US2181825A - Electric fuse - Google Patents

Description

Nov. 28, 1939. M. B, woon 2,181,825

ELECTRIC FUSE Filed March 26, 1936 f 33 30 24 22 36 28 12%?. Il

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Patented Nov. 28, 1939 ELECTRIC FUSE Morris B. Wood, Newburyport, Mass., asslgnor to The Chase-Shawmut Company, Newburyport, Mass., a corporation of Massachusetts Application March 26, 1936, Serial No. 70,943

6 Claims.

This invention relates to electric fuses and fuse links having improved delayed operating characteristics upon overload while at the same time having instantaneous operating characteristics upon short-circuit.

The fusible element or fuse link of an electric fuse with which this invention is particularly concerned comprises a strip of fusible metal such as zinc formed with a plurality of necksor sections of reduced cross-sectional area distributed throughout its length and at which necks or sections the fuse is adapted to fuse or blow upon the occurrence of certain current conditions of the circuit in which the link is included. Said necks or readily fusible sections are usually formed by forming opposed notches in the sides of the link or by perforating the link between its side edges. These notches have commonly been quite large, the necks being approximately V5 of an inch long at least for all sizes of fuse links up to 200 amperes, for voltages up to 600 volts.

One of the objects of the present invention is the provision of a fuse link, and especially one designed for operation on alternating current circuits, so constructed as to interrupt the circuit at least at the first zero point of the circuit interrupting arc resulting from the fusing of a fusible section of the link upon overload.

A further object of the invention is the proi vision of an electric fuse, and particularly a fuse link, having a relatively long period of delay of fusing of a fusible section following the occurrence and continuance of a current that will ultimately fuse the section, by an improved construction of the readily` fusible section or neck of the fuse link.

Another object of the invention is the provision of an electric fuse and fuse link having a 4 plurality of readily fusible sections or necks on each side of the transverse middle of the link,

(Cl. 20o-135) Fig. 2 is a plan view of a fuse link embodying the present invention.

Fig. 3 is a view similar to Fig. 2 but illustratinga somewhat modied form of the link.

Fig. 4 is a view similar to Figs. 2 and 3 of a 5 more or less common fuse link over which the present invention constitutes an improvement.

The electric fuse in which the present invention is incorporated is illustrated in Fig. 1 as comprising a tubular enclosing casing I0 of in- 10 sulating material, as bre, having externally screw-threaded metal ferrules I2 on the ends thereof, and end caps I4 removably screwthreaded on the ferrules. The ferrules I2 have end walls i6 which traverse the ends of the 15 enclosing casing and said walls are provided with openings i8 vthrough which the ends of a fusible element or fuse link 2li are passed, the protruding ends of the `link being bent over the outer faces of the wall i6 and compressed thereagainst 20 by the end walls of the end caps. The casing is vented to the atmosphere in any suitable well known manner. The fuse casing thus illustrated is merely representative of the so-called ferrule terminal type of fuse casing. 25

The fuse link 20 embodying the present invention comprises a strip of suitable fusible metal, usually zinc, of uniform thickness and generally uniform width throughout its length and provided on each side of its transverse me- 30 dian line a-a with a pair of opposed confronting notches 22 that are spaced apart to form a narrow end neck 24 that connects the terminal 28 with the body of the fuse link. 'Ihe fuse link between said notches and the middle part 35 thereof is provided with another pair of confronting spaced notches 28 which provide between them an intermediate neck 30. The two sets of notches 22 thus provide between them a middle section 34 that is wider than the necks 40 I0. Each set of notches 22 and 28 provide between them an intermediate section 24 which is wider than the necks 24 and 30. 'I'he middle section 32 is provided with a pair of confronting y notches I8 that traverse the median line a-a 4" and are spaced apart to provide a neck 38.

A common type of fuse link adapted for the same current and voltage ratings as the link 2n of Fig. 2, is illustrated in the link 20a of Fig. 4. 50 In this link the terminal parts 26a correspond to the terminals 28, the notches 22a and 28a correspond generally to the notches 22 and 28, respectively, and provide necks 24a and 30a, a middle section 32a and intermediate sections 34a. 55

The middle section 34a in this instance is not provided with a notch corresponding to the notch 38. With the link 20a the link usually fuses on an overload that is slightly in excess of its rated current either in the middle of the middle section 32a or at a notch 30a. With the link 20, however, the notches 36 are provided n the median line of the link to provide a neck 38 properly dimensioned to eect the fusing of the neck when traversed by a slight sufficiently continued overload. The provision of a neck 38 or its equivalent to calibrate the fusing zone in the broader middle section a-a is old in the art except with respect to the construction of the neck 38 and its relation with the other necks 24 and 30 as will presently appear.

It will be noted that the notches 22a and 28a in the link 20a are relatively wide so that the necks 24a and 30a are correspondingly long. It is common practice to make the necks 34a at the apeces of the notches 1A; of an inch long for fuse links at least up to 200 amperes current rating and 600 volts voltage rating. In contrast with the wide notches of the fuse link 20a of Fig. 4, the notches 22 and 28 of the fuse link 20 of the present invention are narrow and preferably are not much more than .04 of an inch wide and were it not because of manufacturing and handling difficulties would preferably be narrower at least for the lower voltages. Thus the necks 24 and 30 are correspondingly short. It is also to be noted that the notches have generally parallel side walls instead of angularly related walls as in Fig. 4 and the side walls have substantially equal confronting length, particularly in the link of Fig. 3. This parallel side wall construction has been found to be advantageous to increase the delay of response of the link and to improve circuit interruption although not to such an extent as the narrow notches and correspondingly narrow necks.

One of the advantages of the narrow notches and short necks is in securing the interruption of an alternating current circuit at the first zero point of the current wave following the fusing of the neck and the production of the circuit interrupting arc. I'he neck is so short that the cooling effect of the generally parallel confronting regions of the link on opposite sides of the slot is effective throughout the length of the slot in deionizing the ionized medium therebetween at the time the arc current is at the zero point of its wave, the deionizing effect taking place more rapidly than the increase of potential following the current zero so that the arc cannot be reinitiated. The width of the slot for this purpose is preferably not over about .06 of an inch and preferably is less than this.

I have found also that a short slot and consequently short neck as above described also increases to a very considerable degree and in a highly desirable manner the delay in responsiveness of the link to fusing when traversed by a load current of predetermined value. This increase in the time of response is obtained merely by shortening the length of the necks, together with proper adjustment of their cross-sectional area and without any necessary increase of the width or thickness of the link. The increase in the time of responsiveness in some links is approximately 50% over the link of Fig. 4.

I have found that by shortening the length of the necks I can markedly increase the time of responsiveness and at the same time reduce the width' and consequently cross section of the neck.

This is highly advantageous since with the improved neck, less material is present to be fused and vaporized by the circuit interrupting arc and less gas and less gas pressure is generated within the enclosing casing. There is no lower limit to the width of the slot except that the confronting side walls of the neck must not touch and electrically shunt the neck. With a very narrow slot, however, the Width of the neck becomes so small, particularly with the end necks 24, that the fuse link is mechanically weak and can be relatively easily bent and ruptured particularly with links for the lower current ratings. The narrow slots and hence the narrow necks permit a relation between the necks 24 and 28 that cannot be so well obtained, if attained at all, with a fuse link such as is illustrated in Fig. 4 without the introduction of an objectionable amount of metal in the necks that must be vaporized when the necks fuse.

In accordance with the present invention the neck 38, only, is designed to fuse when the link is traversed by an overload current that will ultimately cause the fusing of the link and does not exceed in value 50% more than the normal current rating of the link, this fusing to occur within two and one-half minutes following the application and continuance of the overload current. The two necks 30, only, are intended to fuse at a higher overload. The two necks 24 and the two necks 30 are intended to fuse simultaneously, the

y neck 38 remaining intact, when the fuse is traversed by a short-circuit current, thereby establishing four circuit interrupting arcs in series between the fuse terminals. It is to be noted that the neck 30 is wider and thus has more crosssectional area than the neck 24 and is farther from an end terminal than the neck 24, since the end terminal is the main source of dissipation of heat from the fuse link, the` heat traveling by conduction endwise of the link to the terminals.

I claim:

l. An electric fuse comprising a conducting link having a fusing section of reduced crosssectional area, said link having generally parallel confronting faces at the ends of said fusing section, and said fusing section and the space between said faces being sufliciently short to enable the link parts on opposite sides of the fused fusing section to deionize the arc space therebetween formed by the fusing of said vfusing section, said space being not materially greater than four hundredths of an inch.

2. An electric fuse comprising a fuse link having an opening therethrough which provides a readily fusible neck connecting the link on opposite sides of the opening, said neck having a length not materially exceeding four hundredths of an inch.

3. An electric fuse comprising a fusible link having a plurality of narrow openings therethrough which provide a plurality of fusible necks of reduced cross-sectional area distributed along the length of the link, said necks on opposite sides of the transverse middle of the link having lengths not materially exceeding four hundredths of an inch.

4. A delayed response electric fuse comprising a fusible link having an intermediate fusing section adapted to rupture upon moderate excessive current and an end fusing section adapted to rupture upon an excessive current, said end fusing section being sufficiently short in length to provide said intermediate fusing sectiony with a long time period of response, the length of said end fusing section being not materially greater than tour hundredths o1' an inch.

5. An electric fuse comprising an enclosing casing, terminals on the casing, and a fusible link in said casing and having end terminals connected with said casing terminals, said link having a pair of fusible necks of reduced crosssectional area located between the terminals and the middle part of the link, said necks having lengths at least not greatly exceeding four hundredths of an inch.

6. An electric fuse comprising an enclomng casing, terminals on the casing, and a fusible link in said casing and having end terminals connected with said casing terminals, said link having an opening therethrough which provides a readily fusible neck connecting the link on opposite sides oi' the opening, said necks having a length not materially exceeding four hundredths of an inch.

MORRIS B. WOOD.

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