US3883838A

US3883838A - High-current current-limiting fuse

Info

Publication number: US3883838A
Application number: US456701A
Authority: US
Inventors: Ernest H Bogert; Peter Kotos
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1974-04-01
Filing date: 1974-04-01
Publication date: 1975-05-13
Anticipated expiration: 1992-05-13

Abstract

A high-current current-limiting fuse comprises an aluminum plate containing two spaced-apart notches extending transversely of the central longitudinal axis of the plate from opposite lateral edges of the plate to points near said axis, thereby dividing the plate into two massive tang portions spaced apart along said axis and a restricted neck portion interconnecting the tang portions. The tang portions constitute fuse terminals, and the neck portion constitutes fusible conducting means for carrying current between the tang portions and for melting in response to overcurrents. The neck portion is integral with the tang portions, and the junctions between the neck portions and the tang portions are free of solder. A housing of insulating material surrounds the neck portion and comprises plates of insulating material at opposite faces of the aluminum plate and portions of insulating material extending into the notches. The insulating plates are secured to said tang portions and to each other, thereby effectively preventing motion of the tang portions relative to each other.

Description

United States Patent [1 1 Bogert et al.

[451 May 13,1975

[ HIGH-CURRENT CURRENT-LIMITING- FUSE [75] Inventors: Ernest H. Bogert, Media; Peter Kotos, Havertown, both of Pa.

[73] Assignee: General Electric Company,

1 Philadelphia, Pa.

[22] Filed: Apr. 1, 1974 [21] Appl. No.: 456,701

[52] U.S. Cl. 337/295; 337/159; 337/290 [51] Int. Cl. H0lh 85/04 [58] Field of Search 337/290, 292, 295, 296,

[56] References Cited UNITED STATES PATENTS 2,734,112 2/1956 Kozacka 337/159 X 2,781,434 2/1957 Swain 337/159 2,861,150 11/1958 Swain 337/159 3,288,968 11/1966 Feenan et al. 337/295 Primary Examiner-J. D. Miller I Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-J. Wesley Haubner; William Freedman [57] ABSTRACT A high-current current-limiting fuse comprises an aluminum plate containing two spaced-apart notches extending transversely of the central longitudinal axis of the plate from opposite lateral edges of the plate to points near said axis, thereby dividing the plate into two massive tang portions spaced apart along said axis and a restricted neck portion interconnecting the tang portions. The tang portions constitute fuse terminals, and the neck portion constitutes fusible conducting means for carrying current between the tang portions and for melting in response to overcurrents. The neck portion is integral with the tang portions, and the junctions between the neck portions and the tang portions are free of solder. A housing of insulating material surrounds the neck portion and comprises plates of insulating material at opposite faces of the aluminum plate and portions of insulating material extending into the notches. The insulating plates are secured to said tang portions and to each other, thereby effectively preventing motion of the tang portions relative to each other.

10 Claims, 5 Drawing Figures v I, 48 '42- i i \42 az F, o l i;

\\J Jlj t if 42@|" v I 48- 37 {6F HIGH-CURRENT CURRENT-LIMITING FUSE BACKGROUND This invention relates to a high-current, currentlimiting fuse that is characterized by exceptional simplicity, exceptionally low heat losses, and exceptional ability to withstand high temperatures without changing its operating characteristics.

The type of fuse that we are concerned with is required to carry current in the kiloampere range on a steady-state basis and to interrupt with current-limiting action circuits where the available current is in the range of hundreds of kiloamperes. The usual fuse that is intended for such duty comprises many separate, very thin fusible elements and two massive end plates of conductive material between which the fusible elements extend electrically in parallel. It is not uncom-' mom for such a fuse to have 50 or more such fusible elements in parallel. These fusible elements are typically joined to the conductive end plates by soldering. The end plates usually are provided with conductive tangs serving as terminals for the fuse that can be bolted to buses or other circuit components.

A problem sometimes present with such a fuse is that the relatively high temperatures developed by high continuous currents passing through the bolted terminal connections and through the end plates, the fusible elements, and the joints therebetween can cause undesirable melting of the solder in the joints, thus forming electric arcs between the fusible elements and the end plates. This is an undesirable location for arcing because the arc-extinguishing medium of the fuse (usually quartz sand) will not be in close contact with such arcs, and they will burn holes into the end plates and expel large amounts of molten metal and vapor into the surrounding region, thus creating an unduly high probability that the fuse will fail to interrupt the current.

Even a higher-than-normal temperature that is well below the melting point of the solder can produce an alloying action between the solder and the metal of the fusible element that lowers the melting point of the fusible element at the joint and increases its resistance at the joint, resulting in undesirable changes in the current-time operating characteristics of the fuse as well as the undesirable arcing condition noted in the preceding paragraph.

SUMMARY ing material or some other attachment, such as the M- effect solder bead. This can be quite expensive when it is considered that there often are 50 or more fusible elements requiring such treatment, and all of these elements must be individually attached to the end plates. Further complicating matters is that in such prior fuses,

the fusible elements are usually relatively thin and fragile, and this necessitates special care in their handling.

The parallel-connected fusible elements in the typical prior fuse are embedded in a suitable filler, such as quartz sand; and the tiller is housed within a large insulating casing. Filling the casing with sand that is tightly packed about the fragile fusible elements requires special care to avoid damage to the fusible elements.

Another object of our invention is to construct the fuse in such a manner that the fusible elements can be drastically reduced in number to only one, and, in particular, to one which is much more rugged than the fragile elements referred to hereinabove.

Still another object is to drastically reduce the quantity of filler needed in comparison to that used in the above-described prior fuses and to reduce the requirements for care in filling the fuse casing.

Still another object is to construct the fusible element in such a manner that it has an exceptionally large conductive mass immediately adjacent its reduced crosssection portion with very low thermal resistance intervening between the mass and the reduced cross-section portion. This enables the mass to act as an effective heat-sink cooler for the reduced cross-section portion, thereby permitting higher steady-state currents without overheating.

In carrying out the invention in one form, we provide a plate of high conductivity metal having a central longitudinal axis and two spaced-apart notches extending transversely of the axis from opposite lateral edges of the plate to points near the axis. The notches divide the plate into two tang portions spaced apart along said longitudinal axis at opposite sides of the notches and a neck portion interconnecting the tang portions and located between the inner ends of the notches. The tang portions serve as terminals of the fuse, and the neck portion serves as fusible conducting means for carrying current between the tang portions and for melting in response to overcurrents. The neck portion is integral with the tang portions, and the junctions between the neck portion and the tang portions are free of solder. The neck portion, the tang portions, and the junctions are all of the same material and formed from the same part. A housing of insulating material surrounds the neck portion and comprises plates of insulating material at opposite faces of the conductive plate and portions of insulating material extending into the notches. The insulating plates are secured to the tang portions and to each other, thereby effectively preventing motion of the tang portions relative to each other. The tang portions have end regions located outside said housing that are adapted to be secured to external circuitry.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a plan view of a current limiting fuse embodying one form of the invention.

FIG. 2 is an exploded perspective view of certain major components of the fuse of FIG. 1.

FIG. 3 is a side view, partly in section, of the fuse of FIG. 1.

FIG. 4 is a sectional view along the line 4-4 of FIG. 1.

FIG. 5 is aside view of another modified embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, the illustrated fuse comprises a unitary plate of highly conductive material, preferably aluminum, from which all of the conductive parts of the fuse are formed. This plate 10 has a central longitudinal axis 11 and flat upper and

lower faces

12 and 13 at its opposite sides. Plate 10 contains two spaced-apart notches 16 extending transversely of its longitudinal axis 11 from opposite lateral edges of plate 10 to points near the axis. These notches 16 are located in aligned positions along the axis 11. Notches l6 divide the plate 10 into two

tang portions

20 and 21 spaced apart along the longitudinal axis 11 and located at opposite sides of the 'notches. Between the inner ends of the notches there is defined a neck portion 24 that interconnects the

tang portions

20 and 21.

As will soon appear more clearly, the

tang portions

20 and 21 constitute fuse terminals, and the neck portion 24 constitutes fusible conducting means for carrying current between the tang portions and for melting in response to overcurrents. The neck portion is integral with the tang portions, being machined or otherwise formed out of the same unitary plate as the tang portions are formed. In view of this integral construction, the junction between the neck portion and each of the tang portions is free of solder and brazing material and is of the same material as the neck portion and the tang portions.

The notches 16 have V-shaped inner ends and, as a result, the neck portion 24 has V-shaped edges and a region of minimum cross section near its center substantially equidistant from the tang portions. As will be apparent from FIGS. 2 and 4, the neck portion 24 is spaced from the upper face 12 of plate 10. Note further that each tang portion contains a recess 32 in its edge facing the other tang portion, and this recess extends from the upper face 12 to the location of the neck portion. As will be apparent from FIG. 2, these recesses 32 are aligned withthe neck portion and thus provide at the end of each recess a reduced thickness region 34 of the tang portion forming an extension of the neck portion 24. The significance of the reduced thickness region 34 will soon appear more clearly.

Surrounding the neck portion 24 is a housing 35 of insulating material. This housing 35 comprises two

thick plates

37 and 38 of insulating material, for example, a melamine resin. These insulating

plates

37 and 38 are located at opposite faces of the conductive plate 10, and one of the plates has a pair of ribs 40 projecting toward the other plate and fitting snugly in the notches 16.

The insulating

plates

37 and 38 are clamped to opposite faces of the conductive plate 10 and to each other by means of bolts 42 extending through registering holes in the plates and nuts 43 threaded on the bolts. Suitable lockwashers are provided between the nuts 43 and one of the insulating plates to maintain the desired clamping pressure despite creepage of the insulating material of the plates 37 38.

As shown in FIG. 2, each of the insulating plates is hollowed out at 46 in the region of the neck portion 24 so that the inner wall of the insulating housing is spaced from the neck portion at all points on the neck portion.

As shown in FIG. 4, the space between housing 35 and neck portion 24 is filled with a granular arcextinguishing material 45, such as quartz sand. A suitable hole 47 is provided in the housing wall to permitintroduction of the filler. The filler material can be tamped through this hole to pack it tightly about the neck portion 24, after which the hole is suitably plugged, as by a screw 49. It will be noted that the filler completely fills the recesses 32 and is therefore in intimate contact with the reduced thickness portions 34 at the ends of the neck portion.

In the form of the invention in FIGS. 1-4, a sealant is provided along the entire interface between the housing 35 and plate 10 to seal the cavity within the housing from the exterior, thereby preventing any filler from escaping and also confining within the cavity the arcing products formed upon fuse operation. A suitable sealant is a room-temperature-vulcanizing silicone cement.

The fuse of FIGS. 1-4 is incorporated in the circuit that it is intended to protect by bolting its

tangs

20 and 21 to the adjacent circuit elements. Suitable holes 48 are provided in the tangs to accommodate the bolts that are used for this purpose. As previously noted, current flows between the

tangs

20 and 21 via neck portion 24. The neck portion remains cool enough to carry rated steady-state current indefinitely without melting. But if the current should abruptly rise, say in response to a short circuit condition, the neck portion will rapidly melt, thus forming an are within housing 35. The surrounding filler 45 reacts with the arc in a known manner to develop a high are voltage that limits the current let through by the fuse to a value substantially below the peak of the available current.

The fuse of FIGS. 14 is capable of carrying steadystate currents in the kiloampere range and of interrupting with current-limiting action available currents in the hundreds of kiloampere range. In one specific embodiment, the plate 10 is of aluminum having 56 percent of the conductivity of copper and is approximately inch thick, 3% inch wide, and 3 l5/l6 inches long. The notches 16 are approximately 0.312 inch in width with their inner ends spaced by about 0.145 inch. Each edge of the neck 24 forms A V-angle of about 50 degrees, and the neck is about inch thick, as measured vertically in FIG. 4. Such a fuse can carry on a steadystate basis a current of 2650 amperes rms and can limit to 100,000 amperes the peak let-through current in a volt rrns 60 Hz circuit with 150,000 amperes rms symmetrical available current. These figures are provided by way of example and not limitation.

The high steady-state current-carrying capability of the fuse is made possible by the fact that the

massive tang portions

20 and 21 are in very close proximity to the neck portion 24 and are integral with the neck portion, with no solder being present at the junctions between the neck portion and the tang portions. This integral construction provides exceptionally low electrical resistance to limit 1 R heating at the junction and also exceptionally low thermal resistance to heat generated at the neck portion and flowing into the tang portions. The absence of solder in the junction regions desirably eliminates any alloying action that could result from exposing the solder to high temperatures. Such alloying typically increases the resistance of the affected metal, resulting in further heating. 3

Another source of heat that can present problems is the bolted joint (not shown) at each fuse terminal. I-Iigh currents passing through this bolted joint will develop heat that contributes to heating of the conductive plate 10. If any solder were present in the neck-to-ta'ng junctions, the temperature of the solder could be elevated sufficiently by such heating to produce alloying and the resultant undesirable increase in electrical resistance at the junctions. By eliminating any solder in these junction regions, we are able to avoid such alloying in the junctions.

The highly effective cooling of the neck portion 24 made possible by the large conductive masses immediately thereadjacent and the low thermal resistance at the neck-to-tang junctions make it possible for us to utilize only a single neck portion for the high continuous currents instead of the many elements in parallel previously used. This greatly simplifies the fuse and eliminates the manufacturing complications resulting from forming, treating, and handling many fuse ele-' ments in a single fuse.

While it is possible to reduce heating under continuous current conditions by making the neck portion 24 large in cross section, it is to be understood that the maximum cross-section is limited by the need for the neck portion to melt in the required times under shortcircuit conditions.

Effective current-limiting action under short circuit conditions depends upon the arc being confined and not allowed to spread over the width of the fuse element. The housing 35 which closely surrounds the neck portion 24 contributes to such confinement. The intimate contact that is present between the arc and the surrounding quartz sand also plays an important role in the current-limiting interrupting process by intensely cooling the arc. By providing the recesses 32 in the edges of the tang portions and 21, we are able to maintain the quartz sand in contact with the arc along its entire length even after its terminals have burned back into the tang portions. In this regard, note that the quartz sand is in intimate contact with the thin regions 34; and even if the arc terminals burn into these thin regions, there is sand immediately adjacent them to react with the arc.

It is to be noted that our fuse of FIGS. 14 uses only a very small amount of arc-extinguishing filler material.

Since the fuse comprises only a single fusible element,

arcing upon fuse operation is confined to a very restricted region and only a small quantity of filler is needed to provide a surrounding body of sand. Where multiple parallel fuse links have been present, it has been customary to provide a single large body of filler in which all the links are embedded.

Using a single fusible element 24 instead of many in parallel means that the fusible element will have a much larger cross-section than each of the parallelconnected elements and will therefore be much less fragile. This enables filler to be introduced into the housing and packed tightly by vibration and tamping with less concern about damaging the fuse element as a result of the associated forces.

In addition to confining the arc, housing 35 serves also to protect the neck portion 24 against possible physical damage as a result of rough handling of the fuse. Since the thick insulating

plates

37 and 38 are tightly clamped to the

rigid tang portions

20, 21 and to To provide an indication of when the fuse has operated, we provide exteriorly of the fuse housing 35 a high resistance indicator wire 55 that extends between two

terminals

58 and 59 respectively secured to the two

tangs

20 and 21. When the fuse operates, current is shunted through this high resistance wire 55 to melt it and indicate fuse operation, all in a conventional manner. By locating the

indicator wire terminals

58, 59 on laterally-opposed sides of the housing 35 instead of on the same side, they are a greater distance apart and there is less likelihood of an electricalbreakdown between them upon melting of the indicator wire. To protect the indicator wire against physical damage prior to its melting, a pair of notches 57 (FIG. 4) are provided in the exposed corners of the housing 35 to receive and shield the wire. The notches 57 serve also to position the indicator wire in a position midway between the heads of bolts 42, thus reducing the likelihood of contact between the wire and boltheads.

Referring to FIG. 1, it will be noted that the tangs project laterally beyond the housing 35, providing exposed portions adjacent the laterally-opposed edges of conductive plate 10 to which the

terminals

58 and 59 can be attached. These exposed portions also serve as fins for improving heat transfer to the surrounding ambient as well as providing additional cross-section for reducing the electrical resistance arid; resultant heating of the tangs.

For much higher current applications than those referred to hereinabove, the fuse assembly of FIG. 5 can be used. This assembly comprises two fuses in parallel, each of the same construction as the fuse of FIGS. l-4 except that one of the insulating plates 38 serves as a portion of the housing 35 of both fuses. Bolts 42 extending through the three insulating

plates

37, 38, 38a and the conductive plates 10 clamp these parts together in fixed relationship to each other. The central insulating plate 38 is hollowed out on both its upper and lower faces to form a part of the cavity surrounding the neck portion of each fuse.

While we have shown and described particular embodiments of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

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

1. A current-limiting fuse comprising:

a. a plate of high-conductivity metal having a central longitudinal axis and two spaced-apart notches extending transversely of said axis from opposite lateral edges of said plate to points near said axis, thereby dividing said plate into: (a,) two tang portions spaced apart along said longitudinal axis at opposite sides of said notches and (a a neck portion interconnecting said tang portions and located between the inner ends of said notches,

b. said tang portions constituting fuse terminals and said neck portion constituting fusible conducting means for carrying current between said tang portions and for melting in response to overcurrents,

c. said neck portion being integral with said tang portions, the junctions between said neck portion and said tang portions being free of solder, and the neck portion, the tang portions, and said junctions all being of the same material and formed from the same part,

(1. each of said tang portions having a relatively large cross-section in any plane therethrough normal to said longitudinal axis compared to the crosssection of said neck portion in any plane therethrough normal to said longitudinal axis,

e. a housing of insulating material surrounding said neck portion andcomprising: plates of insulating material at opposite faces of said conductive plate and portions of insulating material extending into said notches,

f. and means for securing said insulating plates to said tang portions and to each other, thereby effectively preventing motion of said tang portions relative to each other,

g. said tang portions having'end regions located outside said housing adapted to be secured to external circuitry.

2. The current-limiting fuse of claim 1 in which said insulating housing has an inner wall that surrounds said neck portion and is spaced therefrom, and which further comprises: a filler of granular arc-extinguishing material filling the space between said neck portion and said inner wall.

3. The current-limiting fuse of claim 2 in which sealing means is provided between said housing and said conductive plate to confine the filler to the space within said housing and to confine arcing products to said space.

4. The current-limiting fuse of claim 1 in which said integral tang portions and said neck portion are of aluminum.

S. The current-limiting fuse of claim 1 in which said neck portion is spaced from at least one face of said plate; and each of said tang portions, at its edge facing the other tang portion, has a recess therein extending from said one face to the location of said neck portion; said recess being aligned with said neck portion, thereby providing at the end of said recess a reduced thickness region of said tang portion forming an extension of said neck portion.

6. The current-limiting fuse of claim 5 in which said insulating housing has an inner wall that surrounds said neck portion and is spaced therefrom, and which further comprises: a filler of granular arc-extinguishing material filling the space between said neck portion and said inner wall; said filler filling said recesses in the tang portions thereby provide filler in contact with said reduced thickness regions forming extensions of said neck portion.

7. The current-limiting fuse of claim 1 in which said plate is several inches in width as measured between its lateral edge and said neck portion has a minimum crosssection zone less than about /4 inch as measured in the same direction.

8. The current-limiting fuse of claim 1 in which said tangs project laterally outward beyond the boundaries of said insulating housing, thus forming exposed fins adjacent said notches for facilitating cooling of said conductive plate. A

9. The current-limiting fuse of claim 1 in combination with an indicating wire electrically in parallel with said neck portion, located externally of said housing, and connected between two terminal points on said spaced tang portions located adjacent the laterallyopposed edges of said conductive plate.

10. The current limiting fuse of claim 1 in combination with an indicating wire electrically in parallel with said neck portion, located externally of said housing, and connected between two terminal points on said exposed fins at laterally-opposed sides of said housing.

Claims

1. A current-limiting fuse comprising: a. a plate of high-conductivity metal having a central longitudinal axis and two spaced-apart notches extending transversely of said axis from opposite lateral edges of said plate to points near said axis, thereby dividing said plate into: (a1) two tang portions spaced apart along said longitudinal axis at opposite sides of said notches and (a2) a neck portion interconnecting said tang portions and located between the inner ends of said notches, b. said tang portions constituting fuse terminals and said neck portion constituting fusible conducting means for carrying current between said tang portions and for melting in response to overcurrents, c. said neck portion being integral with said tang portions, the junctions between said neck portion and said tang portions being free of solder, and the neck portion, the tang portions, and said junctions all being of the same material and formed from the same part, d. each of said tang portions having a relatively large crosssection in any plane therethrough normal to said longitudinal axis compared to the cross-section of said neck portion in any plane therethrough normal to said longitudinal axis, e. a housing of insulating material surrounding said neck portion and comprising: plates of insulating material at opposite faces of said conductive plate and portions of insulating material extending into said notches, f. and means for securing said insulating plates to said tang portions and to each other, thereby effectively preventing motion of said tang portions relative to each other, g. said tang portions having end regions located outside said housing adapted to be secured to external circuitry.

5. The current-limiting fuse of claim 1 in which said neck portion is spaced from at least one face of said plate; and each of said tang portions, at its edge facing the other tang portion, has a recess therein extending from said one face to the location of said neck portion; said recess being aligned with said neck portion, thereby providing at the end of said recess a reduced thickness region of said tang portion forming an extension of said neck portion.

7. The current-limiting fuse of claim 1 in which said plate is several inches in width as measured between its lateral edge and said neck portion has a minimum crosssection zone less than about 1/4 inch as measured in the same direction.

8. The current-limiting fuse of claim 1 in which said tangs project laterally outward beyond the boundaries of said insulating housing, thus forming exposed fins adjacent said notches for facilitating cooling of said conductive plate.

9. The current-limiting fuse of claim 1 in combination with an indicating wire electrically in parallel with said neck portion, located externally of said housing, and connected between two terminal points on said spaced tang portions located adjacent the laterally-opposed edges of said conductive plate.