US3304389A

US3304389A - Current-limiting fuse having an impact member to sever mechanically ribbon extensions of current-limiting elements

Info

Publication number: US3304389A
Application number: US523312A
Authority: US
Inventors: Sigurd I Lindell
Original assignee: S&C Electric Co
Current assignee: S&C Electric Co
Priority date: 1966-01-27
Filing date: 1966-01-27
Publication date: 1967-02-14
Anticipated expiration: 1984-02-14

Description

Feb. 14, 1967 s. 1. LINDELL 3,304,389 CURRENT-LIMITING FUSE HAVING AN IMPACT MEMBER TO SEVER MECHANICALLY RIBBON EXTENSIONS 0F CURRENT-LIMITING ELEMENTS Filed Jan. 27, 1966 2 Sheets-Sheet 1 Feb. 14, 1967 s. l. LINDELL 3,304,389 CURRENT-LIMITING FUSE HAVING AN IMPACT MEMBER TO Filed Jan. 27, 1966 T/ME (5560mm) cuRREA/TU) SEVER MECHANICALLY RIBBON EXTENSIONS OF CURRENT-LIMITING ELEMENTS 2 Sheets-Sheet 2 United States Patent .EXTENSIONS OF CURRENT-LIMITING ELE- MENTS Sigurd I. Lindell, Northbrook, Ill., assignor to S & C

Electric Company, Chicago, Ill., a corporation of Delaware Filed Jan. 27, 1966, Ser. No. 523,312 17 Claims. (Cl. 200-120) This invention relates, generally, to fuses and it has particular relation to current-limiting fuses for use on alternating current circuits operating at voltages ranging upwardly from 2,400 volts and in which the current flow may range from 3 to 200 amperes. It constitutes an improvement over the current-limiting fuse disclosed in my copending application Serial No. 522,676 filed January 24, 1966 and assigned to the assignee of this application.

In the application above referred to a current-limiting fuse is described having current calibrated fusible element means located in air within one of the fuse tube terminals and connected in series circuit relation with a plurality of parallel connected current-limiting fusible elements located in a filling of granular relatively inert material, such as sand, and having a common terminal at the junction with the current calibrated fusible element. Expressed in magnitude of current the time-current curve for the current calibrated fusible element means lies below the time-current curve for the combined parallel connected current-limiting fusible elements while the time-current curve for an individual one of them lies below the time-current curve for the current calibrated fusible element means. In order to cause the currentlimiting fusible elements to blow one by one on flow of relatively low fault current suflicient to blow the current calibrated fusible element means, spark gaps are provided, one for each current-limiting fusible element. These spark gaps are connected in parallel with the gap that is formed on blowing of the current calibrated fusible element means. One electrode of each spark gap is connected to the junction between a major portion of the respective current-limiting fusible element and a minor portion or extension thereof. One of the spark gaps breaks down at random on blowing of the current calibrated fusible element means and the fault current divides between a major portion of the associated current-limiting fusible element and a minor portion or extension thereof which now is connected in series with the other parallel connected current-limiting fusible elements. As a result, a larger portion of the fault current flows through the extension or minor portion and blows or melts it whereupon the total fault current flows through the major portion of the current-limiting fusible element and causes it to blow or melt at several points of reduced cross section along its length. Then another of the spark gaps breaks down at random and this sequence is repeated until all of the current-limiting fusible elements have blown or melted and the circuit is finally completely opened.

Among the objects of this invention are: In a currentlimiting fuse of the type referred to above to provide for severing the minor portions or extensions substantially simultaneously to disconnect the parallel current-limiting elements simultaneously from the common terminal to which the current calibrated fusible element means is connected to cause the total fault current to flow through one of the major portions at a time in succession immediately following the blowing of the current calibrated fusible element means on relatively low fault current, the arcing over of one spark gap and the severance of all the minor portions or extensions; to provide a spring driven'rne- "Ice chanical impact member having a lost motion to rupture the minor portions or extensions by impact force; to provide mechanical advantage means to facilitate the severance of the minor portions or extensions by the impact member; to position and anchor a straight part of each minor portion or extension laterally and tautly in the path of the impact member to enhance the stress in that part and to facilitate rupture by tensile stress thereby providing the mechanical advantage; to provide a reduced section at the rupture zone, as by forming a perforation in the part, to concentrate the stress at the preferred point of rupture; to sever the minor portions or extensions simultaneously on flow of relatively low fault current before the current flowing there through has had time to melt them in a time interval substantially less than the melting time of an individual minor portion or extension; to provide on the impact member a tooth individual to each minor portion or extension and formed of insulating material; to combine with the impact member an indicator that moves to indicating position when released by blowing of the current calibrated fusible element means; and to coordinate the time-current characteristics of the current calibrated fusible element means with the currentlimiting section so that the time-current'curve of the former on relatively low current is below the long time minimum blowing current for the current-limiting section to release the impact member for initiating the mechanical separation of and blowing of individual current-limiting elements one by one each at the total fault current.

In the drawings: FIG. 1 is a vertical sectional view, substantially at full scale, of a current-limiting fuse embodying this invention. FIG. 2 is a horizontal sectional view taken generally along the line 22 of FIG. 1. FIG. 3 is a vertical sectional view taken generally along the line 33 of FIG. 2. FIG. 4 is a horizontal sectional view taken generally along the line 4-4 of FIG. 1. FIG. 5 is a top plan view of the ceramic guide member shown in FIG. 6. FIG. 6 is a view, in side elevation, of

the ceramic guide member shown in FIG. 5. FIG. 7 is a view, in side elevation, of the combined metallic indicator tube and impact member. FIG. 8 is a bottom plan view of the combined metallic indicator tube and impact member shown in FIG. 7. FIG. 9 illustrates diagrammatically the circuit connections embodied in the currentlimiting fuse shown in FIG. 1. FIG. 10 shows graphically the relation between the melting times of the various current carrying elements of the current-limiting fuse for different magnitudes of current flow therethrough. FIG. 11 shows graphically the current flow when the available short circuit magnitude of alternating current is so high that the current-limiting section melts on rise of current and the current flow is interrupted in a fraction of the first current loop by the current-limiting fuse embodying this invention.

In FIG. 1 the reference character 10 designates, generally, a current-limiting fuse which includes an elongated insulating housing 11 of ceramic material such as porcelain or other suitable insulating material. The elongated insulated housing 11 has a fuse upper end terminal indicated, generally, at 12 and a fuse lower end terminal indicated, generally, at 13. The upper end terminal 12 includes a metallic ferrule 14 that is secured by cement or other means to the upper end of the housing 11. The ferrule 14 has connector plates 15 welded thereto for receiving screws 16 that extend through a radial flange 17 forming an integral part of a metallic terminal cap 18. The end terminal 12 also includes a metallic tube 19 that extends downwardly into the tubular insulating housing 11 and is provided with a radially inwardly extending flange 20. It will be understood that the metallic ferrule 14, metallic terminal cap 18 and metallic tube 19 are solidly interconnected and that the metallic terminal cap 18 is arranged to be positioned in a fuse clip. Gaskets, shown collectively at 21, serve to prevent the egress of relatively inert granular material 22 such as sand, which fills the tubular insulating housing 11.

The lower end terminal 13 includes a metallic ferrule 25 that is suitably secured by cement to the lower end of the insulating housing 11. It has connector plates 26 welded thereto for receiving screws 27 that extend through a flange 28 extending outwardly from a metallic terminal cap 29. The metallic terminal cap 29 ha an opening 30 through which the granular material 22 can be introduced. A closure cap 31 is provided for the opening 30. The lower end terminal 13 also includes a connector plate 32. Gaskets, shown collectively at 33, are employed to prevent the egress of the granular material 22. It will be understood that the ferrule 25, metallic terminal cap 29 and connector plate 32 are solidly interconnected and that metallic terminal cap 29 is arranged to be received by a fuse clip.

Extending centrally of the tubular insulating housing 11 is a ceramic core that is indicated, generally, at 36. The ceramic core 36 is provided with a central opening 37 and has four vertical ribs 38 that are provided with grooves 39 for receiving in spiral fashion preferably four or more current-limiting fusible elements or ribbons 40 which may be formed of silver. For the lower ampere ratings, when it is desirable to use the greatest practical number of elements in parallel, they are formed of materials having a higher resistivity and higher mechanical strength than silver. For example nickel and alloys of silver can be used. Perforations 41 are uniformly spaced along each of the current-limiting fusible elements 48 to provide localized restricted cross-sections to initiate fusion and arcing on flow of fault current. The lower ends of the current-limiting fusible elements or ribbons 40 are connected by terminal clips 42 secured to the connector plate 32 by screws 43. Thus the lower ends of the current-limiting fusible elements or ribbons 40 are solidly connected to the fuse lower end terminal 13.

As shown at the upper end of FIG. 1 and in FIG. '3 the upper ends ofthe current-limiting fusible elements or ribbons 40 extend through metallic terminals 47 that are generally tubular in form and are suitably crimped to provide good contact engagement with the currentlimiting fusible elements or ribbons 40. The terminals 47 extend through and may be secured by cement to sleeves 48, FIGS. -6, that are formed integrally with a flange 49 that forms a part of a guide member, shown generally at 50, which has a central upstanding guide tube 51. A gasket 52 underlies the flange 49 and serves to prevent egress of the granular material 22 from the insulating housing 11.

As shown in FIGS. 1 and 2 the current-limiting fusible elements or ribbons 40 extend horizontally as indicated at 53 from the upper ends of the terminals 47 and into intimate contact with a body portion 54 of a stationary intermediate terminal that is indicated, generally, at 55. Each horizontal portion 53 includes at least one perforation 41 at a definite location to provide a high stress concentration section for a purpose that will be apparent presently. The body portion 54 is suitably crimped to the ends of the current-limiting fusible elements or ribbons 40. The body portion 54 has a depending extension 56 through which a transverse pin 57 extends into registering transverse openings 58, FIGS. 5 and 6, in the guide tube 51. The opening at the lower end of the guide tube 51 is closed by a plug 59 of ceramic material. It will be noted that the guide tube 51 is formed with longitudinally extending slots 60 through which the horizontal portions 53 of the current-limiting fusible elements or ribbon 40 extend into the body portion 54 of the intermediate terminal 55. The inter-mediate terminal 55 also includes a lower f use element terminal 62 to which the lower end of current calibrated fusible element means, shown generally at 63, is connected. It includes a coiled silver fusible element 64 and a strain wire 65 with their upper ends being secured to an upper fuse element terminal 66 that projects through an apertured flange 67 of a metallic indicator tube 68. A nut 69, threaded on the upper end of the upper fuse element terminal 66, serves to hold the assembly in place as long as the current calibrated fusible element means 63 remains unblown.

The upper end of the metallic indicator tube 68 extends into a flange opening 70 in the metallic terminal cap 18. Contact engagement is maintained with the metallic indicator tube 68 when it moves from the nonindicating to indicating position by contact fingers 71 which are secured by screws 72 to the radially inwardly extending flange 20 from the metallic tube 19. The screws '72 project into the upper ends of the vertical ribs 38 of the ceramic core 36. The metallic indicator tube 68 has an enlarged lower end portion 73 which forms a shoulder 74 that is arranged to limit the upward movement of the metallic indicator tube 68 as it is urged upwardly by a coil compression spring 75 on blowing of the current calibrated fusible element means 63. The coil compression spring 75 is positioned between the underside of the apertured flange 67 and a stepped metallic ring 76 which overlies the upper end of the guide tube 51. A fuse tube 77 is secured to the upper fuse element terminal 66 and extends downwardly over the current calibrated fusible element means 63. The fuse tube 77 is formed of fiber or an inorganic material carrying a material which evolves an arc extinguishing medium when subjected to the heat of the are formed on blowing of the current calibrated fusible element means 63. This increases the arc voltage between the

fuse element terminals

62 and 66.

It is desirable that provision be made for blowing the current-limiting fusible elements or ribbons 40 one by one, particularly on flow of relatively low fault current. In order to accomplish this, terminal conducting strips 80 are connected to a junction 81 between a minor portion or extension 82 of the respective current-limiting fusible element or ribbon 40, which includes the horizontal portion 53, and a major portion 83 of the respective current-limiting fusible element or ribbon 40. The arrangement is illustrated diagrammatically in FIG. 9. The terminal conducting strips 80 are held in place by screws 84 that are threaded into the respective vertical rib 38 of the ceramic core 36. Each of the terminal conducting strips 80 is provided at its upper end with an electrode 85 that extends into an opening 86 in the flange 49 of the guide member 50. Above each opening 86 there is a vent opening 87 in the radially inwardly extending flange 20 of the metallic tube 19. This arrangement provides a spark gap 88 between the electrode 85 and the adjacent portion of the flange 20 which constitutes the other electrode.

As disclosed in the above application, on blowing of the current calibrated fusible element means 63 as the result of flow of fault current therethrough, the spring 75 no longer is restrained. It moves the metallic indicator tube 68 upwardly until'the shoulder 74-engages the underside of the top of the metallic terminal cap 18. This indicates that the current limiting fuse 10 has 'blown and should be replaced. This action is accompanied by the formation of a gap between the

fuse element terminals

62 and 66 across which a voltage appears that is applied to the spark gap 88. One of them at random breaks down and an arc 89, FIG. 9, is formed at this spark gap. One-fourth of the fault current then flows through the major portion 83 of the respective current-limiting fusible element or ribbon v40 while three-fourths of it flows through the respective minor portion or extension 82 thereof to the intermediate terminal 55 and thence through the remaining three current-limiting elements or ribbons 40. As a result of the flow of three-fourths of the fault current through the minor portion or extension 82, it may blow to open the circuit to the intermediate terminal 55 and cause the entire fault current to flow through the major portion 83 which melts out at several points therealong at the perforations 41 and interrupts the flow of current therethrough. The succeeding spark gaps break down at random and repeat this process, The circuit interrupting action just described is that which occurs on flow of intermediate values of fault current. It will be apparent that the transfer of the entire flow of fault current to the major portion 83 of the currentlimiting fusible element or ribbon 40 individual to the 'spark gap 88 that has broken down depends upon the rupturing of the respective minor portion or extension 82.

In order to make certain that the minor portions or extensions 82 are ruptured or severed simultaneously at the lowest fault current that causes blowing of the current calibrated fusible element means 63, a mechanical impact member, shown generally at 91 and in detail in FIGS. 7 and 8, is provided. The mechanical impact member 91 preferably is formed of insulating material, such as a ceramic material, and it has a cylindrical body portion 92 that telescopes into the enlarged lower end portion 83 of the metallic indicator tube 68. The cylindrical body portion 92 has an annular groove 93 for receiving an indented portion 94 of the enlarged lower end portion 73. At its lower end the cylindrical body portion 92 has slots 95 to permit application of the mechanical impact member 91 with respect to the horizontal portions 53 of the current-limiting fusible elements or ribbons 4t) and positioning integrally formed teeth 96 the upper impacting edges 97 of which are arranged to engage the undersides of the horizontal portions 53 at the respective perforation 41 under the influence of the coil compression spring 75 to sever them mechanically immediately upon blowing of the current calibrated fusible element means 63. Because of the slots 95, there is lost motion in the movement of the mechanical impact member 91 and it acquires a substantial amount of kinetic energy before the teeth 96 engage the portions 53 at the perforations 41 therein. Preferably the spring 75 is capable of exerting a force of the order of 10 pounds. Since the horizontal portions 53 are anchored at their ends and thus are arranged to be relatively taut and the tension stress is concentrated by the perforations 41, a mechanical advantage arrangement is provided and they are severed readily by the mechanical impact member 91 when it moves upwardly under the influence of the coil compression spring 75 to apply the required rupturing stress at the reduced sections.

Thus it is unnecessary at low fault currents to rely upon the transfer of fault current to the minor portions or extensions 82 of the current-limiting fusible elements or ribbons 40 for blowing them and transferring the flow of fault current back to the major portion 83 in each case. Instead, on blowing of the current calibrated fusible element means 63, the minor portions or extensions 82 are promptly severed mechanically by the mechanical impact member 91 and before they are melted by flow therethrough of relatively low fault current. The rupture is faciliated by the heating of the minor portions or extensions 82 due to flow of fault current therethrough.

The voltage appearing at the gap formedby the blowing of the current calibrated fusible element means 63 is applied across the spark gaps 88 in parallel. One of them at random breaks down and the fault current then flows through the respective major portion 83 until it blows at several points theralong as determined by the locations of the perforations 41. The are voltage across the blown major portion 83 thus appears across the remaining spark gaps 88 and they break down at random followed by the successive blowing of the remaining current-limiting fusible elements or ribbons 40 at total fault current.

Under normal operating conditions with the currentlimiting fuse 10 in the unblown condition, the current path is from the upper end terminal 12 through the con- 6 tact fingers 71 to the metallic indicator tube 68 and thence through upper fuse element terminal 66, the current calibrated fusible element means 63, intermediate terminal 55 and current-limiting fusible elementsor ribbons 40 to the lower end terminal 13. FIG. -1() shows the time-current characteristics for the several fusible elements. Here curve represents the time-current relationship for the current calibrated fusible element means 63. It is so calibrated that it melts or blows as indicated by curve 100 and by its response at the upper or long time end of this curve to determine the ampere rating of the current-limiting fuse 10, which rating is conventionally approximately one-half of the five minute or ten minute melting current. Broken line curve 101 indicates the time-current relationship for the blowing of all four of the'parallel connected current-limiting fusible elements or ribbons 40. It will be observed that this curve is entirely to the right of and substantially above the curve 100 expressed in current magnitude. This indicates that the arrangement is such that the current calibrated fusible element means 63 blows or melts first on flow of fault current followed by blowing or melting of the current-limiting fusible elements or ribbons 40. The succeeding curves, shown by broken lines 102403- 104, show the time-current relationships for the blowing of 3, 2 and 1, respectively, of the current-limiting fusible elements or ribbons 40. To simplify the presentation the curves 104L104 here shown illustrate typical average current values. It is understood that allowance for plus or minus deviations are to be made with sufficient margin for coordination with other series connected current interrupting means and with the sections within the current-limiting fuse 10. A greater number of currentlimiting fusible elements or ribbons 40 in parallel permits the use of smaller cross section individual currentlimiting elements for a given current rating and provides a higher speed of response of each individual currentlimiting element on low current operation.

Curve 105 in FIG. 11 indicates the current in a half loop of alternating current that is available to flow in the event of a high current fault provided some means is not introduced to limit such current flow to a value well below the maximum here indicated. Assuming that the current flow is as indicated at I in FIG. 10 at time t the current calibrated fusible element means 63 blows. This is indicated at t in FIG. 11, and is followed at t by the blowing of the current-limiting fusible elements or ribbons 40 substantially simultaneously. Because they are embedded in the filling of relatively inert granular material 20, such as sand, the resistance of the circuit provided thereby rises rapidly so that the fault current instead of following the curve 185, is rapidly decreased and at time t FIG. 11, is reduced to zero.

As pointed out in the application above referred to particular difliculty is encountered when the flow of fault current is substantially lower than that represented by curve 105 in FIG. 11, and in the region above curve 100 in FIG. 10. For example, it is conventional to rate the current-limiting fuse 10 to blow at a minimum value of approximately twice its normal current carrying rating. Such a fault current is indicated at 1 in FIG. 10. In that case it will be observed that the flow of fault current is not sufiicient to blow all four of the current-limiting fusible elements or ribbons 40. However, it would be suflicient to blow one of them as indicated at 1 along curve 104. By providing the spark gaps 88 in the manner described and making provision for severing all of the minor portions or extensions 82, simultaneously, it is possible successively to blow the current-limiting fusible elements or ribbons 40 promptly at total fault current so that, under the assumed conditions where the current flow is as indicated at I they will be blown one by one.

After the current calibrated fusible element means 63 is blown under conditions of relatively low fault current,

such as the current I at time t in FIG. 10, the spring 75 is released, one of the gaps 88 sparks over, and the mechanical impact member 91 promptly severs all of the minor portions or extensions 82 of the current-limiting fusible elements or ribbons 40, the arrangement being such that a perforation 41 is located in a part of the horizontal portion 53 in the path of the respective impacting edge 97. As described hereinbefore, the voltage developed between the

fusible element terminals

62 and 66 is applied across the spark gaps 88 which are connected in parallel. The arc 89 is formed at random between the electrodes of one of these spark gaps and the respective major portion 83 of the current-limiting fusible element or ribbon 40 is subjected to the flow of fault current I sufficient to blow it as indicated by curve 104. It blows by melting or fusing at the several perforations 41 therealong and opens the circuit therethrough with a current-limiting action in known manner. The remaining spark gaps 88 break down at random and successively, as described, to complete the blowing of the remaining current-limiting elements or ribbons 40 to interrupt the circuit.

From the foregoing it will be apparent that the crosssection of the minor portion or extension 82 is not determined by the amount of low fault current required to melt or blow it. Rather, the mechanical impact member 1 makes certain that the minor portions or extensions 82 are severed as soon as the current calibrated fusible element means 63 blows. Then it is possible to use even larger cross sections for the current-limiting fusible elements or ribbons 40 which is of particular importance in the production of the low ampere rated current-limiting fuses.

What is claimed as new is:

1. A current-limiting fuse including:

an elongated insulating housing with end terminals,

an intermediate terminal in said housing insulated from said end terminals,

fusible element means interconnecting one end terminal and said intermediate terminal,

a plurality of current-limiting fusible elements embedded in a granular filling in said housing and connected in parallel circuit relation between said intermediate terminal and the other end terminal,

a minor portion of each current-limiting fusible element adjacent said intermediate terminal constituting an extension of a major portion thereof,

means providing a spark gap individual to each current-limiting fusible element, one terminal of each spark gap being connected to the junction between the respective extension and the major portion of its current-limiting fusible element, the other terminal of each spark gap including conductor means connected to said one end terminal thereby placing said spark gaps in shunt circuit relation to the gap formed on blowing of said fusible element means on flow of fault current therethrough, and

means operated in response to blowing of said fusible element means for mechanically severing said extensions.

2. The current-limiting fuse according to claim 1 wherefin:

the extensions are located externally of the inert filling in the elongated housing and transversely of said housing, and

the severing means is-a mechanical impact member movable endwise of said housing and has an impact portion individual to each extension.

3. The current-limiting fuse according to claim 2 wherein at least each impact portion is formed of insulating material.

4. The current-limiting fuse according to claim 2 wherein each extension has a reduced cross section and the mechanical impact member engages the extensions at the r duced ra s $9i 9rls thereof.

5. The current-limiting fuse according to claim 2 wherein spring means restrained by the fusible element means bias the mechanical impact member to sever the extensions.

6. The current-limiting fuse according to claim 5 wherein an indicator is movable endwise of the elongated housing conjointly with the mechanical impact member from non-indicating position to indicating position on blowing of the fusible element means.

7. The current-limiting fuse according to claim 5 wherein insulating guide means is stationarily mounted in the elongated housing and the mechanical impact member is telescoped thereover and is guided thereby.

8. The current-limiting fuse according to claim 7 wherein means secure the intermediate terminal to the insulating guide means.

9. The current-limiting fuse according to claim 1 wherein the intermediate terminal is stationarily mounted in the insulating housing.

10. The current-limiting fuse according to claim 1 wherein the intermediate terminal is stationarily mounted in the insulating housing.

11. The current-limiting fuse according to claim 1 wherein the current-limiting fusible elements are formed of metal having a high resistivity and higher strength than silver.

12. In a current-limiting fuse, in combination:

current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected in series circuit relation with said current calibrated fusible element means,

the current values of the long time end of the timecurrent characteristic curve for said current calibrated fusible element means lying below the current values of the long time end of the time-current characteristic curve for said plurality of parallel connected current-limiting fusible elements and above the long time end of the time-current characteristic curve for one of said parallel connected current-limiting fusible elements, and

means responsive to blowing of said current calibrated fusible element means for causing said current-limiting fusible elements to blow one by one including:

a spark gap individual to and connected in series circuit relation with a major portion of each current-limiting fusible element and connected in parallel circuit relation through a minor portion of the respective current-limiting fusible element with said current calibrated fusible element, and

means operated in response to the blowing of said fusible element means for mechanically severing said minor portions.

13. The current-limiting fuse according to claim 12 wherein each current-limiting fusible element is provided with substantially uniformly spaced reduced cross sections to provide locations at which it blows on flow of fault current.

14. The current-limiting fuse according to claim 12 wherein:

the minor portions extend transversely of the housing,

and

the severing means is a mechanical impact member movable endwise of said housing.

15. The current-limiting fuse according to claim 14 wherein spring means restrained by the fusible element means bias the mechanical impact member to sever the minor portions.

16. The current-limiting fuse according to claim 14 wherein the transverse parts of the minor portions are characterized by cooperating with the mechanical impact member at a mechanical advantage.

17. The current-limiting fuse according to claim 14 wherein the mechanical impact member is arranged to rupture the minor portions on flow of predetermining fault 9 10 current before said minor portions are melted by said 2,400,408 5/1946 Haefelfinger 200-118 fault current. 2,502,992 4/1950 Rawlins et a1. 200-120 X 2,934,622 4/1960 Massar 200117 References Cited y h E min r 3,218,517 11/1965 Sankey 31766 UNITED STATES PATENTS 5 BERNARD A. GILHEANY, Primary Examiner. 1,848,152 3/1932 Bleger 200-118 2 200 0 5 /194 Triple; 200 117 H. B. GILSON, Assistant Examiner.

Claims

1. A CURRENT-LIMITING FUSE INCLUDING: AN ELONGATED INSULATING HOUSING WITH END TERMINALS, AN INTERMEDIATE TERMINAL IN SAID HOUSING INSULATED FROM SAID END TERMINALS, FUSIBLE ELEMENT MEANS INTERCONNECTING ONE END TERMINAL AND SAID INTERMEDIATE TERMINAL, A PLURALITY OF CURRENT-LIMITING FUSIBLE ELEMENTS EMBEDDED IN A GRANULAR FILLING IN SAID HOUSING AND CONNECTED IN PARALLEL CIRCUIT RELATION BETWEEN SAID INTERMEDIATE TERMINAL AND THE OTHER END TERMINAL, A MINOR PORTION OF EACH CURRENT-LIMITING FUSIBLE ELEMENT ADJACENT SAID INTERMEDIATE TERMINAL CONSTITUTING AN EXTENSION OF A MAJOR PORTION THEREOF, MEANS PROVIDING A SPARK GAP INDIVIDUAL TO EACH CURRENT-LIMITING FUSIBLE ELEMENT, ONE TERMINAL OF EACH SPARK GAP BEING CONNECTED TO THE JUNCTION BETWEEN THE RESPECTIVE EXTENSION AND THE MAJOR PORTION OF ITS CURRENT-LIMITING FUSIBLE ELEMENT, THE OTHER TERMINAL OF EACH SPARK GAP INCLUDING CONDUCTOR MEANS CONNECTED TO SAID ONE END TERMINAL THEREBY PLACING SAID SPARK GAPS IN SHUNT CIRCUIT RELATION TO THE GAP FORMED ON BLOWING OF SAID FUSIBLE ELEMENT MEANS ON FLOW OF FAULT CURRENT THERETHROUGH, AND MEANS OPERATED IN RESPONSE TO BLOWING OF SAID FUSIBLE ELEMENT MEANS FOR MECHANICALLY SEVERING SAID EXTENSIONS.