US2287553A - Fuse - Google Patents

Description

D. w. FLoTr-:N 2,287,553

FUSE

June 23, 1942.

Filed April 9, 1958 due to their momentary character.

Patented June 23, 1942 UNITED sTATEs PATENT oFFicE FUSE Donald W. Floten, Chicago, Ill., assigner to Chicago Roller Skate Company, Chicago, lll., a corporation oi' Arizona.

Application April 9, 193s, serial No. 201,079 1o claims. (i. zoo- 135) fore unnecessary to have th'e fuse in such cases blow. In fact, it is desirable not to have it blow, since the blowing of the fuse under such circumstances renders no protection to the circuit and merely wastes the fuse element. Also, the consequent waste of time required to replace the fuse and put the circuit back into operation may in .many instances constitute a large loss. While all fuses inherently have some time lag between the time the heavy overload starts and the time when the fuse blows, fuses have been made to possess a predetermined time lag of such extent as to prevent the fuse from being blown by momentary overloads and yet to blow when overloads are sustained for an. appreciable time. Such a fuse gives the necessary protection for which it is primarily designed and also avoids un.-

necessary blowing.

The general object of the invention is to provide a new and improved fuse element having a time lag sufficient to withstand momentary overloads and yet functioning accurately on to aiord the it in which the more or less sustained overloa necessary protection to the ci fuse is placed.

It is also an important object of the invention to provide a fuse element having such a time lag,v i which is so constructed that it may be made of sheet metal of uniform thickness throughout its length and dimensioned so that it may be use in a casing structure of standard size. f

Another object is to provide a fuse element having fusing sections adjacent its respective ends, which sections have sufiicient electrical resistance to generate under a predetermined overload the required amount of heat by being made relatively long, thereby ypermitting the element to' be made of relatively thick metal to give it the desired time-lag characteristics and also give it suflicient physical strength so that it will not be inadvertently bent or distorted in handling. It is also an object to provide a fuse element,

the fusing sections of which attain the desired heat-generating resistance by being elongated, with at least a portion Aof each section extending laterally of the link to reduce the space longitudinally of the fuse required for the fusing section.

A further important object is to provide a fuse element having its fusing sections so constructed that variations in length, forced upon the ele' ment .by expansion due' to heat or by variations in length of the fuse casing, will not cause the element to bow out of its normal plane.

A still further object is to provide a fuse element constructed so that its portion of reduced cross-sectionalarea is subjected to a minimum distortion upon expansion or contraction, thereby avoiding undue crystallization of the metal and any change in size of such portions dueto bending or stretching. Y Y Still another object is'to provide a fuseelement having a fusing section which is constructed s o that any tendency of an arc to form upon the blowing of the fuse, or the arc to be maintained, is reduced to va minimum.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing, in which:

Figure 1 is a sldeview of'a fuse utilizing a fuse eile'ment embodying the features of the invent on. L

Fig. 2 is a longitudinal section through the fuse shown in 1.

Fig. 3 is a view with the fuse in the'same position as in Fig. 1 but showing merely the ltermirial structure with the fuse element connected thereto.

Fig. 4 is a fview showing the fuse element alone and in a flat position.

while features of the invention may be advantageously used in various types' of fuses, many of its features are particularly useful and advantageous in a fuse of the cartridge type meet- A ing the standards both for physical size and characteristlcs of operation of the National Board of Fire Underwriters. For purposes of illustration,

the fuse element embodying the features of the invention has been shown in a cartridge fuse of the knife blade type.

Asshown herein, this fuse comprises a tubular casing l0 made of nbre or the like and having at one end thereof a i'errule'l I'threaded into the end of the casing Ill and staked, as at i2, to

hold it against rotation. The ferrule Il is provided with an inwardly turned flange I3 at its outer end. At the other end' of the casing is a ferrule Il threaded into the casing and similarly staked, as at I5. The outer end of the Iferrule Il is threaded to receive a`cap Ivprovided with an inwardly turned flange I1 similar to the flange I3.

The terminal structure of the fuse illustrated herein comprises a pair of knife blades provided with m'eans for closing the ends of the casing comprising washers 2| secured on the blades. The washers cooperate with the flanges I3 and I'l in the casing structure to close the latter, and further serve to accurately locate the terminal structure within the casing. The knife blades 20 have inwardly extending portions connected to each other by a pair of laterally spaced bridge members 22 made 'of insulating material, the bridge members being attached to the knife blades by metallic bridge supports 23. The fuse element, indicated generally at 24, is attached or anchored at its respective ends to the knife blade terminals by a screw 25. The ar-A rangement shown is such that each end of the fuse element 24 is lclamped between. adjacent faces of the knife blade terminal and one of the bridge supports 23. Preferably the fuse element is slightly bent out of its plane so that it lies against one face of one of the knife blade terminals and against the opposite face of the other terminal. A

- The particular construction of the casing and terminal structure constitutes no part of the invention but is herein shown merely as an example of a casing and terminal ystructure of a type meeting the requirements of 'the' National Board of Fire Underwriters/and suitable for use in connection with a fuse element embodying the features of the invention.

Q by dimensional requirements for the casing, such i masses of metal can be obtained solely`by lncreasing the thickness of the element.

In the present instance, a sheet metal fuse element of uniform thickness throughout its length is utilized. This thickness is such as to provide relatively large masses of metal functioning to absorb heat generated in the sections of -the element of reduced cross-sectional area.

Since a fuse blows by virtue of the heat generated in its section or sections of reduced crosssectional area, or by heat imparted thereto from other sections. the object in designing a fuse element is not to prevent the generation of heat, but rather to generate heat in a controlled manner. In the present instance two types oi' sections of reduced cross-sectional area are employed, one of which may serve under certain operating conditions merely as a heat-generating element, while the other type serves as the point which ruptures or fuses. Under other operating conditions. the first-mentioned type oi' section serves as the point or points of rupture.

'In the fuse element illustrated herein. with particular reference to Fig. 4, a pair of fusing sections. indicated by the reference numeral 30, is provided, each fusing section having a length indicated. by the bracket. These fusing sections are preferably located adjacent the respective ends of the element, said 'ends being anchored anemona termed an intermediate section 3|. The intermediate section 3l may .in turn be provided with a plurality of-sec'tions of reduced cross-sectional area. In the present instance such sections are provided by punching in the intermediate sec'- tion a plurality of longitudinally spaced apertures 32. The apertures 3l are preferably rectangular in form, leaving a plurality of sections 33 of reduced cross-sectional area along both edges of the element. 'I'he apertures 32 also leave large masses of metal, indicated b'y the numeral 34, at the ends of the intermediate section, and other relatively large masses, indicated by the numeral 35, between` the apertures.

The masses 34 and 35 serve to absorb heat `generated within the element. Since the sections of reduced area 33 lie along the edges of the element, verylittle of the current flowing from our section 33 to the next will dip down into the central portion of the adjacent mass 35 because of the increased length of travel. The

masses 35 Athus function chiey as heat-absorbing means, while the bulk of the current will pass along the edges of the element.

When the element is lunder a moderate overload, the heat generated in each fusing section 30 is conducted and absorbed by the terminal structure of the fuse and by the mass 34 on the other side. Under the same operating conditions, the heat generated in the sections 33 is absorbed chiefly by the masses 3 5. lPreferablythe masses 34 are made greater than the masses 35 Thus with the larger heat-absorbing ability of the masses 34 and with the very large heat-absorbing ability of the terminal structure, the fusing sections 30 may be made with greater heatgenerating capacity than the sectionsA 33. Even with such difference in the heat-generating ca- I pacity, a greater amount of the heat generated in the fusing sections 30 will be absorbed than the heat from the sections 33.

Under such conditions the sections 30 merely serve as heating sections, the fusing or rupture taking place at one or more of the sections 33.

f' Under larger overloads, the heat-absorbing capacity of .the masses 34 and 35 is completely taken up, and the flow of heat from the fusing sections 30 into the terminal structure is insuiliciently fast to take up the heat generated therein so that the latter, which are of greater heatgenerating capacity than the sections 33, will become heated and will rupture.

Since, as mentioned above, the fuse element is made of relatively thick metal, the masses 34 and 35 have relatively large heat-absorbing capacity. Thus on momentary overloads,` even though such overloads be relatively large, the masses 34 and 35`are suflicient to absorb enough of the heat generated in the fusing sections 30 and thesections 33 to prevent blowing thereof. It is only when the overload is sustained an appreciable time that the heat-absorbing capacity of the masses 34 and 35 is taken up so that the fuse element will then blow at the desired overload to protect the circuit in which it is installed.

In fuse elements heretofore manufactured with such time-lag characteristics, the large masses of metal for absorbing heat were formed by adding metal to the faces of an ordinary sheet metal link. With the construction ofthe fusing sections of such elements, the element could not be made of particularly thick sheet metal for the reason that by so doing the fusing sections would have to be narrowed'to suchan extent that they would lack Athe requisite physical strength to ditions.

structed, the fusing sections were so shaped that 50 assauts y l 3 expands relative to the terminals, the laterally stand handling. Thus thinner sheet metal was used for the element proper and metal was added to the faces thereof to increase the heat-absorbing capacity.,

The present fuse element is designed so that ksuillcient'physical strength-is maintained in the fusing sections and at the same time the element may be made of relatively thick metal. To this end each fusing section Il attains its heat-generating resistance by being made relatively long. 19

With such length it may have a large crosssectional area giving suillcent physical strength with a thick metal.

In a fuse element ofthe present character, it

is desired to have the intermediate section 3l l5 to provide laterally extending portions in each fusing section 30 so as to give the required length to attain electrical resistance andat the same time occupy but little space longitudinally of the element.

In the preferred form, each fusing section is made by punching a pair of notches 40 opposite each other in the respective edges of the element to form a central longitudinally extending portion 36. A rectangular aperture 31 is also punched in the element in parallel relation to the notches 35 tov form laterally extending portions 38 and the logitudinally extending edge portions 39 connecting with that part of the element which is an- 35 chored to .the terminal structure of the fuse. Thus in each fusing section the portions 38 extend laterally and oppositely to each other to give suilicient'length to provide heat-generating resistance and at the same time to occupy but lit- 40 tie space longitudinally of the element. Thev intermediate section of the element thus is of maximum length.

In any fuse of this character, variations in relative length between the element on the one hand and the casing and terminal structure on the other hand occur duetto expansion or contraction under varying heat and moisture con- In many of the fuses heretofore cona relative increase in length of the element caused the element to bow laterally. The bowing would obviously occur in the fusing sections, since these sections are the weakest physically, so that these sections would be moved toward the casing or bridge structure. The heat generated. in the fusing sections would then tend to burn or char the casing or bridge, both of which are usually made of fibre. Moreover, the bowing would produce rather sharp angular bends in the fusing section which tend to cause the metal therein to stretch and, under repeated bending, to crystallize. In the case of a zinc element the metal becomes quite brittle ywhen heated so that any such distortion would have a bad effect upon the 05 sectional area, thereby reducing its electrical rating.

The form of each fusing section 30 is such that any relative expansion of the element occurs in the plane of the element and the distortion occurring is a minimum. Thus, when the element 76 The metal 20 vof the element is punched in such a manner as extending portions ll will bend so that the central portion .36 moves toward the aperture 31. The expansion is therefore in the plane of the element. Moreover the length of the laterally extending portions 38 causes a very slight angular bend relative tothe central portion 35 and the outer portions 30 so that, while the full amount of expansion of the element is provided for, the actual angular distortion is small. This puts but very little stress upon the metal and thereby avoids the dilculties arising under excessive bending and distortion.

A further advantage is obtained by the structure-of each fusing section 30, namely, that, with the laterally extending portions 38, the current is branched to flow laterally in opposite directions. Such splitting of thecurrent with its opposite direction of flow sets up opposing magnetic fields. On blowing of the fusing section 30 if any arcing occurs, the opposed magnetic fields tend to reduce such arcing. to a minimum.

.From the foregcing it will be evident that l have provided a new fuse element having a time lag sumcient to 'withstand momentary overloads and yet function accurately on :sustained overloads to afford the necessary protection to the circuit in which the fuse is placed. The fuse element is made of sheet metal cf a single thickness, and is uniform in such thickness throughout its length and is dimensioned so that it may be used in a casing structure conforming to the requirements oi' the National Board of Fire Underwriters. The necessary mass of metal in the element to obtain such time lag is attained by the use of relatively thick metal and the various parts of the element are so constructed that they have sufficient physical strength to avoid distortion in handling. The heat-generating resistance in the fusing sections is attained by elongating each of such sections with parts thereof extending laterally of the fuse element to reduce the space longitudinally of the fuse required for such sections. The fusing sections are also so constructed that, upon variation in length of the element, bending out of the normal plane of the element is avoided and a minimum distortion of the metal takes place. With the laterally extending portions oi' each fusing section, opposed magneticfields are set up which tend to prevent excessive arcing when the section blows.

I claim as my invention:

l. A fuse element made of sheet metal of unlform thickness comprising a pair of fusing sections connected by an intermediate section, each fusing section being elongated to provide sufficient electrical resistance with a large crosssectional area, and having at least a part extending laterally of the element to provide an intermediate section of maximum length.

2. A fuse element comprising a strip of relatively thick sheet metal having a pair of fusing sections and an intermediate section therebetween, each fusing section being elongated and having at least a part thereof extending laterally ofthe element whereby both sufficient electrical resistance and physical strength are attained with the thick metal and the fusing section is oi minimum dimension longitudinally of the element.

3. A fuse element comprising a flat strip of sheet metal having a pair of fusing sections and an intermediate section therebetween,each fusing section comprising a central portion and a pair of laterally extending portions providing an elongated fusing section of large cross-sectional area for physical strength and occupying a relaand a transversely extending aperture spaced longitudinally from the notches, the notches and the' aperture providing therebetween laterally extending portions of relatively great length to permit a large cross-sectional area' for physical strength.

5. A fuse element comprising a iiat strip of sheet metal of uniform thickness and having -a pair oi' opposed notches cut in the respective edges of the strip and a transversely extending aperture parallel to the transverse dimension of the notches to form a fusing section comprising a central portion branching laterally and then extending longitudinally of the strip whereby the required electrical resistance is attained by the length of fusing section with suflicient physs ical strength.

6. In a fuse comprising a lcasing and a pair of terminals, the combination therewith of a fuse Velement Within the casing connecting the terminals, said element comprising a strip of sheet metal comprising end sections for attachment to said terminals, an intermediate section, and fusing sections connecting said intermediate section with the respective end sections, said fusing sections having laterally extending elongated portions capable of bending in the plane of the strip upon relative expansion or lcontraction betw'een the casing and the strip to prevent the intermediate section from bending out of the plane of the strip.

7. In a fuse comprising a casing, a .pair of terminals, and a pair of laterally spaced bridge members connecting the terminals, the combination therewith of a fuse element connected at its respective ends to said terminals and located between said bridge members, said fuse element comprising a pair of fusing sections Aand an intermediate section therebetween, said fusing sections having laterally extending elongated portions capable of bending in the plane of the element upon relative expansion or contraction between the bridge members and the element to prevent the intermediate section from bendingv toward said bridge members.

8. A fuse element comprising a-strip of sheet metal having a pair of opposed rectangular notches in the respective edges of the strip to provide a central longitudinally extending tongue, and having 'a rectangular aperture in parallel relation to the notches to lprovide portionsV xtending laterally `from the tongue and portions extending longitudinally from the outer' ends of the laterally extending portions.

9. A fuse element comprising a pair of terminal sections, an intermediate section of relalocated at the respective ends of the intermediate section, each fusing section having at least a part which is elongated and extends laterally of the strip.

f DONALD W. FLOTEN'.

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