US2879354A - Fusible devices - Google Patents

Description

March 24, 1959 v H. H. FAHNOE FUSIBLE DEVICES 2 Sheets-Sheet l Filed May 26, 1954 on n N. wl. mm www ,wm

WITNESSES:

4H. H. FAHNOE FUsIBLE DEVICES y March 24, 1959 2 Sheets-Sheet 2 Filed May 26' 1954 f/ Nmg/ v IIIIIJ.. .llllll I .M mm QN .Nui

United States FUSIBLE DEVICES Application May 26, 1954, Serial No. 432,425

'7 Claims. (Cl. Zoll- 113) This invention relates to circuit interrupters in general, and more particularly to fusible devices.

A general object of my invention is to provide an improved fusible device which comprises a pair of electrically parallel fuse assemblies, with one fuse assembly having relatively low thermal capacity, and the other fuse assembly having relatively high thermal capacity.

A more specific object of my invention is to provide an improved fusible device in which a pair of fuse assemblies are in parallel, one of which has relatively low thermal capacity, and the other fuse assembly having relatively high thermal capacity, in which operation of the fuse assembly with the low thermal capacity during relatively light overloads insures that the other fuse assembly with the high thermal capacity will completely effect interruption.

Another object of my invention is to provide an improved fusible device particularly adapted for applications in which there may be only a temporary inrush ot current. Such an application would be that for motorstarting duty and in the control of such motors. ln this application, as well known, the inrush current may approach 8 times the normal running current, and it is desirable to protect the motor with a fusible device which will not fuse or blow during such motor-starting duty.

A more specific object of my invention is to provide an improved fusible device of the current and voltage limiting type in which fault currents and heavy overload currents are limited in a traction of the irst half-cycle of fault current, vand to adapt such a current limiting fusible device for an application in which temporary or momentary overload currents are encountered, such as motor inrush currents in which, during such temporary overload conditions, it is desirable not to elfect operation of the fusible device.

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings, in which:

Figure l is a longitudinal vertical sectional view through a fusible device incorporating the features of my invention;

Fig. 2 is an exploded perspective view of certain detail items which are utilized in the construction of the upper fuse assembly in Fig. l, that is, the one with the higher thermal capacity;

Fig. 3 is a side elevational view of one of the fusible elements incorporated in the upper fuse assembly of rny device;

Fig. 4 is a top plan view of the fusible element of Fig. 3;

Fig. 5 is an exploded perspective view of detail parts utilized in the lower fuse assembly of Fig. l, that is, the one with the lower thermal capacity;

Fig. 6 is a side elevational view of one of the fusible elements utilized in the construction of thelower fuse assembly of Fig. 1, having the lower thermal capacity; and

Fig.r 7 somewhat diagrammatically illustrates a modied form of the present invention to a simplified fusible arent device in which the immediately surrounding arc-extinguishing material has dierent heat-absorbing characteristics for each of the two electrically parallel disposed fuse assemblies.

Referring to the drawings, and more particularly Fig. l thereof, the reference numeral 1 generally designates a fusible device comprising a pair of electrically parallel disposed fuse assemblies 2, 3, which are electrically and mechanically secured together by conducting braces 4, 5. The conducting braces 4, 5 may be secured, as by brazing, to the respective terminal caps 6, 7, 8, 9 of the fuse assemblies 2 and 3. The line connections are indicated by the reference numerals 10, 11.

Referring more particularly to the upper fuse assembly 2, it will be observed that the terminal caps 6, 8 are secured to the outer ends of a tubular fuse holder 12. lnteriorly of the fuse holder 12, and at the outer ends thereof, are conducting plugs 13, 14 which are preferably threadedly secured in place. Staking pins 15 may be employed to rigidly secure both the conducting plugs 13 and 14 and the terminal caps 6, 8 xedly in place to the tubular fuse holder 12.

Disposed interiorly and longitudinally within the tubular casing 12 is a high thermal capacity fusible section, generally designated by the reference numeral 16, and generally comprising a plurality of longitudinally disposed fusible elements 17, of the type more particularly shown in Figs. 3 and 4 of the drawings. The fusible section 16 more specifically includes a centrally disposed longitudinally extending insulating rod 18, of fiber or the like, having the outer ends thereof threadedly secured to conducting terminals 19, 20, which project respectively through the plug 13 and terminal cap 8 and through the plug 14 and terminal cap 6. Preferably, the outer reduced ends 21, 22 of the conducting terminals 19 and 20 are soldered, as at 23, to the terminal caps 6, 7.

Encircling the insulating rod 18 are a plurality of spaced insulating sleeves 24 having a plurality of parallel disposed slots 25 therein. The slots 25 are preferably aligned, as shown in Fig. l, to accommodate the fusible elements 17, the ends of which may be xedly secured, as by soldering, to the conducting terminals 19, 20.

Disposed between adjacent insulating grooved sleeves 24 are a plurality of metallic sleeves 26 composed of a good heat-absorbing material, such as copper or the like. The fusible elements 17 lie along the outer surface of the metallic sleeves 26 and may be secured thereto by solder 27 or by wrapping binding wire thereabout. Disposed externally of the metallic sleeves 26 are conducting segmental sleeves 28, which collectively with the metallic sleeves 26 form spaced heat wells, generally designated by the reference numeral 29. The heat wells 29 serve to absorb heat which ows lengthwise through the fusible elements 17 with the net result that the fuse assembly 2 is able to temporarily withstand a relatively heavy overload current of a momentary nature without fusing or blowing.

As more particularly set out in U.S. Patent 2,605,371, which issued July 29, 1952, to Harold H. Fahnoe, and assigned to the assignee of the instant application,the provision of the heat wells 29 adapts the fuse assembly 2v for motor-starting duty. Surrounding the fusible section 16 is finely divided inert insulating material 30, such as sand. Also encircling the insulating liber sleeves 24 are liber washers 31, which, as set out in the aforesaid patent, have the desirable function of not only preventing the arcs from getting completely out of the grooves or slots 25, but also increase the arc voltage and provide additional quantities of deionizing gases which assist in effecting circuit interruption.

For the general theory of operation of the fuse assembly 2 and for a detailed description of the function of the reduced portions 32 spaced axially along the fusible elements 17, reference may be had to the aforesaid Fahnoe patent. It may, however, be stated that the fuse assembly 2 is of the current-limiting type n which fault currents are limited to a small fraction of what they would reach by a fusing of the several parallel disposed fusible elements 17 during such a fault condition.

Referring now more particularly to the lower fuse assembly 3 of Fig. 1, it will be observed that the terminal caps 7, 9 are secured in a similar manner to a tubular fuse holder 33, interiorly of which is also disposed sand 30. The fuse assembly 3 comprises an interiorly disposed longitudinally extending fusible section 34, which includes an insulating rod composed of liber, or some other insulating material, which evolves gas upon being contacted by an arc.

As shown, the fiber rod 3S has a plurality of helically disposed grooves or slots 36 machined therein, within which are disposed a plurality of electrically parallel fusible elements 37 having a configuration more clearly shown in Fig. 6 of the drawings.

As more particularly brought out in U.S. Patent 2,496,- 704, which issued February 7, 1950 to Harold H. Fahnoe, and which was assigned to the assignee of the instant application, the fusible elements 37 disposed within the grooves 36 are in intimate engagement with the fiber material constituting the rod 3S. In addition, they are maintained in place by a plurality of liber washers 3l, which are generally similar to those previously described and which have generally the same function.

The ends of the fusible elements 37 are soldered to a terminal plug 38 at one end, and to a conducting positioning plug 39 at the other end. A reduced portion 40 of the terminal plug 38 extends through a metallic plug 41 and also through the terminal cap 9, being soldered thereo as at 42.

A reduced portion 43 of the positioning plug 39 passes through apertures 44 provided in a metallic bracket 45, the other end of which is secured to the other terminal plug 46 in a manner more fully described hereinafter.

Within the bracket is disposed an indicating device, generally designated by the reference numeral 47. The indicating device 47 includes an insulating tube 4S, within which is disposed a coil tension spring 49 and a relatively high resistance fuse wire 50, which is attached to the spring 49 at one end and to an indicating rod 5l at the other end. The outer end of the indicating rod 51 passes through an indicating disc 52, being soldered, as at 53, to the outer side thereof. A compression spring 54 serves to bias the indicating disc 52 to the indicating position, but is maintained in the compressed state by the high resistance fuse wire and the tension spring 49. The outer end of the tension spring 49 is secured to a conducting disc 55, which is electrically connected by a high resistance indicating fuse wire 56 to the terminal plug 38 of the fuse assembly 3.

Consequently, upon blowing or fusing of the several fusible elements 37, the voltage across the fuse assembly 3 causes substantially simultaneous fusing of the fuse wire 56, and the fuse wire S0, thereby releasing the compres sion spring 54 which forces the indicating disc 52 to the indicating position, thereby indicating fuse operation.

Compared with the fuse assembly 2, the fuse assembly 3 has a relatively low thermal capacity. My invention in part, concerned with a parallel two-fuse unit assembly, with each fuse assembly having approximately the same resistance for all currents up to full load current. Each unit, or fuse assembly 2, 3, uses substantially identical fusible materials Which inherently have high temperature coefficient of resistance, but eachunit 2, 3 has radically different thermal capacities. On sustained currents in eX- cess of normal load current rating, the unit or fuse assembly 3 with the lower thermal capacity reaches melting temperature before the other unit and operates iirst, thereby forcing all the current through the other fuse assembly l 2 which, in turn, operates to clear the overload condition.

On a temporary overload condition, such as motorstarting inrush currents, which preferably this type of fusible device must be able to withstand, the combination of the two types/of fuse assemblies 2, 3 retains the greater overload capacity of the thermal well type of fuse assembly 2. Initially, the current divides practically evenly between the two fuse assemblies 2, 3. As soon as the fusible elements 17, 37 start to heat up, due to this overcurrent, the resistance of the fusible elements increases according to the temperature coeiicient of resistance characteristic of the material used. Preferably, the material of the fusible elements 17, 37 is silver. The unit with the lower thermal capacity, that is the fuse assembly 3, reaches a higher temperature than the other, and, consequently, has a higher resistance than the other.

This effects a partial transfer of current from the unit 3 of lower thermal capacity to the unit 2 with higher thermal capacity. This second unit 2 can better withstand this extra heating effect of the higher current, and the rate of rise of temperature of the fusible element 17 is slowed down in accordance with the basic principle of this type of fuse. Neither fuse units 2, 3 reaches an excessive temperature during the normal starting period.

The advantages of combining the two types of interrupting structures in parallel with these different thermal capacities are:

(1) The heat generated during normal load and temporary harmless overloads is more effectively dissipated.

(2) The tremendous arc energy during fault clearing is divided into two structures, thus reducing the duty on each.

(3) By making the first of the parallel units the one which has good clearing characteristics at relatively low values of current, here fuse assembly 3, the possibility of restrike in this unit 3 when the other unit 2 iinally clears is minimized.

(4) By making the rst of the parallel units 3 the one which has the good clearing characteristic approximately double current is then imposed on the other unit 2, which it can more readily and positively clear.

Fig. 7 illustrates the basic principles of my invention, as applied to a simplified fusible device, generally designated by the reference numeral 57, and including a pair of fuse assemblies 58, 59 in electrical parallel. For convenience, let us assume that the upper fuse assembly S8 has the high thermal capacity, and for this reason, we may use a granulated material, such as zirconium 60, within the fuse casing 61. The other fuse assembly 59 may have an arc-extinguishing material of relatively low heat absorbing ability, such as silica, or sand 62. The fuse elements 63, 64 being in immediate proximity to arc-extinguishing material of diierent heat absorbing abilities, or characteristics, function so that the fusible element 64 within the silica will blow first, since the silica 62 has poor heat absorbing characteristics. The other fuse assembly 58 will then serve to clear the circuit.

From the foregoing description of the several features of my invention, it will be apparent that the invention may be widely applied to different structures. A basic requirement is that one of the two parallel fuse assemblies has a lower thermal heat capacity than the other fuse assembly. Whether this is accomplished by having heat wells 29 disposed along the fuse assembly 2, or by using a heat-absorbing material about the fuse Wire, which will readily absorb the heat of the fuse wire, the end result is the same. During temporary overload, such as motor inrush currents, when the fusible device is used particularly for motor-starting applications, the presence of the fuse assembly of higher thermal capacity serves to control the situation, and although the other fuse assembly of lower thermal capacity heats up, thereby diverting more current to the fuse assembly of higher thermal capacity, the presence of the higher thermal capacity fuse assembly prevents blowing of the fuse structure as a whole. During overload currents of a long duration which would damage the equipment, the fuse assembly of lower thermal capacity will blow, thereby immediately forcing all of the current into the fuse assembly of higher thermal capacity, immediately causing the latter to fuse. For heavy fault currents, the presence of both fuses in parallel serves to diminish the duty requirements of each.

Although I have shown and described speciiic structures, it is to be clearly understood that the same were merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the art, without departing from the spirit and scope of the invention.

I claim as my invention:

1. A fusible device including a pair of cooperable spaced fuse assemblies, means electrically connecting the fuse assemblies in parallel, each fuse assembly having one or more fusible elements so that both assemblies share in the carrying of normal load current, each fuse assembly also having an insulatinU casing thereabout enclosing the one or more fusible elements associated with that particular fuse assembly, one fuse assembly having relatively low thermal capacity, the other fuse assembly having relatively high thermal capacity, the interiorly disposed fusible elements within both spaced insulating casings of the pair of fuse assemblies blowing interiorly within the casings during fault current interruption to eliminate external hazardous display, and each fuse assembly having its casing exposed to the ambient for rapid dissipation of heat and more effective clearing on fault current interruption.

2. A fusible device including a pair of cooperable fuse assemblies, means electrically connecting the fuse assemblies in parallel, each fuse assembly having one or more fusible elements so that both assemblies share in the carrying of normal load current, one fuse assembly having relatively low thermal capacity, and the other fuse assembly having one or more thermal heat Wells enabling it to have a relatively high thermal capacity.

3. A fusible device including a pair of cooperable fuse assemblies, means electrically connecting the fuse assemblies in parallel, one fuse assembly having arc-extinguishing material of relatively low heat absorbing ability to result thereby in a relatively low thermal capacity fuse assembly, and the other fuse assembly having arc-extinguishing material of relatively high heat absorbing ability to result thereby in a relatively high thermal capacity fuse assembly.

4. The combination in a fusible device of a pair of cooperable spaced fuse assemblies, means electrically connecting the fuse assemblies in parallel, both fuse assemblies using the same type of fusible elements and having substantially the same resistance characteristic over the current range in which blowing of the fusible device does not occur, one of the fuse assemblies having additional means for absorbing heat energy so that said one fuse assembly has a greater thermal capacity than the other fuse assembly, and each fuse assembly having its casing exposed to the ambient for rapid dissipation of heat and more efective clearing on fault current interruption.

5. A fusible device including a pair of cooperable fuse assemblies, means electrically connecting the fuse assemblies in parallel, one fuse assembly having relatively low termal capacity, and the other fuse assembly having relative high thermal capacity, the rst said fuse assembly having a plurality of electrically parallel fusible elements, the second said fuse assembly also having a plurality of electrically parallel fusible elements, and a plurality of heat wells associated with the second said fuse assembly.

6. A fusible device including a pair of cooperable fuse assemblies, means electrically connecting the fuse assemblies in parallel, one fuse assembly having relatively low thermal capacity, the other fuse assembly having relatively high thermal capacity, bo'th fuse assemblies having a plurality of fusible elements, means electrically connecting the fusible elements in each fuse assembly in parallel with the other fusible elements therein, and the said other fuse assembly having one or more heat wells therein to absorb heat.

7. A fusible device including a pair of cooperable spaced fuse assemblies, means electrically connecting the fuse assemblies in parallel, one fuse assembly having relatively low thermal capacity, the other fuse assembly having relatively high thermal capacity, each fuse assembly having a plurality of fusible elements therein, and each fuse assembly having its casing expo-sed to the ambient for rapid dissipation of heat and more effective clearing on fault current interruption.

References Cited in the tile of this patent UNITED STATES PATENTS 2,496,704 Fahnoe Feb. 7, 1950 2,523,139 Owens Sept. 19, 1950 2,605,371 Fahnoe July 29, 1952 2,658,974 Kozacka Nov. 10, 1953 2,670,418 Kozacka Feb. 23, 1954 2,678,980 Hitchcock May 18, 1954 2,713,098 Swain July 12, 1955 FOREIGN PATENTS 12,210 Great Britain May 18, 1914 236,298 Switzerland lune 1, 1945 384,766 Gerat Britain Dec. 15, 1932 477,152 Great Britain Dec. 22, 1937 516,324 Great Britain Dec. 29, 1939 570,441 Germany Feb. 16, 1933

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