EL LIGHTS AND EMERGENCY LIGHTING SYSTEM FOR HAZARDOUS AREAS

Abstract: An electroluminescent light and an intrinsically safe lighting system for use in hazardous areas. The light has controlled dielectric strength and capacitance, which when matched with the low current and voltage from an intrinsically safe power supply, and the controlled capacitance of each lighting circuit, produces an emergency lighting system which is both safe and effective.

Description: EL LIGHTS AND EMERGENCY LIGHTING SYSTEM FOR HAZARDOUS AREAS INTRODUCTION This invention relates generally to electroluminescent ("EL") lights and an emergency lighting system, particularly for marking an exit path, for use in explosive, flammable and other hazardous areas. This system combines a power source capable of providing AC power of controlled voltage and current to a system of two or more parallel EL lighting circuits of controlled capacitance, providing both an assurance of spark-free, low temperature operation and adequate- brightness for emergency use. BACKGROUND OF THE INVENTION Providing adequate illumination in hazardous work areas, particularly explosive or flammable work areas, has been difficult to accomplish with adequate safety. Most lighting systems utilize circuitry with an inherent spark potential, operate at high temperatures, or both. In addition, large area or long-length lighting systems commonly require power inputs which create the hazard of fire or explosion in these unique environments. This is true for both ambient lighting and emergency lighting. While daylight operation can be an adequate answer to the former, emergency lighting must operate at all times and in adverse atmospheres (smoke, fog, etc.), and hence creates special difficulties. Certain metering and other devices for use in hazardous areas have used what is commonly known as "intrinsically safe" ("IS") circuitry and equipment, where low power input is adequate, such as about one watt or less. IS systems call for the use of low capacitance, current and voltage for safety reasons. Components and circuitry of this type are provided, for example, by the MTL Group of Luton England, but such systems have not been favored for use in lighting systems as acceptable light sources were not available. EL lights have been used for exit path marking systems, such as those provided by Loctite Luminescent Systems, Inc. , Lebanon, N.H. , U.S.A. but have not been used in hazardous environments where the potential for explosions due to electrical sparks has existed. To provide adequate brightness and lit area for effectiveness, EL systems call for the use of high capacitance and voltage. SUMMARY OF THE INVENTION According to the invention herein, it has been found that a specific construction and arrangement of EL lighting circuits can be used in conjunction with a carefully constructed and controlled IS power source to make available an emergency lighting system capable of use in flammable, explosive or like hazardous areas. Specifically the invention contemplates an IS emergency egress lighting system comprising: (a) an IS power supply providing AC power at a maximum current of about- 200 milliamps and a maximum voltage of about 50 volts peak to each of two or more lighting circuits; and (b) said two or more lighting circuits each comprising one or more EL lights and cables connecting said lights to the power supply, each of said circuits having a total circuit capacitance not exceeding about 1000 nanofarads; said EL lights comprising, in sequence, a back electrode having a lead attached thereto, an interlayer comprising an electroluminescent phosphor, and a transparent electrode having a busbar attached thereto and a lead attached to said busbar, said EL light encased within a substantially moisture impermeable casing and said light characterized in that its capacitance, in operation, is from about 2 to about 10 nanofarads per square inch, and its brightness is at least about one footlambert. The invention also contemplates a new class of EL lights having properties and structures as described herein. DESCRIPTION OF THE INVENTION Intrinsically safe circuits, in this case the EL light circuits, are those which can produce no spark or thermal effect capable of causing ignition in a flammable or explosive gas atmosphere. The power input to the lighting circuits herein is provided by an IS power supply. The IS power supply contains an interface consisting of a number of discrete devices such as shunt diode safety barriers or equivalent means of isolation. The shunt diode safety barriers are composed of a combination of resistors and Zener or forward connected diodes. The above described IS power supply can be located in a non-hazardous area or, if suitably enclosed or otherwise protected, can be located within the hazardous area. The key is its capability to provide AC power of controlled amperage and voltage which can be matched with the requirements of the lighting circuits. While the voltage input should be controlled to a peak voltage of about 50V, in many ' environments a commonly accepted preference is a peak voltage of about 42.5V. Amperage should be sufficient to provide power to each of the lighting circuits, but is maintained at a maximum of 200 milliamps per circuit, and preferably a maximum of approximately 130 milliamps per circuit. The frequency of the AC input may vary, but must be sufficient to produce the minimum brightness required in the EL lights, discussed below. Generally, at least about 400 Hertz will be used, preferably at least about 600 Hertz, and most preferably at least about 800 Hertz. The upper limit is not critical, but typically will not exceed about 2500 Hertz. A plurality of lighting circuits is required in order to provide adequate lighting capacity while maintaining controlled system capacitance. While there is no theoretical limit to the number of circuits which can be utilized, system safety and efficiency is best provided with a maximum of 20 such circuits, and preferably not more than about 10 of such circuits. Each circuit however must have a maximum circuit capacitance of not greater than 1,000 nanofarads when the lights are in the lit condition, preferably about 500 nanofarads, and most preferably a maximum of about 250 nanofarads per circuit. While cabling, connectors and other minor components must be accounted for, the bulk of the circuit capacitance is found in the EL lights. As the lights may be considered light emitting capacitors, their construction is important to the successful operation of the system described herein. For a detailed review of the general parameters of EL light construction, reference is made to U.S. Patent #s 4,104,555 + 4,902,567, the details of which are incorporated herein by reference. As show in the above and other references, the EL light is composed of a plurality of layers. A light emitting dielectric layer is placed between two conductive layers, one of which is light transmissive. The first and non-light transmissible layer typically is a thin aluminum, silver or other highly conductive metallic film or foil, whereas the second and light transparent conducting layer typically is composed of a thin, transparent layer of indium tin oxide or like material. The light emitting dielectric layer commonly is composed of several sublayers, including at least one insulating sublayer of high dielectric constant, such as barium titanate in a high dielectric constant resin, such as a cyanoethylated polymer. Also present is a sublayer containing the light emitting substance, typically a zinc sulphide phosphor. The phosphor typically is contained within an organic binder, such as cyanoethylated polymers, above. Each of the conducting layers also has a lead attached thereto, for the purpose of power input to the EL light. The light transmissive conducting layer also commonly is supplemented with a busbar of highly conductive material, such as a silver paint. The busbar generally is placed along at least a portion of the periphery of the layer, but it may be located elsewhere on the conductive layer, if the user desires. The entire EL light thereafter is encased within a substantially moisture impermeable casing, such as by sealing with a transparent fluoropolymer or other barrier film. The preferred choice is the fluoropolymer film sold under the trademark "Aclar" by Allied Chemical Co. Lights constructed as described herein will preferably have a capacitance of at least three, and most preferably at least four, nanofarads per square inch, and preferably exhibit a brightness of about two, and most preferably at least four, footlamberts when subjected to an AC input of a maximum of 50 volts peak, and a frequency of between 400 and 1500 Hertz. While the low capacitance desired for intrinsic safety is obtained with thicker dielectric layers and lower dielectric constant, lamp brightness is reduced when these are used. Accordingly each circuit must be designed to meet the limits described herein in order to provide the required minimum brightness and lit area needed for the workability of the system, while preserving appropriate voltage, amperage and capacitance needed to meet the required safety limits. In order to provide the necessary illuminated surface, typically a minimum of about 20 sq. in. of EL area per circuit, and preferably 30 sq. in., must be provided. Typically elongated lamps of width less than 1/2 in. are used in order to form an illuminated path necessary for emergency guidance. EL lights have been found to provide the highly desirable benefit of high visibility in smoke, fog and other adverse environments, even at relatively low brightness levels. The brightness, however, should be at least 1 footlambert, preferably 2 footla bertε and most preferably 4 footlambertε, in order to provide clear visibility, particularly over extended periods of usage which may cause partial loss of brightness in the system. To accomplish these requirements, the total effective capacitance of the EL lights should be between about 2 and about 10 nanofarads per square inch. For maximum benefits and utility, this capacitance preferably is between 3 and 8, and most preferably between 4 and 7, nanofarads per square inch. Two or more EL lights may be combined within each circuit provided their total capacitance, when aggregated with the capacitance of the cables, connectors and any other components of the circuit, does not exceed the maximum allowable capacitance for the total circuit, defined above. The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements of the invention and still be within the scope and spirit of this invention as set forth in the following claims.

Claims: CLAIMS 1. An intrinsically safe ("IS") electroluminescent ("EL") light comprising,- in sequence, a back electrode having a lead attached thereto, an interlayer comprising an electroluminescent phosphor, and a transparent electrode having a busbar attached thereto and a lead attached to said busbar, said EL light encased within a substantially moisture impermeable casing and said light characterized in that its capacitance, in operation, is from about 2 to about 10 nanofarads per square inch, and its brightness is at least about one footlambert when subjected to an alternating current with a peak voltage of at most about 50 volts at a frequency between 400 and 800 Hertz. 2. The EL light of claim 1 wherein the capacitance of said light is at least about 3 nanofarads per square inch, and the peak voltage of the current is at most about 42.5 volts. 3. The EL light of claim 1 wherein the total capacitance of the light is at least about 4 nanofarads per square inch. 4. The EL light of claim 1 wherein the brightness is at least 2 footlamberts. 5. The EL light of claim 1 wherein the brightness is at least 4 footlamberts. 6. An intrinsically safe ("IS") emergency egress lighting system comprising: (a) an IS power supply providing AC power at a maximum current of about 200 milliamps and a maximum peak voltage of about 50 volts peak to each of two or more lighting circuits; and (b) said two or more lighting circuits each comprising one or more EL lights and cables connecting said lights to the power supply, each of said circuits having a total circuit capacitance not exceeding about 1000 nanofarads; each said EL light comprising, in sequence, a back electrode having a lead attached thereto, and interlayer comprising an electroluminescent phosphor, and a transparent electrode having a busbar attached thereto and a lead attached to said busbar, said EL light encased within a substantially moisture impermeable casing and said light characterized in that its capacitance, in operation, is from about 2 to about 10 nanofarads per square inch, and its brightness is at least about one footlambert. 7_ The IS emergency egress lighting system of claim 6 wherein the total capacitance of each EL lighting circuit does not exceed about 500 nanofarads. 8. The IS emergency egress lighting system of claim 6 wherein the total capacitance of each EL lighting circuit does not exceed about 250 nanofarads. 9. The system of claim 6 wherein the total capacitance of each EL lighting circuit does not exceed about 250 nanofarads and voltage of the current is at most about 42.5 volts. 10. The system of claim 6 wherein the brightness is at least 2 footlamberts. 11. The system of claim 6 wherein the brightness is at least 4 footlamberts 12. The system of claim 6 wherein the frequency of the alternating current is at least 400 hertz. 13. The system of claim 6 wherein the frequency of the alternating current is at least 600 hertz. 14. The system of claim 6 wherein the capacitance of the lights is from about 4 to about 7 nanofarads per square inch. 15. The system of claim 6 wherein the current in each circuit is not greater than about 130 milliamps.

Inventors: DERRICK DONALD E, HARRIS HOLLIS A, MARION ROBERT H, TOWER WILLIAM A, TOWLES CHRISTOPHER, DERRICK, DONALD, E., HARRIS, HOLLIS, A., MARION, ROBERT, H., TOWER, WILLIAM, A., TOWLES, CHRISTOPHER
Applicants: LOCTITE LUMINESCENT SYST, LOCTITE LUMINESCENT SYSTEMS, INC.
Application year: 1992
Main classification: F21V9
IPC codes: F21K7, F21S2, F21V25, G09F13, H05B33