US3723930A

US3723930A - Oil immersible current limiting fuse assembly

Info

Publication number: US3723930A
Application number: US00225232A
Authority: US
Inventors: R Koch
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1972-02-10
Filing date: 1972-02-10
Publication date: 1973-03-27
Anticipated expiration: 1990-03-27
Also published as: GB1399808A; CA960733A

Abstract

An oil immersible current limiting fuse assembly is provided by encapsulating a current limiting fuse in a homogeneous, molded epoxy material and providing resilient seal means adjacent the axial ends of the assembly, between the epoxy encapsulation and the fuse. The fuse assembly is further characterized by incorporating a vent passageway through one of the fuse terminals, which is effective to vent gas from the interior of the fuse during its manufacture in order to assure the maintenance of a fluid-tight sealing relationship between the fuse and the encapsulating epoxy.

Description

2,333,354 11/1943 Andersenetal. 2,713,098

UNITED STATES PATENTS ..337/28O Swain ..337/252 I Umted States Patent 1191 1111 3,723,930 Koch 1 51 Mar. 27, 1973 [54] OIL IMMERSIBLE CURRENT 3,480,898 11 1969 Giegerich ..337 4 LIMITING FUSE ASSEMBLY 3,471,816 10 1969 Giegerich ..337 195 [75] Inventor: Robert E. Koch, P1ttsf1eld, Mass. Primary Examiner Harold Bmome [7 3] Assignee: General Electric Company, New Attorney-Francis X. Doyle et al. York, NY.

[22] Filed: Feb. 10, 1912 [571 3 ABSTRACT [21] APPL Nod 225,232 An oil immersible current limiting fuse assembly is prov1ded by encapsulatmg a current l1m1t1ng fuse m a l homogeneous, molded epoxy material and providing 337/251 resilient seal means adjacent the axial ends of the as- [51] Int. Cl. .HOlh 85/02 semblyy between the epoxy encapsulation and the [58] F'eld Search "337/1581 2481 2491 fuse. The fuse assembly is further characterized by in- 337/204 1951 1861 corporating a vent passageway through one of the fuse terminals, which is effective to vent gas from the in- [56] References Clted terior of the fuse during its manufacture in order to assure the maintenance of a fluid-tight sealing relationship between the fuse and the encapsulating epoxy.

14 Claims, 2 Drawing Figures (mick/ "(IIIIIIIIIII OIL IMMERSIBLE CURRENT LIMITING FUSE ASSEMBLY BACKGROUND OF THE INVENTION relatively severe thermal cycling.

It is fairly common practice to provide various means for sealing cartridge type fuses in fluid-tight housings so that the fuses may be immersed in a liquid. In fact, the use of such fuses in connection with oil switches has been fairly extensive in Europe for many years. Normally in these prior art applications, a properly sealed current-limiting fuse isimmersed in insulating oil, but it is not subjected to any appreciable thermal stresses because the oil in such switches remains at a temperature level consistent with a fairly uniform ambient temperature. At the present time, there is a growing interest in the application of current limiting fuses for protecting distribution transformers against short circuit currents. It has been established that in this new application it is frequently desirable, from an economic design standpoint, to mount such current limiting fuses in a transformer, beneath the surface of the oil or other liquid coolant in the transformer. However, since it is possible that the temperature of oil in a distribution transformer may vary between arange of approximately 40 Centigrade up to +140 Centigrade, any fuse immersed in the oil may be subjected to large mechanical stresses due to thermal cycling.

The type of submersible fuses known in the prior art typically had two major disadvantages. First, the risk of developing a leak in the fuse cartridge seal when it was thermally cycled was sufficiently great to recommend against the use of such fus'es in an environment of extensive thermal cycling, such as that encountered in distribution transformers. Secondly, the cost of manufacturing such fuses was prohibitive for most desired applications. Specifically, prior art types of immersible fuses are known to include fuses having a glazed ceramic cartridge that houses fusible elements in the manner described in U.S. Pat. No. 2,333,354-Anderson et al., which issued Nov. 2, 1943. Such ceramic cartridges do not involve a separate encapsulating housing for a cartridge fuse, but rather use a ceramic shell to which metal end terminals are bonded by a metal-to-ceramic sealing technique. The fuse links are then fastened to the terminals of the fuse. Since such sealing techniques normally require a brazing temperature of 600 C to 700 C, it is necessary in forming such fuses to first manufacture the ceramic-to-metal seals, then in a separate operation mount fusible links into the ceramic cartridge and seal the metal end caps. In addition to'the expense of such an operation, it is necessary in the manufacture of such prior art fuses to individually machine the surfaces of the ceramic fuse cartridges to properly fit them to the metal end terminals, prior to performing the metal-to-ceramic sealing operation. This further process step added a substantial cost to the manufacture of such fuses. A further disadvantage of ceramic fuse cartridges is that they are relatively fragile mechanically, and bulky to handle.

After recognizing such common problems of early forms of immersible fuse casings, a fuse cartridge encapsulating means was developed. This early encapsu lating method involves the provision of a synthetic rubber housing or sleeve bonded to the outer surface of a cartridge type fuse, in the manner disclosed in U.S. Pat. No. 2,681,398-Kozacka et al., which issued on June? 15, 1954. This early form of fuse encapsulating means provides a light-weight and fairly rugged fuse encapsulation; however, it does not afford an adequate solution to the thermal cycling problem encountered when a current limiting fuse is immersed in the coolant of a distribution transformer. Of course, as explained in detail in the above-referenced Kozacka patent, if leakage occurs in an oil immersed current limiting fuse,

it can result in a violent explosion of the fuse when an over-current condition causes the fuse link to rupture. Therefore, it is imperative that any housing means developed to enable a current limiting fuse to be immersed in the oil of a distribution transformer must, in fact, maintain a fluid-tight seal around the fuse during a large range of thermal cycling over an extended period of time. i

Before the present invention, a major obstacle encountered in the manufacture of oil-immersible fuse assemblies was that frequently a seal between a fuse and its associated encapsulated housing would be destroyed during the manufacture of the housing. Often, such destruction is due to the venting of gases from the interior of the fuse as a consequence of the high temperaand an end cap ring that is brazed to the glazed ceramic of the fuse cartridge, it has been found that the dielectric material of the fuse cartridge or an associated fuseelement-supporting core member, might be caused to evolve gas when these components are soldered or welded together. As such evolved gas escapes from the cartridge, it may form small passageways either around the seal between the ceramic housing and the metal end cap ring, or between the end cap ring and the metal terminal bonded to it, so that in use oil could seep through these passageways into the fuse. This problem was particularly acute due to the fact that in prior art fuses, no method was known for effectively determining whether such potential sources of dangerous leakage existed. The present invention provides a solution to these two major problems.

A primary object of this invention is to provide an oil-immersible current limiting fuse assembly that is economically feasible to produce for commercial use, and that overcomes all of the above-identified problems of prior art immersible fuses.

Another object of the invention is to provide an immersible fuse assembly that is rugged in construction, relatively lightweight, and convenient to handle, while at the same time affording. a fluid-tight seal that prevents leakage of oil into the fuse even when it is subjected to thermal cycling in a range of 40 C to +1 50 C.

A further object of the invention is to provide an immersible current limiting fuse that is characterized by having means for accurately testing the impregnability Yet another object of the invention is to provide an immersible fuse assembly with a venting arrangement that assures the formation and maintenance of a fluidtightseal between a fuse and a body of encapsulating material housing the fuse to seal the fuse against impregnation by a liquid in which it may be immersed. 1 Additional objects and advantages of the'invention will become apparent to those skilled in the art from abutment 3a thereby to afford access into the interior the description of it that follows taken in conjunction 1 with the accompanying drawings.

BRIEF SUMMARY OF THE INVENTION An oil-immersible current-limiting fuse assembly having a pair of metal terminals extending from opposite ends thereof is formed by bonding a body of epoxy resin material around a fuse casing and its as sociated end caps, in fluid-tight sealing relationship with respect thereto. A gas vent passageway is formed through one of the terminals of the fuse, and bands of resilientsealing material are provided around the peripheries of both end caps, between the end caps and the. encapsulating epoxy resin, so that a fluid-tight seal is maintained between the fuse casing and the encapsulating resin during the manufacture of the fuse. Moreover, the passageway through the fuse terminal provides'a means of introducing a test gas into the fuse so that it can be tested to assure that the fuse is fluidtight after this passageway is sealed in a succeeding step of the manufacturing procedure used to fabricate the.

fuse assembly.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view, partially in cross section, showing an oil-immersible current limiting fuse assembly that is constructed pursuant to the teaching of the present invention.

FIG. 2 is a side elevation view, in reduced scale, of the immersible fuse assembly shown in FIG. 1, depicting the fuse casing in phantom.

DESCRIPTIONOF A PREFERRED EMBODIMENT Referring now to FIG. 1, it will be seenthat there is shown an oil-immersible current limiting fuse assembly 1 in afo'rm that constitutes a preferredembodiment of my invention. The fuse assembly 1 includes a hollow tubular fuse casing 2 that is formed of rigid dielectric material such as woven and impregnated fiberglass. Of course, any other suitable, conventional fuse casing material may be used to form the casing v2. A first metallic end cap 3 and a second metallic end cap 4 are mounted, respectively, over opposite ends of the casing 2 in sealing relationship therewith. It will be noted that the metallic end caps 3 and 4 have integral abutments 3a and 4a, respectively, in the form of a short cylindrical portion, at their axes. A first elongated terminal,

member 5 and a second elongated terminal member 6 are mounted respectively on the end cap 3 and end cap 4. In this form of the invention, the mounting of ter.

minal members 5 and 6 is attained by brazing them, respectively, to the abutments 3a and 4a. It should be. understood that end caps 3 and 4 are formed of copper and the elongated, generally cylindrically shaped terminal members 5 and 6 are'formed of an electrically conductive copper or bronze. Obviously, other conventional materials may be substituted for these metals, if desired.

of fuse casing 2. The novel function of the passageway 7-7 will be more fully described below. At this point, in order to complete the description of the portion of the fuse assembly associated with elongated terminal member 5, it is only necessary to observe that in its assembled form, a threaded plug 8 is screwed into the wall means of the passageway 7 to at least partially limit a flow of gas through this passageway. In addition, it should be understood that other means for sealing the passageway may be used, in lieu offtheplug For example, instead ofa threaded plug alsjir'npl e" stake can be force-fitted into the passageway 7. Moreover,'pursuant to the present invention, the means. for sealing passageway 7 includes a body of potting compound 9 that fills the interstices between the plug 8and-the walls of passageways 7, as well as filling'the portion ofthe passageway 7 above the plug 8. In this embodim ent of the invention, this second means" for sealing the' passageway 7 comprises a body of solder having a melting point higher than 160 C, so tliat tlrisgoritical fluidtight seal will not be destroyed when the fuse assembly is immersed in the cooling oil of a distribution transformer, which may be subjected to temperatures as high as 150 C, during its operation.

Metallic end cap 4 at the other end of the fuse assembly l is not provided with an aperture similar to aperture 7' in end cap 3. However, as the description of the invention proceeds, it will'be apparent that in some embodiments of the invention it may be desirable to also provide a vent in end cap 4, along with suitable means for preventing the escape of sand or other filler material through such a vent. Returning now to the to a first contact member 11 and a second contact member 12. The contact members 11 and 12 may take any suitable form, but in this embodiment they comprise generally flat .plate members having serrated edges. In this form of the invention, there are, in fact, a pair of fusible links 10 and 13, both of which are connected between the contact members 1 1 and 12. It will be understood that either a large or smaller number of parallel fusible links may be employed in' alternative embodiments of the invention, as is well known in the art. 1

An elongated core member 14 is mounted in the casing 2 in spaced-apart relationship with respect to the inner walls thereof. As shown in FIG. -l, the fusible links 10 and 13 are spirally wound about the core member 14 to hold them in a suitable spaced-apart position, in a manner well known'in the fuse art. As is also'typical in the fuse art, the links 10 and 13 are each provided with a plurality of spaced-

apart apertures

10a, 10b and 13a, 13b, etc., which are used to determine a desired interrupting characteristic for the fuse. It should be appreciated that theopposite ends of the fusible links and 13 may be secured, respectively, to

the contact members 11 and 12 by being mechanically secured to the serrated edges thereof and then soldered, in any suitable conventional manner.

In addition to providing a contact terminal for the fuse links 10 and 13, the contact members 11 and 12 serve an additional function. These members 11 and 12 are electrically connectedrespectively to the end caps 3 and 4 at the outer peripheries thereof, as is clearly shown in FIG. 1, by a tight press-fit, discharge-formed cap connection between the inwardly turned tabs of the connector members 11 and 12 and the associated inwardly (or, as viewed in FIG. 1, downwardly) turned edges of the end caps 3 and 4. The interior of easing 2 is filled with a suitable granular material 15, such as silica sand, or other well known arc-quenching material. In order to assure that the body of filler material 15 will remain tightly compacted around the core member 14, a pad 16 of resilient elastic material is compressed between the end cap 3 and metallic terminal plate member 11, where it exerts a compressive force on the body of granular material 15. A second function of pad 16 is to prevent the escape of sand through the passageway 7-7; prior to the time that the passage way is sealed.

Pursuant to the, present invention, a rigid body of essentially void-free epoxy resin 17 is mounted on the outer surface of the casing 2 of fuse assembly 1 so that the .body of resin 17 completely encapsulates the casing 2 and end caps 3 and 4 of the fuse. The body of epoxy resin 17 is molded around the casing 2 and end caps 3 and 4 so that it is bonded to the tubular casing 2 to form a void-free interface therewith. An important feature of the present invention is that the epoxy resin is selected to have a coefficient of thermal expansion that is sufficiently similar to the coefficient of thermal expansion of the fuse casing 2 and metal end caps 3 and 4 to enable the encapsulated fuse 1 to be thermally cycled between 40 C and +l50 without causing the bond between these members and the epoxy resin 17 to be ruptured. Those skilled in the manufacture of epoxy resins will recognize that various different techniques may be used to make a resin compound having the foregoing desirable thermal characteristics. For exam- .ple, various filler materials having low coefficients of thermal expansion may be added to well known resin bases to lowerthe thermal coefficient of expansion of the resultant encapsulating epoxy resin compound to a desired range. In one preferred embodiment of the invention disclosed herein, such a suitable resin base was formed of the following constituent parts:

190 parts by weight Epoxy resin (such as'Di-glycidyl ether of di-phenylol propane) 150 parts by weight Methyl-endo-methylene tetra hydro phthalic anhydride 2 parts by weight Catalyst (such as a tertiary amine salt) More specifically, the body of epoxy resin 17 is formed of an essentially homogenous compound and it is molded so that at no point does it have a minimum thickness in the direction of a radius extending from the longitudinal axis of easing 2, which is less than Ill 6 of an inchin thickness. It has been found that this minimum thickness is critical in order to assure a consistent freedom from oil-leakage through the epoxy cycle of its manufacture.

resin into the casing 2. When lesser minimum thicknesses are used to form the encapsulating body of epoxy resin 17, there is an appreciable risk that minor cracks may occur in the resin material during a curing Consequently, in the preferred form of the invention, as shown in FIG. 1, a

minimum thickness in excess of 1/16 of an inch is maintained over the entire body of the epoxy resin material 17. Inorder to further assure the impregnability of the immersible fuse assembly 1 to oil, a first band 18 and a second band 19 of resilient elastomeric material mounted, respectively, around the peripheries of end caps 3 and 4 to form fluid-tight seals between them and the body of encapsulating epoxy resin 17. Various means may be used to form the first and second bands 18 and 19 of elastomeric'material; however, in this embodiment of the invention the means defining bands 18 and 19 comprises a pair of annular gaskets (l8 and 19) and also includes a pair of layers of epoxy cement 20 and 21, respectively positioned between each of the gaskets 18 and 19 and their associated end caps 3 and 4, to bond the gaskets to the end caps influid-tight sealing relationship. Alternatively, it has been found that the means defining first and second bands 18 and 19 may comprise first and second thermally cured elastomeric bands that are bonded directly to the end caps 2 and 3, respectively.'Such bands may be cured either before or during the manufacturing in which the body of epoxy resin 17 is cured. I

The foregoing description of the embodiment of the invention shown in FIG. 1 will enable those skilled in the art to'construct an immersible fuse assembly that will afiord the above-enumerated desired objects of the invention. However, as sated above,-during the description of elongated terminal 5 and the passageway 7 through it, this unique form of the invention makes possible an important advantage that has not thus far been fully described. As was mentioned at the outset,

during the discussion of prior art immersible fuse strucvention in a manner that will now be described in detail. v

During the manufacture of the fuse assembly 1, it is assembled as described above, with the body of epoxy resin 17 molded around the casing 2 and end caps 3 and4; however, threaded plug 8 and the body of solder 9 are only positioned in passageway 7 after the epoxy resin 17 of fuse assembly 1 has been fully cured to form fluid-tight seals'with gaskets 18 and 19. Then, the integrity of the seals 18 and 19, and epoxy 17 is tested for possible leakage. This testing is accomplished by flushing a mixture of Helium gas and air through the granular filler 15 in fuse assembly 1. It has been found that a mixture of one part Helium gas to 20 parts air is suitable for this purpose. Following such a flushing operation, the screw 8 is secured in passageway 7 and a body of solder 9 is melted over it, then allowed to cool so close passageway 7.

that an hermetic seal is formed to Finally, the fuse 1 is placed in a vacuum chamber and a vacuum isdrawn on the chamber such that a pressure differential of approximately 14 pounds per square inch of surface area is established between the gases adjacent the interior and exterior surfaces, respectively, of the casing 2 and resin material 17. Then, a mass spectrometer is used to detect the rate of flow of Helium gas from the interior of casing 2 into the evacuated chamber. lt has been found that when a fuse assembly 1 is properly constructed, pursuant tothe present invention, the body of epoxy resin 17 and the fluid-tight seals 18 and 19 are effective to limit a flow of Helium gas therethrough to a rate no larger than 1 X cubic centimeters of Helium per second, under the foregoing pressure differential. Thus, if a faster rate of leakage is detected, the, fuse can be rejected at that point in the manufacturing and test cycle, or it can be examined to In conclusion, reference may be madeto FIG. 2 which depicts the outward appearance of a fuse 1 constructed pursuant to the present invention. As is apparent from the illustration of the fuse assembly 1 in FIG. 2, in its finished form, it is a ruggedly constructed, easily handled assembly comprising a generally smooth outer casing of epoxy resin 17, having only. the pair of determine the source of leakage, which may be due simply to a poor seal around the plug 8 and solder 9 in elongated terminal 5, which could be easily corrected.

In, addition to the above-described critical testing function, a second important function of the passageway 77 through terminal 5 and end cap 3 is that it serves to avoid the risk of undesirable gas pressures being developed within the casing 2 of fuse assembly 1 during a molding operation in which the body of epoxy resin 17 is bonded around casing 2 and end caps- 3 and 4. In the preferred embodiment of the invention, the elongated'core member 14 is formed of a rigid, dielectric material, such as cordierite, steatite,

alumina or other conventional fuse core materialthat does not evolve gas when it is heated up to a temperature of 150 C. However, since the epoxy resin material 17 is subjected to a temperature of approximately 130 C during its molding and curing operation, normal thermal expansion of air trapped in the granular material 15 within casing 2 tends to force the air to expand outwardly from the casing 2. If the passageway 7-7" was not available to act as a vent during such manufacturing operations, pressure might build up within the casing 2, possibly resulting in the formation of undesirable venting passageways around the sealing bands 18, 19 or possibly through interstices of the epoxy resin 17. Thus,

I the novel passageway 7'7" through terminal 5 and end cap 3 helps avoid the risk of potentiallydangerous leaks into the body of granular material 15, as well as fuse, following its manufacture.

During the description of the sealing integrity tests of rigid body of essentially void-free epoxy resin mounted cylindrically shaped elongated terminals 5 and 6 extending from its opposite ends. Of course, if desired, it'

isa simple matter to mold various identifying symbols or other markings in the exterior surface of the resin 17, as is well known in the art. I

While a preferred embodiment of my invention has been described above, those skilled in the art will understahd that various alternative embodiments and obvious improvements of the invention may be made without departing from its true spirit and scope. Accordingly, it is my intention to define the scope of my invention in the following appended claims.

, What I claim as new and desire to secure by Letters Patent of the United States is:

1. An immersible current limiting fuse assembly comprising -a fuse having ahollow tubular casing on the outer surface of said fuse to completely encapsulate it,'and. means defining first and second bands'of resilient 'elastomeric material mounted, respectively,

" around the peripheries of said end caps to form fluidtight seals between them and the body of encapsulating epoxy resin.

2. An invention as defined inclaim 1 wherein the body of epoxy resin is bonded to the tubular casing to form avoid-free interface therewith, and wherein the epoxy resin has a coefficient of thermal expansion that serving as a test means for assuring the integrity of the fuse assembly 1, above, a desirable leakage rate of .1 X

. 10" cubic centimeters per second of Helium gas was mentioned as an acceptable maximum, for use in testing fuses that are designed for immersion in distribution transformer coolants. However, it should be noted that I a broader range of leakage rates has been found to vprovide immersible fuse. structures that are safe for many applications in which a fuse is subjected to temperature cycling in the range from .40 C to +150 C. Specifically, it has been found that when the epoxy resin, 17

' and the-fluid tight seals l8=and 19 are effective to limit a flow of Helium gas therethrough to a rate in range of l X 10" cubic centimeters per second of Helium gas issufficiently similar to the coefficient ofthermal expansion of the fuse casing to enable the encapsulated fuse to be thermally cycled between 40 and'+1'50 without rupturing the bond between the casing and the epoxy resin. a I

3. An invention as defined in claim 2 wherein the body of epoxy resin is essentiallyhomogenous and is,

formed to have a minimumthickness of H16 inch, in

the direction of a radius ofsaid casing.

4. An invention as defined in claim 3 wherein the body of epoxy resin and said fluid-tight seals are effective to limit a flow of Helium gas therethrough to a rate no larger than l X'lO' cubic centimeters per second of Helium gas when a pressure differential of approximately 14 pounds per square inch of surface area is established between the gases respectively adjacent the interior surface of said casing and the exterior surface of the body of encapsulating epoxy resin.

when the pressure between the interior of casing 2 and the exterior of epoxyresin 17 is approximately 14 pounds per square inch of surface area, the fuses will not allow a dangerous rate of oil leakage into the body 5. An invention as defined in claim 4 wherein said epoxyresin and fluid-tight seals are effective to limit a of granular material 15, duringnormal operating life spans forum in transformer coolants.

flow of Helium gas therethrough to a rate in the range of l v 10' and l 10' cubic centimeters per second of vI -Ielium gas whenthe aforesaid pressure differential exists.

- said filler material.

6. An invention as defined in claim 1 including an elongated core member mounted in said casing in spaced apart relationship with respect to the inner walls thereof, said at least one fusible link being spirally wound on said core member, said core member being formed-of a dielectric material that does not evolve gas when heated to a temperature of 150 C.

7. An invention as defined in claim 6 including a body of granular dielectric filler material mounted in said casing around the core member, and including a pad of a resilient elastomeric material that is compressed between said first end cap and the filler material thereby to exert a continuous compressive force on 8. An invention as defined in claim 1 wherein said means defining first and second bands of resilient elastomeric material comprise first and second bands of a thermally cured elastomer that are bonded to said end caps, respectively.

9. An invention as defined in claim 1 wherein said means defining first and second bands of resilient elastomeric material comprise a pair of annular gaskets, and including a pair of layers of epoxy cement, respectively positionedbetween each of said gaskets and their associated end caps, thereby to bond the gaskets to the end caps in fluid-tight relationship.

10. An invention as defined in claim 1 including wall means defining a passageway through said first end cap and said first terminal member, whereby gas may be vented from the interior of the casing to the exterior thereof, in combination with means for sealing said passageway.

11. An invention as defined in claim 10 wherein said means for sealing said passageway comprises a potting compound positioned in said passageway in fluid-tight relationship with the wall means thereof.

12. A invention as defined in claim 10wherein said means for sealing said passageway comprises a body of solder having a melting point higher than C.

13. An invention as defined in claim 11 wherein said means for sealing said passageway includes a plug disposed within the wall means of the passageway thereby to at least partially limit a flow of gas through the passageway.

14. An invention as defined in claim 13 wherein said plug comprises a metal screw that is threaded into said wall means.

Claims

1. An immersible current limiting fuse assembly comprising a fuse having a hollow tubular casing formed of rigid dielectric material, first and second metallic end caps mounted respectively over opposite ends of said casing in sealing relationship therewith, at least one fusible link electrically connected to form a circuit through said casing between the first and second end cap; in combination with the improvement comprising, first and second elongated electrical terminal members mounted, respectively, on the first and second end caps and extending outward from them, a rigid body of essentially void-free epoxy resin mounted on the outer surface of said fuse to completely encapsulate it, and means defining first and second bands of resilient elastomeric material mounted, respectively, around the peripheries of said end caps to form fluid-tight seals between them and the body of encapsulating epoxy resin.

2. An invention as defined in claim 1 wherein the body of epoxy resin is bonded to the tubular casing to form a void-free interface therewith, and wherein the epoxy resin has a coefficient of thermal expansion that is sufficiently similar to the coefficient of thermal expansion of the fuse casing to enable the encapsulated fuse to be thermally cycled between -40* and +150* without rupturing the bond between the casing and the epoxy resin.

3. An invention as defined in claim 2 wherein the body of epoxy resin is essentially homogenous and is formed to have a minimum thickness of 1/16 inch, in the direction of a radius Of said casing.

4. An invention as defined in claim 3 wherein the body of epoxy resin and said fluid-tight seals are effective to limit a flow of Helium gas therethrough to a rate no larger than 1 X 10 6 cubic centimeters per second of Helium gas when a pressure differential of approximately 14 pounds per square inch of surface area is established between the gases respectively adjacent the interior surface of said casing and the exterior surface of the body of encapsulating epoxy resin.

5. An invention as defined in claim 4 wherein said epoxy resin and fluid-tight seals are effective to limit a flow of Helium gas therethrough to a rate in the range of 1 X 10 6 and 1 X 10 6 cubic centimeters per second of Helium gas when the aforesaid pressure differential exists.

6. An invention as defined in claim 1 including an elongated core member mounted in said casing in spaced apart relationship with respect to the inner walls thereof, said at least one fusible link being spirally wound on said core member, said core member being formed of a dielectric material that does not evolve gas when heated to a temperature of 150* C.

7. An invention as defined in claim 6 including a body of granular dielectric filler material mounted in said casing around the core member, and including a pad of a resilient elastomeric material that is compressed between said first end cap and the filler material thereby to exert a continuous compressive force on said filler material.

8. An invention as defined in claim 1 wherein said means defining first and second bands of resilient elastomeric material comprise first and second bands of a thermally cured elastomer that are bonded to said end caps, respectively.

9. An invention as defined in claim 1 wherein said means defining first and second bands of resilient elastomeric material comprise a pair of annular gaskets, and including a pair of layers of epoxy cement, respectively positioned between each of said gaskets and their associated end caps, thereby to bond the gaskets to the end caps in fluid-tight relationship.

12. A invention as defined in claim 10 wherein said means for sealing said passageway comprises a body of solder having a melting point higher than 160* C.