Abstract:
A thermal overload relay has an actuator mechanism that generates an operating force by bending of a main bimetal with a heating wire wound thereon; a switching mechanism that is driven by the operating force from the actuator mechanism; a contact changeover mechanism wherein contacts are changed by the switching mechanism operation; and a casing that contains the actuator, the switching and the contact changeover mechanisms. The casing has main circuit terminals and auxiliary circuit terminals arranged on one side, the main circuit terminals electrically connecting the thermal overload relay to an electric load device, and the auxiliary circuit terminals electrically connecting the thermal overload relay to another electric device. When the casing is viewed in plan, the main circuit terminals are aligned and, the auxiliary circuit terminals are arranged in a position offset from the position of the alignment of the main circuit terminals.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
       [0001]    The present invention relates to a thermal overload relay carrying out switching of contacts when an overcurrent is detected and particularly to an improvement of a casing of a thermal overload relay containing an actuator mechanism, a switching mechanism and a contact changeover mechanism. 
         [0002]    A thermal overload relay has a casing containing therein an actuator mechanism provided with bimetals each having a heating wire (heater) wound thereon, a shifter engaged with each of the open ends of the bimetals to be supported movably in the horizontal direction, a switching mechanism linked with a shifter to be operated by the shift of the shifter and a contact changeover mechanism carrying out changeover of contacts by the operation of the switching mechanism (see JP-A-2004-172122, for example). 
         [0003]    Such a thermal overload relay is an electric device that is connected with an electromagnetic contactor to thereby form an electromagnetic switch, makes the electromagnetic switch connected between an electric load device such as a motor and a power supply, disconnect the electric circuit between the motor and the power supply when an overcurrent is caused in which a current energizing the motor exceeds a predetermined current value, and prevents the motor from being damaged. 
         [0004]      FIG. 9  is a plan view showing an external appearance of a related thermal overload relay. In  FIG. 9 , on the upper face of a casing  50  with an approximately cubic shape, there are disposed an adjusting dial  51  for adjusting a stabilized current and a resetting rod  52  for resetting a contact mechanism. On the one side of the casing  50 , load side main circuit terminals  53  are provided which are connected to an electrical load device. Along with this, from the other side of the casing  50 , there protrude three of an R-phase power supply side terminal  54 R, an S-phase power supply side terminal  54 S and a T-phase power supply side terminal  54 T which are connected to an electromagnetic contactor. Here, an R-phase terminal  53 R, an S-phase terminal  53 S and a T-phase terminal  53 T forming the load side main circuit terminals  53 , are arranged in line at the same height. 
         [0005]    On one side of the casing  50  provided with the load side main circuit terminals  53 , there are provided first auxiliary circuit terminals b 1  and b 2  with normally closed contacts and second auxiliary circuit terminals a 1  and a 2  with normally open contacts all having the same terminal screw sizes as those of the load side main circuit terminals  53 . The first auxiliary circuit terminal b 1  and the second auxiliary circuit terminal a 1  are made positioned on the same line as that of the load side main circuit terminals  53  (the R-phase terminal  53 R, S-phase terminal  53 S and T-phase terminal  53 T). The remaining first auxiliary circuit terminal b 1  and the second auxiliary circuit terminal a 2  are provided at positions ahead of and lower than those of the load side main circuit terminals  53 . 
         [0006]    Normally, the first auxiliary circuit terminals b 1  and b 2  with normally closed contacts are provided so as to be electrically connected to terminals on the side of an electromagnetic coil of an electromagnetic contactor connected to the thermal overload relay and the second auxiliary circuit terminals a 1  and a 2  with normally open contacts are provided so as to be electrically connected to a means such as an alarm device notifying an abnormality by lighting up a lamp. 
         [0007]    [Patent Document 1] JP-A-2004-172122 
         [0008]    Such a thermal overload relay is normally used by mounting it on a wall. However, the thermal overload relay mounted on a wall, when viewed from the upper face side of the casing  50  as shown in  FIG. 9 , has the R-phase terminal  53 R, S-phase terminal  53 S and T-phase terminal  53 S forming the load side main circuit terminals  53  and the first auxiliary circuit terminal b 1  and the second auxiliary circuit terminal a 1  positioned on the same line. This may cause improper wiring when a three-phase motor is connected to the thermal overload relay. 
         [0009]    Moreover, the frequency of using the first auxiliary circuit terminals b 1  and b 2  electrically connected to an electromagnetic contactor is more than that of using the second auxiliary circuit terminals a 1  and a 2 . However, the first auxiliary circuit terminal b 2  as one of the pair of the first auxiliary circuit terminals b 1  and b 2  is positioned on the wall side. Thus, there is a problem that the work of electrical connection with the electromagnetic contactor takes a lot of time. 
         [0010]    Furthermore, within the casing  50 , there are provided three heat element containing spaces in which three bimetals of an actuator mechanism for the R-, S- and T-phases, respectively, in a three-phase system are contained isolatedly from one another, and a contact switching mechanism containing space in which a switching mechanism and a contact changeover mechanism are contained. The casing  50 , however, when the thermal overload relay is to be made downsized, causes the heating by a heating wire in the heat element containing space for the S-phase, positioned between the heat element containing space for the R-phase and that for the T-phase, to be thermally affected by the heating by the heating wires in the other heat element containing spaces to result in an increase in an amount of heating in the heating element containing space for the S-phase. To counter this, there is proposal of setting the heating capacity smaller with respect to the heating wire in the heat element containing space for the S-phase. This, however, results in increase in special process control in manufacturing the thermal overload relay to be problems in heating wire mounting work and in manufacturing cost. 
         [0011]    Accordingly, the invention was made with attention given to the unsolved problems in the above-explained related thermal overload relay with an object of providing a thermal overload relay that facilitates work of electrical connection with an electric load device and an electromagnetic contactor and, along with this, is thermally unaffected by the heating wires even though the relay is to be made downsized to make it possible to facilitate the heating wire mounting work and to reduce the manufacturing cost. 
         [0012]    Further objects and advantages of the invention will be apparent from the following description of the invention. 
       SUMMARY OF THE INVENTION 
       [0013]    In order to achieve the above object, the thermal overload relay according to the invention includes an actuator mechanism that generates an operating force by bending of a main bimetal (with a heating wire wound thereon) in response to a temperature rise in the main bimetal; a switching mechanism that is driven by the operating force from the actuator mechanism; a contact changeover mechanism with its contacts changed over by the operation of the switching mechanism; and a casing that contains the actuator mechanism, the switching mechanism and the contact changeover mechanism. The casing has main circuit terminals and auxiliary circuit terminals arranged on one side, the main circuit terminals electrically connecting the thermal overload relay to an electric load device, and the auxiliary circuit terminals electrically connecting the thermal overload relay to another electric device. When the casing is viewed from its upper face cover side, the main circuit terminals are aligned and, along with this, the auxiliary circuit terminals are arranged in a position different from the alignment of the main circuit terminals. 
         [0014]    According to the invention, the upper face cover of the thermal overload relay mounted on a wall faces a worker carrying out wiring work that connects the thermal overload relay to an electric load device. In this state, the auxiliary circuit terminals are arranged in positions different from the positions of the aligned main circuit terminals. Therefore, a distinction in visual angle is clearly established between the circuits of the main circuit terminals and the circuits of the auxiliary circuit terminals. 
         [0015]    Moreover, in the thermal overload relay according to the invention, the auxiliary circuit terminals are formed of first auxiliary circuit terminals with normally closed contacts and second auxiliary circuit terminals with normally open contacts. When the casing is viewed from the side, the first auxiliary circuit terminals with normally closed contacts are arranged in upper positions with respect to the positions of the second auxiliary circuit terminals with normally open contacts. 
         [0016]    According to the invention, the first auxiliary circuit&#39; terminals with higher frequency of being used for connection with a device such as an electromagnetic contactor are at positions on the side of a worker (on the front side) with respect to the positions of the second auxiliary circuit terminals. This facilitates the connection work on the first auxiliary circuit terminals compared with the connection work at the related thermal overload relay. 
         [0017]    In addition, the thermal overload relay according to the invention has on the upper face cover of the casing, an auxiliary circuit terminal display formed in the vicinity of the auxiliary circuit terminals, for indicating the terminal types and positions of the auxiliary circuit terminals. 
         [0018]    According to the invention, a worker can perform precise connection work between a specified wire and a terminal while observing the auxiliary circuit terminal display on the upper face cover of the casing. 
         [0019]    Moreover, the thermal overload relay according to the invention is, within the casing, provided with a heat element containing space for containing the actuator mechanism having the main bimetal with a heating wire wound thereon, and a contact switching mechanism containing space containing the switching mechanism and the contact changeover mechanism. The thermal overload relay is further provided with vents on both of the side faces of the casing for enabling an interchange of the air in the heat element containing space and the air on the outside of the casing. 
         [0020]    According to the invention, the temperature rise in the heat element containing space can be inhibited so as not to affect the operation of the main bimetal. 
         [0021]    In addition, the thermal overload relay according to the invention is provided with vents on both of the side faces of the casing for enabling an interchange of the air in the contact switching mechanism containing space and the air on the outside of the casing. 
         [0022]    According to the invention, the temperature rise in the contact switching mechanism containing space can be inhibited so as not to affect the operations of the switching mechanism and the contact changeover mechanism. 
         [0023]    Moreover, in the thermal overload relay according to the invention, each of the vents is made to have a ventilation structure in which an opening formed in the casing has a plurality of first bars extendedly provided in parallel with one another at specified intervals so as to partly shut the opening and, in a position on the inside of the casing at a specified distance apart from a plurality of the first bars, a plurality of second bars are extendedly provided in parallel with one another at specified intervals. In the ventilation structure, the first bars and the second bars are alternately arranged so that one first bar blocks a gap between adjacent second bars and one second bar blocks a gap between adjacent first bars  15   b.    
         [0024]    According to the invention, ventilation of the air on the inside and the outside of the casing can be ensured and, along with this, the casing can be prevented from foreign matter entering into it. 
         [0025]    Furthermore, the thermal overload relay according to the invention has on the upper face cover of the casing an operation indication and manual trip operation window formed through which window the contact changeover mechanism can be manually changed over and the operation of the contact changeover mechanism can be recognized. The peripheral wall of the opening of the operation indication and manual trip operation window is formed as an inclined surface with an inclination ascending from the bottom part of the peripheral wall in the direction away from the opening. 
         [0026]    According to the invention, the region within which a worker can observe the operation indication and manual trip operation window becomes enlarged to allow the worker to easily recognize the operation of the contact changeover mechanism. 
         [0027]    According to the invention, in the thermal overload relay, the upper face cover of the thermal overload relay mounted on a wall assumes a state of facing a worker carrying out wiring work of connecting the thermal overload relay to an electric load device. In this state, the auxiliary circuit terminals are arranged at positions different from the positions of the main circuit terminals are aligned. Thus, a distinction in visual angle is clearly established between the circuits of the main circuit terminals and the circuits of the auxiliary circuit terminals. Therefore, improper wiring wherein a worker carries out wiring work, which is to be made on the main circuit terminals for connecting an electric load device, erroneously on terminals other than the main circuit terminals, can be securely avoided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a perspective view showing an external appearance of a thermal overload relay according to an embodiment of the invention as seen from a side provided with load side main circuit terminals and auxiliary circuit terminals; 
           [0029]      FIG. 2  is a perspective view showing an external appearance of the thermal overload relay according to the embodiment of the invention as seen from the side with connecting wires connected to an electromagnetic contactor; 
           [0030]      FIG. 3  is a top view showing the thermal overload relay according to the embodiment of the invention; 
           [0031]      FIG. 4  is a perspective view showing the thermal overload relay according to the embodiment of the invention with a side cover removed; 
           [0032]      FIG. 5  is a perspective view showing the main part of the contact changeover mechanism in the thermal overload relay according to the embodiment of the invention; 
           [0033]      FIG. 6  is a cross sectional view taken along the line  6 - 6  in  FIG. 3 ; 
           [0034]      FIG. 7  is an enlarged view showing an auxiliary circuit terminal display formed on the upper face cover of the thermal overload relay according to the embodiment of the invention; 
           [0035]      FIG. 8  is a cross sectional view taken along the line  8 - 8  in  FIG. 3 ; and 
           [0036]      FIG. 9  is a plan view showing an external appearance of a related thermal overload relay. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    In the following, an embodiment for carrying out the invention (hereinafter referred to as an embodiment) will be explained in detail with reference to the attached drawings. 
         [0038]      FIG. 1  is a perspective view showing an external appearance of the thermal overload relay according to an embodiment of the invention as seen from the side provided with load side main circuit terminals and auxiliary circuit terminals,  FIG. 2  is a perspective view showing an external appearance of the thermal overload relay according the embodiment of the invention as seen from the side with connecting wires connected to an electromagnetic contactor and  FIG. 3  is a top view showing the thermal overload relay according to the embodiment of the invention. 
         [0039]    As shown in  FIG. 1  to  FIG. 3 , a thermal overload relay  1  is provided with a casing  9  including a cubic insulation case  7   a , a side cover  7   b  attached to an opening opened on the side of the insulation case  7   a  and an upper face cover  7   c  of the insulation case  7   a.    
         [0040]    As shown in  FIG. 2  and  FIG. 3 , on the upper face cover  7   c , a resetting rod  43  is provided so as to project therefrom for resetting a contact changeover mechanism  21  that will be explained later with reference to  FIG. 4  and  FIG. 5 , an adjusting dial  11  is provided for adjusting a switching mechanism  20  shown in  FIG. 4  and, along with this, an operation indication and manual trip operation window  44  is provided. 
         [0041]    As shown in  FIG. 1 , on one side of the casing  9  (the insulation case  7   a ), load side main circuit terminals  4  are provided which are electrically connected to a three-phase motor (electric load device not shown). The load side main circuit terminals  4  are an R-phase terminal  4 R, an S-phase terminal  4 S and a T-phase terminal  4 T arranged in line at the same height. 
         [0042]    At positions adjacent to the load side main circuit terminals  4 , there are provided first auxiliary circuit terminals  5   b   1  and  5   b   2  each with a normally closed contact and second auxiliary circuit terminals  5   a   1  and  5   a   2  each with a normally open contact. The first auxiliary circuit terminals  5   b   1  and  5   b   2  are arranged in line at approximately the same height as that of the of the load side main circuit terminals  4 . The second auxiliary circuit terminals  5   a   1  and  5   a   2 , while being arranged at a height below that of the first auxiliary circuit terminals  5   b   1  and  5   b   2 , are arranged in line in the same direction as that in which the first auxiliary circuit terminals  5   b   1  and  5   b   2  are extended. In general, the first auxiliary circuit terminals  5   b   1  and  5   b   2  each with a normally closed contact are electrically connected to terminals of the coil of an electromagnet of an electromagnetic contactor connected to the thermal overload relay  1 . 
         [0043]    As shown in  FIG. 3 , when viewed from the upper face cover  7   c  side, to the positions at which the R-phase terminal  4 R, the S-phase terminal  4 S and the T-phase terminal  4 T of the load side main circuit terminals  4  are arranged, the first auxiliary circuit terminals  5   b   1  and  5   b   2  are at positions a little toward the adjusting dial  11  with a terminal arrangement different from that of the load side main circuit terminals  4  (not on the line of the load side main circuit terminals  4 ), and the second auxiliary circuit terminals  5   a   1  and  5   a   2  are also at positions distant from the positions of the adjusting dial  11  with a terminal arrangement shifted from that of the load side main circuit terminals  4  (not on the line of the load side main circuit terminals  4 ). 
         [0044]    In the vicinity of the first auxiliary circuit terminals  5   b   1  and  5   b   2  on the upper face cover  7   c , an auxiliary circuit terminal display  19  is formed which indicates the arranged positions of the first auxiliary circuit terminals  5   b   1  and  5   b   2  and the second auxiliary circuit terminals  5   a   1  and  5   a   2  as shown in  FIG. 3  and  FIG. 7 , an enlarged view showing the auxiliary circuit terminal display  19  formed on the upper face cover  7   c  of the thermal overload relay  1 . In the auxiliary circuit terminal display  19 , alphabetical letters “NC” are formed for indicating that the normally closed first auxiliary circuit terminals  5   b   1  and  5   b   2  are at positions a little toward the adjusting dial  11  side and alphabetical letters “NO” are formed for indicating that the second auxiliary circuit terminals  5   a   1  and  5   a   2  are at positions distant from the adjusting dial  11  side. Along with this, numerals “95” and “96” are formed for indicating the types and the positions of the first auxiliary circuit terminals  5   b   1  and  5   b   2  and numerals “98” and “97” are formed for indicating the types and the positions of the second auxiliary circuit terminals  5   a   1  and  5   a   2 . 
         [0045]    Moreover, as shown in  FIG. 2 , on the side cover  7   b  attached onto the other side of the casing  9  (the insulation case  7   a ), three terminals of an R-phase power supply side terminal  12 R, an S-phase power supply side terminal  12 S and a T-phase power supply side terminal  12 T are provided so as to project therefrom for connecting the thermal overload relay  1  to an electromagnetic contactor (not shown). On the side cover  7   b , connecting legs  6  are provided which is made engaged with the electromagnetic contactor when the thermal overload relay  1  is attached to an electromagnetic contactor. 
         [0046]      FIG. 4  is a perspective view showing the thermal overload relay  1  as an embodiment according to the invention with a side cover  7   b  being removed. 
         [0047]    As shown in  FIG. 4 , in the insulation case  7   a , there are provided an actuator mechanism  10  utilizing bending deformation due to temperature rise of three main bimetals  2  for three phases, respectively, the switching mechanism  20  moved by following the shift of a shifter  3  engaged with the main bimetals  2 , the contact changeover mechanism  21  in which contacts are made changed over by the operation of the switching mechanism  20  and the resetting rod  43  for resetting the contact changeover mechanism  21 . 
         [0048]    Furthermore, in the insulation case  7   a , a partition  7   d  is formed. A space provided on the right side of the partition  7   d  and containing the actuator mechanism  10  is to be referred to as a heat element containing space, and a space provided on the left side of the partition  7   d  and containing the switching mechanism  20  and the contact changeover mechanism  21  is to be referred to as a contact switching mechanism containing space. 
         [0049]    The actuator mechanism  10  is provided with the three main bimetals  2 , three heaters  2   a  of conductive material spirally wound around the three main bimetals  2 , respectively, and the above explained shifter  3  engaged with the open ends of the three main bimetals  2 . The three heaters  2   a  wound around their respective main bimetals  2  are connected, respectively, to the electrical connection section between the R-phase terminal  4 R and the R-phase power supply side terminal  12 R, the electrical connection section between the S-phase terminal  4 S and the S-phase power supply side terminal  12 S and the electrical connection section between the T-phase terminal  4 T and the T-phase power supply side terminal  12 T. Each heater  2   a  generates heat the quantity of which corresponds to the amount of current flowing in its own electrical connection section. Moreover, the three bimetals  2  are contained in their respective individual spaces formed by partitions  7   e  and  7   f  formed in the heat element containing space in the insulation case  7   a.    
         [0050]    The switching mechanism  20  is provided with an adjusting link  22 , a release lever  23  rotatably supported by the adjusting link  22  and a temperature compensation bimetal  24  secured to the release lever  23  to be made engaged with the shifter  3 . The upper part of the release lever  23  butts against the peripheral surface of an eccentric cam  11   a  of the adjusting dial  11  (see  FIG. 2  and  FIG. 3 ) rotatably disposed on the upper face cover  7   c.    
         [0051]      FIG. 5  is a perspective view showing the main part of the contact changeover mechanism  21  in the thermal overload relay  1  as an embodiment according to the invention. 
         [0052]    The contact changeover mechanism  21  is, as shown in  FIG. 5 , provided with a changeover mechanism support  32 , a link plate  34 , a movable plate  35  and a changeover spring  36 . The link plate  34  is disposed in the vicinity of the changeover mechanism support  32  while being rotatably supported by a supporting shaft  33  provided on the inner wall of the insulation case  7   a . The movable plate  35  has its upper part  35   b  made swayable with a point in its lower part  35   a , which point butts against the changeover mechanism support  32 , made to serve as a support. The changeover spring  36  is formed as a tension coil spring stretched between an engaging opening  35   c  provided on the upper part  35   b  side of the movable plate  35  and a spring support  32   a  provided in the changeover mechanism support  32  positioned under the lower part  35   a  of the movable plate  35 . 
         [0053]    At the top end of the link plate  34 , a trip operation lever  34   a  is formed. The trip operation lever  34   a  can be observed through the above-explained operation indication and manual trip operation window  44  (see  FIG. 3 ) formed on the upper face cover  7   c . With a tool such as a screwdriver, inserted through the operation indication and manual trip operation window  44 , so as to engage the trip operation lever  34   a , the movable plate  35  can be turned through the link plate  34  in the direction of causing the thermal overload relay  1  to assume a tripped state. 
         [0054]    The contact changeover mechanism  21  is provided with a fixed contact and a movable contact each with a normally open contact (a contact) and a fixed contact and a movable contact each with a normally closed contact (b contact). Electric information from the normally open contacts (a contacts) and normally closed contacts (b contacts) is transmitted to terminals of an electromagnetic coil of an electromagnetic contactor through the above-explained first auxiliary circuit terminals  5   b   1  and  5   b   2  with normally closed contacts, by which the electromagnetic contactor is operated to open so as to shut off an overload current in a main circuit. 
         [0055]    While, as shown in  FIG. 1 , below the positions at which the load side main circuit terminals  4  are provided on the casing  9  (insulation case  7 ), a first heat element section vent  15  is formed which leads to the above-explained heat element containing space. Moreover, below the positions at which the second auxiliary circuit terminals  5   a   1  and  5   a   2  are provided, a first auxiliary circuit section vent  16  is formed which leads to the above-explained contact switching mechanism containing space. 
         [0056]    In addition, as shown in  FIG. 2 , also on the side cover  7   b , there are formed a second heat element section vent  17  which leads to the heat element containing space and a second auxiliary circuit section vent  18  which leads to the contact switching mechanism containing space. 
         [0057]    Here in detail, the first heat element section vent  15  and the second heat element section vent  17  lead to the heat element containing space between the partition  7   e  and the partition  7   f  shown in  FIG. 4  (the space containing the main bimetal  2  and the heater  2   a  which are provided between the S-phase terminal  4 S and the S-phase power supply side terminal  12 S to be connected to them). 
         [0058]      FIG. 6  is a cross sectional view taken along the line  6 - 6  in  FIG. 3 . 
         [0059]    The first heat element section vent  15 , as shown in  FIG. 6 , is constructed so as to have a ventilation structure in which a large opening  15   a , formed in the casing  9  and leading to the heat element containing space, has a plurality of first bars  15   b  extendedly provided in parallel with one another at specified intervals so as to partly shut the opening  15   a  and, in a position on the inside of the casing  9  at a specified distance apart from a plurality of the first bars  15   b , a plurality of second bars  15   c  are extendedly provided in parallel with one another at specified intervals. In the ventilation structure, the first bars  15   b  and the second bars  15   c  are alternately arranged so that one first bar  15   b  blocks a gap between adjacent second bars  15   c  and one second bar  15   c  blocks a gap between adjacent first bars  15   b.    
         [0060]    The second heat element section vent  17 , as shown in  FIG. 6 , is also made to have a ventilation structure in which a large opening  17   a , formed in the casing  9  and leading to the heat element containing space, has a plurality of first bars  17   b  extendedly provided in parallel with one another at specified intervals so as to partly shut the opening  17   a  and, in a position on the inside of the casing  9  at a specified distance apart from a plurality of the first bars  17   b , a plurality of second bars  17   c  are extendedly provided in parallel with one another at specified intervals. In the ventilation structure, the first bars  17   b  and the second bars  17   c  are alternately arranged so that one first bar  17   b  blocks a gap between adjacent second bars  17   c  and one second bar  17   c  blocks a gap between adjacent first bars  17   b.    
         [0061]    Although not shown in detail, the first auxiliary circuit section vent  16  and the second auxiliary circuit section vent  18  also have the same structures as those of the first heat element section vent  15  and the second heat element section vent  17 , respectively. 
         [0062]    Moreover, the operation indication and manual trip operation window  44  provided on the upper face cover  7   c  shown in  FIG. 3  is formed in a shape with which the trip operation lever  34   a  shown in  FIG. 5  can be easily viewed. That is, as shown in  FIG. 8 , a cross sectional view taken in the direction of the arrows along the line  8 - 8  in  FIG. 3 , each of three of the four sides of a peripheral walls  44   b , forming a rectangular opening  44   a  of the operation indication and manual trip operation window  44  with their bottom parts, is formed as an inclined surface with an inclination ascending from the bottom part of the peripheral wall  44   b  in the direction away from the opening  44   a.    
         [0063]    Next, the action and effect of the thermal overload relay  1  according to the embodiment will be explained. 
         [0064]    The thermal overload relay  1  according to the embodiment is mounted on the wall with three of the R-phase power supply side terminal  12 R, the S-phase power supply side terminal  12 S and the T-phase power supply side terminal  12 T electrically connected to an electromagnetic contactor. The thus mounted thermal overload relay  1  assumes a state as shown in  FIG. 3  in which the upper face cover  7   c  faces a worker carrying out wiring work. In this state, wiring work is carried out for connecting the thermal overload relay  1  with a three-phase motor (electric load device). 
         [0065]    In the thermal overload relay  1  according to the embodiment, when viewed from the upper face cover  7   c  side as shown in  FIG. 3 , to the terminal arrangement of the R-phase terminal  4 R, the S-phase terminal  4 S and the T-phase terminal  4 T of the load side main circuit terminals  4 , the terminal arrangement of the first auxiliary circuit terminals  5   b   1  and  5   b   2  is shifted upward and the terminal arrangement of the second auxiliary circuit terminals  5   a   1  and  5   a   2  is shifted downward. Therefore, a distinction in visual angle is clearly made among the circuit for the load side main circuit terminals  4 , the circuit for the first auxiliary circuit terminals  5   b   1  and  5   b   2  and the circuit for the second auxiliary circuit terminals  5   a   1  and  5   a   2 . Thus, it is possible to securely avoid improper wiring when a worker carries out wiring work, on the load side main circuit terminals  4  such as connecting a three-phase motor erroneously to terminals other than the load side main circuit terminals  4 . 
         [0066]    Moreover, in the thermal overload relay  1  according to the embodiment, the first auxiliary circuit terminals  5   b   1  and  5   b   2  with higher frequency of being used for connection with a device such as an electromagnetic contactor are at positions on the side of a worker (on the front side) to the positions of the second auxiliary circuit terminals  5   a   1  and  5   a   2 . This facilitates the connection work at the first auxiliary circuit terminals  5   b   1  and  5   b   2  compared with the work at the related thermal overload relay. 
         [0067]    In addition, in the vicinity of the first auxiliary circuit terminals  5   b   1  and  5   b   2  on the upper face cover  7   c , an auxiliary circuit terminal display  19  is formed which indicates the terminal types and positions of the first auxiliary circuit terminals  5   b   1  and  5   b   2  and the second auxiliary circuit terminals  5   a   1  and  5   a   2 . Thus, a worker can perform precise connection work between specified wires and terminals while observing the auxiliary circuit terminal display  19  on the upper face cover  7   c.    
         [0068]    Furthermore, the first heat element section vent  15  provided on one side of the casing  9  and the second heat element section vent  17  provided on the other side of the casing  9  lead to the heat element containing space. 
         [0069]    Thus, as shown in  FIG. 6 , there is produced a flow of cooling air wherein the outside air enters the heat element containing space through, for example, the second heat element section vent  17  and is discharged from the first heat element section vent  15  after removing the heat in the heat element containing space. By the flow of the cooling air, the temperature rise in the heat element containing space due to heat generation of the heater  2   a , wound around each of the three main bimetals  2 , can be prevented so as not to affect the action of the main bimetals  2 . 
         [0070]    The first heat element section vent  15  and the second heat element section vent  17  particularly lead directly to the S-phase heat element containing space (the space between the partition  7   e  and the partition  7   f ) which is provided between the R-phase heat element containing space and the T-phase heat element containing space to be largely susceptible to the thermal influences of the spaces on both sides. Thus, in the thermal overload relay  1  according to this embodiment of the invention, the cooling effect in the S-phase heat element containing space is enhanced. Hence, the thermal influence on the space can be made reduced even though the thermal overload relay  1  is downsized. 
         [0071]    Therefore, in the thermal overload relay  1  according to the embodiment, there is no need for particularly setting the amount of heat generation small for the heater  2   a  contained in the S-phase heat element containing space. This simplifies the mounting work of the heaters to make it possible to reduce manufacturing cost. 
         [0072]    In addition, the first auxiliary circuit section vent  16  provided on one side of the casing  9  and the second heat element section vent  17  provided on the other side of the casing  9  lead to the contact switching mechanism containing space. Thus, there is produced a flow of cooling air wherein external air enters the contact switching mechanism containing space through, for example, the second auxiliary circuit section vent  18 , and is discharged via the first auxiliary circuit section vent  16  after removing the heat in the contact switching mechanism containing space. By this flow of cooling air, a temperature rise in the contact switching mechanism containing space can be prevented so as not to affect the action of the temperature compensation bimetal  24 . 
         [0073]    The first heat element section vent  15  is formed to have a double layer structure with a plurality of the first bars  15   b  arranged directly in the casing  9  in parallel with one another at specified intervals and a plurality of the second bars  15   c  arranged in parallel with one another in a position on the inside of the casing  9  at a specified distance apart from a plurality of the first bars  15   b . In the double layer structure, the first bars  15   b  and the second bars  15   c  are alternately arranged so that one first bar  15   b  blocks a gap between adjacent second bars  15   c  and one second bar  15   c  blocks a gap between adjacent first bars  15   b . This can ensure ventilation of the air on the inside and the outside of the casing  9  and, along with this, prevent foreign matter from entering into the casing  9 . Moreover, each of the second heat element section vent  17  and the first and second auxiliary circuit section vents  16  and  18  is made to have the same structure as above to make it possible to ensure ventilation of the air on the inside and the outside of the casing  9  and, along with this, to prevent foreign matter from entering into the casing  9 . 
         [0074]    Furthermore, in the operation indication and manual trip operation window  44  provided on the upper face cover  7   c , each of three of the four sides of the peripheral walls  44   b , surrounding the rectangular opening  44   a  with their bottom parts, is formed as an inclined surface with an inclination ascending from the bottom part of the peripheral wall  44   b  in the direction away from the opening  44   a . Therefore, the region within which a worker can watch the operation indication and manual trip operation window  44  becomes enlarged. This allows easy recognition of the status of the trip operation lever  34   a  when the link plate  34  is in the initial state. 
         [0075]    In the above embodiment, the operation indication and manual trip operation window  44  has each of three sides of the peripheral walls  44   b , forming the rectangular opening  44   a  with their bottom parts, provided as an inclined surface. Compared with this, even with at least one of the four sides of the peripheral walls  44   b  provided as an inclined surface, the region within which a worker can observe the operation indication and manual trip operation window  44  becomes enlarged. Further, with all of the four sides of the peripheral walls  44   b  provided as inclined surfaces, the region within which a worker can observe the operation indication and manual trip operation window  44  can be made to become best enlarged. 
         [0076]    While the present invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention. 
         [0077]    The disclosure of Japanese Patent Application No. 2009-079398 filed on Mar. 27, 2009 is incorporated as a reference. 
         [0078]    While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.