SURGE ARRESTOR FAIL SAFE THERMAL OVERLOAD MECHANISM

Abstract: A spring member (32) and associated channel member (28) bias a solder billet (22) into engagement with a ground electrode (16) of the surge arrestor. When the solder melts in response to a thermal overload condition the channel member (28) causes molten solder from the billet to flow preferentially along a path establishing a low resistance short circuit between the electrodes. Flow of the solder to desired locations is enhanced by solder flux, which preferably is of rosin type having substantial dielectric properties.

Description: SURGE ARRESTOR FAIL SAFE THERMAL OVERLOAD MECHANISM Field of the Invention This invention relates to surge arrestors for preventing damage to telecommunications and other electronic equipment due to lightning strikes, power line cross-overs and the like. The invention more specifically relates to an improved fail safe thermal overload mechanism for a surge arrestor. Background of the Invention Surge arrestors are well known in the telecommunications and related electronic arts. They commonly consist of a tubular housing having ground and one or two line electrodes spaced along its length. When the arrestor is subjected to a current surge condition over a long period of time, as might occur for instance due to a power line crossing, the heat generated by the arrestor may be sufficient to present a fire hazard. In an effort to prevent the foregoing, it has heretofore been proposed to short circuit the ground and line electrodes when the arrestor is subjected to a thermal overload. A commonly employed means for establishing the short circuit includes a spring that is normally maintained in an inactive position by solder or other meltable material. When a thermal overload condition occurs, the material melts and permits movement of the spring to an active short effecting position. In the aforesaid prior art arrestor the meltable material does not itself form the short circuit. However, U.S. Patent 4,851,946 discloses a different type of fail safe thermal overload mechanism in which molten solder material directly forms a short circuit between ground and line electrodes when the arrestor is subjected to a thermal overload. Summary of the Invention The fail safe thermal overload mechanism of the present invention is similar to that disclosed in the above noted prior patent, in that it employs solder material that melts and directly forms the desired short circuit when the arrestor is subjected to a thermal overload. The fail safe mechanism is of highly reliable in its operation and is relatively inexpensive. In a preferred embodiment the fail safe mechanism includes solder flux upon the outer surface of the housing of the arrestor, a solder billet overlying the arrestor housing, and channel and spring members that overlie the billet and bias it to a location closely adjacent and preferably butting the arrestor body. Solder flux may also be provided upon the inner and/or outer surfaces of the solder billet. A preferred flux is a rosin based one that under normal (i.e., no thermal overload) conditions, cleans and protects the surfaces to which it is applied, and has good dielectric properties and acts as an insulator. When the solder melts under thermal overload conditions, the flux causes the molten solder to thoroughly wet surfaces of the arrestor housing and the channel member of the fail mechanism so as to facilitate preferential flow of molten solder from the solder billet to one or more locations establishing a highly conductive, low resistance short circuit between the arrestor electrodes. When the arrestor housing is of cylindrical shape, as is customary, the channel member is preferably of generally V-shaped configuration and has first and second sections that extend angularly relative to each other and meet at an apex that overlies and extends generally parallel to the central axis of a the arrestor housing. When the arrestor is subjected to a thermal overload, the channel member permits relatively free flow of molten solder from the solder billet in a first direction, which in the illustrative embodiment is generally parallel to the longitudinal axis of the arrestor housing, while limiting flow of the molten solder in a second, transverse direction. Description of the Prior Art In addition to previously noted U.S. Patent 4,851,946, the following U.S. Patents may also be of interest relative to the present invention: 5,029,302, 4,912,592, 4,603,368, 5,027,100, 4,405,967, 4,158,869, 4,402,031, 4,380,036, 4,321,649, 4,212,047 and 4,062,054. Description of the Drawings Other features of the invention will be apparent from the following description of an illustrative embodiment thereof, which should be read in conjunction with accompanying drawings, in which: FIG. 1 is a vertically exploded perspective view of a surge arrestor having a fail safe thermal overload mechanism in accordance with invention; FIG. 2 is a end view of the arrestor and components of the overload mechanism in an assembled condition; FIG. 3 is a side elevational view of the surge arrestor; FIG. 4 is a top plan view of the assembly of Fig. 2, showing in phantom lines a solder billet whose opposite ends are spaced from line electrodes at opposite ends of the arrestor housing; FIG. 5 is a view similar to Figure 4 showing in phantom lines a solder billet whose ends extend to electrodes at opposite ends of the body of the arrestor; FIG. 6 is a fragmentary end view of the arrestor housing and of a solder billet and overlying channel member of the fail safe mechanism; FIG. 7 is a view similar to Fig. 6, but showing the components in positions assumed during a thermal overload; and FIG. 8 is a view similar to Fig. 6 showing gel or other protective sealant material encapsulating the arrestor and components of the fail safe mechanism. Description of the Illustrated Embodiment The surge arrestor 10 shown in the drawings is illustratively of a known type having a cylindrical housing 12 that includes disk-shaped line electrodes 14 at its opposite ends, a disk-shaped ground electrode 16 intermediate the length of the housing, and insulating material 17 intermediate electrode 16 and each of the line electrodes 14. Arrestor 10 may and illustratively does further include pin-type lead elements 18 that extend downwardly from respective ones of the electrodes. At least some, and illustratively substantially all, of the exterior surface of housing 12 is overlaid by a film, foil or coating of solder flux material 20 which is indicated in the drawings by stippling. Flux material 20 is preferably of a rosin-based type that under normal temperatures of housing 12 has strong dielectric properties, and protects the housing and other members engaged thereby from contaminants and other materials such as soft textured encapsulants 44 (e.g., gels, oils, greases, etc.) such as shown in Fig. 8. Under thermal overload conditions the flux greatly facilitates flow of molten solder along the housing and other members engaged thereby. Flux of the foregoing type is sold by M. W. Dunton Co. of Providence, Rhode Island, under the trademark ELECTRO-ROSIN RA50, and is comprised essentially of natural rosin, alcohol and proprietary activators. A channel-shaped solder billet 22 overlies and extends longitudinally of the upper surface of housing 12. Billet 22 is illustratively of inverted V-shaped configuration and has opposite side sections that extend angularly downwardly from each other and from an apex 24 upon the upper surface of the billet. The undersurface of the billet preferably and illustratively has a concave contour complementary to the cylindrical outer surface of housing 12, and may have a film or coating 20-1 of flux 20 thereon. The thickness of billet 22 is greatest in the portion thereof underlying apex 24 and is of a lesser magnitude adjacent the opposite side edges of the billet. The upper surface of the billet has a semispherical protuberance 26 generally centrally thereof, and may have a foil, film or coating 20-2 of flux 20 upon such upper surface. Alternatively or additionally, flux material 20-2 may be present upon the undersurface of a conductive channel member 28 of the thermal overload mechanism. In keeping with billet 22, member 28 is preferably of generally channel-like V-shaped configuration, and has opposite side sections that closely overlie the opposite side sections of billet 22. A centrally located semispherical socket 30 upon the upper surface of member 28 receives billet protuberance 26 and allows limited adjustive movement of billet 22 relative to plate 28 and arrestor housing 12. The aforesaid components of FIG. 1 are secured to each other and to arrestor housing 12 by a generally U-shaped resilient spring member 32. Spring 32 has generally horizontally extending upper and lower legs 34, 36 that extend in parallel relationship to each other from a generally vertically extending section 38. Legs 34, 36 have vertically aligned openings 40, 42 adjacent their free outer ends. The center one of the conductive pins 18 extends downwardly through opening 42 of leg 36 of arrestor housing 12. Opening 40 of upper spring leg 34 receives the protuberance 30 of channel member 28, and permits limited adjustive movement of plate 30 and underlying solder billet 22 relative to arrestor housing 12 and spring 32. Spring forces imposed by spring 32 upon the assembled components bias member 28 and billet 22 downwardly to a position wherein billet 22 is firmly seated upon the upper surface of arrestor housing 12. As is best shown in Figs. 4-6 of the drawings, the opposite side edges of member 28 preferably extend beyond the opposite side edges of the underlying solder billet 22, and the opposite end portions of member 28 preferably extend beyond the opposite ends of billet 28 and the opposite ends of arrestor housing 12. In the embodiment of Fig. 5 the central portion of billet 22 overlies ground electrode 16 of arrestor 10, and opposite end portions of billet 22 overlie respective adjacent ones of line electrodes 14 of arrestor housing 12. The embodiment of Fig. 4 differs from that of Fig. 5 primarily in that the opposite ends of billet 22 portions are spaced axially from, and do not overlie, electrodes 14. Consequently, while the solder flux employed in the Fig. 4 embodiment may be of the previously described flux 20 type, other flux not having the dielectric insulating properties of flux 20 may instead be used in the embodiment of Fig. 5. When the arrestor 10 of Fig. 4 is subjected to thermal overload solder billet 22 melts and molten solder from the billet flows axially, as well as in other directions, along the exterior surface of arrestor housing 12 into engagement with line electrodes 14 so as to thereby establish a dense and highly conductive short circuit between each of such electrodes and the ground electrode 16 underlying the billet. When the solder flux is of the preferred type that causes the molten solder to thoroughly wet housing 12 of arrestor 10, the molten solder will flow not only to the annular surfaces of the electrodes, but also to the outer end surfaces of line electrodes 14. This will normally occur irrespective of the orientation of arrestor housing 12. The axial flow of molten solder from billet 22 is enhanced by the generally V-shaped configuration of channel member 28. As is shown in Fig. 6, the opposite side edges of member 28 preferably extend outwardly beyond the opposite side edges of billet 22, and normally are spaced slightly above the underlying cylindrical surface of arrestor housing 12. When solder billet 22 melts in response to a thermal overload condition, molten solder passes initially from both the opposite ends and the opposite sides of the billet and channel member 28. This initial passage of molten solder from the billet, in conjunction with the downward biasing force imposed upon member 28 by spring 32, causes plate 28 to descend until its opposite side edge portions engage the underlying surfaces of arrestor housing 12. Such engagement restricts, if not altogether stops, the passage of molten solder from beneath the opposite side edge portions of member 28, which in turn causes preferential flow of the molten solder parallel to the central axis of arrestor housing 12 through the opposite ends of the space overlaid by roof 28 and to electrodes 14. While in the illustrative embodiments the solder flux 20 is provided upon substantially all of the exterior surfaces of arrestor housing 12, the flux might instead be applied, in bands or the like, only to selected surfaces of the housing upon which solder is to flow. In lieu of solder flux that is applied separately, the solder flux may be integral with the solder material of billet 22. While preferred embodiments of the invention have been shown and described, this was for purposes of illustration only, and not for purposes of limitation, the scope of the invention being in accordance with the following claims.

Claims: The Claims 1. A fail safe thermal overload mechanism for a surge arrestor having a housing, comprising: a ground electrode and at least one line electrode upon said housing, said electrodes being spaced from each other; solder flux and a solder billet upon said housing; said solder billet melting when said arrestor is subjected to thermal overload conditions and then closing a short circuit between said electrodes. 2. A mechanism as in Claim 1, wherein said solder flux insulates said solder billet from said electrodes when said arrestor is subjected to normal thermal conditions. 3. A mechanism as in Claim 1, wherein under thermal overload conditions said solder flux facilitates the flow of molten solder from said billet to at least one location closing said short circuit. 4. A mechanism as in Claim 3, wherein said solder body has an inner surface confronting said housing, and an outer surface opposite said inner surface, and part of said solder flux is upon said inner surface of said solder billet. 5. A mechanism as in Claim 4, wherein part of said solder flux is upon said outer surface of said solder billet. 6. A mechanism as in Claim 1, and further including mounting means mounting said solder billet upon said housing. 7. A mechanism as in Claim 6, wherein said mounting means includes a resilient spring member connected to said housing and biasing said solder billet to a position adjacent said housing. 8. A mechanism as in Claim 7, wherein said mounting means further includes a channel member carried by said spring member and overlying said solder billet. 9. A mechanism as in Claim 8, and further including connector means upon said plate member and upon said solder body for permitting adjustive relative movement between said solder billet and said channel member. 10. A mechanism as in Claim 1, wherein said solder billet is generally of inverted V-shaped configuration, and has outer side edges. 11. A mechanism as in Claim 10, wherein said channel member is generally of inverted V-shaped configuration and has outer edges spaced outwardly from said outer side edges of said solder billet. 12. A mechanism as in Claim 11, wherein said outer side edges of said channel member at times engage said housing and said engagement causes preferential flow of molten solder from said solder body from beneath opposite ends of said channel member.

Inventors: CURRY WILLIAM JOSEPH, DEBBAUT CHRISTIAN ARTHUR M L, FIEN KENNETH JAMES, JESSUP KIMBERLEY ANN, MULLANEY JULIAN SEAN, CURRY, WILLIAM, JOSEPH, DEBBAUT, CHRISTIAN, ARTHUR, MARIE-LOUISE, FIEN, KENNETH, JAMES, JESSUP, KIMBERLEY, ANN, MULLANEY, JULIAN, SEAN
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1993
Main classification: H01H39
IPC codes: H01R4, H01R13, H01T1, H02H7, H04M3, H04Q1