US2453397A - Fuse link - Google Patents

Description

Nov. 9, 1948; E. H. YoNKERs FUSE LINK Filed April 26, 1947 Patented Nov. 9, 1948 FUSE LINK Edward H. Yonkers, Glencoe, Ill., assigner to Joslyn Manufacturing and Supply Corporation, Chicago, Ill., a. corporation of Illinois Application April 26, 1947, Serial No. 744,061

3 Claims.

The present invention relates to fuse links and more particularly to improvements in fuse links of the character disclosed in applicants copending applications Serial No. 514,030, filed December 13, 1943, Serial No. 533,824, filed May 3, 1944, and Serial No. 637,528, filed December 28, 1945, all assigned to the same assignee as the present invention.

It is an object of the present invention to provide an improved fuse link of sim'ple construction which provides long time overload protection, short time transient overload protection and is self-protected against damage due to lightning surges and the like.

In accordance with a further object of the invention, improved and exceedingly simple facilities are provided in the link for protecting the link against burn-out or damage when subjected to lightning surges or the like.

The present invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings in which the single figure is a fragmentary sectional view illustrating a fuse link characterized by the features of the present invention.

Referring now more particularly to the drawing, the present improved fuse link 5 is there illustrated as comprising a tubular metal casing 3| having an open lower end and a flanged upper end which is closed by a terminal cap 32.

The construction and mode of operation of the fuse link components provided within the metallic casing 3| are substantially the same as disclosed and claimed in applicants co-pending application Serial No. 637,528, iiled December 28,' 1945, and assigned to the same assignee as the present invention, with the important exceptions pointed out below. This casing 3| houses three fusible elements 40, 4| and 42 which are connected in series circuit relationship between the casing 3| and the pigtail conductor 37. The two elements 4| and 42 are both formed of Nichrome wire or another metal wire or ribbon of the desired resistivity and are both of the same cross sectional area, such that they possess substantially identical time-current fusing characteristics. The fusible element 4| is substantially straight throughout the major portion of its length and the lower end portion thereof is centrally disposed Within the upper end portion of a stranded pigtail conductor 31, the overlapping portions of the two elements 4| and 3l being telescoped Within a (Cl. 20D-115) metal assembly sleeve 44. This sleeve is crimped adjacent the lower end thereof, as indicated at 44a, and the lower end of the fusible ,element 4| is brought out through the strands of the conductor 3l and wrapped around the conductor 31 as indicated at 4|b. After the three elements 4|, 3], and 44 have been assembled to occupy the relative positions illustrated, the crimp 44a may be formed around the lower portion of the sleeve 44 for the purpose of providing a rigid mechanical connection between the three named parts. Thereafter, the lower end portion of the sleeve 44, the adjacent portion of the pigtail conductor 3l and the wrapped end portion 4|b of the fusible element 4| may be soldered to provide a rigid connection therebetween.

The fusible element 42 may more properly be designated a combination impedance element and heating element. in that it functions in conjunction with theimproved surge gap facilities described below to prevent surge currents of large magnitude from traversing itself and also acts to heat the fusible element to a fusing temperature when the link is subjected to an overload current for a sustained time interval. In order to perform these two functions the element 42 ls constructed in the form of a helically coiled conductor, and the upper end portion 42a thereof is electrically and mechanically connected to the under side of the contact head 32 by crimping the same between the flange 3 la and the cap 32.

The convolutions of the element 42 are spaced apart axially of the tube 3|, and the spaced apart relationship between the turns is maintained 'by embedding the same in Ia body of dielectric refractory material 45. This ibody is preferably formed of a refractory cement and serves several functions which are pointed out with particularity below. It may, for example, Ibe formed of Portland cement or any ceramic material which is chemically inert, has high specific heat, and is possessed of good electrical insulating properties. In order to increase the inductance of the element 4'2, thereby to enhance the surge current blocking function thereof, particles of magnetic,

material, such, for example, as iron powder or magnetite, may be disposed throughout the body 45, but in no case should the density of .the magnetic particles be such as to provide conductive paths capable of short-circuiting the convolutions of the element 42. Among other functions, the body 45 serves rigidly to position or support the lturns of the element 42 within the tube 3|, and to this end entirely fills the upper portion of the tube. It also serves to support a tubular conductor d@ centrally oi the tube iii, this element being `utilized in the connection. oi the fusible element il with the lower end portion o the coinn hination heating and inductance element [ith lviore specifically, the upper tubular portion .the connecting element it is projected well within the turns of the element in spaced apart relationship therefrom, and is embedded in the it its lower end this connecting element is provided with an outwardly extending flange [lilo which serves to seat the lower turn of the element l2 in a manner such that the tubular portion oi the element it is substantially concentrioally dis= posed within the turns of the element fill. This lower turn o'i the element t2 is electrically and mechanically connected to the flanged portion bito or the element it |by means oi a high iinciting point solder dl, or the like. The upper end of the connecting element d is electrically and mechanically connected to .the upper serpentine end ta of the fusible element fil through the fusible element do, the latter element being in the iorm of a body of alloy solder having a melting point of approximately 365 F. lt is to be noted that the fusible element do, as thus formed within the tubular portion oi the connecting element 46, is disposed well Within the turns oi the element t2 so that heat generated by current conduction through :the latter element may be transferred to the fusible element lo through that portion of the refractory body lili which separates the fusible element Il@ from the adjacent turns of the element [52. Spring rtension imposed upon the free lower end of the pigtail conductor till may be utilized to rapidly withdraw the end lia oi the fusible element di from the connecting element d@ when the named fusi-ble element is heated to a melting temperature, and to widen a ibrealr in the fusible element di occasioned 'by heating this element .to a fusing temperature.

For the purpose oi increasing the heat storage capacity ci the structure including .the fusible element du, a thermal storage element is pro= vided which is arranged in heat transfer relationship with the element dii and the heating element rlihis element is in the form oi a copper or brass rod disposed centrally 0i the tubular casing iii and having its lower end con tacting the exposed upper suriace of the iusible element fill., lt is held in an upright position in axial alignment with the tubular conductor do by virtue of its being embedded in the body During prolonged use oi the link, .the upper por1 tion of the fusible element iii may many times be heated to a fluid state without actual rupture oi the element due to lacie of persistence of the current overload responsible for the excessive heat energy. Also repeated heating land cooling of the body dii may result in the formation or small nterstices therein through which the fluid metal could be dispersed to short-circuit the lower convolutions of the coiled conductor t2. Fthis would result in undesired modification of the tinten-overload characteristic of the linie. In order to obviate this possibility, a ceramic insulating tulbe de is provided as a barrier between the parts lili, tti, and it and the turns of the coiled conductor element ft2. Specifically, this tube is telescoped over the tubular conductor Citi yto rest upon the flange ita and is embedded in 'the body d5 in the manner illustrated. Being oi heat resistant ceramic material, the tube dii is not subject to cracking and hence acts as a leali-prooi barrier between the fusible element dit and the convolutions or the coiled conductor element di. Als-o, since the tube it has approximately the same heat transfer characteristics as the -body its presn ence in the zone oi heat transfer between the ele ments iii and itl and the element lf/2 does not seriously complicate the `problem of producing linirs having substantially the same timemverload characteristics in production quantities. Further, provi-sion of the tube la in the structure facilitates assembly of the component elements oi the structure in the manner explained below.

ln order to maintain the turns or the element it out of contact with the metal casing 3i, to maintain the element di out of contact with the tubular casing 3l and to provide a support for the surge gap facilities described below, the entire assembly within the casing @l and a part of the sleeve 48 are surrounded by a tube itil formed or Bakelite or other suitable insulating material. At its upper end, this tube is provided with a ange 5Ela pressed against the metal casing flange 3io when the cap 32 is fastened to the ange Bia. This tube 5i! fits snugly within and is adhesively 'bonded to the tubular member si, and the lower end portion 50o thereof projects outwardly from the open lower end of the casing 3i. It is counn terbored from the upper end to provide a lrst portion 5de of large internal diameter for rreceiving the parts lit, to, fili, ond portion till; of smaller internal diameter ier receiving the fusible element di. The flange dta oi the 'tubular conductor M is seated upon the step bild between these two portions of the tube 5i).

ln order to assist in producing arc extinction within the tube @il when the linie is ruptured either through fusing oi the element lil or :using of the element Lil, that part oi the tube portion. obb through which the fusible element di i xtends may be lined with a gas evolving material di., Preferably, this lining is in the torni of a layer of long nber cellulose adhesiifely secured to the inner surface o the t be portion hlibbetween the flange bild and the upper end oi the sleeve (irl surround the fusible element lli,

ln accordance with the present invention, in order to prevent lightning surges irorn rupturing the fusible heating element di?, a surge gap having relatively immovable electrodes 'is connected shunt with this elernenu Specifically, the heat storage element id having lower end. in elecw trieal and mechanical contact with the upper ci the fusible body til, is provided with an up er end which is spa-coil apart iroro a member it associated with the cap assembly in a manner so as to deilne a small gap between the member 5t and the adjacent end oi the heat storage member Lid. This gap in combination with the storage element and the fusible body provides a surge path in shunt with the heating element il? for toy-passing surges around this element.

lin considering the inode ot operation of the iuse linie i5, lit may be assumed that the transformer which the link protects is provided with a low voltage secondary load which under normal conditions approximates the full load capacity ot the transformer, and that this secondary load includes motors and other devices which, during the starting periods thereof, are capable of prou ducing heavy transient currents of relatively short duration in both the primary and secondary windings oi the transformer. In this regard, it will be understood that due to the heat radiating capabilities of the transformer parts, the transformer dit and 49, and a secmay be capable of withstanding an overload current of reasonable magnitude, such, for example, as 200 to 300 per cent, for a relatively long time interval; whereas it can withstand current of the order of 500 per cent of normal for only a short time interval. It will also be understood that the transformer is capable of being damaged by a sustained increase in the voltage applied thereto.

'I'he fuse link I5 operates to protect the transformer against damage occasioned by overload currents caused by any one of the mentioned factors. At the same time, the fuse link permits the transformer to be operated under sustained overload current conditions for a period less than that required to damage the transformer, and will not rupture when subjected to the normal and non-injurious high currents which are produced incident to motor starting, or the like. In this regard it will be understood that since the three serially related elements 40, 4| and 42 of the link I5 are traversed by the current flowing through the primary winding of the transformer, they are all heated by current conduction and the temperature of each element varies with changes in the magnitude of this current. The fusible element 40 is also heated by the heat conducted thereto from the element 42 through the walls of the connecting element 46. Heat energy is also transferred from the turns of the element 42 to the fusible element 40 through that portion of the refractory body 45 and the tube 49 which are disposed between the tubular portion of the connecting element 46 and the surrounding turns of the element 42, and also from the thermal storage element 48. Under constant load current conditions and with a constant voltage so long as the load current through the secondary winding of the transformer does not substantially exceed the full load capacity of this transformer, the fusible element 40 is operated at a temperature well below that required to produce fusion thereof. When, however, the secondary load current of the transformer gradually rises and is sustained for a period approachingJr that at which the transformer will be damaged, the temperature of the fusible element 4U is raised accordingly. Thus, as the load current increases, the current traversing the three elements 43, 4I and 42 is correspondingly increased so that more heat is produced in the fusible element 4U by current conduction. Concurrently the amount of heat conducted to this fusible element from the fusible elements 4| and 42 is increased. Also concurrently, the amount of heat transferred from the turns of the element 42 to the fusible element 40 through the refractory body 45 and the tube 49 is increased. A portion of the heat energy accumulating in the element 40 is transferred to the storage element 48 by conduction. After a predetermined time interval, required for the accumulation of heat within the fusible element 23, this element is heated to its fusing temperature and melts. Provision of the storage element 48 materially increases this time interval over that which would obtain in the absence of this element in the combination. When the element 40 is thus ruptured, the fusible element 4|, the sleeve 44, and the upper end of the pigtail conductor 31, are quickly expelled from the lower end of the tube 50, thereby rapidly to break the circuit for energizing the primary winding of the transformer.

During the described separation of the element 4I from the tubular conductor 46 upon fusing of the element 4U, an arc is drawn within the portion 50h of the tube 50. This arc instantly heats the coating BI to gas evolving temperature, with the result that gases are evolved therefrom which accelerate extinction of the arc. It has been found that provision of the gas evolving coating 5I permits the link itself to effectively extinguish heavy current arcs of the order of several hundred amperes even when the link is used in high voltage circuits of several thousand volts.

To consider the action of the fuse link I5 further, it may be pointed out that the refractory body 45 and the tube 49 prevent the fusible element 40 from being heated to its melting temperature when surge currents are produced in the primary circuit of the transformer as a result of motor starting or the like. Such 'surge currents are of short duration, being of the order of only a few seconds. The resulting momentary increase in heat generation within the fusible element 4I) is wholly insufficient to raise the temperature of the element to its melting point. Moreover, those portions of the refractory body 45 and the tube 49 which are disposed between the tubular portion of the connecting element 46 and the storage element 48 and the turns of the heating element 42 dissipate a large portion of the heat resulting from the current surge through the element 42 away from the element 4I. They also delay the transmission of the increased increment of heat produced by the element 42 to the storage element 48 and the adjacent walls of the connecting element 4E for an interval which will normally exceed the duration of the current transient. Accordingly, the increased increment of heat energy arriving at the surfaces of the connecting element 46 and storage element 48 from the element 42 as a result of the momentary high current, effects an insufficient increase in the temperature of the fusible element 40 to cause this element to/melt. In other words, the total heat accumulated in the fusible element 40 as a result of the transient high current is insufficient to heat this element to its fusing temperature. Thus it will be understood that the refractory body 45 and the tube 49, or more exactly the thermal impedance of this composite structure, protects the fusible element 40 against outages occasioned by transient currents of the character which frequently occur in the load pattern of any transformer secondary load. This is accomplished, moreover, Without increasing the thermal capacity of the fusible element 40 to a point such that it will provide no protection for sustained overload currents.

The thermal impedance of the refractory body 45 and the tube 49 also prevents the fusible element 40 from immediately rupturing when the transformer is subjected to a high current, such, for example, as that which is produced when the secondary winding of the transformer is shortcircuited. In the absence of an additional protective element, therefore, the transformer could easily be damaged by an overload current of this character during the period required to transfer suiilcient heat from the heating element 42 to the fusible element 40 to cause the latter element to melt. The second fusible element 4| functions to guard the transformer against damage when subjected to an overload current of this type. Thus, immediately the element 4| is subjected to a transient current of the particular character just referred to, a portion thereof lying between the upper end of the sleeve 44 and the lower end of the fusible element 40 is heated to a fusing temperature, permitting this element to rupture.

While there has been described what is at presandasse t? ent considered to be the preferred embodiment of the invention, it vviil be understood that vari ous modications may be made therein which are within the true spirit and scope oi the invention as defined in the appended claims.

i claim:

l. A fuse link comprising a tubular metal cas ing open at one end and having a terminal can closing the other end, an insulating tube fitting Within and extending through said casing, a coiled heating conductor disposed Within said tube and having its convolutions spaced axially of said tube, a tubular conductor extending within the convolutions of said coiled conductor to receive heat from said coiled conductor, said heating con-7 duct-or being connected in series between said tubular conductor and said casing, a fusible body filling at least a portion of said tubular conductor, a circuit conductor having an end embedded in said fusible body 'and extending out of said insulating tube, an elongated and conductive heat storage element in heat transfer relationship with said fusible body and extending through the convolutions' of said heating conductor toward said terminal cap, means including said'heat storage element for defining within said tube a surge gap shunting said heating conductor, and insulating means disposed within said insulating tube to insulate said tubular conductor and said heat storage element from the convolutions oi said heating conductor.

2. A fuse link comprising a tubular metal casing open at one end and having a terminal can closing the other end, an insulating tube iitting within and extending through said casing, a coiled heating conductor disposed Within said tube and having its convolutions spaced axially of said tube, a tubular conductor extending within the convolutions of said heat from said coiled conductor, said heating conm1 ductor being connected in series between said tubular conductor and said casing, a fusible body iilling at least a portion oi said tubular conductor, a circuit conductor having an end embedded in said fusible body and extending out of said insulating tube, an elongated and conductive heat storage element in heat transfer relationship with said fusible body and extending through the couvolutions or said heating conductor toward said coiled conductor to receive' lil `tubular conductor and said convolutions of said coiled terminal can, said heat storage element being' electrically connected with said fusible body one end and having its opposite end spaced from said terminai can to define a surge gap shunting said heating conductor, and insulating means disn posed Within said insulating tube to insulate said heat storage element from the convolutions oi said heating conductor.

3. A fuse link comprising a tubular rnetal casing open at one end and having a terminal cap closing the other end, an insulating tube fitting Within and extending through said casing, a coiled heating conductor disposed Within said tube and having its convolutions spaced axially of said tube, a tubular conductor extending within the conductor to receive heat from said coiled conductor, said heating conductor being connected in series between said tubular conductor and said casing, a fusible body lling at least a portion of said tubular conductor, a circuit conductor having an end embedded in said fusible body and extending out of said insulating tube, a second tube of insulating material telescoped over said tubular conductor within said coiled conductor to insulate the convolutions of said coiled conductor from said tubular conductor and said fusible body, and an elongated and conductive heat storage element in heat transfer relationship with. said fusible body and cxtending through said second tube toward said terminal cap, said heat storage element being electrically connected with said fusible element at one end and having its opposite end spaced from said terminal cap to dene a surge gap shunting said heating conductor.

EDWARD H. YONKERS.

REFERENCES (Cil'iilEllI The following references are of record in the ie of this patent:

UNITED STATES PATENTS Number

Images