US3465275A

US3465275A - Current limiting fuse for use in rotating machinery

Info

Publication number: US3465275A
Application number: US708028A
Authority: US
Inventors: Kenneth W Swain
Original assignee: Chase Shawmut Co
Current assignee: GOLUD INC A DE CORP; Gould Inc
Priority date: 1968-02-26
Filing date: 1968-02-26
Publication date: 1969-09-02
Anticipated expiration: 1986-09-02

Description

K. w. swAlN Sept. 2, 1969 CURRENT LIMITING FUSE FOR USE IN ROTATNG MACHINERY 2 Sheets-Sheet 1 Original Filed April 13, 1962 Sept. 2, 1969 K. w. swA|N 3,465,275

l CURRENT LIMITING FUSE FOR USE IN ROTATING MACHINERY Original Filed April 13, 1962 2 sheets-sheet 2 Iaweza'orl:

K W'Szhvazw, 'y MJA/ww United States Patent O 3,465,275 CURRENT LIMITING FUSE FOR USE IN ROTATING MACHINERY Kenneth W. Swain, North Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass.

Continuation of application Ser. No. 187,261, Apr. 13,

1962. This application Feb. 26, 1968, Ser. No. 708,028 Int. Cl. H01h 85 04 U.S. Cl. 337-158 f 9 Claims ABSTRACT OF THE DISCLOSURE A fuse structure having a fuse link shaped to be braced against centrifugal forces by the pulverulent arc-quenching filler inside the casing of the fuse. The fuse link assembly includes a back-up plate of a synthetic-resinglass-cloth laminate for the fuse link. The fuse link assembly is designed to minimize let-through currents or let-through 2t values, respectively, and to yield fuse strulctures having high current carrying capacities in spite of their great compactness. The electrical performance characteristics of the fuse link assembly are due to the coaction of a ribbon fuse link having a specific geometrical configuration, and its aforementioned back-up plate of synthetic-resin-glass-cloth laminate. The back-up plate forms an auxiliary brace for the fragile perforated portion of the fuse link.

This application further discloses a blown fuse indicator having a restraining wire acted upon by braking means and being subject to minimal stresses when the fuse is being rotated.

BACKGROUND OF INVENTION This application is a continuation of my abandoned patent application Ser. No. 187,261, iiled Apr. 13, 1962.

The present invention is an outgrowth of the structure disclosed and claimed in U.S. Patent 3,020,370 to Kenneth W. Swain, Feb. 6, 1962, for Protection of Semiconductor Diodes, and of the structure disclosed and claimed in U.S. Patent 3,152,233 to Frederick l. Kozacka, Oct. 6, 1964, for Blade-Type Electric Fuse.

U.S. Patent 3,020,370 discloses a fuse structure intended for use in rotating machinery wherein the fuse link assembly is subjected to intense centrifugal forces. The fuse link assembly disclosed in the above patent is designed to minimize let-through currents and letthrough i2-t values, respectively, but it does not allow to achieve high current carrying capacities without becoming-extremely bulky. An important fea-ture of the structure disclosed and claimed in U.S. Patent 3,020,370 is the provision of fuse link engaging plates of syntheticresin-glass-cloth laminate which contribute to the electrical performance of the fuse and brace the fuse link against the action of centrifugal forces. The structure of the above patent is intended for applications wherein the circuit voltage is relatively low, and wherein the action of the centrifugal forces is at right angles to the longitudinal axis of the tubular casing of the fuse.

U.S. Patent 3,152,233 discloses a fuse having a ribbon fuse link of a metal that has a relatively high conductivity and a relatively high fusing point. The fuse link forms a lattice pattern area including parallel connected necks and serially connected necks. Said lattice pattern area is established by substantially transverse lines of perforations of which each includes a predetermined number of perforations per line. The number of perforations per line is substantially in excess of the number of lines of perforations. The ribbon fuse link has a pair of flexible bends adjacent to the axially outer ends -thereof establishing portions of the fuse link substantially at right angles to the aforementioned lattice pattern area thereof. A plate of insulating material substantially coextensive with the lattice pattern area of the fuse link is aliixed to the fuse link.

The a'bove configuration of the fuse link makes it possible -to control relatively high circuit voltages with a relatively small number of serially connected necks and thus to limit the voltage drop across the fuse when the latter is current carrying. When the fuse blows the current to be interrupted at each point of break is relatively small, due to the presence of a large number of necks of which each forms a point of break. The insulating plate may evolve arc-quenching blasts of gas directed against each point of break, and may have the ability to absorb large amounts of hea-t from each of the parallel arclets and series arclets formed upon blowing of the fuse.

For the aforementioned reason combining the features of U.S. Patents 3,020,370 and 3,152,233 seems to result in a current-limiting fuse suitable for use in rotating machinery, i.e. suitable for back-up protection of solid state diodes in such machinery. It has, however, been found that neither the fuse link assembly of U.S. Patent 3,152,233, nor any other prior art fuse link assembly, is capable of minimizing let-through currents and maximizing current-carrying capacity to the extent required for the protection of rotating diodes within the space limitations to which such fuses are subject, if the current carrying capacity of Ithe diodes is relatively high, e.g. in the order of many hundred amps., and the circuit voltage is relatively high, e.g. in the order of 800 volts, or even higher.

The aforementioned objects are achieved according to the present invention by combining the features ot the structures of U.S. Patents 3,020,270 and 3,152,233, and by adding to this combination a means predicated on the evolution of an arc-quenching blast of gas which increases and prolongs Ithe arc voltage without adversely affecting the current-carrying capacity of the fuse.

The blown fuse indicator according to the present invention is an outgrowth of prior art flapper type blown fuse indicators shown, for instance, in a pamphlet by Voigt & Haeffner A.G., Frankfurt am Main, Germany, titled Druckfreie Hochlestungssicherungen, Publication #650 (Pressureless High Interrupting Capacity Fuses), published in 1937. The above prior art blown fuse indicator is adapted for application in fuses forrotating machinery by reducing the spring bias to which its restraining wire is subjected virtually to zero, and by providing braking means surrounding the restraining wire adjacent one end thereof and acting radially inwardly upon the restraining wire. These braking means absorb a portion of the centrifugal forces to which the flapper or other equivalent blown fuse indicating element is subjected. Thus the center region of the restraining wire is normally not subjected to any spring bias and to but a portion of the centrifugal forces to which the tiapper, or its equivalent, is subjected. When the restraining wire melts the centrifugal forces acting upon Ithe flapper, or its equivalent, overcome the braking action of the aforementioned braking means, and thus the flapper, or its equivalent, is moved under the action of centrifugal forces to the indicating position thereof.

SUMMARY OF INVENTION A current-limiting fuse embodying this invention includes a substantially tubular casing of insulating material and a pair of terminal plugs having axially inner end surfaces provided with tuse-link-receiving grooves and jointly forming with said casing a solid, substantially cylindrical wall. A pulverulent arc-quenching filler is arranged inside the casing. A plate of synthetic-resin-glass-cloth laminate is arranged inside the casing, embedded in the arcquenching filler, and has edges braced against said solid, substantially cylindrical wall. A fuse link of sheet metal arranged inside the casing, and embedded in the arcquenching filler, has axially outer ends projecting into said fuse-link-receiving grooves of said pair of plugs. The fuse link includes a perforated portion extending parallel to, abutting against, and being affixed to said plate of synthetic-resin-glass-cloth laminate, and the fuse link further includes non-perforated portions extending substantially transversely to said plate of synthetic-resin-glass-cloth laminate. The perforated portion of the fuse link has a plurality of transverse lines of perforations each defining a plurality of spaced parallel-connected necks. The number of necks per line of perforations exceeds the number of transverse lines of perforations. The perforated portion of the fuse link further has an additional line of perforations arranged between one of said pair of terminal plugs and an axially outer of said plurality of transverse lines of perforations. The number of necks defined by said additional line of perforations is smaller than the number of necks defined by each of said plurality of lines of perforations. The aggregate width of the necks defined by said additional line of perforations is smaller than the aggregate width of said plurality of necks defined by each of said plurality of lines of perforations.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is in part a top plan view and in part a section of a fuse structure embodying the invention;

FIG. 2 is a side elevation of the structure of FIG. 1 seen from the left side thereof;

FIG. 3 is a side elevation of the structure of FIG. 1 seen from the right side thereof;

FIG. 4 is a section taken along 4-4 of FIG. 3 showing the structure of FIG. 1 on a larger scale;

FIG. 5 is a section taken along 55 of FIG. 3;

FIG. 6 is a bottom plan view of the integral fuse link and fuse-link-support unit forming a part of the structure of FIGS. l and 5; and

FIG. 7 shows diagrammatically the general lay-out of the rotor for an exciter for a brushless electric generator and the way in which current-limiting fuses embodying this invention are arranged therein.

DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION The aforementioned U.S. Patent 3,020,370 shows a wiring diagram of the rotor of an exciter for a brushless electric generator including cell fuses adapted to withstand centrifugal forces of many thousand g (acceleration of gravity), and the current-limiting fuses embodying the present invention are intended to be included into an electric circuit in the same fashion as shown in FIG. 1 of the above patent.

Referring now to the drawings, numeral 1 has been applied to indicate a tubular casing of insulating material, preferably a melamine-glass-cloth laminate, closed on both ends thereof by terminal elements in the form of cylindrical plugs 2 press-fitted into casing 1. Each plug is provided with a knife blade contact 3 forming an integ'ral part thereof. Blade contacts 3 are provided with holes 4 for securing the fuse structure to the rotor of an exciter for a brushless electric generator. Since plugs 2 are subjected to high centrifugal forces it is necessary to firmly secure plugs 2 to casing 1. This is best achieved by radial steel pins extending radially through casing 1 into radial bores provided in plugs 2. These pins have not been shown in the drawings, but FIG. 1 shows clearly radial pin-receiving holes 5 into which the aforementioned pins are inserted. The axially inner surface 2 of each plug 2 is provided with four parallel grooves, namely two radially outer grooves 6 and two radially inner grooves 7. Reference numeral 8 has been applied to indicate a pair of plates of insulating material evolving gas under the heat of electric arcs. Plates 8 may be made, and are preferably made, of a melamine-glass-cloth laminate. The axially outer ends, or edges, of plates 8 abut against and are braced by the axially inner end surfaces 2 of plugs 2. This is achieved by inserting the aforementioned ends, or edges, of plates 8 into the radially outer grooves 6 and firmly holding the same therein by friction. The fuse structure further comprises a pair of ribbon fuse links 9, preferably of silver, who-se length exceeds the spacing between the axially inner end surfaces 2" of terminal elements or terminal plugs 2. The axially outer ends of links 9 are inserted into grooves 7 and the latter are filled with soft solder (not shown). Thus a conductive connection is established between the ends of each of links 9 and plugs 2, and plugs 2 are conductively interconnected by links 9. A portion of each link 9 is bent out of the plane defined by its ends and by the grooves 7 into which its ends are inserted into the plane defined by the insulating plate 8 immediately adjacent thereto. In FIG. 6 reference character 9 has been applied to indicate the ends of link 9 which are intended for insertion into link-receiving grooves 7, and reference character 9 has been applied to indicate the intermediate portion of link 9 situated between ends 9 and bent into the plane of the immediately adjacent plate 8. Each of links 9 is provided with a plurality of transverse lines a, b, c, b, c of substantially circular perforations. Each of lines a, b, c, b, c defines a region of reduced cross-section. Lines b of perforations define two regions of reduced cross-section Where the reduction of cross-section is smallest, i.e. the cross-section relatively large. Lines c of perforations define two regions of reduced cross-section where the reduction of cross-section is higher than at lines b, i.e. the cross-section at the regions of reduced cross-section defined by the lines of perforation c is less than the cross-section at the regions of reduced cross-Section defined by the lines of perforations b. The line of perforations a has been derived from a line of perforations which was initially identical with the lines of perforations c by integrating each of the two laterally outer perforations of such a line into one single substantially U-shaped perforation of increased size. Line a of perforations defines a region of reduced cross-section where the reduction of cross-section is highest, i.e. higher than at lines b and c. In other words, the cross-section at the region of reduced cross-section defined by line a is less than the cross-section at any other point of link 9. The cross-section at line a being smallest, fusion and arcing will be initiated at this point of the link on occurrence of fault currents of significant magnitude. The arc voltage at the point of initial fusion and arcing is sufficient to limit momentarily the rate of rise of the fault current, but not sufficient to bring the fault current to zero. While the arc still burns at the region of line a, fusion and arc initiation occur at the two lines c substantially simultaneously. The combined arc voltage formed at the breaks adjacent lines a, c and c is sufiicient to rapidly bring the current down to zero. The crosssection of link 9 at the two points of lines of perforations b is so large that fusion never occurs at these two points as long as the fuse is used within its rated interrupting capacity. Each transverse line of perforations b comprises but two perforations. An eyelet 10 whose shank has a diameter substantially equal to the diameter of the perforations forming line b is inserted into each of these perforations and project through a registering perforation in one of plates 9. Thus the perforated portion 9 of each fuse link 9 intermediate ends 9 is firmly clamped against one of plates 9 once the four eyelets 10 which project through each of links 9 and through each of plates 8 are upset.

Each of the two lines c of perforations defines four necks which are connected in parallel and have the same width. The line of perforations a defines but two necks which are connected in parallel. The aggregate width of the necks of each of lines c is much larger than the aggregate width of the necks of line a. As a result, the current density at the necks of line a is in excess of the current density at lines c. Because of the immediate proximity of the necks of line a to one of terminal plugs 2 operating as a heat sink and heat dissipator, the high current density and concomitant heat generation at the two necks of line a does not significantly affect, or limit, the currentcarrying capacity of the fuse structure. However, since the two necks of line a fuse first, resulting in a relatively long arcing time at these two points, and since the current density at the two arclets resulting from melting of the two necks is particularly large, the line of perforations a could not achieve its intended purpose if its necks were not backed up by plates 8 capable of effectively absorbing heat and of evolving under the heat of arcs highly effective blasts o-f arc-quenching gases.

The right terminal plug 2 as seen in FIGS. l, 4 and 5 is provided with an abutment 2' for transmitting centrifugal forces from the fuse structure to a cooperating abutment member or bracing member integral with the rotor of an exciter for a brushless electric generator. The function of abutment plate 2' will be explained below more in detail in connection with FIG. 7. The area of plate 2' exceeds the cross-sectional area of casing 1 as determined by the outer diameter thereof. Plate 2 is provided with a groove 11 which is U-shaped in cross-section. A blown fuse indicator element is arranged within groove 11. This indicator element is made of sheet metal and comprises a flat end 12' secured by a rivet 18 to plate 2' and this indicator element further comprises a substantially U-shaped end 12" restrained by fusible wire 13. Since indicator element 12', 12" is connected by rivet 18 to plate 2' of right terminal plug 2 (FIGS. l and 5), and since element 12', 12" is made of sheet metal, this element is in conductive connection with the aforementioned right plug 2.

Fusible wire 13 forms a shunt across fuse links 9 and interconnects conductively the two terminal plugs 2. The left end of fusible wire 13, as seen in FIG. 4, extends into a bore in one of plugs 2 and is frictionally held in position by a resilient cap 14. The right end of fusible wire 13, as seen in FIG. 4, is inserted into a bore 15 provided in the right plug 2 and its abutment plate 2 and extends through a fine bore in the abutment plate 2' coaxial to bore 15 to the outside of the fuse structure. Bore 15 houses impediments to the flow of products of arcing to the outside of the fuse structure. These impediments may take various forms. As shown in FIGS. 4 and 5 the right end of restraining wire 13 is in frictional engagement with a block 16 of an elastomer, e.g. wire 13 may be threaded through such a block, thus restraining the right end of wire 13 in position. The indicator element 12', 12" has a normal position and an indicating position. It has been shown in FIGS. 3, 4 and 5 in its normal position in which it is normally held by restraining wire 13. In the indicating position of element 12', 12" the portion 12" thereof is moved out of groove 11 and its flat highly flexible portion 12' bent. The portion 12" of indicator element 12', 12" which is U-shaped in cross-section is provided with a relatively large aperture and wire 13 is threaded through said aperture. Wire 13 is further threaded through a small hole in a small disc 17 and provided with a knot at the axially outer side of disc 17. Thus disc 17 establishes a conductive connection between wire 13 and indicator element 12', 12".

Though the impediments of the outflow of hot produ-cts of arcing arranged in bore 15 are quite effective, safety considerations require that the diameter of the passage through which wire 13 projects outwardly of the fuse structure, i.e. through plug 2 and plate 2', be as narrow as possible, i.e. that the clearance between wire 13 and that passage be minimized. If indicator 12', 12" were provided with a passage for wire 13 having -a diameter as small as the passage in

parts

2, 2 for wire 13, aligning both passages would involve some difficulties in the manufacture of the device. These difficulties are avoided by providing indicator element 12', 12" with an aperture for the passage of wire 13 whose diameter is relatively large and by superimposing upon indicator element 12', 12" disc 17, the bore of which through which wire 13 is threaded is quite small and which is freely movable in the plane of element 12', 12" so as to be self-adjusting, i.e. to effect automatic registration of its narrow passage for wire 13 with the narrow passage for wire 13 provided in parts 2, 2'.

It will be noted that the two bends of deg. in each fuse link 9 protect the points of reduced cross-section thereof formed by lines of perforations a, b, c, b, c against stresses which might result in the absence of these two bends from 4an accidental change of the spacing between the axially inner surfaces 2' of plugs 2.

It will further be noted that the structure shown in the drawings does not include any spring means imposing a spring bias upon wire 13; nor does the structure shown in the drawings include any other auxiliary source of motive power such as, for instance, a spring for moving indicator element 12', 12" from the normal position shown in the drawings to the indicating position thereof.

Each terminal plug is provided with two holes for filling a pulverulent arc-quenching ller into casing 1 and each of these holes is closed by a resilient metal cap 14. One of the aforementioned filling holes and caps is arranged under the indicator element 12', 12", as clearly shown in FIG. 3. The filling holes and the caps 14 in both plugs 2 are in alignment, but the filling hole through which wire 13 extends is slightly out of alignment with bore 15 for wire 13. Hence the direction in which wire 13 extends is not strictly parallel to the longitudinal axis of casing 1 and plugs 2. This slight deviation from parallelism has been indicated in FIG. l in a slightly exaggerated fashion. The clamping action of cap 14 (see FIG. 4) is not sufficient to establish a good conductive connection between wire 13 and plug 2 and to firmly hold in position the left end of wire 13. Hence wire 13 has an end situated outside of the left plug 2, as seen in FIGS. 1 and 4, and this end of wire 13 is screwed against the axially outer surface of said left plug. FIG. 2 shows the end of wire 13 extending out of casing 1 and terminal or clamping screw 19 clamping wire 13 against plug 2.

As clearly shown in FIGS. 1 and 4, insulating plates 8 subdivide casing 1 into a radially inner chamber'A and a pair of radially outer chambers B communicating with said radially inner chamber A. Casing 1 is filled with a pulverulent `arc-quenching filler, preferably quartz sand. There is, therefore, a mass of quartz sand in the radially inner chamber A and there are additional masses of quartzsand in the radially outer chambers B. These masses of arc-quenching filler, or quartz-sand, perform different functions in interrupting excessive currents, as will be explained below more in detail.

Referring now to FIG. 7, numeral 21 has been applied to indicate the rotor of an exciter for a brushless electric generator. Rotor 21 is mounted on shaft 22 and supports six semiconductor diodes or semiconductor rectifier cells 23. Each cell is associated (serially connected) with a cell fuse 24. Each of the cell fuses 24 is a structure of the type shown in FIGS. l-5 and includes a casing 1, terminal plugs 2, and an abutment plate 2'. The latter is abutting against an abutment 25 integral with rotor 21 tending to support each fuse 24 against the high centrifugal forces acting upon it when rotor 21 rotates at high `angular velocities.

It will be apparent from the foregoing that the structure shown in FIGS. 1-5 protects the weak points of reduced cross-section of fuse links 9 against damage by the action of centrifugal forces. Since the mass of indicator apper 12', 12" is small the centrifugal forces acting upon wire 13 and tending to stretch wire 13 are relatively limited. Since the right end of wire 13 (FIG. 4) is compressed by elastomeric body 16 through which wire 13 is threaded, this arrangement operates as a friction brake, providing further relief to wire 13 from the action of centrifugal forces in a direction longitudinally of wire 13. Since the centrifugal forces upon the fuse struct-ure 24 `are received by abutments 25 the bolts intended to extend through bores 4 in blade contacts 3 are not subjected to any shearing stresses. Excessive currents result in the formation of initial breaks at the lines a of reduced cross-section of link 9 (FIG. 6). As mentioned above, the arc-voltage generated at these initial points of break is not sufficiently high to force the excess current down to zero, but reduces the rate of rise of excess currents. Shortly upon formation of initial breaks at lines a of reduced cross-section additional breaks are formed at the two lines c of reduced cross-section. The aggregate arc voltage formed at all the serially related breaks forces the excess current rapidly down to zero. The arcs formed at lines a and c are driven into chamber A by the joint action of magnetic forces inherent in the geometry of the fuse structure and by gases evolving from insulating plates 8 under the heat of arcs. Some of the mass of quartz-sand in chamber A will fuse and form a fulgurite. The non-fused quartz-sand in chamber A forms a pre-cooler for the products of arcing resulting from blowing of the fuse. The products of arcing upon having been pre-cooled in chamber A flow into chamber B where the temperature thereof is further reduced. In other words, the mass of quartz-sand in chamber A forms a pre-cooler and the masses of quartz-sand in chambers B form after-coolers for the products of arcing, and plates 8 form baflies guiding the flow of products of `arcing inside of casing 1. Wire 13 has a much higher resistance than ribbon fuse links 9 and, therefore, wire 13 does not carry any significant amount of current as long as links 9 are intact. Upon fusion of links 9 wire 13 begins to carry current and fuses rapidly. Now indicator element 12', 12 is free to move axially outwardly under the action of centrifugal forces in that direction. In moving from its normal position to its indicating position indicator element 12', 12 overcomes the friction between the elastomeric braking body 16 and the right end of wire 13 (FIG. 4). Inspection of the rotor 21 by means of a stroboscopic light source having the proper frequency readily reveals whether or not one or more of the fuses 24 carried by rotor 21 have blown.

It will be apparent from the foregoing that I have provided fuse structures for rotating rectifiers that are adapted to be radially mounted on the rotor in positions like the positions of the spokes of a wheel. This makes it possible to provide simple and highly effective means against the destructive action of centrifugal forces and to provide for simple springless blown fuse indicator means including a restraining wire which is not significantly subjected to stresses and indicating means which are moved only by centrifugal action from the normal position to the indicating position thereof and which, therefore, do not require any auxiliary source of motive power for that purpose. The mass of the indicator element ought to be sufiiciently large to cause the same to snap instantly to its indicating position upon fusion of the restraining wire 13. Because of the high angular velocities of rotating machinery this can be achieved with a relatively small mass.

The substantially transverse bends in fuse links 9 have two functions, i.e. they and the pulverulent arc-quenching filler provide jointly an effective bracing action against centrifugal forces, and furthermore the transverse bends impart such flexibility to the

fuse link assembly

9, 8 as necessary to preclude undue stresses in the fragile perforated portion of the fuse link on account of tolerances and thermal expansion and contraction. The bracing action of the aforementioned transverse bends is missing in case that the packing of the pulverulent arc-quenching filler is not sufficiently dense. Then each of plates 8 operates as a standby to brace the fuse link 9 with which it is associated.

The performance of the fuse depends critically upon the geometry of the fuse links 9 which has been described above and the presence and the nature of back-up and bracing plates y8. The lines of perforation a of fuse links 9 generate arc voltages prior to the point of time when arc voltages are generated at the lines of perforations c. Be-

cause of the positioning of the lines of perforations a relative to the adjacent terminal plug 2, the heat generated at the former is largely absorbed and dissipated by the latter. Thus the presence of line a of perforations has but a small limiting effect upon the current-carrying capacity of the fuse. The tendency of the relatively small arc voltages generated at lines of perforations au to decay rapidly is effectively counteracted by plate 8 as described above in detail. These plates have the dual function of stabilizing the arc voltage at points of highest current density and longest arc duration, and of bracing the fragile perforated portion of the fuse link against the action of centrifugal forces.

It will be understood that I have illustrated and described herein a preferred embodiment of my invention, and that various alterations may be made therein without departing from the spirit and scope of the appended claims.

I claim as my invention:

1. In a current-limiting fuse structure for use in rotating machinery the combination of:

(a) a substantially tubular casing of insulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the ends of said casing, said pair of plugs having axially inner end surfaces provided with fuselink-receiving grooves and jointly forming with said casing a solid substantially cylindrical wall;

(c) a pulverulent arc-quenching ller inside said caslng;

(d) a plate of synthetic-resin-glass-cloth laminate arranged inside said casing, embedded in said arcquenching filler and having edges braced against said solid substantially cylindrical wall; and

(e) a fuse link of sheet metal arranged inside said casing, embedded in said arc-quenching filler and having axially outer ends projecting into said fuselink-receiving grooves of said pair of plugs, said fuse link including a perforated portion extending parallel to and abutting against and being affixed to said plate of synthetic-resinglass-cloth laminate and said fuse link further including non-perforated portion extending substantially transversely to said plate of synthetic-resin-glass-cloth laminate, said perforated portion of said fuse link having a plurality of transverse lines of perforations each defining a plurality of spaced parallel-connected necks, the number of necks per transverse line of perforations exceeding the number of transverse lines of perforations, said perforated portion of said fuse link further having an additional line of perforations arranged between one of said pair of terminal plugs and an axially outer of said plurality of transverse lines of perforations, the number of necks defined by said additional line of perforations being smaller than the number of necks defined by each of said plurality of lines of perforations, and the aggregate width of said necks defined by said additional line of perforations being smaller than the aggregate width of said plurality of necks defined by each of said plurality of lines of perforations.

2. A current-limiting fuse structure as defined in claim 1 wherein said perforations of said plurality of lines of perforations are circular, and wherein said additional line of perforations includes a circular center perforation and a pair of substantially U-shaped lateral perforations jointly defining with said center perforation a pair of parallelconnected necks.

3. A current-limiting fuse structure as specified in claim 1 including (a) a blown fuse indicator element having a normal position and an indicating position and arranged adjacent the outer end surface of one of said pair of P ugs;

(b) a fusible wire substantially free from any spring bias arranged inside said casing and conductively connecting said one of said pair of plugs to the other of said pair of plugs by the intermediary of said indicator element, said fusible wire -being attached to said indicator element and normally restraining said indicator element in said normal position thereof; and

(c) brake means inside said one of said pair of plugs immediately adjacent said indicator element surrounding said fusible wire and exerting a radially inward braking action upon said fusible wire tending to reduce stresses adjacent the center of said fusible wire resulting from centrifugal forces acting upon said indicator element.

4. A current-limiting fuse structure for use in rotating machinery exerting centrifugal forces in a direction longitudinally thereof, said fuse structure including (a) a substantially tubular casing of insulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the ends of said casing, said pair of plugs having grooves on the axially inner end surfaces thereof;

(c) a pulverulent arc-quenching filler inside said cas- (d) a plate of synthetic-resin-glass-cloth laminate arranged inside said casing extending in a direction longitudinally thereof, embedded in said arc-quenching filler and having axially outer ends braced against said axially inner end surfaces of said pair of terminal plugs; and

(e) a fuse link of sheet metal arranged inside said casing, embedded in said arc-quenching filler and having axially outer ends projecting into said grooves on said axially inner end surfaces of said pair of plugs, said fuse link including a perforated portion extending parallel to and Iabutting against and being affixed to said plate of a synthetic-resin-glass-cloth laminate and said fuse link further including non perforated portions extending substantially transversely to said plate of synthetic-resi-n-glass-cloth laminate, said perforated portion of said fuse link having a plurality of transverse lines of perforations each defining a plurality of spaced parallel-connected necks, the number of necks per transverse line of perforations exceeding the number of transverse lines of perforations, said perforated portion of said fuse link further having an additional line of perforations arranged between one of said pair of terminal plugs and an axially outer of said plurality of transverse lines of perforations, the number of necks defined by said additional line of perforations being smaller than the number of necks dened by each of said plurality of lines of perforations, and the aggregate width of said necks defined by said additional line of perforations being smaller than the aggregate width of said plurality of necks defined by each of said plurality of lines of perforations.

5. A current-limiting fuse structure as defined in claim 4 wherein said perforations of said plurality of lines of perforations are circular, and wherein said additional line of perforations includes a circular center perforation and a pair of substantially U-shaped lateral perforations jointly defining with said center perforation a pair of parallel connected necks.

6. A current-limiting fuse structure as specified in claim 4 including (a) a substantially non-spring biased fusible wire arranged inside said casing and having one end attached to one of said pair of plugs;

(b) a block of an elastomer surrounding and frictionally engaging the end of said fusible wire remote from said one of said pair of terminal plugs; and

(c) a blown fuse indicator element having a normal position and an indicating position arranged adjacent the outer end surface of the other of said pair of terminal plugs, the end of said fusible wire remote from said one pair of terminal plugs being attached to said indicator element and normally restraining said indicator element in said normal position thereof, and said indicator element having a suiciently large mass to be moved upon fusion of said fusible wire by centrifugal action against the frictional action of said block of an elastomer from said normal position to said indicating position thereof.

7. A current-limiting fuse structure as specified in claim 5 wherein said indicator element is adapted to be moved from said normal position to said indicating position solely by the action of centrifugal forces acting in a direction substantially longitudinally of said fusible wire without the presence of any other source of motive power.

`8. A current-limiting cell fuse for rotating rectiliers comprising:

(a) a tubular casing of insulating material;

(b) a pair of terminal plugs press-fitted into the ends of said casing, one of said pair of plugs having a narrow axial bore therein;

(c) a plate means of insulating material inside said casing directly supported by said pair of plugs;

(d) a ribbon fuse link inside said casing conductively interconnecting said pair of plugs, said link defining a region of reduced cross-sectional area situated between the ends thereof and aflixed to said plate means;

(e) a blown fuse indicator element of sheet metal arranged on the axially outer surface of one of said pair of plugs and having a relatively large aperture therein;

(f) a disc superimposed upon said indicator element having a relatively small aperture therein; and

(g) a fusible wire substantially free from any spring bias shunting said link and conductively interconnecting said pair of plugs, said wire being threaded through said bore in said one of said pair of plugs, said relatively large aperture in said indicator element and said relatively small aperture in said disc and forming a knot at the axially outer surface of said disc.

9. A current-limiting fuse structure for use in rotating machinery exerting centrifugal forces in a direction longitudinally thereof, said fuse structure including (a) a substantially tubular casing of insulating material;

(b) a pair of terminal plugs arranged adjacent to and closing the ends of said casing, said pair of plugs having grooves on the axially inner end surface thereof;

(c) a body of quartz s-and inside said casing;

(d) a plate of glass-cloth-melamine arranged inside said casing, extending in a direction longitudinally thereof, embedded in said body of quartz-sand and having axially outer ends braced against said axially inner end surfaces of said pair of terminal plugs;

(e) a fuse link of sheet silver arranged inside said casing, embedded in said body of quartzsand and having axially outer ends projecting into said grooves on said axially inner end surfaces of said pair of terminal plugs, said fuse link including a perforated portion extending parallel to and abutting against and being aixed to said plate of glass-cloth-melamine and said fuse link further including non-perforated portions extending substantially transversely to said plate of glass-cloth-melamine, said perforated portion of said fuse link having a plurality of transverse lines of perforations each defining a plurality of parallel connected necks, the number of necks per line of perforations exceeding the number of transverse lines of perforations, said perforated portion of said fuse link further having an additional line of perforations arranged between one of said pair of terminal plugs and an axially outer of said plurality of transverse lines of perforations defining a plurality of necks, the number of necks of said additional line of perforations being smaller than the number of necks defined by each of said plurality of lines of perforations, and the aggregate width of said plurality of necks defined by said additional line of perforations being smaller than the aggregate width of said plurality of necks dened by each of said plurality element surrounding said fusible wire and exerting a radially inward braking action upon said -fusible wire tending to reduce stresses adjacent the center of said fusible wire resulting from centrifugal forces upon said indicator element.

of lines of perforations;

(f) a blown fuse indicator element having a normal position and an indicating position and arranged adjacent the outer end surface of one of said pair of References Cited UNITED STATES PATENTS plugs. 10 2,826,660 3/1958 KOZaCka 337-158 X (g) a fusible wire substantially free from any spring acqbs '"1" 337-158 X bias arranged inside said casing and conductively 30804 3 5/1963 Wamket a' 37-1581X connecting said one of said pair of plugs to the other 32757g1 951966 ggg; a "ggwggyg ofsaid pair of plugs by the intermediary of said indicator element, said fusible wire being attached to 15 said indicator element and normally restraining said v indicator element in said normal position thereof;

and

(h) brake means of an elastomer inside one of said pair of plugs immediately adjacent said indicator 20 337-159. 227, 295

BERNARD GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner U.S. C1. X.R.