness section  70  of shunt  36 . In a preferred embodiment, the transition zones  100  from the full thickness “t” parts of the shunt to the reduced thickness “r” section  70  are gradual slopes. However, shunt  10  may also be constructed with no transition zones  100 . That is, the transition from full thickness “r” to reduced thickness section  70  is a sharp decrease. The distance from full thickness point A to full thickness point B is designated by “y”. Also, the thickness “r” of the fully reduced thickness section  18  is equal to t−x, where “x” is the amount of conductive material removed from the full thickness “t” of the shunt. Bimetallic strip  34 , shown in phantom, contacts a surface  102  of reduced thickness section  70  of shunt  36 . Surface  102  is formed on a side of shunt  36  opposite the side from which conductive material is removed. Shunt  36  and strip  34  are in contact over a distance “z” along surface  102 . Conductive heat transfer from shunt  36  to bimetallic strip  34  is made across this portion of surface  102 . It can be seen that the distance “y” and the distance “z” are overlapping. That is, a portion of the reduced thickness section  70  (A-B) is in contact with bimetallic strip  34 . In the embodiment shown, the distance “y” is approximately equal to the distance “z”. However, the distance “y” can range from 3% to 200% of the distance “z”. 
     FIG. 5 is a side view of a bimetallic strip  34  attached to shunt  36  at the reduced thickness area  70 . The full-line position of bimetallic strip  34  shown in FIG. 5 is the unheated or low level heat condition commensurate with no current flow through shunt  36 . Bimetallic strip  34  is normally spaced a predetermined distance “d” from arm  42  of the circuit breaker operating mechanism  30  (see FIG.  1 ). When electrical current flows through shunt  36 , heat from shunt  36  transfers to bimetallic strip  34  via the connection between shunt  36  and bimetallic strip  34  at area  70 . When the temperature of the bimetallic strip  34  reaches a predetermined limit, the bimetallic strip  22  deflects from the full line position to the dashed line position to contact arm  58 , thereby causing the circuit breaker to open and prevent a circuit overload. The amount of heat, and hence the degree of deflection of bimetallic strip  34 , is a function of the temperature distribution through shunt  36 . 
     The addition of reduced section  70  to shunt  36  results in a “hot spot” of increased localized temperature in the shunt at section  70 . This increased temperature translates directly into an increase in the deflection of bimetallic strip  34  for any given current level. This increased temperature and increased deflection occur for both steady state and transient current flow in shunt  36 . The increased temperature is localized to reduced section  70 , and lower temperatures prevail in the remainder of shunt  36 . Thus, the shunt of the present invention is a clear improvement over the prior art in that the shunt of the present invention reduces the temperature hot spots at the extreme ends of the shunt and contains the hot spot in a preferred location. 
     The increased deflection of bimetallic strip  34  resulting from the increased temperature of hot spot  70  results in a greater range of deflection and/or a greater actuating force for a given current flow. Therefore, the steady-state distance “d” between the bimetallic strip  34  and arm  42  can be increased. This reduces nuisance tripping. Also, the localized hot spot of the reduced section  70  has the unexpected result of reducing trip time on first operation and in surge conditions. 
     FIG. 6 is a graph showing circuit breaker trip time as a function of rated current for various shunt designs. Multiples of a 250 amp rms rated current are plotted on the X axis, and trip time in seconds is plotted on the Y axis. Curve  4  represents the trip time for a prior art shunt having a uniform thickness of 1.8 to 2.2 millimeters. Curve  3  represents the trip time for a shunt of the present invention having a thickness of 1.8 to 2.2 millimeters, a dimension “y” (as shown in FIG. 4) of 6 millimeters, and a dimension “x” (as shown in FIG. 4) of 0.5 millimeters. Curve  2  represents the trip time for a shunt of the present invention having a thickness of 1.8 to 2.2 millimeters, a dimension “y” (as shown in FIG. 4) of 6 millimeters, and a dimension “x” (as shown in FIG. 4) of 1 millimeter. Curve  1  represents the trip time for a shunt of the present invention having a thickness of 1.8 to 2.2 millimeters, a dimension “y” (as shown in FIG. 4) of 8 millimeters, and a dimension “x” (as shown in FIG. 4) of 1 millimeter. All of the shunts represented by curves  1 - 4  are constructed of the same material. The chart of FIG. 5 shows that the shunt of the present invention is a clear improvement over the prior art in that the shunt of the present invention reduces the amount of time to trip the breaker on an overcurrent condition. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.