US2496704A - Circuit interrupter - Google Patents

Description

Feb. 7, 1950 H. H. FAHNOE CIRCUIT INTERRUPTER Filed March 15, 1945 w M w ml4\. m Y Trm. M 4 m ...R 8 vu. www m T 6 M MA 4 .4 O v 3 .Hwmo 4 am. 1m 6 64 0 7 a H3.

.tateni'ed Feb. 7, 1950 CIRCUIT INTERRUPTER Harold H. Fahnoe, Wilkinsburg, Pa., assignor to Westinghouse` Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 15, 1945, Serial No. 582,867

13 Claims. (Cl. 20o- 120) This invention relates. in general to electric circuit interrupting devices," and more particularly to high-voltage fuses of the type especially adapted to limit the current and voltage at least upon interruption of heavy overloads and short circuits.

In the copending application of H. L. Rawlins and myself on Circuit interrupters, Serial No.

514,502, flied December 16, 1943, and assigned to the same assignee of this invention, there is disclosed a 3element current limiting fuse structure which includes, (1) current limiting fuse elements, (2) a. shunt resistance and i' 3) a series clean-up fuseelement. At the time the invention of this copending application was made, it was considered that the three elements enumera ated were all essential. Thus, the lpurpose of the shunt resistance was to limit the voltage surge on blowing of the current limiting fuse elements, and, of course, a series fuse element was necessary to interrupt the current flowing in the shunt resistance.

Subsequent experimental and development work on the current limiting fuse structure of the above-mentioned copending application, divorced from the shunt resistance and series fuse element, shows that this structure does not produce harmful surges during interruption, and that it is of itself capable of efficiently interrupting current flow, at least for certain applications, such as motor starting and the like, where the fuse is required to afford protection only against relatively high overlowds and short circuits. The

reason for this unexpected performance is not known with certainty, but it is apparently due to the particular structure associated with the fuse elements which subjects the arc formed on melting of the fuse element or elements, to a plurality of different influences, as will be hereinafter pointed out.

One object of this invention, therefore, is the provision of a novel current limiting fuse structure wherein there is in eil'ect but a single conducting path through the fuse structure, comprising a single fusible means which may consist of one or more parallel fuse elements.

Another object of this invention is to provide a simplified current limiting fuse structure which ls relatively small in size, yet is emcient in operation.

In certain types of circuits where relatively light prolonged overloads may be encountered, it may still be desirable to employ a series fuse element calibrated primarily to melt on such overloads, in order to provide adequate protection 2 against this type of-overload where current limitation is not necessarily a factor.

Accordingly, still another object of this invention is to provide a novel current limiting fuse structure having a, single conducting path comprising serially arranged fusible means having different fusing characteristics, with one fusible means adapted to fuse only on relatively high currents being particularly constructed to limit the current interrupted.

These and other objects of this invention will become more apparent upon consideration of the following description of preferred embodiments thereof, when taken in connection with the attached drawing, in which:

` Figure l is a substantially central longitudinal section through a fuse constructed in accordance with the invention, with certain of the parts shown in elevation; i

Fig. 2 is an enlarged, partial, transverse crosssectional view of the fuse, shown in Fig. 1, taken substantially on the line II-lI thereof Fig. 3 is an enlarged, partial, longitudinal, sectional view taken through one of the slots of the fuse supporting member shown in elevation in Fig. l; and l Fig. 4 is a substantially central longitudinal sectionthrough a modified form of fuse constructed in accordance with this invention, with certain of the parts thereof shown in elevation.

The particular embodiment of the invention shown in Fig. 1 is illustrated as being mounted within a tubular fuse holder 2 of insulating material, such as fiber ora synthetic resin. The

tubular fuse holder 2 is provided with terminal caps 4 and 6 adapted to be secured over thevopposite ends of the tubular holder. Terminal caps 4 and 6 may be of any desired conducting material` such for example as copper or the like.

and they are preferably secured in position by having indentations 8 rolled therein and into the ymaterial of fuse holder2.

the current limiting unit disclosed in the abovementioned copending application of H. L. Rawlins and myself. This unit comprises a rod 20 of yan insulating material which is capable of evolving an arc-extinguishing gas when in proximity to an electric arc, such for example as ber or a synthetic resin. The rod 2li has a plurality of longitudinally extending slots 22 cut in the periphery y thereof, for the reception of one or more parallel fuse wires 2l. Thefuse wires are adapted to be electrically connected at their ends with terminal discs 26 and 28, respectively, in any desired manner, such as by soldering or the like. Rod 20 has a plurality of spaced barrier discs 34 provided thereon, for a purpose to be described, and the entire unit i2 is supported within holder 2 in spaced relation to tube 2 and the end terminals 4 and 6.

Opposite ends of rod 20 are provided with bores for receiving a reduced extension 21 on a terminal rod 3!! at one end, and for receiving a positloning rod 29 at the other end. Terminal rod 30 has a reduced extension lil adapted to extend through an aperture provided in a plug I6 and in terminal cap 4, to be secured to the' terminal cap in any desired manner, such for example as by solder Ii. pose of confining within tubular holder 2 a mass of finely divided inert insulating material 38 such for example as sand, or the like. Both plug I6 yis secured to a plug 42 and terminal cap B by means of a shouldered plug 32 extending through central apertures in plug 42 and end cap 6, and secured in position by a nut 40 threaded on the outer end thereof. Plugs i8 and 42 are adapted to be threadedly engaged in opposite ends of fuse tube 2.

As previously stated, the unit structure I 2 thus far described, is substantially identical with the current limiting structure disclosed in the previously mentioned copending application. As described in the aforesaid copending application, current limitation is accomplished in this structure by reason of the following factors. The

fuse wire 24 is of special construction, and may be of silver or other material having a high temperature coeihcient of resistance, and is initially chosen to be of substantially the same size as that required to carry the rated current without undue heating, and yet melt on currents above the rated current. Such a wire is then provided with a plurality of reduced sections 36 (Fig. 3). It has been found that in such a wire the fusion time is speeded up to correspond to that of a wire of substantially the same diameter as the reduced sections, at least for high values of current. This speeding up eiect has been found to be roughly proportional to the ratio between the area of the large portions of the fuse wire 24 and reduced portions 36. As a practical mata ter, this ratio may be made as high as possible, being limited only by mechanical strength considerations. Such a fuse wire retains all the normal time current characteristic desired in a fuse for time values in excess of approximately two cycles without appreciable reduction in current carrying ability. Reduction in current carrying ability varies roughly directly in proportion to the length of sections 36, so that it is desirable that these sections be made as short as possible. For short times of less than one-fourth cycle, fuse wire 24 has an exceptionally fast melting time.

Thus, with a fuse wire having a diameter of .038 inch and reduced portions of .0145 inch it has been found that the time current characteristic on low currents up to about 400 amperes is substantially the same as those of a fuse wire of uniform diameter equal to the large diameter portion of wire 24. However, on currents above Plug I6 is provided for the pur- 4 400 amperes, fuse wire 24 is much faster with its characteristics on these higher currents being substantially coincident with the characteristics of a fuse wire of uniform diameter which is substantially the same as the diameter of reduced sections It. This means that fuse wire 24 has its continuous current carrying ability substantially unimpaired, by provision oi' reduced section Il, and even its time current characteristic on light overloads will be substantially unchanged. However, on heavy overloads, where itisdesiredtolimitthecurrent,iusewire24 operates much faster than it would without the reduced sections It, which means that it is psible to initiate the current limiting actim at a very early point in the first hal! cycle of fault current.

As pointed out in the previously mentioned copending application, actual current limitation is effective by providing a high arc voltage upon fusion of fuse wires 24, which arc voltage b built up substantially instantaneously, to thereby exert a limiting or choking effect on the current and prevent its rise beyond a predetermined value. The particular structure disclosed provides a high arc voltage per unit length of unit i2, which means that the overall length of the fuse is small. One way of providing such a high arc voltage per unit of arc length is to provide a relatively large number of reduced sections It in fuse wire 24 so that when these reduced sections melt, a plurality of serially related arcs. will be formed and the summation of the arc voltages of these series arcs will then be greater than the arc voltage across a single arc. Generally the arc voltage per unit of length is directly proportional to the number of series arcs or restrictions 36 per unit of length. This suggests that maxlmum arc voltage per unit length may be obtained by providing as many restrictions 36 per unit of length as is physically possible. However. as the number of restrictions 36 per unit of length is increased. a point is eventually reached where a further increase results in a decrease in current carrying ability. It has been found that at least for voltages above 600 volts, at least two restrictions 36 per inch of fuse wire 24, or a total of ten should be provided to obtain an efiectiverisein arc voltage withafuse wire which is not of excess length. This corresponds approximately to a spacing of restrictions 3l not to exceed about 15 times the large-diameter oi' the fuse wire. It is desirable, however, to use the maximum number of restrictions possible without substantially impairing the current carrying ability. The most desirable number ot restrictions is dependent on the sine oi' the fuse wire, and appears to correspond roughly to a spacing of about 3% times the largest diameter of the wire. This is an optimum spacing, since spaclngs as low as about twice the largest diameter oi the fuse wire may be employed with good results.

. Another reason why the arc voltage is rapidlybuiltuptoavaluiehighenoughtoexerta current limitingeiIecLlsthe-narrowslobnin may be made no more than .010 inch than the fuse wires Preferably, the somewhat deeper than the diameter of wires,sothatthewireswillcloselycontact slot on three sides, so as to physically thearcformeduponmeltingofthefnse and thereby raise the arc voltage at the instant the arc is formed.

Because fuse wires 24 are in close proximity to the walls of slots 22, it will be apparent that the arc formed upon interruption will cause the evolution of arc-extinguishing gas from the walls through the arc to perform three functions, all oi' which act to increase the arc voltage and to extinguish the arc. First, the blast of arc-extinguishing gas acts to sweep metal vapor out oi the arc stream andvout of slots 22 into the material 38 in which the metal vapor becomes dispersed and condenses into separated particles insulated from each other so that a high resistance path is maintained outside the slots. Secondly, the blast of arc-extinguishing gas also acts to supply unionized gas to the arc path to further increase the resistance of the arc path and to extinguish the arc at current zero. A third function of the transverse gas blast is to cause the series arcs to be looped outwardly toward filling material 38, thereby lengthening the arc path and, consequently, increasing the resistance thereof and the voltage drop across it. One function of washers 34 on supporting rod will now be apparent, as preventing escape of the arcs from slot 22 and, consequently. from proximity with the gas evolving material and from the restricting action from the narrow slot, .and thus preventing the series arcs from restriking as a single arc outside the slots.

From the foregoing, it is apparent that efficient current limitation of the iirst half cycle of the arcing current may be obtained with the structures disclosed, because (l) the fusible wires themselves are capable of melting to establish an arc at least on such high currents which it is desired to limit, in a very short time, that is, in a very small fraction oi' a half cycle, (2) as soon as the arc is established it is subjected to all of the factors enumerated above to create an arc voltage high enough to prevent any i'urther rise of fault current.

It was previously thought that the arc voltage produced by unit I 2 would be so high as to create undesirable surges in the circuit. and consequently a shunt resistance was provided for n limiting the voltage, and a series fuse was then necessary to iinally interrupt the circuit. However, experiments on unit I2 unexpectedly demonstrate that the voltage may be controlled so as not toexceed a dangerously high value, prefof the slot, and this gas will blast laterally erably not to exceed 11/2 times restored circuit voltage. I2 is itself capable of interrupting the circuit at the iirst current zero so that no other means 'is necessary other than unit I2, in order to obtain current and voltage limitation, and emcient arc extinction. The reasons for these unexpected results are not definitely known, but may be due to several factors. One discovery which has been made is that the arc voltage. and the ef'- ficiency and speed of current limitation and interruption appear to be in some measure dependent upon the number and thickness of washers 34. It appears that an increase in the number of washers 34 increases the arc voltage while increasing current limitation and interruption speed. Moreover, the -washers must be thick enough as not to be eroded by the action oi an arc. One explanation for this may be arrived at by considering that fuses are known in the art rlhese experiments also show that unit which comprise one or more silver wires embedded in sand and that these fuses obtain current limitation by the formation of a semi-conductive core about the wire due to the action ofthe heat of the arc on the sand. However,

such 'fuses must have a much longer arc path 'short in length. The provision of barrier .plates 34, which themselves are of a material capable oi' evolving an arc-extinguishing vgas when subjected to the heat of an arc, thus appear to provide spaced deionized zones along the length of unit I2, which would explain why an increase in number of barriers 34 `would provide an ihcreasein arc voltage, and also explains why a relatively high arc voltage per unit length is obtained with unit I2. Moreover, this action of barrier plates 34 undoubtedly contributes to rapid extinction of the arc.

While there may be other reasons for the unexpected results obtained, tests show a structure such as unit I2, of and by itself, is capable of limiting high overload currents in the first part of the first half cycle thereof, without producing voltage surges substantially greater than 11/2 times restored circuit voltage, and it is effective to interrupt the circuit at the first or second current zero.

In order to provide an indication of the operation of unit I2r to interrupt the circuit, an indicator disc 68 is provided secured to a connecting rcd 58 extending through shouldered plug 32, to be normally held adjacent end terminal cap 6 by a high resistance fuse wire 48 which is connected to a supporting disc 46" by means of a small tension spring 52. A coil compression spring 'I0 is provided between indicator disc 68 and cap 8 to bias the disc outwardly away from 4the end of the fuse. Supporting disc 46 for spring 52 is adapted to be supported Within fuse tube 2 by a4 small inner tube 44 of insulating material, such' as liber or the like, which is threadedly mounted on the inner end of shouldered plug 32. Innertube 44 not only acts to support disc 46. but also prevents insulating material 38 from interfering with movement of rod 50. The inner end of fuse wire 48 is connected to the terminal rod 30 by means of a high resistance fusible wire 58. High resistance fuse wires 48 and 56 are preferably of -such a high resistance as compared with fuse wire 24 that they normally do not carry any appreciable amount of current. However, as soon as wires 24 are melted, the arc voltage developed in unit I2 forces suicient current through wires 56 and 48 to fuse these Wires at substantially the same time, tov release indicator disc 68 and `permit it to be moved'by spring 'HI outwardly to a clearly visible indicating position.

Fusible wires 48 and 56 are of such high resistance relative to wires 24, on the order of several hundred times the resistance of wires 24, that they do not ailect the operation of unit I2 in any appreciable way either before or during a circuit interrupting operation. l

The fuse illustrated' in Fig. 1 is particularly designed for applications where the only overload currents encountered are thoseof high enough value that current limitation is desirable. In Fig. 4 there is illustrated a fuse which is adaptable for use where overloads of low value, as Well acca-:o4

as those of high val'ue are encountered. This fine has certain parts identical with certain of the parts of the fuse illustrated in Pigs. 1 to 3, and like reference numerals are used to identify such parts. These include a current and voltage limiting and arc-extinguishing unit I2 which is identical in structure with thatv described above in connection with Figs. 1 to 3, and it is supported in fuse tube 2 with terminal plate 26 adapted to abut against end plug I6, with a relatively short positioning member B1 adapted to extend through central apertures in plug I6 and end cap l, and having a reduced extension 69 adapted to enter the bore at the adjacent end of supporting rod 2l, to properly position unit I2 substantially centrally in fuse tube 2. At the Other end f unit I2, a terminal extension 82 is provided with a reduced portion 88 adapted to be received in the end opening provided in supporting member 2l, and terminal 82 is adapted to be secured to a strip 60 of fusible material, preferably by means of a low melting point solder 62, or the like.

The unit I2 disclosed in Fig. 4 is supported at a considerable distance from end terminal cap i, with this space being occupied by fusible element 60 and parts associated therewith. A winding of metallic screening 18 or the like is provided between end cap 6 and a supporting disc III, and the disc 80 is adapted to support a tube 54 of insulating material, in the opposite end oi' which is threaded a supporting plug 64 adapted to abut against terminal disc 28 of unit l2. Mounted within tube 54 are one or more annular blocks 58 of an insulating material which is capable of evolving an arc-extinguishing gas when in proximity with an electric arc, such for example as boric acid, fiber or a synthetic resin. The openings in blocks 58 provide a bore for receiving fusible strip 60, with the outer end of the strip being biased toward end terminal cap l by means of a coil tension spring I6, which reacts between fusible strip 80 and indicator disc 88 on the outside of terminal cap 5. A hollow plug I2 is mounted in a central aperture in end cap l instead of the shouldered plug l2 shown in Fig.

l, and fusible strip lll is electrically connected with plug 'I2 by means of flexible shunts 18 to relieve spring 66 from the current path. Plug I2 is held against inward movement by a nut N threaded on the outer end thereof.

In the operation of the fuse shown in Fig. 4, the fusible material 62 will determine the minvimum melting current and long time melting characteristic of the fuse, because melting of this material is caused by heatconducted thereto from element 60. The arrangement is such that fusible material 62 will melt on lower overload currents than fuse wires 2l, and will melt on such currents with an inverse time current characteristic. On such light overloads which are not of a magnitude sufficient to melt fuse wires 24, fusible material B2 will eventually melt to permit spring 66 to move fusible strip 6l outwardly through the bore in blocks 58 to elongate the arc and cause the liberation of an arc-extinguishing gas from material 62 to extinguish the arc. It is preferred that the gas evolved by blocks 58 be condensable so that upon discharge outwardly it will contact metallic screen 18, which is a good heat conducting material, to be condensed, and thereby avoid building up of excessive pressure within fuse holder 2. When the fuse illustrated in Fig. 4 is subjected to overload currents high enough to melt fuse wires 2l, fusible material 62 or fuse strip 68 will melt substantlally at the same time, with spring Il operating to extend the arc. and form an isolating gap in the circuit as before. At the same time. melting of the fuse wires 24 exerts a voltage and current limit effect, as well as an arcextlnguishing effect in the same manner as described in connection with unit I 2 in Figs. l to 3, sothat the arc-extinguishing action of unit I2 is assisted by the arc-extinguishing action of the bore through blocks 58, and afterthe arc has been extinguished, an isolating gap is provided between the unfused terminals of strip Il to prevent restriking of the arc when circultvoltage is restored.

The fuse constructions described above provide a simplified form of fuse which may be made relatively small in size because it includes in a single unit both current and voltage hunting means, together with arc-extinguishing means. In the fuse structure disclosed herein, there is but a single effective circuit through the fuse, and this circuit passes through unit I2 so that this unit constitutes the only current and voltage limiting means for the fuse. In addition, in the embodiment shown in Fig. 1, unit I2 also comprises the sole arc-extinguishing means. Since fuse wires 24 of unit I2 are preferably of silver, which may be heated to a relatively high temperature before melting and thus cause deterioration of the material of supporting rod 28, on relatively low and continuing overload currents, it is desirable that a series fusible element be provided as in Fig. 4, where such relatively light continuing overloads are to be encountered. This series fuse also assists in the arc-extinguishing operation when the fuse blows on high currents which are to be limited in unit I2, in the manner described above.

Having described preferred embodiments of the invention in accordance with the patent statutcs, it is desired that the invention be not limf ited to these particular structures, inasmuch as it will be obvious to persons skilled in the art that many modiilcations and changes may be made in these particular structures without departing from the broad spirit and scope of this invention. Accordingly, it is desired that the invention be interpreted as broadly as possible and that it be limited only as required by the prior art.

I claim as my invention:

1. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having in proximity to one side thereof for at least a substantial portion of the length thereof means of an insulating material which is capable of evolving an arc extinguishing gas when in proximlty to an electric arc, a ilnely divided inert insulating material in proximity to the side of each conductor opposite said gas evolving material, so that the arc formed upon fusion of each conductor is subjected to a transverse gas blast from sain gas evolving material through the arc and into said inert material, said conductors being adapted to fuse in response to currents above a predetermined value a'nd said conductors and material comprising the sole current and voltage limiting and arc extinguishing means of said fuse for currents above said predetermined value.

2. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and Il voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having in proximity to one side thereof for atleast a substantial portion of the length thereof solid means of an insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc,

a finely divided inert insulating material in proximity to the side of each conductor opposite said gas evolving material. so that the arc formed upon fusion of each conductor is subjected to a transverse gas blast from said gas evolving material through the arc and into said inert material, said conductors being adapted to fuse in response to currents about a predetermined value and said conductors and material comprising the sole current and voltage limiting and arc extinguishing means of said .fuse for currents above said predetermined value.

3. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, solid means of an insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc forming a narrow passage for each conductor which is open at one side, a finely divided inert insulating material in proximity to the open side of each passage, so that the arc formed upon fusion of each conductor is subjected to a transverse gas blast from said gas evolving material through the arc and into said inert material, said conductors being adapted to fuse in response to currents above a predetermined value, and said conductors and material comprising the sole current and voltage limiting,r and arc extinguishing means of said fuse for currents above said predetermined value.

4. In a fuse, a fuse holder of insulating matei0 and said conductors and material comprising the sole current and .voltage limiting and arc extinguishing means of said fuse for currents above said predetermined value.

6. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having in proximity to one side thereof for at least a substantial portion of c the length thereof means of an insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc, a nely divided inert insulating material in proximity to the side of each conductor opposite said gas evolving material, so that the arc formed upon fusion of each conductor is subjected to a transverse gas blast from said gas evolving material through the arc and int-o said inert material, a plurality of longitudinally spaced, transversely extending barriers of a solid insulating material positioned at least in said inert insulating material to prevent the formation by the heat of an 'are of any continuous conductor or semi-conductor in said inert material. said conductors being adapted to fuse in response to currents above -a predetermined value, and said conductors .and material comprising the sole current and rial having spaced terminals thereon, current andvoltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusiblefconductors, a finely divided inert insulating material positioned in proximity to -a substantial length of at least a portion of each conductor, a plurality of longitudinally spaced, transversely extending barriers of a solid insulating material positioned at least in said inert insulating material to prevent the formation by the heat of an arc of any continuous conductor or semi-conductor in said inert material, said conductors being adapted to fuse in response to currents above a predetermined v-alue, and said conductors and material comprising theA sole current and voltage limiting and arc extinguishing means of said fuse for currents above said predetermined value.

5. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, a finely divided inert insulating material positioned in proximity to a substantial length of -at least a portion of each conductor, a plurality of longitudinally spaced, transversely extending barriers of a solid insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc, said barriers positioned at least in said inert insulating material to prevent the formation by the heat of an arc of any continuous conductor or semi-conductor in said inert material, said conductors being adapted to fuse in response tocurrents above a predetermined value,

voltage limiting -and arc extinguishing means of said fuse for currents above said predetermined value.

7. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, solid means of an insulating material which is capable oi' evolving an arc extinguishing gas when in proximity to an electric arc forming a narrow passage for each conductor which is open at one side, a finely divided inert insulating material in proximity to the open side of each passage, so that the arc formed upon fusion of each conductor is :subjected to a transverse g-as blast from said gas are extinguishing means of said fuse for currents ,v

above said predetermined value.

8. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals adapted to fuse in response to currents above a predetermined value to limit such currents and interrupt the circuit, said limiting fusible means comprising the sole current and voltage limiting means of said fuse, additional fusible means serially connected with said limiting fusible means between said terminals, and said additional fusible means being adapted to fuse in response to overload currents below said predetermined value with an inverse time-current characteristic.

9. In a fuse, a fuse holder oi' insulating material having spaced terminals thereon, current .and voltage limiting fusible circuit interrupting means connected between said terminals adapted to fuse in response to currents above a predetermined value to limit such currents and interrupt the circuit, said limiting fusible means comprising the sole current and voltage limiting means of said fuse, additional fusible means serially connected with said limiting fusible means between said terminals, partition means on said holder for physically separating said fusible means, said additional fusible means being adapted to fuse in response to overload currents below said predetermined value with an inverse time-current characteristic, means for elongating the arc formed when said additional fusible means fuses, and means of insulating material forming` an arc passage for said arc having inner wall portions of a material capable of evolving an arc extinguishing gas when in proximity to an electric arc.

10. In a fuse. a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals adapted to fuse in response to current above a predetermined value to limit such currents and interrupt the circuit, said limiting fusible means comprising the sole current and voltage limiting means of on relatively heavy overloads and to provide a corresponding number of spaced series arcs upon fusion of said reduced area sections, each of said fusible conductors having in proximity to one side thereof for at least a substantial portion of the length thereof means of an insulating material which is capable of evolving an arc extinguishing gas when in proximity to an electric arc, a nely divided inert insulating material in proximity to the side of each conductor opposite said gas evolving material, so that the arc formed upon fusion of each conductor is subjected'to a transverse gas blast from said gas evolving material through the arc and into said inert material, and said conductors and material comprising the sole current and voltage limiting means of said fuse.

13. In a fuse. a fuse holder of insulating material having spaced terminals thereon, current limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having a cross-section area of a size capable of carrying the rated current said fuse, additional fusible means serially connected with said limiting fusible means between said terminals, said additional fusible means being adapted to fuse in response to overload currents belw said predetermined value with an inverse time-current characteristic, and adapted to fuse substantially at the same time as said limiting fusible means in response to currents above said predetermined value, and means for separating the terminals of said additional fusible means when it fuses to extinguish any arc drawn and provide a safe gap in the circuit after interruption has occurred.

11. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuitinterrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having a cross-section area of a size capable of carrying the rated current of said fuse but which will fuse on currents above said rated current, and each conductor having a plurality of spaced sections reduced in area to render said fuse faster acting at least on relatively heavy overloads and to provide a correspondini;r number of spaced series arcs upon fusion of said reduced area sections, and means positioned to act on said arcs to limit the arc current and voltage, there being no other current or voltage limiting means.

12. In a fuse, a fuse holder of insulating material having spaced terminals thereon, current and voltage limiting fusible circuit interrupting means connected between said terminals, comprising one or more parallel fusible conductors, each of said fusible conductors having a. cross-section area of a size capable of carrying the rated current of said fuse but which will fuse on currents above said rated current, and each conductor having a plurality of spaced sections reduced in area to render said fuse faster acting at least of said fuse but which will fuse on currents above said rated currents, and each conductor having a plurality of spaced sections reduced in area to render said fuse faster acting at least on relatively heavy overloads and to provide a corresponding number of spaced series arcs upon fusion of said reduced area sections, a finely divided inert insulating material positioned in proximity to a substantial length of at least a portion of each conductor, a plurality of longitudinally spaced, transversely extending barriers of a solid insulating material positioned at least in said inert insulating material to prevent the formation by the heat ot an arc of any continuous conductor or semiconductor in said inert material, and said conductors and material comprising the sole current and voltage limiting means of said fuse.

HAROLD H.- FAHNOE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date i 1,540,119 Glowacki June 2, 1925 1,545,550 Coates July 14, 1925 1,849,875 Kees Nov. 15, 1932 1,927,905 Stepian Sept. 26, 1933 1,996,901 McMahon July 17. 1934 2,013,427 Starr Sept. 3, 1935 2,157,906 Lohausen May 9, 1939 2,230,207 Thommen et al Jan. 28, 1941 2,309,013 Rawlins et al. Jan. 19,' 1943 2,337,495 Rawlins Dec. 2l, 1943 2,343,224 Powell Feb. 29, 1944 2.354.134 Ludwig et al July 18, 1944 2,414,344 Suits Jan. 14, 1947 FOREIGN PATENTS Number Country Date 20,409 Great Britain 1911 450,970 Great Britain July 27, 1936

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