The present invention relates to a thermal overload/open-phase tripping device mounted on a circuit breaker such as an auto-breaker. 
With an auto-breaker as an example, FIGS.  5(a) and 5(b) show a structure of a circuit breaker with a thermal (bimetal type) overload/open-phase tripping device mounted thereon.  A conventional thermal overload/open-phase tripping device has a structure shown in FIG.  6 (refer to Japanese Patent Publication (Kokai) No.  2002-298723). 
In FIGS.  5(a) and 5(b), reference numeral 1 denotes a case of the circuit breaker; reference numeral 2 denotes a power supply side terminal; reference numeral 3 denotes a load side terminal; reference numeral 4 denotes a switching operation handle; reference numeral 5 denotes a switching mechanism; reference numeral 6 denotes a current cutoff section formed of movable and stationary contacts and an arc-extinguishing chamber; reference numeral 7 denotes an electromagnetic instantaneous tripping device; and numeral 8 denotes a thermal overload/open-phase tripping device. 
As shown in FIG.  6, the thermal overload/open-phase tripping device 8 is formed of three main bimetallic elements 9, a differential shifter mechanism, and a tripping lever 13.  The main bimetallic elements 9 correspond to respective phases (R, S, and T phases) of a main circuit, and are arranged in a row in the lateral direction, i.e. a direction of bending and restoration thereof.  The differential shifter mechanism is formed of a push shifter 10, a pull shifter 11, and a differential lever 12, and contacts the main bimetallic elements 9.  The tripping lever 13 functions as a compensating bimetallic element, and transmits a force due to an operation of the differential shifter mechanism to a latch receiver 5a incorporated in the switching mechanism 5, thereby tripping the switching mechanism 5. 
The push shifter 10 and the pull shifter 11 of the differential shifter mechanism are arranged with the main bimetallic elements 9 in between, and are slidable along a row of the main bimetallic elements 9 in a direction that the main bimetallic elements 9 bend and restore.  The push shifter 10 has arms 10a projecting in a direction perpendicular to the row of the main bimetallic elements 9 at positions corresponding to the main bimetallic elements 9, respectively.  The pull shifter 11 has arms 11a projecting in a direction perpendicular to the row of the main bimetallic elements 9 at positions corresponding to the main bimetallic elements 9, respectively.  With this structure, the main bimetallic elements 9 are held between the arms 10a and 11a at the positions corresponding to the main bimetallic elements 9 in the direction that the main bimetallic elements 9 bend and restore. 
The differential lever 12 is connected to the push shifter 10 and the pull shifter 11.  More specifically, the differential lever 12 has one end at a side of the pull shifter 11 pivotally supported on an upper surface of the pull shifter 11 through a coupling pin 14.  The differential shifter 12 is connected to the push shifter 10 through a link pin 15 fitted into a guiding long hole 10b (long hole extending in a direction perpendicular to a sliding direction of the shifter) formed in the push shifter 10 with a plate shape.  The differential lever 12 has an action end 12a projecting toward and facing the tripping lever 13 at the other end thereof opposite to the coupling pin 14. 
The tripping lever 13 has a pivot 13a on a center line thereof, so that the tripping lever 13 is supported to be rotatable around the pivot 13a as a lever device.  The tripping lever 13 has one end facing the action end 12a of the differential lever 12 and the other end facing the latch receiver 5a of the switching mechanism 5. 
Japanese Patent Publication (Kokai) No.  2002-298723 has described an operation of the overload/open-phase tripping device 8 in detail.  When an overload current flows in the main circuit while supplying power to a load, the main bimetallic elements 9 of the respective phases bend in a specific direction.  As a result, as shown in FIG.  7, the main bimetallic elements 9 push the push shifter 10 to move in a direction of leftward arrows.  Then, the differential lever 12 connected to the push shifter 10 through the link pin 15 fitted in the guiding long hole 10a follows the push shifter 10 to move in the direction of the leftward arrows, so that the movement is transmitted to the tripping lever 13 through the action end 12a.  
In the process of the movement, the pull shifter 11 connected to the differential lever 12 through the coupling pin 14 moves in the arrow direction while following the movement of the push shifter 10.  Accordingly, the differential lever 12 moves in parallel in the arrow direction while maintaining an initial posture to push the tripping lever 13 leftward with a thrust force f1.  When the tripping lever 13 rotates clockwise around the pivot 13a to press the latch receiver 5a to an open position, the switching mechanism 5 (see FIG.  5) performs a tripping operation, thereby opening the main circuit contacts of the current cutoff section 6 to shut off the overload current. 
When an open-phase occurs in the main circuit (S phase, for example) while power is supplied to a load, the overload/open-phase tripping device 8 operates as follows with reference to FIG.  8.  When the S phase becomes the open-phase, a temperature of the main bimetallic element 9 corresponding to the S phase returns to a room temperature.  As a result, the main bimetallic element 9 returns to an initial state (non-energized state) with no deformation to push back the pull shifter 11 in the rightward arrow direction. 
Since currents continue to flow in the R and T phases, the main bimetallic elements 9 corresponding to the R and T phases keep pushing the push shifter 10 in the leftward arrow direction.  Accordingly, the differential lever 12 rotates clockwise around the coupling pin 14, so that the action end 12a pushes the tripping lever 13 leftward with a thrust force f2.  As a result, similar to the tripping operation upon the overload shown in FIG.  7, the tripping lever 13 drives the latch receiver 5a into the open position, so that the circuit breaker performs the tripping operation. 
The overload/open-phase tripping device with the conventional structure described above has the following functional problems.  In the operation, upon the overload shown in FIG.  7, the main bimetallic elements 9 in the R, S and T phases cooperatively push the push shifter 10.  The differential lever 12 follows the main bimetallic elements 9, and the action end 12a pushes the tripping lever 13 with the thrust force f1.  On the other hand, in the operation, upon the open-phase shown in FIG.  8, the main bimetallic elements 9 in the R and T phases push the push shifter 10, and the main bimetallic element 9 in the S phase does not push.  Accordingly, the action end 12a of the differential lever 12 pushes the tripping lever 13 with the force f2 smaller than the force f1 in the operation upon the overload by about 40% (f1>f2). 
Further, the differential lever 12 applies a force to the tripping lever 13 at the same action point in the operations upon both the overload and the open-phase.  As a result, a moment of the force applied to the tripping lever 13 is L1×f1 upon the overload and L1×f2 upon the open-phase, wherein L1 is a length between the pivot 13a and the action point (action end 12a of the differential lever 12; see FIG.  6).  Accordingly, the operation upon the overload generates the moment larger than that in the operation upon the open-phase. 
When the switching mechanism 5 performs the tripping operation; it is necessary to apply a specific constant load to the latch receiver 5a.  Incidentally, in the operation upon the open-phase, the action end 12a of the differential lever 12 moves for a distance greater than that in the operation upon the overload.  Accordingly, in the conventional tripping device, in order to obtain a stable tripping operation of the circuit breaker upon the open-phase and the overload, it is necessary to increase a size of the main bimetallic elements 9 so that the differential shifter mechanism can apply a sufficient force to the tripping lever 13.  Further, it is necessary to manufacture and assemble the differential shifter mechanism with high accuracy and provide fine adjustment. 
However, when the main bimetallic elements in the circuit breaker have a large size, it is difficult to reduce a size of the circuit breaker.  Further, the differential shifter mechanism needs to be produced and assembled with high accuracy, thereby increasing cost. 
In view of the problems described above, the invention has been made, and an object of the invention is to provide an overload/open-phase tripping device for a circuit breaker in which a differential shifter mechanism is optimized to generate a proper force with high efficiency, thereby obtaining a stable tripping operation upon the overload and the open-phase.