US2672540A - Banded multiple element fuse - Google Patents

Description

March 16, 1954 Q G DEWEY BANDED MULTIPLE ELEMENT FUSE Filed March 20, 1953 a q x m n w A? t Ve A mud INu/ H c y b r IAN IIIIA Length of fuse Patented Mar. 16, 1954 BANDED MULTIPLE "ELEMENT FUSE Clyde G. Dewey, -Philadelphia,.Pa;, assignur to.-

General Electric Company, .a ..corporation xof Application March-20, 1953, SeriaI-No.343,718

(Cl. 200-121)- a '3 Claims. 1 The invention relates to fuse and more :particularly to power fuse units of the currentzlimiting type having a plurality of fusibleelements operating in parallel current interrupting .re-

lation.

Suchpower fuse units are extensivelyused in .lieu. of the relatively more-expensive andlarger sizecircuit breakers forprotecting relatively high voltage power circuits upon the occurrence of faults, particularly short circuits. In such short circuit protective service, the multi-element current limiting type of fuse has the outstanding advantage over circuit breakers of initially limiting the excesscurrent to a value which .is only a fraction of that which the power circuitis .1

otherwise capable of supplying.

To obtain such current limiting action, the multiple fuse: elements ordinarily are in therform of relatively long fine Wires of areadilyvaporizable metalsuch as silver or zinc andv embedded in intimate heat. exchange relation throughout their activelength in an inert, granular refractory-arc extinguishing material of high dielectric strength, such as sand, or relatively finely dividedguartz. The embedded .elements are enclosed. in an insulatin fuse casing or housing having terminals mounted thereon for connecting, the. fusible elements in -.parallel current .inrterrupting relation.

Upon subjection of suchaiuse unit to fault currents of a high voltage circuit suchas produced' by a shortcircuit, the spaced apartfusible elements, if all conditions are equal, will each attain fusin temperature and dissipateor vaporize throughout a fusible. section of. considerable length, whereupon parallel. arcing will v ccur' and the metal vapors rapidly. expand. The force of {such expansion will throw the metal vapors out of the arc paths and into .theflspaces between the granules of the inert 'fillermaterial separating the fusible elements. As. .a result, these. vapors rapidly condense upon thesurface of the relatively coolgranules and no longeriare available for current conduction so that each arc is confined to the elongated and restricted path previously occupiedby the correspondin fusible element. The intimate heat exchange relation betweenthe spaced apart parallel elongatedhot arc=columns and the relatively'cool granules will, if all conditions are equal, cause an exceedingly rapid transfer of heat fromeach arc column'to the adjacent granulesso that the arc energy is quickly dissipated by this heat transfer with practically"negligible net pressure on i the fusing housing; 7

,.As a result of the fusin of the parallel el'ements. in the-manner outlined above there-willbe in effect; suddenly inserted into the pathpf -the currentat the instantof vaporization 'of thepar -allel fusible elements a high resistanceiformed by the elongatedjparallel. arc columns whichiwill initially limit the current to'a magnitude which is 'onl-ya fraction of thecurrent which theishort circuit is otherwise capable of producing. IIhe inserted. resistance will cause rapid. currerit decay and the quick -O001il'lg .of. the arcs. will-pro.- .duce 'a subsequent interruption-of :the current flow in short circuits within the interruptinglicaiipacity of the fuse unit. But whenever suchzinterrupting capacity is exceeded or'wheneverrall conditions are not equal the multiple element .fuse unitmay fail to extinguish some one-'ormore of the parallel. arcs resulting from the fusing-of the parallel-elements and therefore failato :successfully interrupt the short circuit.

As the use of. such current :limiting .powerifu'se units-becomes extended to even highermvoltage power circuits the problem :of, correspondingly increasing the voltage interrupting ability uf;=a multiple element fuse unit with-aconstruction ofeconomicsize and=manufacturing cost becomes more-acute.

Thus, the principal object of the present invention is to provide an improved means :forincreasing the voltage: interrupting ability of amultiple element 2 current limiting fuse unit without increasing its size or length or -materially in- .cr.easing. its cost.

ln accordance with the present invention, this is accomplished by providing conducting members or bands .for interconnecting :the' elongated fusible elementsin parallel current'equalizingrrelation' at spaced. intervals along. their length. Since these elements :are connected in parallel current interrupting relation by the fuse ter=- minalsat the endsof the elements the eifect'of the additionalintermediate conductingmembers or bands causethe current rtodivide :more evenlyfbothfibetoreand during circuit interrupt.- ing conditions. in thesectionsof, thewfusible elements betweenthe. bands thanit'mayinthe end sections. Such current equalization produces more equal distribution: of .theinterruptingduW between. the elements. .Thi has been established'by'test as the successful voltagerinterrupting ability ofa multiple element current .limitmg fuse has'been substantially improved-solely by the provision of the equalizin conducting members or bands interconnecting .the fusible elements at. spaced intervals along thirdength A further effect of the bands is to cause each current limiting fuse element to produce more voltage drop at any value of current since the voltage gradient is always higher at the ends of an arc than at the intermediate portions of the arc. Thus, the bands of the present invention in effect provide additional arcing terminals intermediate the end' terminals of the multiple elements of the fuse. In this way the total are drop resistance of the fuse unit is materially increased.

But upon operation of a current limiting fuse in a highly inductive circuit, the sudden insertion of a relatively high are drop resistance and rapid decay of the current may result in a voltage surge which under some conditions ma'y attainfuse-element with sections of different fusing current capacity. Thusthefuse elements may be provided with sections of different crossflsec- 'tional area or with sections of different resistance characteristics such that sequential or stepby- .step fusing and dissipation of the fusible sections of each element is secured. In either case, when the different sections of the elements are formed to fuse in sequence, the interconnecting conducting members or bands of the present invention preferably are applied to interconnect the elements at the junctions of the sections and preferably with the section that will first fuse intermediate the bands so as more effectively to produce the joint fusing of the different sections of all elements in the desired sequence.

Further objects and advantages of the invention will become apparent from the following descrlption of the accompanying drawings and the features of novelty which characterize the invention will be pointed out with particularity in the appended claims.

In the drawing,

Fig. 1 is'a side view partly in section of a typical multiple element current limiting power fuse unit embodying the improvements of the present invention;

Fig. 2 is a cross section view of the fuse unit shown in Fig. 1 on the sectional line 2-2 with the inert arc resistance material omitted;

Fig. 3 is a chart indicating the relative increase in voltage interrupting capacity of multiple element fuse units attained by means of the improvements of the present invention.

A typical multiple element current limiting fuse unit construction is illustrated in Fig. 1

as comprising an enclosing tubular casing H) formed of suitable insulating material such as fiber or glass and having the fuse terminals II and I2 mounted on the opposite ends thereof by means of suitable sealing material I3. As shown "in Figs. 1 and 2, three symmetrically spaced fusible element insulating and supporting cores I5, I8, and ll, of ceramic material are mounted inside the casing II! to extend substan tially the full length of the casing. As illustrated, each core has four projecting ribs I8 so as to form an elongated square frame or reel upon which a plurality of fusible elements I 9 are wound in uniformlyspaced apart relationto'be separatelyembedded in the granular refractory material It, as shown three fusible elements I9 individual current limiting'and arc extinguishing "dependent upon the maximum continuous current carrying rating of the fuse. The core construction and theWindingofthe fusible elements in spaced apart relation-thereon preferably is such that the elements are'uniformly distributed throughout the availablespace inside of the fuse casing Ill.

Thusyvhen the casing is filled with the finely dividdsand or other inert arc resistance material l4, each fusible element will throughout its activelength be embedded in intimate heat conducting relation withthis material and sufficientlyisolated from the adjacentelements to obtain action of the parallel elements.

Asshown, the three cores I5, I6, and II, are mounted in symmetrically spaced apart relation inside casing I0 by means of a spacer plate 20 at each end and secured in position by the holding screws 2i. The multiple fusible elements 19 on each core are interconnected in parallel currentinlterrupting' relation at the ends thereof by mean of a common terminal 22 which extends through suitable openings'inthe spacer plate 20 and is joined as by welding to one of the fuse terminals II or I2. In this way, all of the elements I9 on all of the cores are connected between terminals I I and I2 in parallel current interrupting relation.

In accordance with the present invention, the current equalizing conducting members Or bands 24 and 25 are provided for interconnecting all of the fusible elements IE1 at spacedintervals intermediate their ends to increase the voltage interrupting ability of the fuse unit. As shown, the three fusible elements I!) wound on each core are interconnected in current equalizing relation by the bands 24 and 25 at equally spaced intervals intermediate their ends and the corresponding bands on each core are then interconnected by suitable jumpers 21 so as to electrically interconnect all of the elements on all of the cores. If desired, the separate bands 24 and 25 on each core may be formed simply by winding andwelding a few turns of bare silver or zinc wire to interconnect all the. fusible elements thereon and twisting the ends of the wire together and leaving these ends sufficiently long for convenient joining as by welding with the ends of thecurrent equalizing bands on thead-w jacent cores.

When the fusible elements are provided with sections of different fusing current capacityand arran ed so that the middle section will fuse before either of the end sections, then the bands. and 25 are located adjacent to or overlying the junctions between the different sections of the fusible elements so as to thereby interconnect each of the corresponding sections of each of th elements in current equalizing relation. I

It has been found by test wherein the test circuit voltage is increased that a multiple element current limiting fuse unit provided with current equalizing bands constructed and arranged'asdescribed above will have an increased voltage interrupting ability over that provided by the same fuse unit without the bands. Fig. 3 has been plotted from the results of these tests to indicate the substantial increase in successful interrupting ability that was obtained solely by applying the current equalizing bands in accordance with the present invention. As indicated in Fig. 3, the length of a multiple element fuse unit normally required for successful interruption will increase as indicated by the dotted line A as the voltage of the circuit is increased. Thus the over-all length of the fuse unit of the same internal construction becomes a fairly accurate indication of the successful voltage interrupting ability thereof. The solid line B shows that when a pair of current equalizing bands are applied to the multiple element fuse unit, a substantial increase in successful voltage interrupting ability thereof is obtained without any increase whatever in over-all length of the unit.

While there may be various technical theories as to Why the provision of the current equalizing bands intermediate the ends of the multiple element fuse in accordance with the present invention provide the improved results indicated by Fig. 3, this can be explained most satisfactorily on the basis of the equalization of the current in all sections of all of the elements. Such current equalization more effectively insures that each section of each element will participate in and share the joint circuit interrupting duty. In this way the bands effectively prevent a disproportionate share of the circuit interrupting duty becoming imposed upon some one element so that the arc thereof persists and the fuse unit fails to interrupt the short circuit.

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

1. A multiple element current limiting fuse unit having in combination, a casing formed of insulatin material having arc extinguishing material therein and having a pair of current conducting terminals mounted on the casing in spaced apart relation, a plurality of fusible elements in intimate heat exchange relation with the are extinguishing material and connected in parallel current interrupting relation between the terminals, and means including a conducting member interconnecting the elements intermediate the ends thereof for equalizing the distribution of the current between the elements to increase the voltage interrupting ability of the fuse unit,

2. A multiple element current limiting fuse unit having in combination, a casing formed of insulating material having arc extinguishing material therein and having ,a pair of current conducting terminals mounted on the casing in spaced apart relation, a plurality of fusible elements in intimate heat exchange relation with the arc extinguishing material and connected in parallel current interrupting relation between the terminals, and means including at least a pair of conducting members interconnecting the elements at substantially equally spaced intervals intermediate the ends thereof for equalizing the distribution of the current between the elements to increase the voltage interrupting ability of the fuse unit.

3. A multiple element fuse unit having a plurality of fusible elements connected in parallel current interrupting relation and each element having sections of diiferent fusing current capacity to produce sequential fusing thereof, and means including a conducting member interconnecting the junctions between the sections of the elements for equalizin the distribution of the current to increase the voltage interrupting ability of the fuse unit.

CLYDE G. DEWEY No references cited.

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