Abstract:
A safety device for monitoring heat in electrical installations includes a first connecting element in thermal relationship with an electrical connection to be monitored, a second element designed to be connected to ground, and an electrical connecting structure that connects the two elements and can adopt two states, one an insulating state in normal operating conditions and the other an interrupting state wherein there is contact and hence grounding of the first element with the second when a critical temperature is reached. The electrical connecting structure may include a fusible ring that releases a piston, or an insulative thermoretractable sheet that retracts.

Description:
Priority is claimed from French Application 99/16556 filed Dec. 28, 1999 through PCT Application PCT/FR00/03684 filed Dec. 26, 2000, by means of 35 U.S.C. §371. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device for thermal monitoring for an electrical installation and more particularly a device sensitive to heating of the connection. 
     There are known electrical installations provided with different safety devices and particularly devices sensitive to increases in the intensity of a flowing current (fusible and disconnectable) and devices for the detection of the loss of current to the ground (differential devices). 
     Experience has however shown that a number of accidents have taken place by heating of connections in a bad condition. These faulty connections are not protected by present safety devices and are substantially undetectable both by electricians and by the control agencies particularly during the reception of work. This risk is greater because the quality of connections can only deteriorate with time. 
     SUMMARY OF THE INVENTION 
     The present invention has for its object to overcome these drawbacks by providing a thermal monitoring device which is sensitive to the heating produced by a defective connection. 
     The present invention thus has for its object a thermal monitoring device for a connection of an electrical installation, characterized in that it comprises a first connecting element in heat and electrical relation with a connection to be monitored, a second connection element in electrical relationship with the ground of the installation, electrical connection means which interconnect the two connection means and which are adapted to have two conditions, namely a first or normal condition of operation in which they are electrically insulated, and a second rupture condition in which they become electrically conductive when they reach a critical temperature. 
     In one embodiment of the invention, the electrical connection means are such that when they reach the critical temperature, they pass, irreversibly, from a non-conductive condition to a conductive condition. 
     The connection means could be constituted by a conductive piston in electrical connection with the first connection, which is urged by resilient means toward the second connection, against fusible retention means which melt when the connection to be monitored reaches the critical temperature. 
     The connection means could also be constituted by an insulating element of the thermoretractable type, whose one surface, called the recto, is in contact with the first connection and the second surface, called the verso, is in contact with the second connection, this element being such that, when the temperature of the connection to be monitored reaches the critical temperature, it retracts thereby exposing a contact surface of the first connection with the second. 
     In a modification of this embodiment of the invention, the thermal monitoring device comprises an electrical sensor and conductor which is electrically connected to a second connecting element and whose one free end comes into contact with the verso of the thermoretractable insulating element in the contact surface adapted to be exposed. 
     The thermal monitoring device can be disposed in parallel to the terminals of the connection to be monitored but also can be totally integrated into the latter. In such an embodiment, the first connection element will be constituted by a mechanical constituent of the connection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There will be described hereafter, by way of non-limiting examples, various embodiments of the present invention, with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal and transverse cross sectional view of a first embodiment of a thermal monitoring device according to the invention. 
     FIG. 2 is a longitudinal and transverse cross sectional view of a second embodiment of a thermal monitoring device according to the invention. 
     FIG. 3 is a longitudinal and transverse cross sectional view of a third embodiment of a thermal monitoring device according to the invention which is integrated into the connection to be monitored. 
     FIG. 4 is a transverse cross sectional view of a fourth embodiment of a thermal monitoring device according to the invention which is integrated with the connections to be verified of a socket. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The thermal monitoring device shown in FIG. 1 is constituted by a housing  1  made of an insulating material such as for example a plastic material, which is pierced at a first end with a cylindrical recess  3  which is prolonged toward the other end by a cylindrical recess  5  on the same axis and of greater diameter. The second end of the housing  1  comprises a radial boss  7  pierced with the radial opening  9 . 
     The recess  3  permits receiving an electric wire  11  whose conductor  12  is thermally and electrically connected with a connection to be monitored located in immediate proximity and not shown in the drawing. The sheath  13  of the electric wire  11  is omitted over a certain length so as to expose the conductor  12 , which receives a metallic cable terminal  14  which is fixed to it by clamping. This cable terminal terminates at one end  15  that is transversely bent. The end  15  is pierced with an axial hole  17  which receives a rod  19  of a piston  21 , this piston being provided with a head  23  of greater diameter and an intermediate portion  25 . 
     The portion  25  is surrounded by a coil compression spring  27  which, on the one hand, bears against the end portion  15  of the cable terminal  3  and, on the other hand, against the head  23  of the piston  21 , such that it exerts on the head of this piston a force in the direction of the arrow F. The free end of the rod  19  of the piston  21  is hollow with a circular groove  29  in which is disposed a ring  31  of fusible material. This fusible material  31  ensures holding the piston  21  in position against the force of compression exerted by the head  23  of the latter via the spring  27 , such that, when the holding force of the ring  31  is overcome, the piston  21  is expelled in the direction of the arrow F. The fusible material constituting the ring  31  is such that at a given temperature, called the critical temperature, the force exerted by the spring  27  on the piston head  23  is greater than the holding force exerted by the fusible ring  31 , whereby the piston is freed. 
     The second end of the housing  1  receives a metallic member  35  of cylindrical shape which comprises an axial recess  37  receiving an electrical conductor  39  which is connected to the ground of the installation. The element  35  is pierced with a screw threaded hole receiving a locking screw  36  which enters the recess  9  of the boss  7 , and which ensures the connection of the piece  35  with the conductor  39 . 
     The holding in position of the piece  35  is ensured on the one hand by a fitting  41  at its upper portion in a corresponding recess provided in the housing  1  and by resilient bosses  43  provided at the end of the latter. 
     Under these conditions, the operation of the thermal monitoring device takes place as described above. When the connection (not shown in the drawing) to which is connected the electrical conductor  12  heats up (e.g., due to a defective connection), the heat thus produced is transmitted by the conductor and the cable terminal  13  to the thermofusible ring  31 . When the temperature to which the ring  31  is subjected is below the critical temperature, the ring  31  ensures the holding of the piston  21  against the force of the spring  27 . When this temperature increases and reaches the critical temperature, the fusible ring  31  no longer performs its holding function, and the spring  27  presses the piston  21  in the direction of the arrow F to come into contact with the element  35 . Under these conditions, an electrical connection is ensured between the connection and the ground of the circuit of the connection in question, thereby giving rise to cutting the current upstream by the triggering of the differential device. 
     If the user has not done what is necessary to overcome the problem connected with his connection, he cannot reestablish the current in the portion of his installation protected by the differential device in question. 
     The present invention thus permits detecting and signaling to the user any abnormal temperature elevation in the connections of his installation which are provided with the thermal monitoring device according to the invention. 
     In the embodiment shown in FIG. 2, the housing  44  is constituted by a non-conductive material and comprises a front cylindrical portion  45  and rear portion forming a boss  46 . As before, the portion  45  comprises an axial cylindrical recess  3  which receives one end of the electrical wire  11  whose conductor  12  is connected to an electrical connection (not shown in the drawing) located adjacent the device. The end of the electrical wire  11  is provided with a cable terminal  47  which is secured to the end of the conductor  12  of the wire  11 . The cable terminal  47  comprises a front and upper end which extends toward the interior of the recess  3  and comes into abutment against an internal protuberance  49  of the housing  44 . The cable terminal  47  is blocked on its rear portion by resilient abutments  51 . The internal surface of the cable terminal  47  is covered with a sheet  48  of the thermoretractable type, which is to say sheet which, when it is brought to a predetermined temperature, retracts. 
     The internal portion of the boss  46  is hollow with a recess  53  which receives a metallic piece  55  of complementary shape, which is hollowed by a longitudinal recess  57  which receives an electrical conductor  59  which is connected to the ground. 
     As in the previous embodiment, the connection between the piece  55  and the conductor  59  is ensured by a set screw  61 . The forward and lower portion of the element  55  is hollowed by a cavity  63  which receives a resilient metal element  65  which is radially incurved in the direction of the cable terminal  47 , such that its free end comes into contact with the external surface of the thermoretractable sheet  48 . 
     Under these conditions, the operation of the present thermal monitoring device takes place as described above. In normal operation, which is to say when the connection to be monitored (not shown in the drawing) is in good condition, and hence does not heat up, the thermoretractable sheet  48 , by reason of its insulating qualities, prevents the contact of the element  65  with the cable terminal  47 . When heating of the connection to be monitored takes place, the heat produced is transmitted by the conductor  12  to the cable terminal  47  and from the latter to the thermoretractable sheet  48 , which, when the heating temperature reaches the critical temperature, retracts sufficiently that the free end of the element  65  will come into contact with the cable terminal  47 . Under these conditions, the connection becomes connected to the ground, leading to interruption of the current upstream by the triggering of the differential circuit breaker. 
     Of course any other device could be provided between the connection and the conductive wire, connected to the ground and adapted to be triggered when the connection reaches the critical temperature. 
     There could according to the invention be provided a thermal monitoring device which will be integrated into the connection itself. There is shown in FIG. 4 such a device in the drawings at  80 , which comprises at its space a parallelopipedal cavity  81  in which is fitted to be resiliently held a metallic domino  82  of a complementary shape which is pierced with an axial recess  84  adapted to receive two conductive elements  11 ,  11 ′ to be connected, which are held in good contact with the domino  82  by the help of a set screw  83 . The body  80  comprises an upper cavity in which is disposed a cylindrical metallic cable terminal  85  pierced with an axial recess  86  adapted to receive a conductor  39 ′ which is held by a screw  83 ′, this conductor being connected to the ground of the installation. There has been arranged, at the base of the cable terminal  85 , a metallic leaf spring  88  which is contact with its free end with the upper surface of the domino  82 . As before, the upper surface of this latter is covered with a thermoretractable sheet  90  which, under normal operation of the connection, ensures electrical insulation between the domino  82  and the blade spring  88 . The resilient force of this latter blade spring can be improved by the contact of an abutment  94  provided in the body  80  which exerts a pressure against the spring. Under these circumstances, as before, when a heating of the connections of the domino  82  gives rise to heating, the thermoretractable film  90  contracts, thereby exposing a free contact of the blade spring  88  with the upper surface of the domino  82 , thereby triggering the differential device as described above. 
     There is shown in FIG. 3 a modification of the embodiment of the invention, in which the thermal surveillance device is integrated with a socket. 
     There is shown schematically in this figure, in double hatched lines, the pins  60  and  62  secured to a plug adapted to be inserted in a socket and, in single hatched lines, a terminal  64  of a socket which is connected to the ground. Although the thermal monitoring device according to the invention can be disposed on each of the pins  60  and  62 , it has been shown in FIG. 3 only relative to a pin  60 . This socket also comprises resilient metallic element  66  adapted to ensure good electrical contact with the pin  60  and its resilient metallic elements are generally fixed or secured to a piece  67  provided with a recess permitting receiving the conductor  71  and the set screws  69  which ensures holding of the connection of the wire  71 . 
     The thermal monitoring device is here integrated in the connection during clamping or riveting of the elements  66  and  67 , and is constituted by a resilient conductive probe  70  whose shape and securement of the element  66  is held in contact on the ground contact  64 . Along this probe  70 , on the undersurface and during assembly, has been deposited a thermoretractable film  77  or the like, such that at normal temperature, it electrically insulates the probe  70  and the ground terminal  64 . It will be understood, under conditions that, the connections existing between the elements  66  and the pin  60 , either between the domino  67  and the conductor  68 , are of good quality, there is no heating of these connections and that, under these circumstances the thermoretractable film  72  insulates the ground terminal from the elements  66  with good quality, and there is no heating of these connections, and that, under these circumstances, the thermoretractable filament  72  insulates the pin  64  from the elements  66 . When one of these mentioned connections heats up, the heat is transmitted via the conductive rivets  68  to the probe  70 , so that under these conditions the thermoretractable film contracts, thereby establishing electrical contact of the probe  70  with the ground pin  64 . Under these conditions, as mentioned above, the differential device is activated, thereby ensuring cutting the current and the safety of the installation.