US2542738A - Circuit interrupter - Google Patents

Description

CIRCUIT INTERRUPTER Filed Feb. 19, 1947 n a w w ww Q3 Mm 8% H E Akawww x lNVENTORS James M. Wallace and Hndrew W. Edwards. W M

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ATTOR Y Patented Feb. 20, 1951 .23:

*cmcUrr INTERRUPTER James M. Wallace, East McKeesport, and Andrew W. Edwards, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh,

- Pa., acorporation of Pennsylvania Application February 19, 1947, Serial'No. 729,454

Our invention relates, generally, to electric circuit interrupting devices, and more particularly to fusible interrupting devices adapted to auto-- matically interrupt a circuit upon the passage therethrough of currents of a predetermined magnitude.

A primary object of our invention is to provide a novel interrupting device which is adapted to carry currents produced by high voltage surges of short duration which arise in electrical circuits, such as may be caused by nearby lightning strokes, without interrupting the circuit.

Another object of our invention is to provide :a novel fuse, highly'resistant to currents due to high voltage surges, for the primary protection ofsdistribution transformers, which combines with the high resistance to these surges, a dual type of protection, i. e., the fuse is adapted to automatically interrupt a circuit substantially instantaneously in response to heavy overload and short circuit currents, and will also operate with a, time lag to interrupt relatively lighter but continuing overloads.

Another object of our invention is to provide in a fuse for automatically interrupting a circuit in response to currents of a predetermined magnitude, high resistance to currents due to high voltage surges, associated in a novel manner with a simple fuse link construction of high mechanical strength.

j Another object of our invention is to provide a novel fuse link of high resistance to currents due to high voltage surges in a form and size "adapted for insertion into a standard fuse holder.

These and other objects of our invention will become apparent, upon consideration of the 'following specification, and attached drawing, in

which,

Figure "1 is a vertical sectional view of a fuse holder showing one of my improvedrefill units in :operative position therein; r

Fig. 2 is an enlarged partial vertical sectional "View of the refill unit shown in 1, taken at right angles to Fig. 1; and

Fig. 3 is an enlarged partial vertical sectional view of the refill unit shown in Fig. l, and is taken atright angles to Fig. 2.

1' In' the embodiment of .our invention'shown in Figs; 1 to 3,1 have shown a fuse holder which includesthe elongated insulating tube 2 of any desired insulating material such, for example, as fiber, having a terminal ferrule 4 threadedly engaged with the upper end thereof, and a lower terminal collar 5 around the outside of the'insulating tubeadjacent th lower end. thereofland 8 Claims. (Cl. 200123) secured in position on the tube by the set screw 8.. An upper terminal cap it is threadedly engaged with an extension of the terminal ferrule I, for clampingan u-pperfuse refill terminal head 12 between the inner wall of the cap it and the upper terminal ferrule 4. The upper terminal head I2 forms the upper terminal for the fuse refill 1.4. The lower terminal of the fuse refill 1 4 includes a flexible lead 16, which may be made from a plurality of twisted copper wires, depending from the lower end of the refill and extending out of the lcwerend of the fuse tube 2 and having its outer end connected to the lower terminal collar 6, as by the set screw I8. The refill upper terminal head 12, which may be made from tinned copper str-in, has 2. depending integral flat terminal portion 20.

An upper heating element 24 and a lower heating element 26 are preferably made from con-- ducting wire-material having a relatively high electrical resistance and high melting point,.such,, for example, as a nickel-chromium or nickelchromium-aluminum resistance alloy in the form of helically wound coils having in the case of the upper element, a hooked upper end which extends through aperture 28 in the terminal portion 29.. The hooked upper end of the upper heating element '24 is soldered to the terminal portion where it passes through the aperture 28 in said terminal portion. Sufficient solder is there applied so that the joint has a relatively low resistance. pi

The upper end'of the flexible lead It loops over the hooked lower end of the lower heating element 2E, is doubled back upon itself a short distance, and is secured to the said lower end of the lower heating element as with solder. A coil spring 30 "made preferably from high carbon spring steel wire has three portions of varying diameter. It has an enlarged. portion 32 of a size to engage the lower end of a fuse refill tube 22 which is made from any desired insulating material. preferably fiber or the like, an intermediate portion 34 of reduced diameter slightlv smaller than the inside diameter of the fuse refill tube 22, and an upper end collar portion 36 of still smaller diameter, which forms a close fitting col lar about an upper portion of the flexible lead 16 The flexible lead 16 is secured to the spring 30 within the collar portion 36, as by soldering. Fig. '3 also serves to show more clearly than Fig. 1 how a mechanically strong soldered joint is made between the collar portion 36 of the spring 30, the lower end of the lower heating element 28 and the flexible lead [6. Solder is flowed throughthe entire collar portion 36 of the spring 32 which closely surrounds the flexible lead 16. A strong joint is thus assured between the collar portion 35 and the lead 46. The upper end of the flexible lead l6 after passing over the hooked end of the lower heating element 25 is brought back upon itself and soldered. A unitary mechanically strong joint is thus obtained between the three parts. The fuse refill tube 22 is biased against the bottom side of the upper refill terminal head [2 by the spring 30. A pair, upper and lower, of T-shaped fiat conducting strain members 40 and 42, which are preferably made from a conducting material such a tinned copper strap, are soldered to each other, by a low melting point solder such as a bismuth-lead, a bismuth-lead-tin or tin-lead solder, to form a slip connection over a relatively large area; the area of this soldered joint is made large enough s that it can carry a mechanical load at ordinary room temperatures at least as great as that which can be carried by the strain members 40 and 42 themselves. The lower end of the upper heating element 24 is joined as by spot-welding to the upper strain member 40 as at 25, near the lower end of said upper strain member, and the upper end of the lower heating element 26 is joined as by spotwelding to the lower strain member 42 as at 21, near the upper end of said lower strain member. Shoulder portion 4| of upper strain member 40 and shoulder portion 43 of lo er strain member 42 serve to prevent the helically wound portions of heating elements 24 and 26 from unwinding under tension.

Upper washer 43, positioned between the shoulcler portion 4! of upper strain member 40 and upper heating element 24, together with upper insulating sleeve 44, positioned between the upper portion of the helically wound portion of upper heating element 24 and the upper portion of the stem part of upper strain member 40, serve to insulate the upper end of the helically wound portion of heating element 24 from the upper end of strain member 40. Lower washer 50 positioned between the shoulder portion 43 of lower strain member 42 and lower heating element 26, together with lower insulating sleeve 46 positioned between the lower por ion of the helically wound portion of lower heating element 25 and the lower portion of the stem part of lower strain member 42 serve to insulate the lower end of the helically wound portion of heating element 26 from the lower end of strain member 42.

The current path through the circuit arrangement of the fuse link shown in Figs. 1 to 3 inclusive may be traced in series sequence as follows: beginning with the upper terminal head [2, through the terminal portion 20, through the upper heating element 24, through the overlapping portions of strain members 40 and 42, through the lower heating element 26 and ending with the flexible lead P5.

The operation of the fuse link as embodied in Figs. 1 to 3 inclusive is as follows: Upon the occurrence of light overloads in the circuit which continue for a sufiicient length of time, the heating elements 24 and 25 become heated. and in turn heat, largely by conduction, the soldered connection between the strain members 40 and 42. If this heating is continued for a sufficient length of time, the solder will become weakened or will melt and the spring 30 which is held under tension will operate to separate the strain members 40 and 42, and to draw lower strain member 40 toward the lower end of the refill tube 22, H

' capacity per unit of length is increased.

Upon the occurrence of heavier overloads or a short circuit, the heating elements 24 and 26 will almost immediately be ruptured by melting. Should a rupture occur first in the helically wound portion, an arc will be established at the break and momentarily later a rupture will occur in at least one of the end portions, which are stressed by the spring 30, and the spring will operate to separate the opened circuit as before.

Fuses which provide for the dual protection above described are known in the art. An important difliculty encountered with such fuses of low current rating, particularly those rated at less than 10 amperes, is that their thermal capacity is small and as a result they are often fused by a large current of very short duration which flows through them during surges, such as those due to lightning strokes. This high current flow of short duration is not harmful to the electrical equipment which the fuse is designed to protect, since invariably lightning arresters are disposed in the circuit to prevent such harm, and accordingly, fuse outages caused by such surges obviously result in unnecessar interference with the continuous operation of the equipment.

In order to achieve high resistance to these surges and thereby avoid the above difiiculty, we employ heating elements 24 and 26 made from wire of a relatively large diameter. This is done because the surge capacity of the fuse varies increasingly as the current carrying capacity (which is directly proportional to the cross-sectional area) of the part of the circuit having the highest ratio of electrical resistance to thermal The parts of the circuit having the highest ratio of electrical resistance to thermal capacity per unit of length in our fuse link, as described, are the heatin elements 24 and 26. Th use of wire of a relatively large diameter thus provides high surge capacity. By way of example, we have found that resistance wire comprising percent nickel and 20 percent chromium by weight for heating elements 24 and 26, of 0.040 inch diameter for use in a fuse link having a current rating of 1 ampere and 0.051 inch diameter for use in a fuse link having a current rating of 2 amperes, provides the necessary high surge capacity. The surge capacity, using as a measure thereof the current required for fusing the heating element in 1,000 microseconds, of the aforesaid fuse, rated at l ampere and employing 0.040 inch diameter 80-20 nickel-chromium wire, is approximately 2,000 amperes; and that of the aforesaid fuse, rated at 2 amperes and employing 0.051 inch diameter 80-20 nickel-chromium wire is approximately 2,500 amperes. These surge capacities are about five to six times the surge capacities of respectively similarly rated fuses known prior to our invention.

We, and others frequently, in referring, as above, to the ratings of fuses to which our invention has especial connection; i. e., to fuses rated at less than 10 amperes, understand that a fuse having a rating of X amperes is such a fuse as will blow upon carrying a continuous load of 2X amperes for approximately 5 minutes.

For a given fuse current rating, the use of large diameter wire heating elements results in heating element lengths so great, however, that their mechanical arrangement in a fuse refill tube, such as refill tube 22 of Figs. 1 to 3 which tube for universal cable type fuses for the primary protection of transformers has an industry standard inside diameter of only inch, and has a length of approximatel 7 inches-tor *vcltage ratings p to $1500, presents a ditl'lcult problem of fitting all parts into the refill tube. To solve this problem we have devised the novel coil-ed heater elements and a novel simple mechanical structure therefor which prevents the coiled elements from unwind- .ing under tension, :as shown in Figs. 1 to 3 and as previously described.

important aspect of our invention is that we locate the soldered joint connecting the strain members 40 and 42, approximately midway between the heating elements 24 and :25 because this area is the highest temperature zone created byzthe heatin effects of the heating elements, which "positioning enables us to obtain a'rnaxim-um ratio of surge capacity to rated current car wing capacity of the fuse. The correctness of tile latter statement is obvious when it is considcred that for a given wire diameter employed in the heating elements 24 and 26, and therefore for a. given surge capacity as supplied by such wire diameter, the positioning of the soldered joint in the highest temperature zone provides the lowest possible current rating for the link, and accord-'- ingly provides the maximum ratio of surge capacity to rated current carrying capacity. It is also obvious that the center zone, with respect to its length, of our coiled heating means, attains a higher temperature than the end zones because of greater heat conduction to terminals at the end zones.

While we have described and illustrated our inventicnvlargel with respect to its use in universal cable type fuses for the primary protection-of transformers, it is obvious that it can be used in circuit interrupters generally, and we accordingly, desire as broad a construction of our invention as possible in the light of the prior art.'

We claim as our invention:

1. A fuse refill unit including a pair of heating coils, at least the .major portion of each being helically wound and formed of wire having a cross-sectional area large enough to resist currents up to at least substantially 2,000 amperes for 1,000 microseconds without melting, an I- shaped strain element comprising two T-shaped portions lap-joined by a low melting point solder to provide a slip connection, said strain element having the central stem portion thereof interiorly located of said helically wound portions of said heating coils and the shoulder portions thereof exteriorly located of the ends of said helically wound portions of said heating coils, means insulating at least an outer part of each of said helically wound portions of each of said heating coils from the strain element, and each of said heating coils being oonductively connected at their innerends to the respective adjacent one of said T-shaped portions.

2. A'fus e refill unit including a pair of heating coils formed of wire having a cross-sectional area large enough to resist surge currents of at least substantially 2,000 amperes for 1,000 microseconds without melting, at least the major portion of each coil being helically wound, a strain element comprising two T-shaped portions lapjoinedthrough a part of the stem portions thereof by a low melting point solder to provide a slip connection, said strain element extending through said helically wound portions of said heating coils and having the cross portions thereof located in restraining engagement with the outer ends of said helically wound portions of said heating coils, thereby preventing said helically wound portlons of said heating coils from unwinding under tension, and said strain element being; conductively connected between the inner ends of said pair of heating coils.

3. A .fuse link construction includin a Pair of axially aligned heating coils, at least the major portion of each being helically wound and formed of wire having a cross-sectional area large enough to withstand currents up to at least substantially 2000 amperes for 1000 microseconds without melting, a strain element extending through the helically wound portion of said heating coils comprising two members lap joined by a low melting point solder to provide a slip connection at a point located near the adjacent ends of said coils, said strain element having integral shoulder portions in restraining engagement with the outer ends of said helically wound portions of said heating coils but electrically insulated therefrom, and said strain element bein conductively connected in series between the inner ends of said pair of heating coils,

4. In combination with a fuse having a tube with a pair of contacts, a replaceable fuse refill unit in said fuse tube having a pair of terminals each connected respectively, to one of said econtacts, said fuse refill unit includin an insulating tube of a size to be spaced from the inner walls of said fuse tube, and also including with-insaid insulating tube and spaced from the inner walls thereof, the structure of a fuse link construction including a pair oi axiallyialigned heating coils, at least the .major portion or each being helically wound and termed of wire having a cross-sec:

tional'area' large enough to withstand currentsup to at least substantially 2000 amperesfor 1000 microseconds without melting, a strain element extending through the helically wound portion of said heating coils comprising two members lap joined by .a low melting point solder to provide a slip connection at a point located near the adjacent ends of said coils, said strain element having integral shoulder portions in restraining engagement with the outer ends of said helically wound portions of said heatin coils but electrically insulated therefrom, and said strain element being conductively connected in series between the inner ends of said pair of heating coils.

5. A fuse link construction including a pair of axiall aligned heating coils at least the major portion of each being helically wound and formed of wire having a cross-sectional area large enough to withstand currents up to at least substantially 2000 amperes for 1000 microseconds without melting, a strain element extending through said helically wound portion of said heating coils, said strain element comprising two members lap joined by a low melting point solder at their inner ends to provide a slip connection at a point located near the adjacent ends of said coils and having integral shoulder portions engaging the outer ends of but electrically insulated from, said helically wound portions of said heating coils, to prevent said heating coils from unwinding under tension, and resilient means tensioning said heating coils away from each other.

6. In combination with a fuse having a tube with a pair of contacts, a replaceable fuse refill unit in said fuse tube having a pair of terminals each connected respectively, to one of said contacts, said fuse refill unit including an insulating tube of a size to be spaced from the inner walls of said fuse tube, and also including within said insulating tube and spaced from the inner walls thereof, the structure of a fuse link construction including a pair of axially aligned heating coils at least the major portion of each being helically wound and formed of wire having a cross-sectional area large enough to withstand currents up to at least substantially 2000 amperes for 1000 microseconds without melting, a strain element extending through said helically wound portion of said heating coils, said strain element comprising two members lap joined by a low melting point solder at their inner ends to provide a slip connection at a point located near the adjacent ends of said coils and having integral shoulder portions engaging the outer ends of, but electrically insulated from, said helically wound portions of said heating coils, to prevent said heating coils from unwinding under tension, and resilient means tensioning said heating coils away from each other.

7. A fuse link construction including a pair of axially aligned heating coils of substantially equal length, at least the major portion of each being helically wound and formed of wire having a cross-sectional area large enough to withstand currents up to at least substantially 2000 amperes for 1000 miscroseconds without melting, a

strain element extending through said helically wound portions of said heating coils, said strain element comprising two members lap joined by a low melting point solder to provide a slip connection at a point located near the adjacent ends of said coils, and having integral shoulder portions engaging the outer ends of said helically wound portions of said heating coils to prevent said heating coils from unwinding under tension, each of said heating coils being conductively connected at its inner end to the respective adjacent one of said members of said strain element, means insulating the outer ends of said heating coils from said strain element, and resilient means tensioning said heating coils from each other.

8. In combination with a fuse having a tube with a pair of contacts, a replaceable fuse refill unit in said fuse tube having a pair of terminals each connected respectively, to one of said contacts, said fuse refill unit including an insulating tube of a size to be spaced from the inner walls of said fuse tube, and also including within said insulating tube and spaced from the inner walls thereof, the structure of a fuse link construction including a pair of axially aligned heating coils of substantially equal length, at least the major portion of each being helically Wound and formed of wire having a cross-sectional area large enough to withstand currents up to at least substantially 2000 amperes for 1000 microseconds without melting, a strain element extending through said helically wound portions of said heating coils, said strain element comprising two members lap joined by a low melting point solder to provide a slip connection at a point located near the adjacent ends of said coils, and having integral shoulder portions engaging the outer ends of said helically wound portions of said heating coils to prevent said heating coils from unwinding under tension, each of said heating coils being conductively connected at its inner end to the respective adjacent one of said members of said strain element, means insulating the outer ends of said heating coils from said strain element, and resilient means tensioning said heating coils from each other.

JAMES M. WALLACE. ANDREW W. EDWARDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 874,798 Sachs Dec. 24, 1907 1,122,478 Cole Dec. 29, 1914 2,281,795 Pittman et a1 -c May 5. 1942 2,342,310 Taylor Feb. 22, 1944 2,343,723 Wallace -c Mar. 7, 1944 2,453,397 Yonkers Nov. 9, 1948

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