US3213242A

US3213242A - Current limiting fuse

Info

Publication number: US3213242A
Application number: US175103A
Authority: US
Inventors: Frank L Cameron
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1962-02-23
Filing date: 1962-02-23
Publication date: 1965-10-19
Anticipated expiration: 1982-10-19

Description

Oct. 19, 1965 F. CAMERON CURRENT LIMITING FUSE 3 Sheets-Sheet 1 Filed Feb. 25, 1962 INVENTOR Frank L. Cameron BY M 4. WCLLQL ATTORNEY Oct. 19, 1965 Filed Feb. 23, 1962 TIM E IN SECONDS L. m u a m 5 lllllll ix: '0: In in F. L. CAMERON 3,213,242

CURRENT LIMITING FUSE 3 Sheets-Sheet 2 Fig. 4.

IllllFl lll llll I000 2600 4600 liooo Oct. 19, 1965 F. L. CAMERON CURRENT LIMITING FUSE 3 Sheets-Sheet 3 Filed Feb. 23, 1962 my v mm b mm hm United States Patent C 3,213,242 CURRENT LIMITING FUSE Frank L. Cameron, North Huntingdon Township, Westmoreland County, Pa, assignor to Westinghouse Eleotric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 23, 1962, Ser. No. 175,103 4 Claims. (Cl. 200-420) This invention relates, generally, to electric circuit interrupting devices and, more particularly, to interrupters of a type known as current limiting fuses which are connected to limit the current upon interruption of heavy overloads and short circuits.

For effective operation as a current limiting interrupter a fuse must melt on the steeply rising slope of the current wave before the first current peak of the alternating current wave is reached. The rate of rise of this current wave and the arc voltage generated and maintained by the fuse are the essential ingredients of its successful operation.

In accordance with standards established by Electrical Manufacturers, specific melting characteristics are required of a power fuse to qualify it for an E-rating and assure its electrical interchangeability. The fuse must also have the ability to interrupt the considerably lower melting currents associated with the E-rating characteristic.

An object of this invention is to provide a power fuse which is simple in construction, which meets the requirements of standards established by Electrical Manufacturers, which is suitable for relatively high voltage application and which has a high full load current rating.

Current limiting fuses are frequently utilized on lowcost transformer applications where the total space provided for the fuse is small, and where the cost of the fuse must be low. A further requirement, somewhat at variance with the preceding, is that the fuse must have a high interrupting rating.

Another object of the invention is to provide a current limiting fuse which is compact, low in cost, and has a high interrupting rating.

Still another object of the invention is to provide a fuse having specific melting characteristics and a good low current clearing ability.

Other objects of the invention will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with one embodiment of the invention, a desirable current or E-rating characteristic and the ability to interrupt the considerably lower melting currents associated with the E-rating characteristic are obtained by sandwiching a layer of CaCO (calcium carbonate) in a fuse between layers of a silica sand of controlled grain size. The fuse elements are notched silver ribbons prebent in a zig-zag manner to make them fatigueproof. In another embodiment of the invention, the fuse element is a stepped or graduated silver wire constructed to generate a specific arc voltage and obtain a high interrupting rating. A layer of CaCO is utilized between layers of sand in this fuse to obtain specific melting characteristics and a good low current clearing ability.

For a better understanding of the nature and objects of the invention, reference may be had to the following 3,213,242 Patented Oct. 19, 1965 detailed description, taken in conjunction with the ac companying drawings, in which:

FIGURE 1 is a view, partly in elevation and partly in section, of a fuse constructed in accordance with one embodiment of the invention;

FIG. 2 is a view, in plan, of an end terminal and fuse element space-r utilized in the fuse of FIG. 1;

FIG. 3 is an edgewise view of a portion of one of the fuse elements utilized in the fuse;

FIG. 4 is a group of time-current characteristic curves for the fuse of FIG. 1;

FIG. 5 is a view, partly in section and partly in elevation, of a fuse constructed in accordance with another embodiment of the invention, and

FIG. 6 is a view, in elevation, of a stepped wire utilized as the fuse element in the fuse of FIG. 5, the difference in wire diameters being exaggerated.

Referring to the drawing, the fuse shown in FIG. 1 is generally similar to the one described and claimed in my copending application Serial No. 85,872, filed January 30, 1961, which issued May 26, 1964, as US. Patent 3,134,874, and which is assigned to the same assignee as the present application. It comprises a tubular fuse holder 4 having

end caps

5 and 6 secured over opposite ends of the tube 4 and a current limiting and are extinguishing unit 7 disposed inside of the tube 4 which may be composed of a glass melamine material. The

end caps

5 and 6 are preferably composed of copper and they are silver plated over their entire outer surface.

The unit 7 comprises a tubular rod 3 composed of insulating material, such as ceramic, with end terminal members 10, 11 on opposite ends of the rod 8. The ends of the rod 8 have reduced portions 9, 9 each adapted to be received within a centralized mating aperture in one of the terminal members 10, 11. The end terminals may be composed of any conducting material, preferably copper. The rod 8 may be attached to the terminals 10, 11 in any suitable manner, as by an adhesive.

As shown in FIG. 2, each terminal member 10, 11 is a disc of sufiicient diameter to extend substantially across the inside of the tubular holder 4. Each disc is provided with a pair of spaced

conductor studs

12, 12, preferably of 4 inch copper, extending perpendicularly to the disc for mating relationship with

suitable apertures

13, 13 in the

end caps

5, 6. The studs may be connected to the end caps at the

apertures

13, 13 in any suitable manner, but preferably the studs are electrically connected to the end caps or ferrules by means of a mechanical spin-press fit which is then soldered to forestall any possibility of corrosion at the mechanical joint.

Each terminal disc 10, 11 is provided at its outer periphery with twelve radially disposed, equally spaced narrow slots 14. Each slot 14 in one terminal disc is aligned with .a slot 14 in the opposite terminal disc, each pair of aligned slots being provided with a fusible element 15 extending therethrough. The ends of the fusible elements 15 are fitted in the aligned slots and bent over at the ends which may be soldered to the copper end termi nal if desired.

Each fusible element 15 is ribbon-like in shape and is preferably composed of silver. The ribbon is segmented by V-notching on both edges periodically along its length to provide a series of restricted areas which fuse to provide a series of arcs, the summation of which provides a high are voltage as is known in the art. By way of example, the silver ribbon element may be inch wide by .005 inch thick and provided with V-notches having an included angle of about 36 at 4 inch intervals along its length. It will be understood that the length of the fusible elements depends upon the voltage of the system in which the fuse is to be utilized.

Referring now to FIG. 3, there is shown the zig-zag appearance of the fusible element 15 resulting from preindexing or prebending of the element periodically along its length each successive bend being equal and opposite to the adjacent bends relative to a central axis along the fusible element, Investigation with non-indexed ribbon type fusible elements have shown that expansion and contraction of the fusible element effected pronounced flexing at one or two points along the element, ultimately causing the element to fracture or break under fatigue failure.

Preindexing of the fusible element, as shown in FIG. 3, effects uniform distribution of the flexing among the various indexed points with the result that variation in stress intensity at any one point is kept within the endurance range of the metal, and the element does not break. Preferably, the fusible element should be bent as sharply as possible at the points of indexing without damaging the surface of the wire. It is apparent that the minimum number of indexed points is dependent upon the length of the fusible element and the amount of expansion and contraction anticipated for the particular material. As an example of fatigue proofing of a fusible element through prebending or preindexing as shown, a silver strap of the type hereinbefore described, having a length of ten inches, was indexed at about A inch intervals to an included angle of about 120 resulting in a fuse having fatigue proof elements.

The construction of the fuse as hereinbefore described wherein the terminals and 11 may be attached to the support rod 8 and the fuse elements 15 as a self-supporting subassembly permits the testing of the interrupting unit 7 alone before insertion into the tubular holder 4. This is an important advance in manufacturing as it permits the visual inspection and a culling out of any subassemblies which may be defective, before final assembly within the holder 4.

In order to provide a visual indication of fuse operation, an operation indicator 17 is disposed at one end of the tube 4. The indicator 17 comprises a cylindrical housing 18 having a shoulder 19 at one end abutting the terminal disc 11 and having a tubular portion extending beyond the shoulder 19 within the previously described centralized aperture in the disc 11. The opposite end of the housing 18 is open and extends outwardly of disc 11 into abutting relationship with the inside edge of an aperture 20 in the end cap 6, the aperture 20 having a diameter slightly smaller than the inside diameter of the housing 18 for reasons which will be immediately apparent. Slidably mounted within the housing 18 is a cylindrical plastic indicator 21 having a diameter slightly less than the diameter of aperture 20 and having a flange portion 22 at the inner end thereof having a diameter substantially the same as that of the inside of housing 18 and greater than the diameter of aperture 20 in the end cap 6. The length of the cylindrical indicator 21 is such that it may be fully enclosed within the housing 18, thus indicating the absence of fuse operation. A compression spring 23 is mounted in the housing 18 between the shoulder 19 and the flange 22 of the indicator 21 to bias the indicator outwardly of the housing 18 whereupon the flange 22 engages the inside edge of aperture 20, thus indicating the operation of the fuse by the extension of indicator 21 outwardly of the housing 18 into view of an inspector.

Normally, the indicator 21 is held within the housing 18 out of view of the inspector by means of a fusible conductor 24 fixed to an upper stud 12 and extending downwardly through the central aperture of tubular support rod 8, through the centralized aperture of terminal disc 11, through a tapered neck opening 25 in housing 18, an eye 26 on the inner end of indicator 21, back through the opening 25 and between the outside of the shoulder 19 and the underside of the disc 11. Thus, the conductor 24 is pressed into conducting relationship with the disc 11 when the fuse is assembled. A tapered plug 27 is tightly fitted in the tapered opening 25, thereby serving as an anchor for the end of the conductor 24 to hold the indicator 21 in the retracted position against the biasing action of the spring 23. The housing 18 and the plug 27 are composed of insulating material.

In operation, when the main fusible elements 15 fuse, an increased current is passed through conductor 24 causing it to fuse, thereby releasing the indicator 21 which ex tends outwardly of housing 18 under the biasing force of spring 23. Inasmuch as the lower looped portion of conductor 24 extending between the plug 27 and the eye 26 is in air rather than in an arc extinguishing means, as is the remaining portion of the conductor, the described looped portion fuses first rather than the remaining portion to effect release of the indicator 2i.

Heretofore, the practice has been to fill fuses of the present type with sand. In order to obtain the desired current or E-rating characteristic in the present fuse and the ability to interrupt the lower melting currents associated with the E-rating characteristic, a layer of CaCO (calcium carbonate) is sandwiched in the fuse between layers of a silica sand of controlled grain size. The minimum current required to cause melting of a given fuse ele: ment can be controlled by changing the thickness and/ or position of the layer of CaCO The most effective control is achieved by varying the thickness of the layer. As an example, a fuse built with three current limiting elements each composed of a notched silver strap .005 of an inch thick by inch wide is a 65E ampere fuse when constructed with a three inch layer of CaCO This same fuse has an -E ampere rating when the CaCO layer is reduced to two inches in thickness, an example of E rating curves is shown in FIG. 4.

This change in melting characteristic is achieved because of the modified heat flow which results from use of the CaCO layer. The very fine CaCO powder is so compacted during the fuse assembly that the heat generated by the fuse element in the section blanketed by the CaCO layer is less effectively dissipated. This in effect lowers the current required to cause melting of the fusible element.

Equally important is the manner in which the CaCO aids in low current interrupt-ions. At a temperature slightly below the melting point of silver the CaCO decomposes and gives off a gas which aids in are interruption. A fuse built with a layer of CaCO has the ability to effectively interrupt currents well below those which can be interrupted by a sand filler alone. Since the CaCO decomposes instead of fusing, no fulgurite is formed. Furthermore, CaCO has a peculiar advantage over other materials, such as gypsum and boric acid, in that it will begin to decompose only at a temperature significantly higher than that at which either one of the aforementioned materials decomposes. Thus, the gas is given off by the CaCO at a time when it is most effective in interrupting the are.

In building the fuse, the interrupter and the indicator subassemblies are combined and the lower ferrule or end cap press-fitted into place. The glass melamine tube is next inserted into the ferrule and the tube is partly filled with sand 16 to a desired height, such as the distance A in FIG. 4. Next, CaCO indicated by 16', is put into the tube to provide a layer having a desired thickness such as the distance B which encompasses the longitudinal center of the fusible element. Then, sufficient sand 16 is put into the tube to fill it, as indicated by the distance A. An opening 28 is provided in each terminal disc to aid in getting the filler material into the tube 4.

Assurance of an adequate and compact filling may be attained by utilizing a vibrator-filler which completely fills the fuse, except for the indicator. The top ferrule is next fitted over the tube and press-fitted to the upper end studs. Final soldering of the ferrule-stud connection is followed by a baking operation which cures the cement used between the ferrules and the glass melamine tube. The fuse is then ready for final inspection and checking.

As explained hereinbefore, a current limiting fuse constructed in the foregoing manner has a high full load current rating and it has the ability to interrupt the relatively low melting currents associated with the current rating charateristic.

The fuse illustrated in FIG. 5 is also of the current limiting type. It comprises a tube or holder 31, two end caps or ferrules 32, a core 33, a fusible element 34 wound on the core 33 and filler material comprising two layers of sand 35 with a layer 36 of CaCO sandwiched between the two layers of sand. The tube 31 is preferably composed of borosilicate glass having heat resisting properties. The ferrules 32 are preferably composed of copper and they may be cemented onto the ends of the tube. The core 33 is of a relatively low cost and it may be made by coating filament glass tape with a layer of sodium silicate (water glass). The fusible element 34 is a composite silver wire having stepped or graduated diameters. Thus, the fuse contains no organic material.

In order to generate a specific arc voltage during interruption, the composite fusible element 34 comprises three sections E, F and G, each section being of different length and having a different diameter. As shown in FIG. 6, the three sections may be welded together. By way of example, the section B may have a length of 8% inches, sect-ion F a length of 4 inches and section G a length of 5% inches, thereby providing a composite Wire 18 inches in length. The diameter of section E may be .0045 of an inch, that of section P .0056 and that of section G .005 of an inch. Thus, each section has a different fusing characteristic.

As each diameter of the wire melts in turn, the molten wire continues to act as a conductor. There is a considerable increase in resistance as the element changes from a solid to a liquid. There is consequently a large increase in the energy developed at the moment the fuse melts causing the entire section of the element to vaporize. The space formerly occupied by the wire is now filled with a metal vapor of high temperature which is momentarily an insulator. With the sudden insertion of this insulator in the circuit, a high voltage is developed in the inductance of the circuit, and the voltage causes an arc to be formed across the vapor column. The fuse arc voltage now increases rapidly because of the refractory :material in the fuse which both confines and deionizes the arc.

As shown in FIG. 5, a pin 37 is attached to each end of the core 33 by means of a rivet 38. The fuse wire 34 is wound on the core and each end of the wire is soldered to one of the pins 37. A ferrule 32 is placed over one end of the core with a pin 37 extending through an opening in the end of the ferrule. One end of the tube 31 is pressed into the ferrule and cemented in place. The tube is then filled with two layers of sand with a layer of CaCO sandwiched between the layers of sand. The other ferrule is cemented onto the other end of the tube with the pin 37 extending through the ferrule. The pins are soldered to the ferrules, thereby sealing the openings through which the pins extend. After soldering, the ends of the pins may be cut off next to the solder.

As explained hereinbefore, the thickness of and the location of the band of CaCO provide specific melting characteristics and a good low current clearing ability. The present fuse is relatively small in size and low in cost. By way of example, a fuse suitable for 5 kv. service and rated at 2 amperes has an overall length of 7 inches. The layer of CaCO at the center, designated by B is 2 inches thick. Each layer of sand, designated by A,

d is approximately 2 inches thick. Thus, the CaCO encompasses the middle section of the fusible conductor. This fuse is particularly suitable for use on transformer applications where the total space for the fuse is small and it is desirable that the cost be low, but the fuse must have a high interrupting rating.

From the foregoing description, it is apparent that the utilization of a layer of CaCO sandwiched between two layers of silica sand in a fuse has definite advantages as set forth in the foregoing. The utilization of fusible elements of the type described in conjunction with the CaCO makes it possible to build fuses having specific operating characteristics.

Since numerous changes may be made in the abovedescribed construction and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A fuse comprising, a tubular fuse holder, spaced terminal means supported by said holder, a ribbon-like fusible conductor having periodically spaced restricted portions along its length connected between said terminal means, said conductor being prebent in a zig-zag manner across the plane of the conductor along substantially its entire length to distribute flexing of the conductor among the zig-zagged portions of the conductor, and arc-quenching means comprising a layer of calcium carbonate powder disposed in the central portion of said fuse holder and finmly packed between two layers of silica sand around the conductor in the holder.

2. In a fuse, in combination, a tubular fuse holder, an insulating core centrally disposed inside the holder, a ferrule on each end of the holder, a terminal member at each end of the core, each terminal member being electrically connected to one of the ferrules, a three-section stepped fusible conductor wound around said core and connected between the terminal members, the diameter of the middle section of the conductor being greater than the diameter of either one of the end sections, and arcquenching means comprising a layer of compacted calcium carbonate powder disposed in the central portion of said fuse holder and sandwiched between two layers of silica sand surrounding the conductor in the holder.

3. In a fuse, in combination, a tubular fuse holder, an insulating core centrally disposed inside the holder, a ferrule on each end of the holder, a terminal member at each end of the core, each terminal member being electrically connected to one of the ferrules, a three-section stepped fusible conductor wound around said core and connected between the terminal members, the diameter of the middle section of the conductor being greater than the diameter of either one of the end sections, a layer of compacted calcium carbonate powder disposed in the central portion of said fuse holder and substantialy encompassing the middle section of the conductor, and a layer of silica sand substantially encompassing each one of the end sections of the conductor in said holder.

4. A fuse comprising, a tubular fuse holder, spaced terminal means supported by said holder, a ribbon-like fusible conductor having periodically spaced restricted portions along its length connected between said terminal means, said conductor being prebent in a zig-zag manner across the plane of the conductor along substantially its entire length to distribute flexing of the conductor among the zig-zagged portions of the conductor, and arc-quenching means comprising a layer of calcium carbonate powder disposed in the central portion of said fuse holder and firmly packed between two layers of silica sand around the conductor in the holder, said tubular fuse holder being composed of borosilicate glass.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Cole 200-120 Lemmon 200-135 Hill 200-120 Lohausen 200-120 X Lohausen 200-120 Triplett 200-135 Hart 200-135 Lohausen 200-131 Suits et a1 200-131 X Dryer 200-131 Kozacka 200-120 Kozacka 200-120 Burt et a1.

Fister.

Brown 200-120 10 BERNARD A. GILHEANY, Primary Examiner.

Claims

1. A FUSE COMPRISING, A TUBULAR FUSE HOLDER, SPACED TERMINAL MEANS SUPPORTED BY SAID HOLDER, A RIBBON-LIKE FUSIBLE CONDUCTOR HAVING PERIODICALLY SPACED RESTRICTED PORTIONS ALONG ITS LENGTH CONNECTED BETWEEN SAID TERMINAL MEANS, SAID CONDUCTOR BEING PREBENT IN A ZIG-ZAG MANNER ACROSS THE PLANE OF THE CONDUCTOR ALONG SUBSTANTIALLY ITS ENTIRE LENGTH TO DISTRIBUTE FLEXING OF THE CONDUCTOR