US 3911385 A
An outdoor current limiting fuse structure is characterized by a generally tubular, electrically insulating casing having terminal means disposed at opposite ends thereof, one or more fusible elements connected between the terminal means and disposed within the casing and an epoxy resin on the exterior surface of the casing.
Claims available in
Description (OCR text may contain errors)
United States Patent 11 1 Blewitt et al.
[4 1 Oct. 7, 1975 1 OUTDOOR CURRENT LIMITING FUSE  Inventors: Donald D. Blewitt, Pittsburgh; Frank L. Cameron, North l-luntingdon; Charles H. Vondracek, Murrysville,
all of Pa.
 Assignee: Westinghouse Electric Corporation,
 Filed: May 7, 1974 211 Appl. No.: 467,699
 U.S. Cl. 337/202; 337/186', 337/227; 337/246  Int. Cl. 01H 85/02  Field of Search 174/137 B, 179; 337/186, 337/202, 227, 228,246, 231
 References Cited UNITED STATES PATENTS 2,972,031 2/1961 Cox 337/231 X 3,511,922 5/1970 Fisch et a1 174/137 B 3,614,699 10/1971 Fister 337/281 X 3,673,305 6/1972 Mashikian et a1 174/137 B 3,701,069 10/1972 Belcher 337/231 X FOREIGN PATENTS OR APPLICATIONS 1,133,006 11/1968 United Kingdom 174/179 1,233,310 5/1971 United Kingdom.l 174/179 Primary Examiner-J. D. Miller Assistant ExaminerFred E. Bell Attorney, Agent, or FirmL. P. Johns  ABSTRACT An outdoor current limiting fuse structure is characterized by a generally tubular, electrically insulating casing having terminal means disposed at opposite ends thereof, one or more fusible elements connected between the terminal means and disposed within the casing and an epoxy resin on the exterior surface of the casing.
3 Claims, 1 Drawing Figure US. Patent Oct. 7,1975
OUTDOOR CURRENT LIMITING FUSE BACKGROUND OF THE INVENTION 1 their use in outdoor installations. Where a fuse .does not employ some sort of conventional dropout mechanism to physically disengage one end of the fuse from its mounting after interruption, it must be capable of withstanding full circuit voltage (after interruption) for an indefinite period against all the insulation-degrading effects of the outdoor environment. Usually, some degradation has occurred prior to the interruption and continues-to occur afterwards. I
For that reason, the quality ofthe tube, or housing of the outdoor non-dropout current limiting fuse is significant. The properties of the outer surface of the tube determine whether or not the electrical insulation of the tube will be destroyed by weathering. So far as is known, the only fuses of this type currently available incorporate either glass or ceramic tubes. The primary disadvantage of such tubes is their brittleness. Glass or ceramic tubing is subject to damage during manufacture of the fuse, as well as in shipping and during installation. Moreover, the inelastic nature of the tube limits the interrupting rating of the fuse since higher levels of input energy can cause the tube to crack or shatter with catastrophic results. Finally, the use of such tubing complicates its fabrication due to difficulty in attaching the end terminals, or ferrules, to the tube to provide an adequate weatherproof seal. Accordingly, there has been a need for an outdoor current limiting fuse structure which can be manufactured with high quality and that not only withstands the shock of interruption but also endures the the ravages of outdoor environment.
SUMMARY OF THE INVENTION It has been found in accordance with this invention that the foregoing problems may be overcome by providing a fuse structure comprising an electrical insulating casing or tubing comprising a glass-melamine mate-- rial, terminal means or ferrules at each end of the tubing, one or more fusible elements disposed within the tubing and connected between the terminal means, and a coating of a cycloaliphatic epoxy resin on the exterior of the surface of the tubing. In addition, a sealant is preferably in the joint between each ferrule and the tubing.
The advantage of the fuse structure of this invention is primarily the glass melamine tube with the outer surface coated with a special weather-resistant epoxy resin which has been found superior to other epoxy and silicone resin systems when under electrical stress in an outdoor environment.
BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawing is an elevational view partly in section of an outdoor non-dropout current limiting fuse showing the features of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing a current limiting fuse structure is generally indicated at 10 and is particularly adapted for high voltage applications, such as 15.5 KV. The fuse structure 10 includes a casing or tube 12, terminal end caps or electrically conducting ferrules 14 and 16, and a plurality of fusible elements l8 extending between the ferrules. The casing or tube '12 is formed from a suitable electrically insulating material'that has sufficient structural strength to withstand the thermal condition and internal pressures that may result during interruption of the fuse structure. A layer or coating 20 covers the exterior surface ofthe casing 12.
The casing 12 is a glass-fiber reinforced melamine tube having a diameter of approximately 3 inches and a length of about 24 inches. More particularly, the glass melamine material forming the casing 12 comprises fiber glass cloth which is preliminarily dipped in a melamine resin and then wrapped upona mandrel and subsequently cured. As such the tube 20 has superior thermal,'arc, and shock resistant properties.
The layer or coating 20 comprises an epoxy resin, a flexibilizing agent, a filler, asbestos as a thickener or thixotroping agent, and if desired, a catalyst or reaction accelerator for expediting the curing reaction. The epoxy resin is substantially cycloaliphatic. The flexibilizing agent comprises polyazelaic polyanhydride having an approximate molecular weight of 2100-2500 and hexahydrophthalic anhydride which in combination is contained in the coating at a ratio of 30-115 parts by weight per 100 parts or resin; for example, parts by weight of polyazelaic anhydride and 12 parts by weight of hexahydrophtlalic anhydride and parts cycloaliphatic epoxy resin. The filler comprises aluminum trihydrate or naturally occurring magnesite, which filler promotes arc'and track resistance. The thickner comprises asbestos of short fiber or fine grain size such as a thixotroping agent. The thickness of the coating 20 may vary from 3 to 20 mils with a preferred thickness of about 8 mils.
The ferrules l4 and 16, being composed of an electrically conductive material, such as metal, serve as enclosures for the casing 12 as well as being terminal means for opposite ends of the several fusible elements 18 the ends of which are disposed between the ends of the casing 12 and the respective ferrules in a conventional manner.
Moreover, at the joint between the coating 20 and the ferrules 14 and 16 a bead 22 of sealant such as a silicone rubber compound, is provided.
Preliminary testing was conducted using the inclined plane tracking test. Several candidate coatings were coated on the glass reinforced melamine laminates and tested for track resistance. Also, identical pieces were first subjected to 100 hours in the weatherometer and then tested for track resistance. Several of these candidate materials were coated on glass melamine tubes then made into dummy fuses and tested in a standard salt-fog chamber. The voltage stress was maintained at 15.5 KV and both 0.1% and 0.05% salt solutions were used. The tests were run until tracking or leakage currents became excessive. The end point was the time required to produce three overcurrents of sufficient magnitude to blow the fuses in the high voltage circuit. Initial tests on the coatings for track resistance using the inclined plane test method showed wide differences in track resistance. However, this particular cycloaliphatic epoxy resin system was at least twice as good as the next best material. After exposure in the weatherometer the coating showed no loss in track resistance while all. others tested showed a 40 to 60% loss in track resistance. Results on dummy fuses tested in the saltfogchamber showed the same cycloaliphatic epoxy resinsystem to be significantly better. This system would be expected to last several months under full voltage stress after having been exposed to several years of outdoor environment.
Accordingly, the outdoor current limiting fuse structure of this invention satisfies a long existing need for a special weather-resistant fuse which is suitable for general purpose outdoor applications.
What is claimed is:
l. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element disposed within the casing and connected between said terminal means, a coating of an cycloaliphatic epoxy resin on the exterior surface of the casing and having a thickness of from about 3 to 20 mils, and the coating comprising a polyazelaic polyanhydride having an approximate molecular weight of 2100-2500 and hexa-hydrophtholic anhydride which in combination is contained in the coating at a ratio of -1 15 parts by weight per 100 parts of resin, a filler selected from a group consisting of aluminum trihydrate and magnesite, and a thixotroping agent.
2. The fuse structure of claim 1 in which the casing comprises a glass fiber-reinforced melamine resinous material.
3. The fuse structure of claim 1 in which the terminal means comprises a cup-shaped member fitting telescopically over each end of casing and forming an annular joint therewith, and a sealant in the joint and comprising a silicone rubber compound.