Abstract:
A self heating thermal protector located in close proximity to a remotely located incandescent light fixture for controlling the flow of current to the lighting fixture by measuring the rate of heat flow from a dedicated heat source. Current is permitted to flow to the lighting fixture when the temperature of the self heating thermal detector is sufficient to avoid deterioration of electrical components such as plastic parts and wire insulation in the associated fixture. The thermal detector interrupts the flow of current to the associated fixture when the temperature of the self heating thermal protector increases to a temperature that is not safe.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 10/745,852, filed Dec. 26, 2003, now U.S. Pat. No. 6,921,884 which is a continuation of application Ser. No. 10/218,227, filed Aug. 13, 2002 now U.S. Pat. No. 6,670,587. 

   BACKGROUND OF THE INVENTION  
   1. Field of the Invention 
   The present invention relates generally to thermal protectors and more specifically to a self heating thermal protector for use in protecting recessed incandescent lighting fixtures. 
   2. Background of the Invention 
   Recessed fixtures provide architecturally desirable illumination in homes, offices and commercial buildings. Offices and commercial buildings are generally designed for hung ceilings to accommodate electrical conduit and heating and ventilating ducts. In the past the space above the ceiling has been either empty or filled with the aforementioned equipment with air spaces between the equipment. Under these conditions the greatest danger has been overlamping, a condition which exists when a lamp of larger wattage than the fixture is rated for is installed in the fixture. When a larger wattage lamp is used, the heat generated in the fixture is not dissipated quickly enough and the fixture temperature increases to a level that is higher than intended by the fixture designer. If the fixture is operated with the larger lamp, the resulting fixture temperature will rise to a point where charring of plastic parts and wire insulation within and about the fixture can occur. Eventually, deterioration of the fixture components may result in a fire. Fires from overheated lighting fixtures have caused extensive property damage. 
   Currently, because of increased concern with energy conservation, local and federal agencies are sponsoring programs in which homes and commercial buildings are being insulated as they are built. Existing homes and commercial buildings are normally insulated by forcing thermal insulation into the spaces which are to be insulated. 
   Building codes require that a barrier be constructed around recessed fixtures to prevent thermal insulation from coming into contact with the fixture. For new structures the foregoing requirement is costly but feasible. The installation of barriers around recessed fixtures in existing structures is relatively expensive and difficult to achieve. 
   Materials used in the construction of recessed fixtures are thermally rated for the application. Such materials are lamps, sockets, wire and insulation to name a few. The overheating problem due to overlamping and of thermal insulation installed in contact with the fixture exists with all type of fixtures but may be particularly serious with respect to recessed incandescent fixtures. 
   The possibility of relying upon the heat generated by the lamp in its fixture and its rate of dissipation as an indicator of safe operation has been considered. However, because of the variety of fixtures and lamp sizes and the cost involved of adapting a thermal protector to each specific application, a universal solution of using a self heating thermal protector described herein is proposed. 
   Safe operation of a recessed fixture depends upon the fixture dissipating a predetermined quantity of heat to its surroundings. The required heat flow is obtained by the air in contact with the fixture removing the heat. The air in contact with the fixture will heat up as it absorbs heat and the heated air, being lighter, rises allowing cooler air to flow in. This continual movement of air dependably cools the fixture. If air movement is restricted by thermal insulation, heat build up will occur resulting in a dangerous situation. 
   SUMMARY OF THE INVENTION 
   The present invention is a self heating thermal protector for controlling the flow of current to a lighting fixture by measuring the rate of heat flow from a dedicated heat source located in close proximity to the incandescent light fixture. Current is permitted to flow to the lighting fixture when the temperature of the self heating thermal detector is sufficient to avoid deterioration of electrical components such as plastic parts and wire insulation in the associated lighting fixture. The thermal detector interrupts the flow of current to the associated fixture when the temperature of the self heating thermal protector increases to a temperature that is not safe. 
   Specifically, the self heating thermal protector of the present invention uses a resistor as a heat source, a bimetallic element as a switch located within a thermally conducting enclosure located proximate the heat source for controlling the flow of current to a lighting fixture and thermally conductive potting compound which encapsulates both the heat source and the bimetallic element. The potting compound provides good thermal conductivity between the resistor, the bimetallic element and the surrounding air. A mounting member for mounting the thermal detector to a junction box is composed of thermally insulating material and provides dual functions, one of mounting the thermal detector to a junction box, and a second of thermally isolating the thermal protector from the junction box. 
   The foregoing has outlined, rather broadly, the preferred feature of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention and that such other structures do not depart from the spirit and scope of the invention in its broadest form. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claim, and the accompanying drawings in which: 
       FIG. 1  illustrates the present invention operatively connected to a recessed fixture; 
       FIG. 2  is a cross sectional view of a self heating thermal protector in accordance with the principles of the invention; 
       FIG. 3  is a schematic diagram of electrical connections of the self heating thermal protector, and 
       FIG. 4  is another schematic diagram of electrical connections of the self heating thermal protector. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Self heating thermal protectors are used in recessed lighting fixtures to prevent fires which result, either directly or indirectly from overheated recessed lighting fixtures. The space around recessed lighting fixtures must be kept free of thermal insulation to allow some of the heat generated by the recessed lighting fixture to dissipate. In instances where the space around a recessed lighting fixture is filled with insulation, such as when insulation is added to a home to reduce drafts or heat loss, the transfer of heat away from the recessed lighting fixture and the self heating thermal protector will be reduced and the temperature of the fixture and the thermal protector will increase. This increase in temperature will cause a switch in the thermal protector to open which removes power from the recessed lighting fixture. This prevents the recessed lighting fixture from overheating and damaging the wiring insulation and surrounding building materials. At some time after power to the fixture has been stopped, the self heating thermal protector will cool sufficiently to allow the switch to close and power will be restored to the fixture. At some instant thereafter the fixture will again become too hot, the switch in the thermal protector will open and power will be removed form the fixture. This on-off cycling of the fixture will cause the light in the fixture to turn on and off at a very slow rate as long as power is applied to the fixture. The blinking light is an indication that a problem exists with the fixture installation. With a proper installation, no insulation surrounds the fixture and the thermal conductivity of the self heating thermal protector transfers sufficient heat to the surrounding air to keep the bimetallic switch from opening and the light in the fixture continuously on. 
   In this invention, the self heating thermal protector comprises a resistor as a heat source, a bimetallic element as a switch for controlling the flow of current to a fixture positioned proximate the heat source and thermally conductive potting compound encapsulating both the heat source and the switch. The encapsulating material provides good thermal conductivity between the resistor, the bimetallic element and the surrounding air. A mounting member of thermal insulating material is coupled to an end of the self heating thermal protector. The mounting member has a dual function, one of providing a convenient method for mounting the thermal protector and a second function of thermally isolating the thermal protector from the mounting structure. In a typical application, the thermal protector is mounted to an electrical junction box which is usually made of metal and, therefore, is a good thermal conductor. Thermal isolation from the junction box is desired to minimize it&#39;s influence on the thermal protector so that it remains primarily responsive to the surrounding air. 
     FIG. 1  illustrates an application of the present invention and is generally designated  10 . The application  10  shows a typical hung ceiling arrangement wherein a hung ceiling  12  supports a high hat fixture  14 . High hat fixture  14  has the generally cylindrical shape of recessed fixtures. Fixture  14  has a ring  22  which supports the fixture  14  as it is centered on an opening  24  and provides fixture  14  with access to the area below the hung ceiling  12 . The end of fixture  14  remote from the ring  22  is capped by a fixture end cover  16 . The fixture end cover  16  can be seen to support a lamp socket  18  which has contained therein a lamp  20 . When lamp  20  is on, it illuminates the area below the hung ceiling  12 . Contained within the fixture end cover  16  is a cable opening  26  which allows a first cable  28  access to the interior of high hat fixture  14 . The first cable  28  is connected to the lamp socket  18  and supplies power to the lamp  20 . The other end of the first cable  28  enters a junction box  30 . Junction box  30  has a support flange  32  which is used to attach the junction box  30  to the hung ceiling  12 . Shown entering the junction box  30  is a second cable  34  which brings power to the lamp  20 . A third cable  36  shown to the left of cables  28  and  34  connects a switch (not shown) which is typically mounted in the space below the hung ceiling  12  to control the flow of electricity to the lamp. 
   Projecting from the front of the junction box  30  is a self heating thermal protector  38 . The self heating thermal protector  38  is attached to the junction box  30  by means of a mounting member of thermally insulating material. The self heating thermal protector consists of a heat source such as a resistor located proximate a bimetallic switch element, each of which is encapsulated within thermally conducting potting compound which is fire retardant and may be electrically insulating. 
     FIG. 2  shows a side cut away view of a self heating thermal protector in accordance with the principles of the invention. A resistive type of heating element  60  and a bimetallic switch  62  located in close proximity to the heating element are encased within thermally conductive potting compound such as Cast-Coat CC#-301 AD-FR with H-7 hardener to form the self heating thermal protector. The potting compound is fire retardant and may also be electrically insulating. The thermal protector can have a cylindrical shape and supports, at one end thereof, a mounting member  46 . The mounting member  46  has a central opening  48  for receiving electrical conductors  64  which connect the heating element and the bimetallic switch embedded within the potting compound to externally located circuitry. The mounting member  46  is adapted to be attached to an electrical junction such as junction box  30  of  FIG. 1 . To facilitate this attachment, the outer surface of the mounting member  46  is provided with a threaded surface adapted to engage a nut  50 . The mounting member is composed of a thermosetting phenol material which provides thermal isolation between the supporting electrical junction box which is normally composed of metal and the body of the self heating thermal protector  38 . 
   The mounting member  46  has a dual function. It provides a convenient method for mounting the self heating thermal protector and, in addition, provide thermal isolation of the thermal protector from the mounting structure (normally a metal electrical junction box which is usually a good thermal conductor). Thermal isolation of the thermal protector from an electrical junction box is desired to minimize the influence that the junction box will have on the thermal protector so that it remains primarily responsive to the surrounding air. 
   The heating element can be of carbon or metal composition, a bare wire wound on a core or wire encased within its own protective covering, either of which can be encapsulated within the potting compound. When the heating element is a bare wire, the potting compound should be electrically insulating. The bimetallic switch is located within a thermally conductive protective enclosure which is then encapsulated by the potting compound. The enclosure prevents the potting compound from interfering with the operation of the bimetallic elements of the switch. 
     FIG. 3  illustrates a schematic of one embodiment of the electrical connection of the present invention together with the lamp it serves. Within the dashed lines of  FIG. 3  is an electrical circuit for interconnecting the various components. Power input is from a first input terminal  76  and a second input terminal  88  connected to a source of input power. The power output from the thermal protector is from output terminals  77  and  87 . Within the self heating thermal protector  38  is bimetallic switch  62  having terminals  78 ,  82  and a resistor  60 . Terminal  78  is connected to terminal  76 ; and terminal  82  is connected directly to terminal  77  and through resistor  60  to terminals  87  and  88 . A fixture lamp  20  is connected across terminals  77 ,  87 . The heat generating resistor  60  is located proximate the bimetallic switch  62 . 
     FIG. 4  illustrates a second schematic wiring diagram of an electrical circuit for the present invention together with the lamp that it controls. Shown within the dashed line is the wiring schematic of a second embodiment of the self heating thermal protector of the present invention. Power to the fixture lamp  20  is obtained from terminals  91  and  102 . The current which enters the output terminal  91  is obtained from an input terminal  90  which is connected directly to a power source as is input terminal  102 . Disposed between terminals  90 ,  91  is the self heating thermal protector  38 . Within the thermal protector is a bimetallic switch  62  having an input  92  directly connected to terminal  90 . Output terminal  96  from the bimetallic switch  62  is connected to one end of a heat generating resistor  60 . The other end of the heat generating resistor  60  is connected to terminal  91 . 
   The operation of the self heating thermal protector of the present invention will be better understood from the following discussion taken together with the drawings. 
     FIG. 1  depicts the situation that exists when the high hat fixture  14  is installed in a hung ceiling  12 . Typically, all electrical connections are made inside the junction box  30 . The installation  10  shows the high hat fixture  14  surrounded by air thereby safely dissipating its internally generated heat. The temperature at the junction box  30  which is physically separate from the fixture is therefore low compared to the temperature of the high hat fixture  14 . If the fixture  14  and the box  30  become surrounded or covered by insulation, a pocket is formed within which fixture  14  and box  30  fit, and the heat dissipated by the fixture  14  will heat the air in the pocket. This situation is a distinct possibility if thermal insulation is blown or foamed into the space within a hung ceiling. When this conditions exist, the temperature at the heat flow detector  38  will increase. The heat generated by the resistor  60  will be prevented from being dissipated to the surrounding air which will cause a further temperature increase at the heat flow detector  38 . At this time the bimetallic switch  62  will open to disconnect the electricity from the lamp when the temperature in the hung ceiling area near the high hat fixture  14  may cause damage to the insulation on the wire or lead to a fire. The bimetallic switch is a mechanical switch which employs a bimetallic element to sense temperature. The bimetallic element distorts as the temperature is raised. A moderate bow assumed by the bimetallic element becomes more pronounced with an increase in temperature. As the bow increases, the contacts of the switch move away from each other and the electrical circuit to the lamp is opened. 
   Because of the relatively good thermal conductivity that exists between the heating element and the bimetallic switch, a minimal thermal gradient exists in the self heating thermal protector disclosed. Therefore, the surface temperature of the device is similar to the temperature of the bimetallic switch. With appropriate operating characteristics of the heater and the bimetallic switch, the convection cooling obtained from the surrounding air will keep the self heating thermal protector below the operating temperature of the bimetal switch and, therefore, prevent the switch from opening. But, if the surrounding air has restricted convection, as can occur if insulation is positioned adjacent to the fixture, the transfer of heat from the self heating thermal protector to the air will be reduced and the temperature of the self heating thermal protector will increase to a value that is above the temperature at which the contacts of the switch will open. The opening of the contacts causes the power to be removed from the lamp in the fixture to limit or prevent overheating of the fixture and the associated hazards of overheating such as insulation breakdown and fire. 
   The embodiment of the present invention herein described and disclosed are presented merely as examples of the invention. Other embodiments, forms and structures coming within the scope of this invention will readily suggest themselves to those skilled in the art, and shall be deemed to come within the scope of the appended claims.