Abstract:
The disclosed circuit breaker comprises a first connection terminal, a second connection terminal, a heat generating part having conductivity and disposed between the first connection terminal and second connection terminal, an igniting part igniting depending on a cut-off signal, an expandable elastic member capable of applying a force to the heat generating part so as to be departed from between the first connection terminal and second connection terminal, a container accommodating the heat generating part, igniting part and elastic member, and a retaining part for retaining the elastic member in compressed state. Herein, at least a portion of the retaining part is provided in the container, and as the igniting part ignites depending on the cut-off signal and the heat generating part generates heat, and the retaining part releases the elastic member, the elastic member applies the force to the heat generating part, and the heat generating part is departed from between the first connection terminal and second connection terminal, so that the conductive state between the first connection terminal and second connection terminal is cut off.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit breaker, and more particularly to a circuit breaker for cutting off an electric circuit in a short time. 
     In an electric equipment system installed in a vehicle, if any abnormality should occur due to load of power window or the like, or if any abnormality should occur in the wire harness or the like composed of plural wires connecting the battery and each load, a large-current fuse interposed between the battery and the wire harness is melted to cut off between the battery and the wire harness, thereby preventing the loads and wire harness from burning down. 
     In the electric equipment system using such large-current fuse, however, if any abnormality should occur in the load such as power window, or if abnormality should occur in the wire harness of the like connecting the battery and each load, it is set so as not to melt down unless a current large than the preset allowable value flows in the large-current fuse. 
     Recently, various protective devices have been developed for cutting off between the battery and wire harness by detecting when a large current close to the allowable value is flowing continuously. 
     SUMMARY OF THE INVENTION 
     According to the investigation by the present inventor, a protective device as shown in FIG. 36 is considered. 
     FIG. 36 is a sectional view showing an example of protective device using a bimetal. 
     Such protective device comprises a housing  103  made of an insulating resin or the like, and forming a fuse compartment  102  at the upper side, a lid  113  for opening and closing the fuse compartment  102  of the housing  103 , a power source terminal  105  disposed at the lower side of the housing  103  so that its upper end portion projects into the fuse compartment  102 , and the lower end exposed outside, with the portion exposed outside connected to a positive terminal of a battery  104 , and a load terminal  109  disposed at the lower side of the housing so that its upper end portion projects into the fuse compartment  102 , and the lower end exposed outside, with the portion exposed outside connected to a load  108  through a wire  107  for composing a wire harness  106 . 
     Such protective device further comprises a fusible element  110  made of a low melting point metal disposed in the fuse compartment  102 , with one end connected to the upper end of the power source terminal  105 , and other end connected to the upper end of the load terminal  109 , an intermediate terminal  111  disposed at an intermediate position of the power source terminal  105  and load terminal  109 , with the lower end disposed at the lower side of the housing  103  so as to be exposed outside, and the portion exposed outside connected to a negative terminal of the battery  104 , and a bimetal  112  made of two long plate members of different metals glued together, with the lower end side connected to the upper end of the intermediate terminal  111  and the upper end side being bent in an L-shape and disposed oppositely to the fusible element  110 . 
     In such constitution, by manipulation of an ignition switch or the like of the vehicle, while a current is flowing in a route of positive terminal of battery  104 , power source terminal  105 , fusible element  110 , load terminal  109 , wire  107  of wire harness  106 , load  108 , and negative terminal of battery  104 , if any abnormality should occur in the load  108  or the wire harness  106  connecting the load  108  and the protective device  101 , and a current exceeding the allowable value should flow in the fusible element  110 , it is heated and melted down. 
     As a result, the circuit is cut off, and the load  108  and wire harness  106  are protected. 
     Or when a large current flows in the fusible element  110  due to some abnormality occurring in the load  108  or the wire harness  106  connecting the load  108  and protective device  101 , if it does not exceed the allowable value, the fusible element  110  is heated by the current flowing in the fusible element  110 , and the bimetal  112  begins to deform. In a specified time after a large current begins to flow in the fusible element  110 , the leading end of the bimetal  112  contacts with the fusible element  110 , and a large short-circuit current flows in the fusible element  110  in a route composed of positive terminal of battery  104 , power source terminal  105 , fusible element  110 , intermediate terminal  111  and negative terminal of battery  104 , so that it is melted down. 
     As a result, if lower than the allowable value, when a certain current flows longer than a preset time, the circuit is cut off, and the wire harness  106  and load  108  are protected. 
     Aside from such protective device  101 , a protective device  121  shown in FIG. 37 is also devised by the present inventor. 
     The protective device  121  shown in FIG. 37 comprises a housing  122  made of insulating resin or the like, a power source terminal  124  buried at one side of the housing  122 , with the lower end portion connected to a positive terminal of a battery  123 , and a load terminal  128  buried at other side of the housing  122 , with the lower end portion connected to a load  127  through a wire  126  composing a wire harness  125 . 
     Moreover, one end of a wire  131  composed of a fusible conductor  129  made of a low melting point metal or the like formed in a U-shape and a heat resistant covering  130  formed to cover the fusible conductor  129  is connected to the upper end of the power source terminal  124 , and other end is connected to the upper end of the load terminal  128 . This wire  131  has a coil  132  made of a shape memory alloy, showing a shape being wound around the wire  131  as shown in FIG. 9 when it is in martensite phase, and returning to the mother phase in a shape of tightening the wire  131  when heated to temperature of 120° C. to 170° C. 
     Further, outside of the housing  122 , there is an external terminal  133  with the upper end connected to one end of the coil  132  and lower end connected to a negative terminal of the battery  123 . 
     In such constitution, by manipulation of an ignition switch or the like of the vehicle, while a current is flowing in a route of positive terminal of battery  123 , power source terminal  124 , fusible element  129  of wire  131 , load terminal  128 , wire  126  of wire harness  124 , load  127 , and negative terminal of battery  123 , if any abnormality should occur in the load  127  or the wire harness  125  connecting the load  127  and the protective device  121 , and a current exceeding the allowable value should flow in the fusible element  129 , it is heated and melted down. 
     As a result, the circuit is cut off, and the load  127  and wire harness  125  are protected. 
     Or when a large current flows in the fusible conductor  129  due to some abnormality occurring in the load  127  or the wire harness  125  connecting the load  127  and protective device  121 , if it does not exceed the allowable value, the fusible conductor  129  is heated by the current flowing in the fusible conductor  129 , and the temperature of the coil  132  climbs up. In a specified time after a large current begins to flow in the fusible conductor  129 , when the temperature of the coil  132  reaches 120° C. to 170° C., the coil  132  is shifted from the martensite phase to the mother phase, and bites into the heat resistant covering  130  softened by heat, an contacts with the fusible conductor  129 , and a large short-circuit current flows in the fusible conductor  129  in a route composed of positive terminal of battery  123 , power source terminal  124 , fusible conductor  129 , coil  132 , external terminal  133  and negative terminal of battery  123 , so that it is melted down. 
     As a result, if lower than the allowable value, when a certain current flows longer than a preset time, the circuit is cut off, and the wire harness  125  and load  127  are protected. 
     In these protective devices  101  and  121 , however, the following problems have been disclosed. 
     First, in the protective device shown in FIG. 36, since flow of large current in the fusible element  110  is detected by using the bimetal  112  gluing two kinds of metals differing in the coefficient of thermal expansion, if the magnitude of the current flowing in the fusible element  110  changes, the bimetal  112  is deformed, and the time until cutting off the circuit varies. 
     Accordingly, in the event of such an abnormality that a large current flows intermittently, the temperature of the fusible element  101  does not rise higher than a certain point, and the protective device  101  may not cut off the circuit appropriately. 
     On the other hand, in the protective device  121  shown in FIG. 37, since flow of large current in the fusible conductor  129  is detected by using the coil  132  made of shape memory alloy, if the magnitude of the current flowing in the fusible conductor  129  changes, the coil  132  is deformed, and the time until cutting off the circuit varies. 
     Accordingly, in the event of such an abnormality that a large current flows intermittently, the temperature of the fusible conductor  129  does not rise higher than a certain point, and the protective device  121  may not cut off the circuit appropriately. 
     Or, in the protective device shown in FIG.  36  and FIG. 37, the heat reaction time of heat deforming conductive members such as bimetal  112  and  132  may vary depending on the passing current. A preferred operating temperature for bimetal or shape memory alloy is about 100° C., but the environmental temperature of the vehicle is about 120 to 125° C., and it is not a preferred environment. It is further predicted that the heat reaction of the heat deforming conductive member may not take place timely in the event of abnormality such as passing of overcurrent. 
     The invention is devised in the light of the above background, and it is hence an object thereof to present a circuit breaker capable of protecting electric parts by securely cutting off the circuit in a short time in the event of input of a failure signal of a vehicle. 
     The circuit breaker of the invention comprises a first connection terminal, a second connection terminal, a heat generating part having conductivity and disposed between the first connection terminal and second connection terminal, an igniting part igniting depending on a cut-off signal, an expandable elastic member capable of applying a force to the heat generating part so as to be departed from between the first connection terminal and second connection terminal, a container accommodating the heat generating part, igniting part and elastic member, and a retaining part for retaining the elastic member in compressed state. Herein, at least a portion of the retaining part is provided in the container, and when the retaining part retains the elastic member in compressed state and the heat generating part is positioned between the first connection terminal and second connection terminal, the conductive state between the first connection terminal and second connection terminal is maintained, and as the igniting part ignites depending on the cut-off signal and the heat generating part generates heat, and the retaining part releases the elastic member, the elastic member applies the force to the heat generating part, and the heat generating part is departed from between the first connection terminal and second connection terminal, so that the conductive state between the first connection terminal and second connection terminal is cut off. 
     In this constitution, when the igniting part ignites by a failure signal from outside, the heat generating part generates heat, and by this heat the holding part is melted. As a result, the compressed elastic member is expanded to kick up the heat generating part, and the electric connection between the heat generating part and the first connection terminal and second connection terminal is cut off. As the electric connection between the first connection terminal and second connection terminal is cut off, the circuit is securely cut off in a short time, so that the electric parts can be protected. 
     Herein, the retaining part includes a resin part, preferably, and the resin part is melted as the igniting part ignites depending on the cut-off signal and the heat generating part generates heat, so that retaining of the elastic member is released quickly. 
     Specifically, the retaining part is a fixing part for tightening and fixing the heat generating part to the container so that the elastic member may be maintained in compressed state in a simple constitution, and more specifically the fixing part preferably comprises a first threaded part formed in the heat generating part and a second threaded part formed in the container so as to be engaged with the first threaded part. 
     Alternatively, the retaining part may be also a protrusion formed in the container so as to resist the force of the elastic member. More specifically, the container has an upper case and a lower case fitting to the upper case, and the protrusion is provided in the inner wall of the upper case at a position confronting the upper surface of the heat generating part, so that the elastic member may be maintained in compressed state securely in a simple constitution. 
     On the other hand, a side wall is formed at the end of the heat generating part, and the end of the first connection terminal and the side wall, and the end of the second connection terminal and the side wall may be individually joined with low melting point materials. 
     In such constitution, usually, the conduction between the first connection terminal and second connection terminal is improved by the low melting point material, and in case of abnormality, the low melting point material is melted securely by the heat generation of the heating agent, and the electric connection between the first connection terminal and second connection terminal is cut off by the force of the elastic member. Usually, meanwhile, since no force is applied to the low melting point material, the reliability of junction between the first connection terminal and second connection terminal is enhanced. 
     Alternatively, between the end of the first connection terminal and the end of the second connection terminal, a conductive member may be provided under pressure welding. 
     In such constitution, usually, the conduction between the first connection terminal and second connection terminal is improved by the conductive member. Moreover, since no force is usually applied to the junction of the first connection terminal and second connection terminal, the reliability of junction between the first connection terminal and second connection terminal is enhanced. 
     When attaching, in order to position the first connection terminal, second connection terminal and heat generating part securely, and prevent from turning when tightening with screw, a bent part is respectively provided in the first connection terminal and second connection terminal, and the heat generating part is preferred to have a groove to be fitted with such a bent part. 
     On the other hand, in order that force may not be applied to the first connection terminal and second connection terminal more securely, a side wall is formed at the end of the heat generating part, and preferably a protrusion is respectively formed in the first connection terminal and second connection terminal so as to press the side wall. 
     Also when attaching, in order to absorb the reaction of screw tightening, notch or concave and convex part should be preferably formed in the first connection terminal and second connection terminal. Similarly, to absorb the reaction of screw tightening, a gap may be formed between the first connection terminal and second connection terminal and the side wall of the heat generating part, respectively. 
     The low melting point material is preferred to be one selected from the group consisting of Sn, Pb, Zn, Al and Cu. 
     The heat generating part includes the heating agent, and the heating agent is preferred to include a thermite compound mixing powder of metal oxide and powder of aluminum because the thermite reaction heat can be securely generated by the thermite reaction. 
     In other words, the heat generating part contains the heating agent, and the heating agent contains at least one metal powder selected from the group consisting of B, Sn, Fe, Si, Zr, Ti and Al, and at least one metal oxide selected from the group consisting of CuO, MnO 2 , Pb 3 O 4 , PbO 2 , Fe 3 O 4 , Fe 2 O 3  and Cr 2 O 3 . 
     Further, the heating agent may also contain additives having alumina, bentonite or talc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view before cut-off of a circuit breaker in a first embodiment of the invention. 
     FIG. 2 is a top view of the circuit breaker of the embodiment. 
     FIG. 3 is a detailed view of threaded parts formed in a thermite case and an outer case of the embodiment. 
     FIG. 4 is a sectional view after cut-off of the circuit breaker of the embodiment. 
     FIG. 5 is a sectional view before cut-off of a circuit breaker in a second embodiment of the invention. 
     FIG. 6 is a top view of the circuit breaker of the embodiment. 
     FIG. 7 is a sectional view after cut-off of the circuit breaker of the embodiment. 
     FIG. 8 is a sectional view of A—A of a circuit breaker in a third embodiment of the invention. 
     FIG. 9 is a top view of the circuit breaker of the embodiment. 
     FIG. 10 is a sectional view of B—B of the circuit breaker of the embodiment. 
     FIG. 11 is a top view of a circuit breaker in a fourth embodiment of the invention. 
     FIG. 12 is a sectional view of thermite case and bus bar of the embodiment. 
     FIG. 13 is a diagram explaining positioning by fitting of the thermite case and bus bar of the embodiment. 
     FIG. 14 is a detailed view of the thermite case of the embodiment. 
     FIG. 15 is a top view of a circuit breaker in a fifth embodiment of the invention. 
     FIG. 16 is a sectional view of thermite case and bus bar of the embodiment. 
     FIG. 17 is a diagram explaining bonding of the thermite case and bus bar of the embodiment. 
     FIG. 18 is a top view of a circuit breaker in a modified example of the fifth embodiment of the invention. 
     FIG. 19 is a sectional view of thermite case and bus bar of the same. 
     FIG. 20 is a detailed structural view of the bus bar of the same. 
     FIG. 21 is a top view of a circuit breaker in a modified example of the fifth embodiment of the invention. 
     FIG. 22 is a sectional view of thermite case and bus bar of the same. 
     FIG. 23 is a diagram explaining positioning of the thermite case in the vertical and lateral directions of the same. 
     FIG. 24 is a diagram explaining the force applied to the thermite case and low melting point metal by rotation of screw tightening of bus bar. 
     FIG. 25 is a top view of a circuit breaker in a sixth embodiment of the invention. 
     FIG. 26 is a perspective view of thermite case and bus bar of the embodiment. 
     FIG. 27 is a sectional view of thermite case and bus bar in a first modified example of the circuit breaker in the sixth embodiment of the invention. 
     FIG. 28 is a sectional view of thermite case and bus bar in a second modified example of the circuit breaker in the sixth embodiment of the invention. 
     FIG. 29 is a top view of a third modified example of the circuit breaker in the sixth embodiment of the invention. 
     FIG. 30 is a perspective view of thermite case and bus bar of the same. 
     FIG. 31 is a perspective view of thermite case and bus bar in a fourth modified example of the circuit breaker in the sixth embodiment of the invention. 
     FIG. 32 is a sectional view of a circuit breaker in a seventh embodiment of the invention. 
     FIG. 33 is a top view of cap of the embodiment. 
     FIG. 34 is a side view of the cap of the embodiment. 
     FIG. 35 is a sectional view of A—A of the cap of the embodiment. 
     FIG. 36 is a sectional view showing an example of a protective device using a bimetal. 
     FIG. 37 is a sectional view showing other example of the protective device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, each of preferred embodiments of the invention is described in detail below. 
     A first embodiment of the invention is described below. 
     In the circuit breaker shown in FIG. 1 to FIG. 4, a first bus bar  11  of a long plate form is made of copper or copper alloy, and a round hole  12  to be connected to a battery or the like is formed in this first bus bar  11 . A leading end  13  of the first bus bar  11  is bent downward nearly at right angle. 
     A second bus bar  19  of a long plate form is also made of copper or copper alloy, and a round hole  20  to be connected to a load or the like is formed in this second bus bar  19 . A leading end  21  of the second bus bar  19  is also bent downward nearly at right angle. 
     Between the first bus bar  11  and second bus bar  19 , a cap  14  having a cavity  22  is disposed at the upper side, and an outer case  15  having a case step  15   a  is disposed at the lower side. The cap  14  and outer case  15  constitute an outer container of insulating material, and a thermoplastic resin is preferably used. 
     A thermite case  25  of a lid form is contained in the outer case  15 , and this thermite case  25  is filled with a heating agent  27 . The thermite case  25  is made of metal excellent in thermal conductivity and not melted by the heat generation of the heating agent  27 , and brass, copper, copper alloy, stainless steel or the like may be used. 
     The thermite case  25  is disposed at a position of a nearly same height as the height of the first bus bar  11  and second bus bar  19 , and the thermite case  25  has a left side wall  25   a  and a right side wall  25   b.    
     The left side wall  25   a  is joined to the leading end  13  of the first bus bar  11  by means of a low melting point metal  23  as low melting point material with melting point about 200 to 300° C. As such low melting point material, solder is used. The right side wall  25   b  is joined to the leading end  21  of the second bus bar  19  by means of low melting point metal  23 . Accordingly, the first bus bar  11  and second bus bar  19  can be electrically connected with each other through the low melting point metal  23  and thermite case  25 . 
     As the low melting point metal  23 , at least one metal selected from the group consisting of Sn, Pb, An, Al and Cu is preferably used. 
     The heating agent  27  is a thermite compound composed of powder of metal oxide such as iron oxide (Fe 2 O 3 ) and powder of aluminum, which generates high heat by inducting thermite reaction by heat generation of the led wire  31 . As the metal oxide, instead of iron oxide (Fe 2 O 3 ), chromium oxide (Cr 2 O 3 ) or manganese oxide (MnO 2 ) may be used. 
     The heating agent  27  may be also a mixture composed of at least one metal powder selected from the group consisting of B, Sn, Fe, Si, Zr, Ti and Al, at least one metal oxide selected from the group consisting of CuO,MnO 2 , Pb 3 O 4 , PbO 2 , Fe 2 O 4  and Fe 2 O 3 , and at least one additive selected from the group consisting of alumina, bentonite and talc. By using such heating agent, it is easily ignited by the igniting part  29 , and the low melting point metal  23  can be melted in a short time. 
     As a fixing part for tightening and fixing the thermite case  25  to the outer case  15 , a threaded part  28  is disposed at the lower side of the thermite case  25 . The thermite case  25  can be contained in a thermite compartment  15   b  formed in the case step  15   a  of the outer case  15  as shown in FIG.  3 . 
     The threaded part  28  consists of a thermite threaded part  28   a  forming male threads as a first threaded part formed in the thermite case  25 , and an outer case threaded part  28   b  as a second threaded part forming female threads in the case step  15   a  to e engaged with this thermite threaded part  28   a . At least one threaded part of the thermite threaded part  28   a  and outer case threaded part  28   b  is made of a resin member. 
     In the lower part of the thermite case  25 , an igniting part  29  is disposed, and the igniting part  29  is contained in the outer case  15 , and has an igniting agent, and the igniting agent is ignited by heat generation caused by the current flowing in a lead wire  31  in case of abnormality of vehicle such as vehicle collision accident, and a thermite reaction heat is generated in the heating agent  27 . 
     The igniting part  29  has a groove  29   a , and between this groove  29   a  and the bottom of the outer case  15 , there is a compression spring  34  as an expandable elastic member, and this compression spring  34  is compressed in the state shown in FIG.  1 . 
     In the state after cut-off of the circuit, as shown in FIG. 4, when the low melting point metal  23  and threaded part  28  are heated and melted by the thermite reaction heat of the heating agent  27 , the compression spring  34  expands, and the thermite case  25  bounces up to the cap  14 . 
     In thus constituted circuit breaker of the embodiment, the operation is described below. 
     Usually, the first bus bar  11  and second bus bar  19  are electrically connected with each other through the low melting metal  23  and thermite case  25 , and a current is supplied from a battery to a load (neither shown). 
     If the vehicle collides against an obstacle or tumbles off a cliff or the like, the abnormality of the vehicle is detected by a collision sensor or the like. By detection of such abnormality of vehicle, a current flows into the igniting part  29  through the lead wire  31 . 
     As a result, by heat generation by the current, the igniting part  29  ignites, and the heating agent  27  which is the thermite compound generates thermite reaction heat in the following reaction formula. 
     
       
         Fe 2 O 3 +2Al→Al 2 O 3 +2Fe+386.2 (kcal) 
       
     
     By this thermite reaction heat, the thermite case  25  is heated, and by the heat generation of the heating agent  27  and the heat of the thermite case  25 , the low melting point metal  23  joining the bus bar leading end  13  and the left side wall  25   a  of the thermite case  25 , and the low melting point metal  23  joining the bus bar leading end  21  and the right side wall  25   b  of the thermite case  25  are heated and melted. At the same time, the threaded part  28  made of resin member for fixing the thermite case  25  to the outer case  15  by screwing is melted by this heat. 
     As a result, the tightening force of the thermite case  25  to the outer case  15  becomes weak, and the compressed compression spring  34  expands, and the thermite case  25  and igniting part  29  bounce up as shown in FIG. 4, so that the thermite case  25  is contained in the cavity  22  in the cap  14 . 
     Accordingly, the electric connection between the thermite case  25  and the first bus bar  11  and second bus bar  19  is cut off. That is, the first bus bar  11  and second bus bar  19  are electrically cut off, and the electric circuit of the vehicle is cut off. 
     In the circuit breaker of the first embodiment, by input of failure signal from the vehicle, and ignition of the igniting part  29 , thermite reaction induced by the heating agent  27 , and the low melting point metal  23  and the threader part  28  are melted by this thermite reaction heat, so that the compression spring  34  pops up instantly. 
     Therefore, the electric circuit of the vehicle can be cut off securely in a short time, and the electric parts can be protected. Moreover, by using the thermite reaction heat of the heating agent  27 , the circuit breaker in a simple structure can be presented. 
     Since the threaded part  28  arrests upward expansion force of the compression spring  34 , spring force is not applied to the low melting point metal  23  at the junction of the first bus bar  11  and second bus bar  19  and the thermite case  25 , so that the reliability of the junction may be enhanced. Further, since the compression spring  34  is used, it is inexpensive, and designing and assembling of the circuit breaker will be easier. 
     Further, in the case of bonding by using low melting point metal such as solder, at the junction, generation of crack due to diffusion of components of low melting point metal is suppressed. Because of low melting point metal, it is possible to bond at a relatively low temperature, and diffusion temperature when bonding can be suppressed low, and formation of intermetallic compound which is likely to lead to generation of cracks can be effectively reduced. 
     A circuit breaker in a second embodiment of the invention is described below. 
     In this embodiment, parts shown in FIG. 5 to FIG. 7 are identified with same reference numerals in the parts corresponding to the first embodiment shown in FIG. 1 to FIG. 4, and their detailed description is omitted. 
     In the circuit breaker of the embodiment shown in FIG. 5 to FIG. 7, a conductive resin  24  formed by insert molding is pressure welded and disposed between a first bus bar  11  and a second bus bar  19 , and this conductive resin  25  is a resin made of a mixture of metal fiber, low melting point metal, flux and synthetic resin. As the metal fiber, copper fiber, brass fiber, aluminum fiber, stainless steel fiber or the like may be preferably used. 
     In the lower part of the conductive resin  24 , there is a thermite case  25  filled with a heating agent  27 , and an igniting part  29  is disposed in the lower part of the thermite case  25 . The conductive resin  25 , thermite case  25 , and igniting part  29  are contained in an outer case  15 . 
     For tightening and fixing the thermite case  25  to the outer case  15 , a threaded part  28  made of a resin member is disposed at the lower side of the thermite case  25 , and this threaded part  28  consists of a thermite threaded part  28   a  and an outer case threaded part  29   b  to be engaged with this thermite threaded part  28   a  same as shown in FIG.  3 . 
     In thus constituted circuit breaker of the embodiment, the operation is described below. 
     Usually, the first bus bar  11  and second bus bar  19  are electrically connected with each other through the low melting metal  23  and thermite case  25 , and a current is supplied from a battery to a load (neither shown). Herein, the current mainly flows into the conductive resin  24 . 
     If the vehicle collides against an obstacle or tumbles off a cliff or the like, the abnormality of the vehicle is detected by a collision sensor or the like. By detection of such abnormality of vehicle, a current flows into the igniting part  29  through a lead wire  31 . 
     As a result, by heat generation by the current, the igniting past  29  ignites, and the heating agent  27  which is the thermite compound generates thermite reaction heat. By this thermite reaction heat, the thermite case  25  is heated, and by the heat generation of the heating agent  27  and the heat of the thermite case  25 , the threaded part  28  for fixing the thermite case  25  to the outer case  15  by screwing is melted. 
     As a result, the tightening force of the thermite case  25  to the outer case  15  becomes weak, and the compressed compression spring  34  expands, and the conductive resin  24 , thermite case  25  and igniting part  29  bounce up as shown in FIG. 7, so that the conductive resin  24  and thermite case  25  are contained in the cavity  22  in the cap  14 . 
     Accordingly, the electric connection between the conductive resin  24  and the first bus bar  11  and second bus bar  19  is cut off. That is, the first bus bar  11  and second bus bar  19  are electrically cut off, and the electric circuit of the vehicle is cut off. 
     In the circuit breaker of this embodiment, therefore, the electric circuit of the vehicle can be cut off securely in a short time, and the electric parts can be protected. Moreover, by using the thermite reaction heat of the heating agent  27 , the circuit breaker in a simple structure can be presented. 
     Since the threaded part  28  arrests upward expansion force of the compression spring  34 , spring force is not applied to the junction of the first bus bar  11  and second bus bar  1  and the conductive resin  24 , so that the reliability of the junction may be enhanced. Further, since the compression spring  34  is used, it is inexpensive, and designing and assembling of the circuit breaker will be easier. 
     A circuit breaker in a third embodiment of the invention is described below. 
     In this embodiment, parts shown in FIG. 8 to FIG. 10 are identified with same reference numerals in the parts corresponding to the first embodiment shown in FIG. 1 to FIG. 4, and their detailed description is omitted. 
     In the circuit breaker of the embodiment shown in FIG. 8 to FIG. 10, an upper case  14   a  and a lower case  14   b  are disposed between a first bus bar  11  and a second bus bar  19 . The upper case  14   a  has a square groove  37 , and this groove  37  is engaged with a first protrusion  39  formed in the lower case  14   b . In the lower case  14   b , a thermite case  26  made of copper or copper alloy is contained, and this thermite case  26  is filled with a heating agent  27 , and an igniting part  29  is also contained therein. 
     A left side wall  25   a  formed in the thermite case  26  is joined to a bent part  8  of the first bus bar  11  with a low melting point metal  23  as low melting point material, and a right side wall  25   b  is jointed to a bent part  10  of the second bus bar  19  with the low melting point metal  23 . 
     A compression spring  34  is disposed between the thermite case  26  and lower case  14   b , and this compression spring  34  pushes the thermite case  26  upward. The lower case  14   b  has a second protrusion  41  made of a resin member, and this protrusion  41  pushes the upper surface of the thermite case  26  so as to arrest upward move of the thermite case  26  by the spring force of the compression spring  34 . 
     In thus constituted circuit breaker of the embodiment, the operation is described below. 
     Usually, the first bus bar  11  and second bus bar  19  are electrically connected with each other through the low melting metal  23  and thermite case  26 , and a current is supplied from a battery to a load (neither shown). 
     If the vehicle collides against an obstacle or tumbles off a cliff or the like, the abnormality of the vehicle is detected by a collision sensor or the like. By detection of such abnormality of vehicle, a current flows into the igniting part  29  through a lead wire  31 . 
     As a result, by heat generation by the current, the igniting part  29  ignites, and the heating agent  27  which is the thermite compound generates thermite reaction heat. By this thermite reaction heat, the thermite case  26  is heated, and by the heat generation of the heating agent  27  and the heat of the thermite case  26 , the low melting point metal  23  joining the bent part  8  and the left side wall  25   a  of the thermite case  25 , and the low melting point metal  23  joining the bent part  10  and the right side wall  25   a  of the thermite case  25  are heated and melted. At the same time, the second protrusion  41  of resin member formed in the lower case  14   b  is heated by the heat. 
     Then, the compressed compression spring  34  expands, the thermite case  26  containing the igniting part  29  pops up, so that the thermite case  26  is put into the cavity  22  (in FIG. 8, reference numeral  26 ′ shows the thermite case after moving upward). 
     Accordingly, the electric connection between the thermite case  26  and the first bus bar  11  and second bus bar  19  is cut off. That is, the first bus bar  11  and second bus bar  19  are electrically cut off, and the electric circuit of the vehicle is cut off. 
     In the circuit breaker of this embodiment, therefore, the electric circuit of the vehicle can be cut off securely in a short time, and the electric parts can be protected. Moreover, by using the thermite reaction heat of the heating agent  27 , the circuit breaker in a simple structure can be presented. 
     Moreover, since the second protrusion  41  formed in the lower case  14   b  arrests upward expansion force of the compression spring  34 , spring force is not applied to the low melting point meal  23  at the junction of the first bus bar  11  and second bus bar  19  and the thermite case  26 , so that the reliability of the junction may be enhanced. Further, since the compression spring  34  is used, it is inexpensive, and designing and assembling of the circuit breaker will be easier. 
     A circuit breaker in a fourth embodiment of the invention is described below. 
     The circuit breaker of this embodiment is characterized by bonding of thermite case and bus bars in order that positioning may be easy when joining the thermite case and bus bars with low melting point metal. Herein, therefore, mainly the thermite case and bus bars are described. 
     In this embodiment, the constitution except for the thermite case and bus bars shown in FIG. 11 to FIG. 14 is same as the constitution of the third embodiment shown in FIG. 8 to FIG. 10, and the circuit cut-off operation is also same as in third embodiment, and their description is omitted. 
     As shown in FIG. 11 to FIG. 14, a round hole  12  is formed at one end of the first bus bar  11   a , and a round hole  20  is formed at one end of the second bus bar  19   a , and other ends of the first bus bar  11   a  and second bus bar  19   a  are arranged so as to enclose the thermite case  26   a , and the bent parts  8 ,  10  of the bus bars and the thermite case  26   a  are bonded with low melting point metal  23 . 
     The thermite case  26   a  is composed of a rectangular parallelepiped  50   a  and a tube  50   b , and recesses  51  are formed near four corners of the lower side of the rectangular parallelepiped  50   a  for positioning of the bus bars and thermite case  26   a . At the upper side of the thermite case  26   a , an opening  53  is formed for accommodating the heating agent  27 , and this opening  53  is sealed by an upper lid  42  of a disk form. 
     In the bent part  10  of the second bus bar  19   a  shown in FIG. 12, both sides are notched, and at the notched ends, a pair of convex parts  56  are formed to be fitted with a pair of concave parts  41  at the right side of the thermite case  26   a , and the constitution of the first bus bar  11   a  is same as that of the second bus bar  19   a , and a pair of convex parts  56  are also formed in the second bus bar  19   a.    
     In thus constituted circuit breaker of the embodiment, by pushing the thermite case  26   a  until the convex parts  56  formed in the first bus bar  11   a  and second bus bar  19   a  are engaged with the convex parts  51  formed in the thermite case  26   a , it is easy to position the first bus bar  11   a  and second bus bar  19   a  with the thermite case  26   a  when bonding with the low melting point metal  23 . 
     Of course, the positioning constitution of the circuit breaker of this embodiment can be applied, as required, to the first and second embodiments. 
     A circuit breaker in a fifth embodiment of the invention is described below. 
     The circuit breaker of this embodiment is characterized by assuring double reliability of the low melting point metal and connection by pressing the side walls of the thermite case with protrusions of the bus bars. 
     In this embodiment, the constitution except for the thermite case and bus bars shown in FIG. 15 to FIG. 17 is same as the constitution of the third embodiment shown in FIG. 8 to FIG. 10, and the circuit cut-off operation is also same as in third embodiment, and their description is omitted. 
     In FIG. 15 to FIG. 17, a round hole  12  is formed at one end of the first bus bar  11   b , and a round hole  20  is formed at one end of the second bus bar  19   b , and other ends of the first bus bar  11   b  and second bus bar  19   b  are arranged so as to enclose the thermite case  26   b , and the bent parts  8 ,  10  of the bus bars and the thermite case  26   b  are bonded with low melting point metal  23 . 
     The thermite case  26   b  is composed of a rectangular parallelepiped  61   a  and a tube  61   b , and four long grooves  62  are formed at side walls of the rectangular parallelepiped  61   a . In the second bus bar  19   b , a pair of curved parts curved outward in a semicircular form are formed in the bent part  10 , and a slightly projecting semicircular protrusion  59  is formed at the leading end of the curved part  57 . 
     The leading end of the curved part  57  and the protrusion  59  are designed to be fitted with the long groove  62  formed in the thermite case  26   b.    
     The constitution of the first bus bar  11   b  is same as that of the second bus bar  19   b , and its description is omitted. 
     In thus constituted circuit breaker of the embodiment, by inserting the leading ends of the bent parts  57  formed in the first bus bar  11   b  and second bus bar  19   b  into the long grooves  62  formed in the thermite case  26   b , the protrusions  59  press the long grooves  62 . That is, the side walls of the thermite case  26   b  are pressed by the protrusions  59  of the first bus bar  11   b  and second bus bar  19   b.    
     Therefore, in this embodiment, while connecting by the low melting point metal  23 , the electric connection of the first bus bar  11   b  and second bus bar  19   b  with the thermite case  26   b  is assured by the protrusions  59  of the first bus bar  11   b  and second bus bar  19   b , so that the connection reliability may be doubled. 
     Of course, the positioning constitution of the circuit breaker of this embodiment can be applied, as required, to the first and second embodiments. 
     A modified example of the circuit breaker in the fifth embodiment of the invention is described below. 
     The circuit breaker of this modified example shown in FIG. 18 to FIG. 20 is also characterized by assuring double reliability of the low melting point metal and connection by pressing the side walls of the thermite case with protrusions of the bus bars. 
     A round hole  12  is formed at one end of the first bus bar  11   c , and a round hole  20  is formed at one end of the second bus bar  19   c , and other ends of the first bus bar  11   c  and second bus bar  19   c  are arranged so as to enclose the thermite case  26   c , and the bent parts  8 ,  10  and the thermite case  26   c  are bonded with low melting point metal  23  same as in the fifth embodiment. 
     A pair of L-shaped parts  64  are formed at the bent part  19  of the second bus bar  19   c , and a slightly projecting hemispherical protrusion  65  is formed at the inner wall of the L-shaped part  64 . The L-shaped part  64  and the protrusion  65  are pressing the side wall of the thermite case  26   c.    
     At the lower side of the L-shaped part  64 , a wedgeshaped projecting stopper  66  is formed for positioning the thermite case  26   c  in the vertical direction. The constitution of the first bus bar  11   c  is same as in the second bus bar  19   c , and the description is omitted. 
     In thus constituted modified example of the circuit breaker, by enclosing the side walls of the thermite case  26   c  by the L-shaped parts  64  formed in the first bus bar  11   c  and second bus bar  19   c , the protrusions  65  press the side walls of the thermite case  26   c.    
     In this modified example, too, the electric connection of the first bus bar  11   c  and second bus bar  19   c  and the thermite case  26   c  is assured by the protrusions  65 . Together with the connection by the low melting point meal  23 , the reliability of the connection is assured in double. Further, since the stopper  66  is provided, the thermite case  26   c  may be positioned easily in the vertical direction. 
     A similar stopper is applied in a modified example of the circuit breaker of the fourth embodiment. 
     The modified example of the circuit breaker in the fourth embodiment shown in FIG. 21 to FIG. 23 is characterized by easy positioning of the thermite case and bus bars in the vertical and lateral direction. 
     That is, other ends of the first bus bar  11   d  and second bus bar  19   d  are arranged so as to enclose the thermite case  26   d , and the bent parts  8 ,  10  and the thermite case  26   d  are bonded with low melting point metal  23 . 
     The thermite case  26   d  is composed of a rectangular parallelepiped  68   a  and a tube  69   b , and square grooves  69  are formed near four corners of the lower side of the rectangular parallelepiped  68   a . In the bent part  10   d  of the second bus bar  19   d , a pair of stoppers  71  cutting off part of the bus bar and projecting like a wedge are formed in the lateral direction, and the pair of stoppers  71  abut against the pair of grooves  69  formed in the thermite case  26   d.    
     Herein, as shown in FIG. 23, the distance a between the pair of stoppers  71  and the distance b between the pair of grooves  69  is set nearly equal to each other. 
     The constitution of the first bus bar  11   d  is similar to that of the second bus bar  19   d , and the description is omitted. 
     In this constitution of the modified example, when the thermite case  26   d  is pressed downward, the four grooves  69  formed in the thermite case  26   d  abut against the four stoppers  71  formed in the first bus bar  11   d  and second bus bar  19   d.    
     Therefore, in this modified example, too, positioning of the thermite case  26   d  and first bus bar  11   d  and second bus bar  19   d  is easy in the vertical and lateral direction. 
     A circuit breaker in a sixth embodiment of the invention is described below. 
     In this embodiment, the constitution except for the thermite case and bus bars shown in FIG.  25  and FIG. 26 is same as the constitution of the fourth embodiment shown in FIG. 11 to FIG. 14, and its circuit cut-off operation is also same as explained in the fourth embodiment, and their description is omitted herein. 
     In FIG. 24 corresponding to FIG. 11 explained in the fourth embodiment, when tightened by turning screws clockwise by inserting screws into a round hole  12  formed in the first bus bar  11   a  and a round hole  20  formed in the second bus bar  19   a , force F by rotation for tightening the screw as indicated by arrow is applied to part A and part B. Such force may be applied to the junction of the low melting point metal  23  to have adverse effects on the durability of the low melting point metal  23  or tightening of the thermite case. 
     Accordingly, in the circuit breaker of the sixth embodiment and its modified examples, between the screw tightening portion of the bus bars and the thermite case, notches are provided to absorb stress and vibration from the bus bars, and transmission of stress from the bus bars to the junction of the low melting point metal  23  is suppressed, thereby preventing loss of durability of low melting point metal  23  and adverse effects on cut-off by tightening of the thermite case. 
     First, in the circuit breaker of the sixth embodiment shown in FIG.  25  and FIG. 26, a round hole  12  is formed at one end of the first bus bar  11   e  and a round hole  20  is formed at one end of the second bus bar  19   e , and other ends of the first bus bar  11   e  and second bus bar  19   e  are arranged to enclose the thermite case  26   a , so that the bent parts  8 ,  10  and thermite case  26   a  are bonded by the low melting point metal  23 . 
     Further, in the first bus bar  11   e , a pair of triangular notches  73  are formed between the round hole  12  and low melting point metal  23 , and also a pair of triangular notches  73  are formed between the round hole  20  and low melting point metal  23  in the second bus bar  19   e . Herein, pairs of notches  73  are deviated by one position each to the upper side and lower side of the bus bars. 
     In this constitution, for example, when the screw is passed in the round hole  12  and tightened clockwise in FIG. 25, the clockwise rotating force acts to close the upper side notch  73  and open the lower side notch  73 . As a result, the stress by screw tightening rotation is absorbed by the pair of notches  73 , and therefore transmission of screw tightening stress to the junction of the low melting point metal  23  is suppressed, and electric faulty contact can be prevented. 
     Of course, the constitution of the circuit breaker of this embodiment can be applied, as required, to the first to third and fifth embodiments and their modified examples. 
     In a first modified example of the circuit breaker of the sixth embodiment shown in FIG. 27, a wavy curved part  74  is formed at a position slightly remote from the low melting point metal  23  in the first bus bar  11   f , and also a wavy curved part  74  is formed at a position slightly remote from he low melting point metal  23  in the second bus bar  19   f . The positions of the curved parts  74  are near the low melting point metal  23 , so that the bending radius R may not be less than the plate thickness of the bus bars  11   f ,  19   f.    
     According to this constitution, in this modified example, too, if unnecessary force is applied when tightening the screw due to deformation of bus bar or deviation of mounting position, it is absorbed by the wavy curved parts  74  same as in the sixth embodiment, and therefore transmission of screw tightening stress to the junction of the low melting point metal  23  is suppressed, and loss of durability of the low melting point metal  23  can be prevented. 
     In a second modified example of the circuit breaker of the sixth embodiment shown in FIG. 28, a round hole  20  is formed at one end of the second bus bar  19   g , and a pair of notches  75   a ,  75   b  cut off from both right and left sides are formed in the bent part  10  bent nearly at right angle to the surface of the bus bar including the round hole  20 . The bus bar leading end  21   g  at the leading end portion of the bent part  10  is bonded to the thermite case  26  by low melting point metal  23 . The first bus bar  11   g  is constituted same as the second bus bar  19   a , and its explanation is omitted. 
     According to the modified example of such constitution, for example, when the screw is tightened clockwise or counterclockwise by passing the screw into the round hole  20 , the stress by screw tightening rotation is absorbed by the deformation of the pair of notches  75   a ,  75   b  in the rotating direction, and therefore transmission of screw tightening stress to the junction of the low melting point metal  23  is suppressed, and electric faulty contact can be prevented. 
     In a third modified example of the circuit breaker of the sixth embodiment shown in FIG.  29  and FIG. 30, a round hole  20  is formed at one end of the second bus bar  19   h , and a pair of notches  75   a ,  75   b  cut off from both right and left sides are formed in the bent part  10   h  (corresponding to the shell) bent nearly at right angle to the bus bar part (corresponding to one end) including the round hole  20 . 
     The right side leading end portion of the bent part  190   h  is bent nearly at right angle in the opposite direction of the round hole  20  side, and composes a bus bar leading end portion  21   h  (corresponding to other end), and this bus bar leading end portion  21   h  and bent part  10   h  are nearly L-shaped. The bus bar leading end portion  21   h  is nearly same in the vertical size as the upper side rectangular parallelepiped of the thermite case  26 , and is bonded to the thermite case  26  with low melting point metal  23 . 
     Moreover, the gap between the right side wall of the thermite case  26  and the bent part  10   h  becomes larger as the position of the bent part  10   h  is away from the bus bar leading end portion  21   h , and the stress by screw tightening rotation is absorbed by such gap. The first bus bar  11   h  is constituted same as the second bus bar  19   h , and its explanation is omitted. 
     According to the modified example of such constitution, for example, when the screw is tightened clockwise or counterclockwise by passing the screw into the round hole  20 , the stress by screw tightening rotation is absorbed by the pair of notches  75   a ,  75   b.    
     Or, as shown in FIG. 29, when the screw is tightened clockwise, the stress by screw tightening rotation is transmitted to the bent part  10   h , and this bent part  10   h  is a bout to rotate clockwise (arrow direction in FIG.  29 ). At this time, since a wedge-shaped gap is formed between the right side wall of the thermite case  26  and the bent part  10   h , the stress is absorbed in this gap, and therefore the stress absorbing effect is greater than the effect of the circuit breaker in the second modified example. 
     A fourth modified example of the circuit breaker of the sixth embodiment shown in FIG. 31 is an application example of the third modified example. That is, the thermite case  26   i  is a cylindrical structure, and conforming to the circular shape of the cylindrical structure of the thermite case  26   i , the bus bar leading end portion  21   i  is formed in an arc, and the bus bar leading end portion  21   i  and the thermite case  26   i  are bonded with low melting point metal  23 . The other constitution of the circuit breaker in the fourth modified example is same as the constitution of the circuit breaker in the third modified example. 
     In the circuit breaker of such fourth modified example, the same effects as in the circuit breaker of the third modified example are obtained. 
     Finally, a circuit breaker in a seventh embodiment of the invention is described below. 
     In this embodiment, In FIG. 32 to FIG. 35, same parts as in the parts in the third embodiment shown in FIG. 8 to FIG. 10 are identified with same reference numerals, and their detailed description is omitted, and mainly the characteristic feature of the embodiment is explained below. 
     A cap  14   d  is an upper case, which is put cover a lower case  14   b . An upper lid  81  is put on the thermite case  26  filled with a heating agent  27 , and this upper lid  81  is tightened by a bolt  82 . 
     A protruding stopper  83  is formed at a position opposite to the upper lid  81  of the thermite case  26  of the inner wall of the cap  14   d , and this stopper  83  is formed in a cross form on the inner wall of the cap  14   d  as shown in FIG.  33 . At the existence of such stopper  83 , an air layer  84  is formed between the inner wall of the cap  14   d  and the upper lid  81 . 
     The protruding shape of the cap  14   d  is not limited to the cross form as shown in FIG. 33, but may include circle, curve, two-dimensional shape or straight line. 
     At both sides of the cap  14   d , protrusions  85  are formed, and inside of the protrusions  85 , further, square grooves  37  are formed so as to be fitted to the first protrusion  39  formed in the lower case  14   b.    
     In this constitution, in the circuit breaker of this embodiment, the circuit is cut off according to failure detection of vehicle, same as in the circuit cut-off operation of the circuit breaker in the third embodiment. At this time, the thermite case  26  pops up to abut against the stopper  83  formed in the cap  14   d , but since the stopper  83  is formed in a cross form in the cap  14   d  and the air layer  84  is formed between the cap  14   d  and thermite case  26 , deformation of the cap  14   d  due to heating of the thermite case  26  can be suppressed. At the same time, drop-out of the cap  14   d  due to deformation of the cap  14   d  is prevented. 
     Further, when the circuit is cut off, since the thermite case  26  is pressed to the cap  14   d  by the compression spring  34 , if vibration is applied to the vehicle, the thermite case  26  is not lowered, and therefore the thermite case  26  will not contact with the bus bar, and the circuit cut-off state can be maintained. 
     Moreover, since the thermite case  26  is covered with the cap  14   d  and the cap  14   d  is fixed to the lower case  14   b  by the groove  37 , when the circuit is cut off, risks of pop-up of thermite case  26  or burn by heat can be lowered. 
     Of course, the constitution of the circuit breaker of this embodiment can be applied, as required, to the first, second, and fourth to sixth embodiments and their modified examples. 
     The invention is not limited to the illustrated embodiments and examples at all. 
     In the first embodiment, for example, forming threaded parts  28  and low melting point metal  23 , the circuit is cut off when the threaded parts  28  and low melting point metal  23  are melted, but it is also possible to constitute, by using the threaded parts  28  only without forming low melting point metal  23 , to cut off the circuit when the threaded parts  28  are melted. 
     Similarly, in the third embodiment, the circuit is cut off when the second protrusion  41  and low melting point metal  23  are melted, but it is also possible to constitute, by using the second protrusion  41  only without forming low melting point metal  23 , to cut off the circuit when the second protrusion  41  is melted. 
     Besides, the invention may be further changed and modified in various forms within the technical scope thereof.