Patent Publication Number: US-6664884-B1

Title: Dual-circuit switch structure with overload protection

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a switch, and in particular to a dual-circuit switch having an overload protection mechanism for operation safety. 
     BACKGROUND OF THE INVENTION 
     A switch is operable between an ON (connected) state and an OFF (disconnected) state for control of power supply or electrical signal transmission. For a power switch, overheating and burning caused by overload resulting from undesired shorting is one of the major concerns of operation safety. Some switches available in the market are provided with safety mechanism that automatically cuts off power supplied therethrough in order to eliminate the potential risk of overheating and burning. Such switches, however, have complicated structures, making costs high and manufacture difficult. 
     The electricity system of some areas, such as Europe, is a dual-circuit system comprised of two electrical circuits individually and independently supplying power to an electric appliance. With the conventional overload protection mechanism, when an overload occurs, it is very likely that only one of the two circuits is open while the other one still maintains the electrical supply. This leads to some disadvantages: 
     (1) Since the power supplied through the switch is maintained by the circuit that is not broken by the overload protection mechanism, risk caused by overloading of the electrical appliance to which the power is supplied cannot be properly controlled. 
     (2) Operators that intend to resume supply of electricity by release the overload protection mechanism may be electrically shocked if the circuit that is still maintained is not cut off first. 
     (3) If the circuit that is open due to overload is not timely resumed its operation condition, power supplied to the electrical appliance through the switch may not be sufficient to properly operate the electrical appliance and thus causing undesired problems. 
     It is thus desirable to have a dual-circuit switch structure having an overload protection mechanism that overcomes the above problems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a dual-circuit switch comprising an overload protection mechanism that operates to opens both circuits simultaneously in order to completely cut off power supplied through the switch. 
     Another object of the present invention is to provide a dual-circuit switch comprising an overload protection mechanism that ensures operation reliability in cutting off power supplied through the switch and eliminates incorrect operation caused by material fatigue. 
     A further object of the present invention is to provide a dual-circuit switch comprising an overload protection mechanism that can be adjusted to provide best response of the overload protection mechanism. 
     To achieve the above objects, in accordance with the present invention, there is provided a switch comprising a casing inside which two electrically isolated circuits are arranged. Each circuit comprises first and second conductive blades fixed inside the casing. A conductive strip made of a material that bends when subject to a temperature rise is fixed to the first blade and has a free end. A conductive plate is arranged inside the casing and in electrical connection with the second blade and movable between an engaged position where the conductive plate engages the free end of the conductive strip to form an electrical connection between the first and second blades and a disengaged position where the conductive plate disengages from the conductive strip to electrically disconnect the second blade from the first blade. When an overload occurs, an excessive current flows through the conductive strips, causing the strips to bend from a normal operation condition to a breaking condition that separates the conductive strip from the conductive plate. A coupler made of insulation material is coupled between the conductive strips to ensure both conductive strips move to the breaking condition at the same time. A leaf spring is pivoted between the casing and one of the conductive strips to retain the conductive strips in the breaking condition until the conductive strip is manually forced to the normal operation condition to ensure operation reliability. The leaf spring is pivotally connected to the casing via a bolt that allows for adjustment of the leaf spring with respect to the conductive strip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be apparent to those skilled in the art by reading the following description of preferred embodiments thereof, with reference to the attached drawings, in which: 
     FIG. 1 is an exploded perspective view of a dual-circuit switch constructed in accordance with a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the switch in an OFF condition; 
     FIG. 3 is a cross-sectional view of the switch in an ON condition; 
     FIG. 4 is a cross-sectional view of the switch in a breaking condition; 
     FIG. 5 a  is a cross-sectional view of a coupler of the switch of the present invention; 
     FIG. 5 b  is a cross-sectional view similar to FIG. 5 but showing the coupler coupling conductive strips of two circuits of the switch together; 
     FIG. 6 is a cross-sectional view of a link coupled between a seesaw plate and a conductive strip; 
     FIG. 6 a  is another cross-sectional view of the link showing the coupler is received in a slot of the link; 
     FIG. 7 is a cross-sectional view similar to FIG. 6 but showing a variation thereof; 
     FIG. 7 a  is a cross-sectional view similar to FIG. 6 a  but showing a variation thereof; 
     FIG. 8 is an exploded view of a dual-circuit switch constructed in accordance with a second embodiment of the present invention; 
     FIG. 9 is a cross-sectional view of the switch in an OFF condition; 
     FIG. 10 is a cross-sectional view of the switch in an ON condition; and 
     FIG. 11 is a cross-sectional view of the switch in a breaking condition. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings and in particular to FIGS. 1-3, a dual-circuit switch constructed in accordance with the present invention comprises a casing  1  forming an interior space (not labeled) and having opposite side walls (not labeled) defining a top opening  11  in communication with the interior space. Aligned holes  111  are defined in the sidewalls. A rotation button  2  is partially received in the opening  11  and has opposite pivot pins  21  rotatably received in the holes  111  of the casing  1  whereby the button  2  is rotatable between first and second positions respectively associated with ON and OFF conditions of the switch as shown in FIGS. 3 and 2. 
     Two driver assemblies  22  are formed on an underside of the button  2  and extend into the interior space of the casing  1 . Each driver assembly  22  comprises a cylinder  221  extending from the underside of the button  2  inside which a cap  222  is partially and movably received. A biasing element  223 , such as a helical spring, is mounted between the cylinder  221  and the cap  222  for biasing the cap  222  away from the cylinder  221 . The helical spring  223  is received and retained in both the cylinder  221  and the cap  222 . 
     Two conductive blade pairs  3 ,  4 , each forming a circuit of the switch electrically isolated from each other inside the casing  1 , are received in and retained by slots (not labeled) in a bottom of the casing  1  and are spaced by a first partition (not labeled) formed on the underside of the casing  1 . Each pair  3 ,  4  comprises a first conductive blade  31 ,  41  and a second conductive blade  32 ,  42  which are spaced from each other by a second transverse partition (not labeled) formed on the underside of the casing  1 . All the conductive blades  31 ,  32 ,  41 ,  42  have tails (not labeled) extending beyond the bottom of the casing  1  for external connection. 
     An opening  321 ,  421  is defined in each second blade  32 ,  42 . A conductive strip  33 ,  43  made of a conductive material, such as an alloy or a bimetal, that bends when subject to heat (and thus having a temperature rise) is arranged inside. the casing  1  and has an end attached to each first blade  31 ,  41  and a second, free end extending through the opening  321 ,  421  of each second blade  32 ,  42 , forming a cantilever beam. The opening  321 ,  421  of each second blade  32 ,  42  is large enough to accommodate the bending and deformation of the associated conductive strip  33 ,  43  without any physical engagement therebetween. 
     Each second blade  32 ,  42  defines a notch  322 ,  422  in a top edge (not labeled) thereof. A seesaw plate  34 ,  44  made of a conductive material is arranged inside the casing  1  and has a concave configuration and forms a bottom projection (not labeled) fit in the notch  322 ,  422  of each second blade  32 ,  42  whereby the seesaw plate  34 ,  44  seesaws about the top edge of the second blade  32 ,  42 . The bottom projection of the seesaw plate  34 ,  44  is formed by pressing the plate  34 ,  44  and thereby making a recess  341 ,  441  on a top side thereof and the recessed portion of the plate  34 ,  44  forms the projection. A movable contact  342 ,  442  is mounted to a first end of each seesaw plate  34 ,  44 . A stationary contact  331 ,  431  is mounted to the free end of each conductive strip  33 ,  43  and corresponds to the movable contact  342 ,  442 . 
     The driver assemblies  22  of the rotation button  2  are arranged to respectively and operatively correspond to the two seesaw plates  34 ,  44 . The cap  222  of each driver assembly  22  engages the topside of the corresponding seesaw plate  34 ,  44  and is slidable along the seesaw plate  34 ,  44  to seesaw the seesaw plate  34 ,  44 . When the button  2  is rotated to the first position (the ON condition, FIG.  3 ), the caps  222  are simultaneously moved to the first ends of the seesaw plates  34 ,  44  close to the movable contacts  342 ,  442  whereby the seesaw plates  3 ,  4  are simultaneously moved to an engaged position where the movable contacts  342 ,  442  are respectively brought into engagement with the corresponding stationary contacts  331 ,  431  of the conductive strips  33 ,  43 . Thus, an electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) of each blade pair  3 ,  4 , through the conductive strip  33 ,  43 , the contacts  331 ,  342  ( 431 ,  442 ) and the seesaw plate  34 ,  44 , is simultaneously formed. 
     When the button  2  is rotated to the second position (the OFF condition, FIG.  2 ), both caps  222  are simultaneously moved to second ends of the seesaw plates  34 ,  44  away from the movable contacts  342 ,  442  whereby the seesaw plates  34 ,  44  are moved to a disengaged position by rotation about the notches  322 ,  422  of the second blades  32 ,  42  to separate the movable contacts  342 ,  442  from the stationary contacts  331 ,  431 . Thus, the electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) is broken simultaneously. 
     In sliding along the seesaw plates  34 ,  44  between the first and second ends thereof, the caps  222  are forced toward the button  2  when the caps  222  pass the edges of the second blades  32 ,  42  by deforming the biasing elements  223 . If desired, the caps  222  may be partially received in the recesses  341 ,  441  defined in the topside of the corresponding seesaw plates  34 ,  44  to be guided thereby. 
     Also referring to FIG. 4, when an overload happens, an excessive current flows through the conductive strips  33 ,  43 , causing a significant temperature rise. The conducive strips  33 ,  43  that are upward concave in the normal operation condition bends in a direction away from the seesaw plates  34 ,  44  to a downward concave configuration to separate the stationary contacts  331 ,  431  from the movable contacts  342 ,  442  thereby breaking the electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) and cutting off the current supplied through the conductive strips  33 ,  43 . This opens the circuit associated with each conductive blade pair  3 ,  4 . 
     Also referring to FIGS. 5 a,    5   b,  to ensure that the conductive strips  33 ,  43  can bend at the same time to break both circuits simultaneously, a coupler  5  is connected between the conductive strips  33 ,  43 . The coupler  5  comprises a lower plate  51  and an upper plate  52  spaced from and connected to the lower plate  51  by a neck (not labeled) to define slits  50  on opposite sides of the neck for each receiving an edge of a corresponding conductive strip  33 ,  43 . By means of the coupler  5 , when one of the conductive strips  33 ,  43  bends due to overloading, the other conductive strip is caused to bend simultaneously. This ensures that both conductive strips  33 ,  43  can bend and move to the breaking condition at the same time thereby breaking both circuits simultaneously. 
     Referring back to FIG.  1  and further referring to FIGS. 6 and 6 a,  a link  6  made of insulative materials extends in a longitudinal direction between the first end of one of the seesaw plates (for example seesaw plate  34  in this case) and the free end of the corresponding conductive strip  33  and is interconnected to the seesaw plate  34  and the conductive strip  33  (as well as the coupler  5  to which the edge of the conductive strip  33  is attached). To allow the coupler  5  to properly connect to the link  6 , a cutoff  53  is defined in the coupler  5  to partially accommodate the link  6 . The link  6  has a first slot  61  and a second slot  62 . One of the slots  61 ,  62  is extended in the longitudinal direction. In the embodiment shown in FIGS. 1,  6  and  6   a,  the second slot  62  is extended and has a predetermined longitudinal dimension defined by upper and lower ends (both not labeled). The first end of the seesaw plate  34  is received in the first slot  61  and is thus attached to the link  6  with a limited rotation with respect to the link  6 . The free end of the conductive strip  33  is received in the second slot  62  and is movable between the upper and lower ends of the second slot  62 . When the button  2  is rotated to the first position (the On condition, FIG.  3 ), the link  6  is moved by the seesaw plate  34  relative to the free end of the conductive strip  33 , causing the free end of the conductive strip  33  to engage and be stopped by the upper end of the second slot  62  of the link  6 . On the other hand, when the button  2  is rotated to the second position (the OFF condition, FIG.  2 ), the link  6  is moved by the seesaw plate  34  in an opposite direction, causing the free end of the conductive strip  33  to engage and be stopped by the lower ends of the second slots  62  of the link  6 . In this respect, the second slot  62  has a longitudinal dimension or a moving distance (ΔS) substantially corresponding to the movement stroke of the movable contact  342  that is mounted to the first end of the seesaw plate  34  toward the stationary contact  331  that is mounted to the free end of the conductive strip  33  whereby no constraint is imposed to the movement of the seesaw plate  34  with respect to the conductive strip  33  by the link  6  during a normal operation. 
     The dimension of the second slot  62  of the link  6  and the dimension of the opening  321  of the second blade  32  are sized so that when an overload occurs during an ON condition with electrical current supplied through the conductive strips  33 ,  34 , the conductive strip  33  bends away from the seesaw plate  34  or is caused to bend away from the seesaw plate  34  by the bending of the conductive strip  43 , the longitudinal dimension of the second slot  62  allows the free end of the conductive strip  33  to move away from the first end of the seesaw plate  34 . The movement of the free end of the conductive strip  33  is stopped by the lower end of the second slot  62  of the link  6  and is not allowed to contact the opening  321  of the second blade  32 . 
     To return to the normal operation from the breaking condition, the button  2  is moved to the OFF condition. The seesaw plate  34  is moved to the OFF position and the free end of the conductive strip  33  is forced to move in unison with the seesaw plate  34  by means of the link  6 . The seesaw plate  44  is moved to the OFF condition simultaneously with the seesaw plate  34  by the button  2  and the conductive strip  43  is moved in unison with the conductive strip  33  by the coupler  5 . Thus, the switch is back to the OFF condition and is ready for next actuation. The button  2  may then be moved to the ON condition to engage the movable contacts  342 ,  442  with the stationary contacts  331 ,  431  for resuming electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) of each pair  3 ,  4 . 
     The link  6  and the coupler  5  ensure that the free end of the conductive strip  33  and thus the free end of the conductive strip  43  can be brought back to their unbent positions for next actuation of the switch. Even when the mechanical property of the conductive strips  33 ,  43  deteriorate due to aging or other reasons, the link  6  and the coupler  5  still provide means for simultaneously returning the conductive strips  33 ,  43  back to their unbent positions. 
     FIGS. 7 and 7 a  show a variation of the example illustrated in FIGS. 6 and 6 a.  In the variation of FIGS. 7 and 7 a,  the first slot  61 , rather than the second slot  62 , of the link  6  is extended in the longitudinal direction. Similarly, due to the longitudinal dimension of the first slot  61 , the movement of the conductive strip  33  is not subject to any constraint caused by the link  6  while the link  6  is able to bring the conductive strip  33  from a bent condition (caused by overload of the switch) back to the normal operation condition. 
     A U-shaped leaf spring  7  has opposite legs of which a first one is pivotally connected to the casing  1  and a second one pivotally coupled to the free end of the conductive strip  33 . The second leg of the leaf spring  7  defines an opening  71  and the free end of the conductive strip  33  forms an extension having barbed end  332 . (In the embodiment illustrated, there is no need for the other conductive strip  43  to form the barbed extension. However, to simplify the manufacturing, the conductive strip  43  may have exactly the same structure as the conductive strip  33  and thus having a barbed extension  432 .) The extension  332  is received in the opening  71 , forming the pivotal coupling between the conductive strip  33  and the leaf spring  7 . The pivotal connection of the first leg of the leaf spring  7  to the casing  1  allows the second leg of the leaf spring  7  to move with the free end of the conductive strip  33  when the conductive strip  33  is moved to the breaking condition due to overloading. 
     The leaf spring  7  is preloaded and applies a force to the free end of the conductive strip  33  in a direction pointing from the pivotal connection of the first leg to the pivotal coupling of the second leg. When the conductive strip  33  is in a normal operation condition, the pivotal coupling of the second leg is located above the pivotal connection of the first leg. The spring force of the leaf spring  7  acts in such a direction to retain the conductive strip  33  in an upward concave condition which leads to the normal operation of the switch (see FIGS.  2  and  3 ). The conductive strip  43  is also maintained in the upward concave condition by means of the coupler  5 . When an overload occurs, either one of the conductive strips  33 ,  43  bends to a downward concave condition and the other one of the conductive strips  33 ,  43  is forced to bend at the same time due to the coupler  5 . The second leg of the leaf spring  7  is thus moved by the conductive strip  33  and the movement of the second leg of the leaf spring  7  moves the pivotal coupling of the second leg to be below the pivotal connection of the first leg whereby the spring force of the leaf spring  7  acts on the free end of the conductive strip  33  in such a direction to retain the conductive strip  33  and thus the conductive strip  43  in the breaking condition (see FIG.  4 ). 
     The spring force of the leaf spring  7  is overcome by a driving force provided by the movement of the link  6  to the conductive strip  33  when the button  2  is manually switched to the OFF condition. Thus, the conductive strip  33  can be moved back to the normal operation condition against the leaf spring  7 . The leaf spring  7  ensures operation reliability of the conductive strips  33 ,  43  in both the normal operation condition and the breaking condition. 
     A bolt  101  is threadingly received in an inner-threaded hole  10  defined in the housing  1 . A circumferential groove  1011 , preferably having a V-shaped cross section, is defined in a free end of the bolt  101 . The U-shaped leaf spring  7  has a flange (not labeled) extending from the first leg of the spring  7  and receivingly engaging the groove  1011  of the bolt  101  for pivotally connecting the first leg of the leaf spring  7  to the casing  1 . The pivotal connection of the first leg of the leaf spring  7  inside the casing  1  is position-adjustable by turning the bolt  101  to change relative position of the bolt  101  with respect to the casing  1 . 
     FIGS. 8-10 show a switch constructed in accordance with a second embodiment of the present invention, comprising a casing  1  forming an interior space (not labeled) and having opposite side walls (not labeled) defining a top opening  11  and a side opening  12  both in communication with the interior space. A cover  13  is fit into the top opening  11  and is fixed to the casing  1 . A hole  130  is defined in an inside surface (not labeled) of the cover  13 . A Z-shaped bar  131  has a major central section and two minor end sections extending from opposite ends of the central section in opposite directions. One of end sections of the bar  131  is fit into the hole  130  whereby the bar  131  is attached to the inner surface of the cover  13 . 
     A pushbutton  2 ′ is movably received in the interior space of the casing  1  through the side opening  12 . A guide block  24  having a polygonal configuration is formed on topside of the pushbutton  2 ′ defining a multi-section channel  23  surrounding the block  24 . The channel  23  forms a closed loop path or route having stop points A and B. The second end section of the bar  131  is movably received in the channel  23  and is guided to move along the route. The pushbutton  2 ′ is linearly movable with respect to the casing  1  between an outer position (FIG. 9) and an inner position (FIG.  10 ). By repeatedly pushing the pushbutton  2 ′, the end section of the bar  131  is moved along the channel  23  between the stop points A and B. When the pushbutton  2 ′ is pushed once and moved to the inner position, the end section of the bar  131  is moved to the stop point B and trapped there for retaining the pushbutton  2 ′ at the inner position. When the pushbutton  2 ′ is pushed again and is thus moved to the outer position, the end section of the bar  131  is moved to the stop point A. The outer and inner positions of the pushbutton  2 ′ respectively associated with OFF and ON conditions of the switch as shown in FIGS. 9 and 10. The pushbutton  2 ′ is spring-biased for helping movement between the stop points A and B. 
     Two driver assemblies  22  are formed on an underside of the pushbutton  2 ′ and extend into the interior space of the casing  1 . Each driver assembly  22  comprises a cylinder  221  extending from the underside of the pushbutton  2 ′ inside which a cap  222  is partially and movably received. A biasing element  223 , such as a helical spring, is mounted between the cylinder  221  and the cap  222  for biasing the cap  222  away from the cylinder  221 . The helical spring  223  is received and retained in both the cylinder  221  and the cap  222 . 
     Two conductive blade pairs  3 ,  4 , each forming a circuit of the switch electrically isolated from each other inside the casing  1 , are received in and retained by slots (not labeled) in a bottom of the casing  1  and are spaced by a first partition (not labeled) formed on the underside of the casing  1 . Each pair  3 ,  4  comprises a first conductive blade  31 ,  41  and a second conductive blade  32 ,  42  which are spaced from each other by a second transverse partition (not labeled) formed on the underside of the casing  1 . All the conductive blades  31 ,  32 ,  41 ,  42  have tails (not labeled) extending beyond the bottom of the casing  1  for external connection. 
     An opening  321 ,  421  is defined in each second blade  32 ,  42 . A conductive strip  33 ,  43  made of a conductive material, such as an alloy or a bimetal, that bends when subject to heat (and thus having a temperature rise) is arranged inside the casing  1  and has an end attached to each first blade  31 ,  41  and a second, free end extending through the opening  321 ,  421  of each second blade  32 ,  42 , forming a cantilever beam. The opening  321 ,  421  of each second blade  32 ,  42  is large enough to accommodate the bending and deformation of the associated conductive strip  33 ,  43  without any physical engagement therebetween. 
     Each second blade  32 ,  42  defines a notch  322 ,  422  in a top edge (not labeled) thereof. A seesaw plate  34 ,  44  made of a conductive material is arranged inside the casing  1  and has a concave configuration and forms a bottom projection (not labeled) fit in the notch  322 ,  422  of each second blade  32 ,  42  whereby the seesaw plate  34 ,  44  seesaws about the top edge of the second blade  32 ,  42 . The bottom projection of the seesaw plate  34 ,  44  is formed by pressing the plate  34 ,  44  and thereby making a recess  341 ,  441  on a top side thereof and the recessed portion of the plate  34 ,  44  forms the projection. A movable contact  342 ,  442  is mounted to a first end of each seesaw plate  34 ,  44 . A stationary contact  331 ,  431  is mounted to the free end of each conductive strip  33 ,  43  and corresponds to the movable contact  342 ,  442 . 
     The driver assemblies  22  of the pushbutton  2 ′ are arranged to respectively and operatively correspond to the two seesaw plates  34 ,  44 . The cap  222  of each driver assembly  22  engages the topside of the corresponding seesaw plate  34 ,  44  and is slidable along the seesaw plate  34 ,  44  to seesaw the seesaw plate  34 ,  44 . When the pushbutton  2 ′ is moved to the inner position (the ON condition, FIG.  10 ), the caps  222  are simultaneously moved to the first ends of the seesaw plates  34 ,  44  close to the movable contacts  342 ,  442  whereby the seesaw plates  3 ,  4  are simultaneously moved to an engaged position where the movable contacts  342 ,  442  are respectively brought into engagement with the corresponding stationary contacts  331 ,  431  of the conductive strips  33 ,  43 . Thus, an electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) of each blade pair  3 ,  4 , through the conductive strip  33 ,  43 , the contacts  331 ,  342  ( 431 ,  442 ) and the seesaw plate  34 ,  44 , is simultaneously formed. 
     When the pushbutton  2 ′ is moved to the outer position (the OFF condition, FIG.  9 ), both caps  222  are simultaneously moved to second ends of the seesaw plates  34 ,  44  away from the movable contacts  342 ,  442  whereby the seesaw plates  34 ,  44  are moved to a disengaged position by rotation about the notches  322 ,  422  of the second blades  32 ,  42  to separate the movable contacts  342 ,  442  from the stationary contacts  331 ,  431 . Thus, the electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) is broken simultaneously. 
     In sliding along the seesaw plates  34 ,  44  between the first and second ends thereof, the caps  222  are forced toward the button  2  when the caps  222  pass the edges of the second blades  32 ,  42  by deforming the biasing elements  223 . If desired, the caps  222  may be partially received in the recesses  341 ,  441  defined in the topside of the corresponding seesaw plates  34 ,  44  to be guided thereby. 
     Also referring to FIG. 11, when an overload happens, an excessive current flows through the conductive strips  33 ,  43 , causing a significant temperature rise. The conducive strips  33 ,  43  that are upward concave in the normal operation condition bends in a direction away from the seesaw plates  34 ,  44  to a downward concave configuration to separate the stationary contacts  331 ,  431  from the movable contacts  342 ,  442  thereby breaking the electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) and cutting off the current supplied through the conductive strips  33 ,  43 . This opens the circuit associated with each conductive blade pair  3 ,  4 . 
     To ensure that the conductive strips  33 ,  43  can bend at the same time to break both circuits simultaneously, a coupler  5  is connected between the conductive strips  33 ,  43 . The coupler  5  comprises a lower plate  51  and an upper plate  52  spaced from and connected to the lower plate  51  by a neck (not labeled) to define slits  50  on opposite sides of the neck for each receiving an edge of a corresponding conductive strip  33 ,  43 . By means of the coupler  5 , when one of the conductive strips  33 ,  43  bends due to overloading, the other conductive strip is caused to bend simultaneously. This ensures that both conductive strips  33 ,  43  can bend and move to the breaking condition at the same time thereby breaking both circuits simultaneously. 
     Referring back to FIG. 8, a link  6  made of insulative materials extends in a longitudinal direction between the first end of one of the seesaw plates (for example seesaw plate  34  in this case) and the free end of the corresponding conductive strip  33  and is interconnected to the seesaw plate  34  and the conductive strip  33  (as well as the coupler  5  to which the edge of the conductive strip  33  is attached). To allow the coupler  5  to properly connect to the link  6 , a cutoff  53  is defined in the coupler  5  to partially accommodate the link  6 . The link  6  has a first slot  61  and a second slot  62 . One of the slots  61 ,  62  is extended in the longitudinal direction. In the embodiment shown in FIG. 8, the second slot  62  is extended and has a predetermined longitudinal dimension defined by upper and lower ends (both not labeled). The first end of the seesaw plate  34  is received in the first slot  61  and is thus attached to the link  6  with a limited rotation with respect to the link  6 . The free end of the conductive strip  33  is received in the second slot  62  and is movable between the upper and lower ends of the second slot  62 . When the pushbutton  2 ′ is moved to the inner position (the On condition, FIG.  10 ), the link  6  is moved by the seesaw plate  34  relative to the free end of the conductive strip  33 , causing the free end of the conductive strip  33  to engage arid be stopped by the upper end of the second slot  62  of the link  6 . On the other hand, when the pushbutton  2 ′ is moved to the outer position (the OFF condition, FIG.  9 ), the link  6  is moved by the seesaw plate  34  in an opposite direction, causing the free end of the conductive strip  33  to engage and be stopped by the lower ends of the second slots  62  of the link  6 . In this respect, the second slot  62  has a longitudinal dimension or a moving distance substantially corresponding to the movement stroke of the movable contact  342  that is mounted to the first end of the seesaw plate  34  toward the stationary contact  331  that is mounted to the free end of the conductive strip  33  whereby no constraint is imposed to the movement of the seesaw plate  34  with respect to the conductive strip  33  by the link  6  during a normal operation. 
     The dimension of the second slot  62  of the link  6  and the dimension of the opening  321  of the second blade  32  are sized so that when an overload occurs during an ON condition with electrical current supplied through the conductive strips  33 ,  34 , the conductive strip  33  bends away from the seesaw plate  34  or is caused to bend away from the seesaw plate  34  by the bending of the conductive strip  43 , the longitudinal dimension of the second slot  62  allows the free end of the conductive strip  33  to move away from the first end of the seesaw plate  34 . The movement of the free end of the conductive strip  33  is stopped by the lower end of the second slot  62  of the link  6  and is not allowed to contact the opening  321  of the second blade  32 . 
     To return to the normal operation from the breaking condition, the pushbutton  2 ′ is moved to the outer position (the OFF condition). The seesaw plate  34  is moved to the OFF position and the free end of the conductive strip  33  is forced to move in unison with the seesaw plate  34  by means of the link  6 . The seesaw plate  44  is moved to the OFF condition simultaneously with the seesaw plate  34  by the pushbutton  2 ′ and the conductive strip  43  is moved in unison with the conductive strip  33  by the coupler  5 . Thus, the switch is back to the OFF condition and is ready for next actuation. The pushbutton  2 ′ may then be moved to the inner position (the ON condition) to engage the movable contacts  342 ,  442  with the stationary contacts  331 ,  431  for resuming electrical connection between the first and second blades  31 ,  32  ( 41 ,  42 ) of each pair  3 ,  4 . 
     The link  6  and the coupler  5  ensure that the free end of the conductive strip  33  and thus the free end of the conductive strip  43  can be brought back to their unbent positions for next actuation of the switch. Even when the mechanical property of the conductive strips  33 ,  43  deteriorate due to aging or other reasons, the link  6  and the coupler  5  still provide means for simultaneously returning the conductive strips  33 ,  43  back to their unbent positions. 
     A U-shaped leaf spring  7  has opposite legs of which a first one is pivotally connected to the casing  1  and a second one pivotally coupled to the free end of the conductive strip  33 . The second leg of the leaf spring  7  defines an opening  71  and the free end of the conductive strip  33  forms an extension having barbed end  332 . (In the embodiment illustrated, there is no need for the other conductive strip  43  to form the barbed extension. However, to simplify the manufacturing, the conductive strip  43  may have exactly the same structure as the conductive strip  33  and thus having a barbed extension  432 .) The extension  332  is received in the opening  71 , forming the pivotal coupling between the conductive strip  33  and the leaf spring  7 . The pivotal connection of the first leg of the leaf spring  7  to the casing  1  allows the second leg of the leaf spring  7  to move with the free end of the conductive strip  33  when the conductive strip  33  is moved to the breaking condition due to overloading. 
     The leaf spring  7  is preloaded and applies a force to the free end of the conductive strip  33  in a direction pointing from the pivotal connection of the first leg to the pivotal coupling of the second leg. When the conductive strip  33  is in a normal operation condition, the pivotal coupling of the second leg is located above the pivotal connection of the first leg. The spring force of the leaf spring  7  acts in such a direction to retain the conductive strip  33  in an upward concave condition which leads to the normal operation of the switch (see FIGS.  9  and  10 ). The conductive strip  43  is also maintained in the upward concave condition by means of the coupler  5 . When an overload occurs, either one of the conductive strips  33 ,  43  bends to a downward concave condition and the other one of the conductive strips  33 ,  43  is forced to bend at the same time due to the coupler  5 . The second leg of the leaf spring  7  is thus moved by the conductive strip  33  and the movement of the second leg of the leaf spring  7  moves the pivotal coupling of the second leg to be below the pivotal connection of the first leg whereby the spring force of the leaf spring  7  acts on the free end of the conductive strip  33  in such a direction to retain the conductive strip  33  and thus the conductive strip  43  in the breaking condition (see FIG.  11 ). 
     The spring force of the leaf spring  7  is overcome by a driving force provided by the movement of the link  6  to the conductive strip  33  when the pushbutton  2 ′ is manually switched to the OFF condition. Thus, the conductive strip  33  can be moved back to the normal operation condition against the leaf spring  7 . The leaf spring  7  ensures operation reliability of the conductive strips  33 ,  43  in both the normal operation condition and the breaking condition. 
     A bolt  101  is threadingly received in an inner-threaded hole  10  defined in the housing  1 . A circumferential groove  1011 , preferably having a V-shaped cross section, is defined in a free end of the bolt  101 . The U-shaped leaf spring  7  has a flange (not labeled) extending from the first leg of the spring  7  and receivingly engaging the groove  1011  of the bolt  101  for pivotally connecting the first leg of the leaf spring  7  to the casing  1 . The pivotal connection of the first leg of the leaf spring  7  inside the casing  1  is position-adjustable by turning the bolt  101  to change relative position of the bolt  101  with respect to the casing  1 . 
     Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.