Abstract:
A layer of thermal fusible material is placed on one surface of a metal oxide varistor (MOV) to monitor the heating of the MOV due to applied voltage spikes. The thermal fusible material is part of the electrical circuit which includes the MOV and melts at a predetermined temperature. In the presence of a severe or a number of voltage spikes the MOV heats up and the heat transferred to the thermal fusible material causes it to open the electrical circuit to the MOV to prevent overheating and thermal runaway. In another form, the MOV is separated into two halves and the thermal fusible material layer is placed between the ends of the MOV halves.

Description:
This Application is based on U.S. patent application Ser. No. 09/074,069 filed May 6, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to metal oxide varistors (MOVs) having thermal protection, and more particularly, to MOVs which contain fusible materials which melt before the MOVs can begin thermal runaway. 
     2. Description of the Prior Art 
     In one known prior art device, thermal cut-off fuses are mounted electrically in series with the MOVs and adjacent one face of the MOVs. When the MOV heats up, due to the flow of current through the MOV, it causes a rise in temperature at such face which melts the thermal cut-off fuse which opens the electrical circuit to the MOVs. The thermal cut-off fuse being separated from the MOV surface can be erroneously heated by other nearby components, such as resistors, or erroneously cooled by convection currents in the surrounding housing. 
     In a further known prior art device, thermal cut-off fuses are located remote from the surface of the MOVs they are to protect and are connected to terminals which engage one face of the MOVs. Based upon the heating the terminals sense, their associated thermal cut-off fuse may be caused to operate. The terminals must have the desired response to heat and the factors of extraneous heating and cooling are also present. 
     Another device provides protection by utilizing varistors having a relatively low initial conduction voltage and using more of them, in parallel and in conductive relationship with a heat sink, for dissipation of the energy load imposed by multiple lightning strikes, for example. 
     Still another device uses current limiting fuses between the MOVs and ground. If the current through the fuse is sufficient, the fuse blows and actuates diagnostic circuitry. 
     SUMMARY OF THE INVENTION 
     An MOV protection device according to the invention provides a fusible member in intimate contact with the MOV it is to protect and when operated by the heat produced by the MOV opens the electrical path to the MOV. In a first embodiment, a thermal fuse is formed by thermal fuse material on substantially all of one face of the MOV and a lead of the MOV is connected to such thermal fuse. When the MOV temperature reaches the operating temperature of the thermal fuse, it melts and opens the circuit to the MOV. In other embodiments only a portion of one face of an MOV is covered by thermal fuse material and this material is connected to an MOV lead. A further embodiment divides the MOV into two segments and joins them by means of a layer of thermal fuse material. When this layer melts the circuit of the MOV is interrupted. 
     In all cases the thermal fuse material is in intimate contact with the MOV and is able to directly operate in response to the heating of the MOV. There is little possibility that the thermal fuse material will be influenced by the heat generated by other components or the cooling effects of convection currents in any housing. It is an object of this invention to provide a novel MOV protection device. 
     It is another object of this invention to provide a novel MOV protection device which protects an MOV against thermal runaway. 
     It is another object of this invention to minimize the dangers from MOV failures. 
     It is still another object of this invention to provide a novel MOV protection device which is intimate contact with one face of an MOV. 
     It is yet another object of this invention to provide a novel MOV protection device which is wired into the circuit with an MOV and upon failure of the protection device opens the circuit of the MOV. 
     It is still another object of this invention to provide a novel MOV protection device which employs thermal fusible material. 
     It is yet another object of this invention to minimize the danger of MOV failures. 
     It is yet another object of this invention to reduce the fire hazard and minimize or eliminate damage to surrounding components and/or nearby personnel caused by MOV burning or explosion when overheated. 
     It is another object of this invention to provide a novel MOV protection device which employs a thermal fusible material in intimate contact with a MOV and which is in the conductive path to such MOV. 
     Other objects and features of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principles of the invention, and the best modes which are presently contemplated for carrying them out. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings in which similar elements are given similar reference characters: 
     FIG. 1 is a front elevational view of a first embodiment of a MOV thermal protection device constructed in accordance with the concepts of the invention. 
     FIG. 2 is a side elevational view, partly in section, of the device of FIG. 1, taken along the line  2 — 2 . 
     FIG. 3 is a front elevational view of another MOV thermal protection device constructed in accordance with the concepts of the invention. 
     FIG. 4 is a front elevational view of yet another MOV thermal protection device. 
     FIG. 5 is a front elevational view of still another MOV thermal protection device with its insulating layer removed to be able to view the components of the MOV protection device. 
     FIG. 6 is a top plan view of the device of FIG. 5 taken along the line  6 — 6 . 
     FIG. 7 is a front elevational view of a further embodiment of the MOV protection device. 
     FIG. 8 is a top plan view of the device of FIG.  7 . 
     FIG. 9 is an exploded, perspective view of another embodiment of a MOV thermal protection device constructed in accordance with the concepts of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The desirability of protecting loads from short term over-voltage conditions due to lightning strikes or circuit switching or the like is well known. One class of voltage limiting protection can be provided by devices whose electrical resistance varies non-linearly under applied voltage so that conduction therethrough is slight at normal power voltages but disproportionately high at high voltages. The devices are known as varistors since the resistance can vary. Some varistors are made from sintered discs of zinc oxide or silicon oxide with other lesser materials and are identified as metal oxide varistors or MOVs. When exposed to a high voltage condition it clamps the circuit to be protected to a safe voltage and directs the remainder to ground. MOVs of the type described are available from Siemens-Part No. SIOK130, General Electric Company, McGraw-Edison and Panasonic. 
     If the high voltage occurrences are spaced in time the MOV will have sufficient time to cool down to its desired operating temperature. If not, the MOV will be at an elevated temperature when the next lightning strike hits and it will heat further. The hot MOV will conduct more current and the additional heating will permit more current to flow through the MOV resulting in thermal runaway and destruction of the MOV. One way suggested to protect the MOV is a thermal protection device which is wired in series with the MOV and positioned adjacent one face of the MOV. The melting point of the thermal protection device is at a temperature below what is required to put the MOV in thermal runaway. As the temperature at the face of the MOV rises, a point is reached at which the thermal protection melts and opens one lead to the MOV which no longer receives current. In all known prior art devices the thermal protection device is adjacent but spaced apart from the face of the MOV surface. It can thus be influenced by other heat sources, e.g. resistors, and cooled by any circulating air or through conduction to the surrounding air changing the time of response of the thermal protection device. This may permit the MOV to enter thermal runaway. 
     Turning now to FIGS. 1 and 2, a first embodiment of a thermal protection device  10  constructed in accordance with the invention is shown. On one face  14  of the MOV disc  12  is placed a layer of thermal fusible material  16 . The fusible material layer  16  is thermally and electrically conductive. Thermosetting materials are preferred, such as epoxy resins readily available in granular or powder form that will become a rigid solid when heated and cured in the normal maimer. The fusible material is then attached to face  14  of MOV disc  12  by the use of adhesives, bonding or the like. As stated above, the fusible material  16  will melt at a much lower temperature than is required to cause MOV  12  failure. An insulation layer  20  covers the exposed portion of face  14 . The insulation layer  20  may be constructed from non-electrically conductive material suitable for high temperature operation. The heating of the layer  20  could be caused by sustained over-voltage when the MOV is shunting current. One material which could be employed is a thin layer of ceramic. The connection tail  18  of the fusible material layer  16  extends over the top of insulation layer  20  where it can be easily connected to a first lead  22 . Explosive destruction of the MOV often results in extensive damage to surrounding components and can also be a fire hazard or cause injury. A second lead  24  is connected to the other face  26  of the MOV device  12 . 
     Thermal energy results from current flow due to a voltage surge which results in an increase in the temperature of the MOV. If the voltage surges due to lightning strikes, switching of power, etc. are well spaced the MOV can cool down between the events. However, if the events are closely spaced the MOV does not have enough time to cool down. Instead the heating of the MOV allows more current to flow which raises the temperature and this continues until the MOV is destroyed by the thermal runaway. Explosive destruction of the MOV often results in extensive damage to the surrounding components and can also be a fire hazard or cause injury. To prevent thermal runaway, the layer of thermal fusible material  16  is employed. The layer of thermal fusible material  16  is in intimate contact with face  14  of the MOV  12 . It also has a connection tail  18  to which is connected a lead  22 . Current is normally passed through the path of lead  24  to the face  26  of the MOV  12 , the MOV  12  itself, the thermal fusible material layer  16  to the connection tail  18  and the lead  22 . If the current flowing through this circuit rises due to lightning strikes, load switching, etc. resulting in the heating of the MOV then the fusible material  16  melts and opens the path to the connection tail  18  and the lead  22 . This takes the MOV  12  out of the circuit, thus protecting it from excessive heating which could cause the MOV  12  to fracture and explode sending parts of the MOV  12  in all directions. 
     The thermal fusible material layer does not have to extend over substantially all of a face of an MOV. It can extend over a lesser portion of such face as is shown in FIGS. 3,  4 ,  7  and  8 . Referring to FIG. 3, a thermal protection device  30  is shown. The front face of MOV  32  has a generally circular layer of thermal fusible material  34  having a diameter approximately equal to the radius of the MOV  32 . A connection tail  36  extends outwardly over a circular layer of insulation  38 . A conductor  40  is fastened to the connection tail  36  and a second conductor  42  is fastened to the other side of the MOV  32  (not visible in the figure). The entire device is covered with a coating of epoxy or similar insulation (not shown) except for the portion of conductors  40  and  42  that extend from MOV  32 . The operation of the device  30  of FIG. 3 is the same as described above with respect to device  10  in FIGS. 1 and 2. 
     Referring now to FIG. 4, a further thermal protection device  50  is shown. One surface of the MOV  52  has placed thereon a layer of thermal fusible material  54  in the general shape of a rectangle. A connection tail  56  extends over a thick layer of insulation  58  and is coupled to a conductor  60 . A second conductor  62  is coupled to the opposite face of MOV  52  (not visible in the figure). The remainder of the face  64  of the MOV  52  is covered with a coating of Epoxy or similar material applied at the factory. A channel or space is preserved in the coating to allow room for the fusible material layer to run off during a thermal runaway condition (a non-explosive, non-short-circuited type of failure). FIGS. 7 and 8 show a thermal protection device  70  where the thermal fusible material  78  occupies only a portion of face  74  of the MOV  72 . The difference in this embodiment over those of FIGS. 1 to  4  is that the conductor  80  is coupled directly to the thermal fusible material layer  78  without the use of the intermediate connection tail. Conductor  82  is coupled directly to the rear face  76  of the MOV  72  and the entire device is covered with a coating of insulation (not shown) such as epoxy or similar material except for the portion of conductors  80  and  82  that extend from MOV  72 . The operation of the devices  50  and  70  are the same as that described above with respect to device  10  of FIGS. 1 and 2. 
     Turning now to FIGS. 5 and 6, a further form of a thermal protection device  90  is shown. The MOV  92  is made up of two halves  94  and  100  which are joined and spanned by a region of thermal fusible material  106 . A conductor  112  is coupled directly to rear face  98  of half  94  and a second conductor  114  is directly coupled to front face  102  of half  100 . The layer of insulation  108  (not shown in FIG. 5 to permit a better understanding of device  90 ) completely surrounds the device  90 , except for conductors  112  and  114  which extend from the MOV  92  and gap  110  and exists adjacent the thermal fusible material  106 . The gap  110  permits the run-off of fusible material layer as set forth above and any gases, produced when the thermal fusible material melts, to escape. With the thermal fusible material  106  in place a complete electrical path through the MOV  92  exists. The path goes from conductor  112  to MOV half  94 , through thermal fusible material  106  to MOV half  100  and conductor  114 . When the thermal fusible material band  106  melts, the path between the halves  94  and  100  is opened cutting off any current flow. 
     The thermal protection device  120  of FIG. 9 shows a further type of device. A MOV  122  has a disc of insulation  126  in the center of face  124 . The insulation  126  is thermally conductive but non-electrically conductive. A layer of conductive material  128  with a central cutout  130  is positioned over face  124  of MOV  122 , so that central cutout  130  is over insulation  126 . A cruciform insert  132  is fit into the central cutout  130 . The cruciform insert  132  is made of thermal fusible material and its lobes are in contact with the wall of conductive material layer  128  that defines the central cutout  130 . A first conductor  134  is connected to the insert  132  and a second conductor  136  is connected to the second face of MOV  122  (not visible in the figure). A current path is established from conductor  136  through the MOV  122  to conductive layer  128  to the insert  132  and the conductor  134 . The melting of the insert  132  interrupts the flow of current to conductor  134  by opening the circuit. 
     While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, as are presently contemplated for carrying them out, it will be understood that various omissions and substitutions and changes of the form and details of the devices illustrated and in their operation may be made by those skilled in the art, without departing from the spirit of the invention.