US2557926A - Time-delay electric fuse - Google Patents

Description

June 1951 K. w. SWAIN ETAL 2,557,926

TIME-DELAY ELECTRIC FUSE Filed March 1, 1949 Currenf-A IN V EN TORS -)/owin 9 fibre-52602145 14 10 1 BY M Patented June 28, 1951' 2,557,986 TIME-DELAY ELECTRIC FUSE Kenneth w. H. Taylor,

Swain, Hampton Falls, and Elmer Newbury'port, Mass assignorl to The Chaoe-Shavnnut Company, Newburyport, Mala, a corporation of Massachusetts Application March 1. 1949, Serial No. 78,986

This invention relates to improvements in fusible protective devices for electrical circuits. More particularly it relates to improvements in electric fuses of the general type wherein substantial time delay is provided on moderate overloads without recourse to mechanical cut-out mechanisms which add materially to production costs. The invention utilizes known thermal characteristics of diilerent fusible materials, combining two such materials in a fusible element in a manner to attain substantially more time delay on moderate overloads as compared with prior comparable fusible elements.

Time delay in electric fuses of the general type to which the invention relates is attained to some extent on moderate overloads as a natural and unavoidable result of the design of their fusible elements which, in the form of strips or fuse links, ordinarily have one or more fusing regions of reduced cross-section with adjacent regions of larger cross-section which necessarily provide some time delay by absorbing heat generated at the reduced regions. The amount of time delay may be increased by increasing the cross-sectional area of the regions adjacent to a reduced cross-section region of fusing and this has been accomplished sometimes in the past by addition of a heat-absorbing mass to the fusible element. However, every commercial fuse is required to have a rating which is determined in accordance with its ability to carry indefinitely a current equal to 110% of its rating, and its ability to interrupt the circuit within one hour when sub- Jected to a current equal to 135% of its rating. Hence, if a particular fuse, having a particular rating, provides an inadequate amount of time delay on moderate overloads, the natural step toward increasing time delay is to increase the cross-section of the fusible element adjacent to its fusing region or regions, or to add heat-absorbing material to the fusible element adjacent to its fusing regions. This accomplishes increase in time delay but, heretofore, it has also changed the rating of the fusible element. It then has been necessary to reduce the cross-sectional area at the fusible region or regions if the modified fusible element is to have the same rating as the original element, and this necessarily reduces the time delay which had been built up. But, a major difllculty designing a fusible element which will have a predetermined rating and yet provide a desired amount of time delay on moderate overloads derives from the fact that there are l mits below which the fusible regions of fusible elements may not practicably be reduced in efforts to attain a particular rating in con- Junction with particular time delay characteristics. And, conversely, there are limits above which the heat-absorbing mass characteristics of a fusible element may not be raised when the 9 Claims. '(ci. zoo-12s) 2. element'is required to have a particular rating. Because of these limits, especially in fusible elements having relatively low ratings, such as thirty amperes and below, for example, the attainment of desirable substantial time delay on moderate overloads heretofore has required introduction of relatively expensive mechanical cut-out mechanism for interrupting a circuit on moderate overloads.

It is among the objects of the present invention to provide fusible protective devices wherein a fusible element of one material has a mass of another material associated therewith for providing substantial time delay on moderate overleads, and wherein the fusible element has the same rating before and after association of the said mass therewith. According to the invention, a fuse strip or link of one material, such as zinc or silver or copper, is designed to have a particular rating, and a mass of another fusible material, such as tin or cadmium, or alloys thereof, is attached to the fuse strip or link at a reduced cross-section region thereof to substantially delay fusing of the strip or link on moderate overloads without increasing the rating of the strip or link above its initial particular rating.

Another object is to provide a fusible protective device wherein a fusible element of fusible material having one melting temperature is designed to have a predetermined rating, and wherein a mass of another fusible material of substantially lower melting temperature is attached to the fusible element at a reduced cross-section region thereof, said attached mass having the character that it substantially delays fusing of the element in response to moderate overloads without appreciably increasing the time of fusing of the element in response to overloads up to of the rating of the fusible element Without the attached mass thereon. The invention utilizes the known principle of associating fusible alloyable materials of different melting temperatures to efiect a fusing of the higher melting temperature element at a temperature substantially below its said melting temperature in response to moderate overloads, but the time-delaying mass has the character whereby its addition to the fusible element leaves the rating of the element the same before and after the addition.

Yet another object of the invention is to provide a fusible protective device wherein a fusible strip is designed to have a predetermined rating and has at least one region of reduced crosssection, and wherein a mass of another fusible material, of lower melting temperatur than the element, is fixed on the element at a said reduced cross-section region thereby to provide substantially delayed fusing of the element in response to moderate overloads without appreciably affecting the ability of the element to interrupt, within an hour, a current equal to or greater than 135% of the said predetermined rating of the element.

A still further object is to provide a fusible element for electric fuses wherein a fusible element has at least one region of reduced crosssection, and wherein a mass of another fusible material, which is alloyable with the element material and of lower melting temperature, is mounted on the element at a said region of reduced cross-section, .for delaying fusing of the element in response to moderate overloads, and wherein a fluxing agent intervenes between adjacent surfaces of said element and said mass for further delaying fusing of the element in response to moderate overloads.

It is, moreover, our purpose and object generally to improve the structure and fusible characteristics of fusible protective devices and more particularly such devices having substantial time delay on moderate overloads.

In the accompanying drawing:

Fig. 1 is a top plan view of a plug-type fusible protective device embodying features of the invention;

Fig. 2 is a medial cross-sectional view on line 2-2 of Fig. 1; r

Fig. 3 is a perspective view of the fusible element of the device of Figs. 1 and 2 prior to addition of the time delay mass thereto;

Fig. 4 is a view similar to Fig. 3 but with the time delay mass mounted on the element;

Fig. 5 is a fragmentary cross-sectional view on line 55 of Fig. 4, on a larger scale;

Fig. 6 is a top plan view of the central portion of a fuse link having a time delay mass mounted thereon but having additional material intervening between the adjacent surfaces of the fuse link and mass;

Fig. 'l is a fragmentary cross-sectional view on line of Fig. 6; and

Fig. 8 represents two comparative fusing curves of two 20 ampere elements of the varieties of Figs. 3 and 4, with the area of increased time delay in the moderate overload zone cross-hatched for emphasis.

Referring to the drawing, a fusible element I0 is shown in Figs. 1 and 2 as it may be mounted within an insulating casing of a conventional type of plug fuse, with one end of'element I0 connected to the center contact I2 and its other end connected, in a well known manner, to screw shell contact I4. It should be understood, however, that the illustrated fuse casing is intended to be merely representative of fuse casings generally, whether of the plug or cartridge types, wherein a fusible element lllextends between two terminal contacts ,which are insulated from each other on the casing. As herein shown, two insulating casing sections I6, I8 fit together and are held together by the screw shell contact [4, in conjunction with a soldered connection 20 of the shell contact one end of fusible element l0. Element l0 e rtends upward within the casing from the centeijlcontact l2, having a substantial central portion laid over upon an in- .sulating disk or plate 22, whence the element extends between the casing sections to its solder connection to the; shell contact at 20. Casing section l8 preferably 'gwill be of glass so that the laid over portiorrof' element l0 will be visible through the top wall'of the section.

According to thelinvention, the fusible element In will be a link or strip of fusible material having a mass 30 of another material mounted thereon at a predetermined location.

In what presently is considered a preferred embodiment, mass 80 is in the form of a rivet made of material having a melting temperature substantially less than the melting temperature of the link material, and having physical properties whereby the link material will become alloyed with the rivet material when the latter melts. But, it is a feature of the invention that the mass 30, in rivet or other form, is added to a link having a particular rating, and that the addition of the mass 30 substantially increases time delay in the moderate overload range with,- m changing the said particular rating of the While it has been proposed heretofore to employ a low melting temperature material, such as cadmium, on a fuse link made of material having a substantially higher melting temperature, such as copper, with the cadmium adhering to the copper and adapted to form an alloy with the copper upon melting of the cadmium, so as to enable fusing of the link in the cadmium-copper alloy region at a temperature sub.- stantially below the melting temperature of copper, all of the prior proposals of this general nature, so far as we are aware, have been directed toward reducing the temperature of fusing of relatively high melting temperature fuse links.

The invention utilizes this known alloying principle to effect ultimate circuit interruption on moderate overloads at a temperature substantially below the melting temperature of the link material but, the primary function of the mass 30 in the present invention is to provide substantial time delay in the moderate overload response of a fuse link whose rating is the same both with and without the mass 30, the alloying characteristic being a contributing factor in the attainment of ultimate circuit interruption. The invention makes it practically feasible, for the first time, to provide substantial time delay in' relatively low cost fuses by merely adding the time delay characteristics to an ordinary fuse link of the desired ultimate rating without changing that rating.

One commercially practical embodiment of the invention employs a flexible strip of zinc for the fusible element or link N] as shown in Fig. 3. This strip or link ID of Fig. 3 may be a 20 ampere fuse link, for example, which is suitable for embodiment in a conventional commercial fuse which provides no substantial time delay on moderate overloads. It has a hole 26 therein located at or near the mid-point between the ends of the link, and has two larger holes 28 equally spaced at opposite sides of hole 28: When this link of Fig. 3, assumed to have a 20 ampere rating, is employed in a conventional manner to provide circuit protection without any substantial amount of time delay on moderate overloads, fusing on moderate overloads ordinarily occurs at the central hole 26 whereas fusing on short circuit provides quick interruption at one or both of the holes 28.

Assuming now that it is desired to provide a 20 ampere link which will provide substantial time delay on moderate overloads, this readily may be accomplished, in accordance with the invention, by taking a Fig. 3 20 ampere link and applying to it, at the region of its central hole 26, a mass of fusible material 30, as illustrated in Figs. '4 and 5. Mass 30 preferably will be tin or a high-tin-content alloy. Cadmium and high-cadmium-content alloys may be used but awmae we have found that our novel results and effects are definitely more pronounced when mass 20 is tin or has large tin content.

The mass 30 may be associated with the link In in various ways but the illustrated rivet-form of mass 30 has the practical advantage that it quickly and effectively may be secured in place in the central hole 26 of a link H), with its shank filling the hole 26. Such a 20 ampere zinc link "I, with a rivet or comparable mass 30 of tin thereon, as in Figs. 4 and 5, retains its character as a 20 ampere fuse link but acquires substantial time delay in the important moderate overload range, yet it provides the same quick action on short circuit that it provides without the mass 30 thereon. Actually, the link with mass 30 thereon fuses on moderate overloads outward of the mass, usually at one of the holes 28. The mass 30 acts, to some extent at least, as a heat-absorber but, when the mass material and the link material are intimately in contact with each other, as in Figs. 4 and 5, the size of the mass 30 apparently is not too critical because numerous tests have revealed that the mass 30 may be varied considerably in size without appreciably affecting either the amount of time delay or the rating of the element. The action on moderate overloads appears to be that the mass 3!] acts as a heat-absorbing mass until it reaches its melting temperature, after which the tin and zinc form an alloy at some region of the link where fusing ultimately occurs at a temperature below the melting temperature of the zinc of the link.

When the tin mass 30 and the link material are intimately in contact with each other, as in Figs. 4 and 5, the amount of time delay attained frequently varies considerably in the individuals of any lot of fusible elements. We have found that greater uniformity is attained, and substantially increased time delay, by introducing an agent in the nature of a fluxing agent between the adjacent surfaces of mass 30 and the link. Such an agent is represented at 32 in Figs. 6 and 7. Rosin is a recommended agent for this purpose, and the rosin may be applied to the mass 30 in powder form by dusting it over the mass surfaces, or rosin dissolved in alcohol may be applied as a liquid to the surfaces of mass 30. Another agent, known commercially as alpha acid, has been dusted over the mass surfaces in dry powder form with good results, but rosin presently is considered preferable.

Introduction of rosin or a comparable agent 32 between the adjacent surfaces of mass 30 and link ID, as in Figs. 6 and '7, considerably modifies the fusing action on moderate overloads as compared with the action of the Figs. 4 and element having no agent 32. Substantially increased time delay is attained as compared with the Figs. 4 and 5 element, and fusing on moderate overloads occurs in practically every instance, at one of the holes 28, with definitely improved results as regards uniformity of time delay. But the element In of predetermined ratin has the same rating both before and after addition of mass 30 and agent 32 thereto and its short circuit response is the same both before and after addition of mass 30 and'agent 32. However, the introduced ag nt 32 apparently considerably modifies the heatabsorbing effect of mass 30, because variation of the size of mass 30 in an element having agent 32 introduced varies the amount of time delay whereas no appreciable variation in time delay has been observable as accompanying change of size of mass 30 in the Figs. 4 and 5 embodiment.

It is not fully understood why the mass 20. agent 32 and link material react as they do in response to moderate overloads, but it is notable that addition of mass 30 and agent 32 to a rated fuse link l0 like that of Fig. 3 does not increase the rating of link l0 even though the size of mass 30 may vary considerably. It also is a notable fact, demonstrated by numerous tests, that if mass 20 becomes melted and the overload drops off or the circuit is opened before fusing of element [0 occurs, the element, with mass 30 re-solidifled, continues to have its original rating and generally the same time delay characteristics. While some alloying of the tin and zinc may have occurred, apparently this has no material affect in subsequent service of the element. The element continues to function, with substantially unchanged time delay characteristics, until such an alloying occurs at one of the holes 28.

In any event, the invention provides inexpensive fusible protective devices having substantial time delay on moderate overloads whose ratings are predeterminable apart from the means which provide the substantial time delay, and which will have the same rating before and after addition of the time delay means.

The substantial amount of time delay provided by the invention and its provision in the important moderate overload region where time delay is wanted, will be apparent from an examination of Fig. 8 wherein comparative fusing curves are shown, the curve 34 representing the .fuslng times, at the indicated currents, of a 20 ampere zinc fuse link like that of Fig. 3, which may be a stock link designed for conventional use without substantial time delay on moderate overloads. Curve 36 represents the fusing times, at the indicated currents, of a 20 ampere zinc fuse link like that of Fig. 3 but equipped, accordin to the invention, with a tin rivet or mass 30 which preliminarily had been dusted with an agent 32 (rosin) a in the Figs. 6 and 7 embodiment of the invention. The cross-hatched region at 38 shows the important moderate overload region and the very substantial amount of increase in time delay effected by the invention in this important region.

While zinc has been mentioned as a desirable link material, silver or copper links equipped with rosin-dusted rivets 30 provide time delay on moderate overloads comparable to what has been described in connection with zinc fuse links, but the ratings of such silver or copper links, before andafter addition of the rivets, will not be the same because of the substantially higher melting temperatures of silver and copper as compared with zinc. But the rosin-dusted tin rivets make it practicably feasible to employ either silver or copper fuse links and to provide time delay characteristics comparable to those of the described zinc links, in the mentioned lower ampere range.

It is presently believed that the agent 32 must have the character of acting as a fluxing agent between the mass 30 and the link material, such a rosin, alpha acid and comparable agents.

It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable-novelty exist in the invention disclosed.

We claim as our invention:

1. In an electric fuse, a fuse link made of fusible material and having a hole therethrough at a central location along the link and two other regions of reduced cross-section equally spaced at opposite sides of said hole, a mass of a lower melting temperature fusible material apsome plied to the link at said hole and filling the hole. said being adapted to substantially delay fusing of the link in response to predete overloads, and being of material having the char acter that, when melted, it will form an alloy with the adjacent link material, and a fluxing agent associated with said mass for delaying the for= mation of said alloy, said link having the rating before and after applying of said mass and fluxing agent thereto.

2. In an electric fuse, a fuse link e of fusible material and having at least two it overload fusing regions of reduced cross section equally spaced at opposite sides of a. central location between the ends of the link at which central location the link is adapted to fuse in response to a moderate overload of predeter= mined character, there being a hole through the link at said central location, a heat-absorbing rivet secured in said hole at said central location and adapted, upon melting in response to a said moderate overload, to form an alloy with the linls material adjacent to a said high overload fusing region whereby the link fuses at a said high overload fusing region, with substantial time delay, in response to a said moderate overload of predetermined character, said rivet having a melting temperature lower than that of the link ma= terial and said moderate overload fusing occurring at a temperature'below the melting temperature of the link material, and a during agent associated with said rivet for delaying the to a tion of said alloy, said link having the same rat= ing before and after addition of said rivet and nursing agent thereto.

3. In an electric fuse, a fuse link made of fusible material and having at least two high overload fusing regions of reduced cross-section equahy spaced at opposite sides of a central lo cation between the ends of the link at which central location the link is adapted to fuse in response to a moderate overload of predeter= mined character, there being a hole through the link at said central location, a heat-absorbing rivet secured in said hole at said central location, a fluxing agent intervening between adjacent surfaces of said rivet and link, said rivet being adapted, upon melting in response to a said moderate overload, to form an alloy with the link material adjacent to a said high overload fusing region whereby the link fuses at a said high overload fusing region, with substantial time delay, in response to a'said moderate over= load of predetermined character, said rivet hav= ing a melting temperature below that of the linlr material, and said fiuuing agent delaying a said moderate overload fusing at a said alloy region of the link, the moderate overload fusing ultimately occurring at a temperature below the melting temperature of the link material.

a. A time delay fusible element, comprising a fuse linls made of a strip of fusible material hav ing two high overload fusing regions and an in= tervening moderate overload fusing region, there being a hole through the link at said moderate overload fusing region, a rivet of a lower melting temperature fusible material secured in said hole and adapted to shift the moderate overload fus lng region to one of said high overload fusing regions, said rivet being adapted to substantially delay moderate overload fusing and to form an alloy with the adjacent link material, and a hu ing agent intervening between adjacent link and rivet surfaces for further delaying moderate overload fusing, the ultimate of the fuse link, in rcspo to a moderate overload. occurrinsatasaidyregionofthelinkatatemperature substantially below the melting temperature of the link ma.

5. A time delay fusible element, comprising a fuse link made of fusible strip material and de-= signed to have a particular rating, a rivet of lower melting temperature fusible material socured to the link and adapted to absorb substantial amounts of heat generated in the link in response to moderate overloads, thereby to provide substantial time delay on moderate overloads, said rivet material having the character that it forms an alloy with the adjacent link material when the rivet melts. and a during agent inter= vs :1 between the adjacent link and rivet sur= f for further delaying fusing of the in response to moderate overloads, said link he the same rating before and after addition of the rivet thereto. 1

s. an electric fuse, comprising a fuse 0 having to rn-= a s thereon, a fusible element es= tending within the casing and connected to said terminals, said fusible element comprising a fuse link made of a rivet of tin secured on the link, and a rosin fiuxing agent intervening be tween the adjacent rivet and link. surfaces.

7. A fusible element for electrical protective devices, comprising a fuse link of zinc, a tin rivet secured at a mid-location along the linlr, and a rosin a agent intervening between the ad= jacent rivet and linlr saces.

8. A fusible element for electrical protective devices, comprising a fuse link, a mass of fusible material having a lower melting temperature than the fuse link material mounted on said link, said mass of lower melting temperature terial being adapted, when elted, to form an alloy with the adiacent material of the fuse linls there by to provide an alloy region in the linlr at which fusing of the iinlr occurs at a temperature below the melting temperature of the linlr material, and a nursing agent interveg between adla= cent surfaces of said link and said mass for delay= ing the said alloying of the link and mass ma terials.

a. A fusible element for electrical protective devices, comprising a strip of fusible material having a region of reduced cross-section therein, a mass of lower melting temperature material mounted on the strip at said region of reduced cross-section, said mass being adapted, when melted, to form an alloy with the adjacent strip material thereby to provide an alloy region in the strip at which the strip will fuse at a temperature below the melting temperature of the strip ma= terial, and a during agent intervening between the adjacent strip and mess surfaces.

KENNETH W. swans. ms Fr. H. TAYLOR.

commences @ET'LED The following references are of record in the Number Name Date l,dl3,2dd Feldlramp Nov. 6, 1923 2,343,135 Triplett May 27, 1941 2,313,281 Taylor Mar. 9, recs FORHGN PAS Nber Ca mr Date 242,338 Great Bri Nov. 5, 1925; 5%,232 Great Britain July 25, w ll behold Great Britain 5.--- 0st. 133, 1%3

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