US3123693A

US3123693A - Time-lag fuses of the blade contact type

Info

Publication number: US3123693A
Authority: US
Publication date: 1964-03-03
Anticipated expiration: 1981-03-03

Description

March 3, 1964 F. J. KozAckA 3,123,693

TIME-LAG FUSES OF THE BLADE CONTACT TYPE Filed June 12, 1961 4 sheets-sheet 1 March 3, 1964 Filed June 12, 1961 F. J. KozAcKA 3,123,693

TIME-LAG FUsEs oF THE BLADE CONTACT TYPE 4 Sheets-Sheet 2 March 3, 1964 F. J. KozAcKA 3,123,693

T1ME-LAG FusEs oF THE BLADE coNTAcT TYPE Filed June l2, 1961 4 Sheets-Sheet 5 HI 'r i (sec) l l F g 7 ll l A INV EN TOR.

Frede rick J. Kozock o www@ TTY,

March 3, 1964 F. J. KozAcKA 3,123,693

TIME-LAG EusEs oF THE BLADE CONTACT TYPE Filed June l2, 1961 4 Sheets-Sheet 4 C 23|.sc

l, l, time (sec) q .^f. 23|.ac

t t t|me(sec) (sec) 2 3 current (amps) lo l I l I l 500 600 700 800900 |000v |500 2000 INVENTOR.

Frederick J. Kozacka United States Patent O 3,123,693 TIME-LAG FUSES F THE BLADE CGNTACT TYPE Frederick J. Kozacka, Southampton, Mass., assigner to The Chase-Shawmut Company, Newburyport, Mass. Filed June 12, 1961, Ser. No. 116,365 17 Claims. (Cl. 200--120) This invention relates to time-lag fuses. It is one object of this invention to provide improved time-lag fuses.

Another object of this invention is to provide time-lag fuses having a high interrupting capacity and longer delay times than prior art fuses having an interrupting capacity of the same order.

This invention is more particularly concerned with timelag fuses having a very substantial interrupting capacity and a very substantial time-lag, the latter being suihciently high to make the fuses applicable in motor circuits involving high starting currents and in like situations involving high inrush transients which must be carried by the fuse without blowing.

It is another object of this invention to improve the fuse structures disclosed in my patent application Ser. No. 764,293, tiled September 30, 1958 for Time-Lag Fuses, now United States Patent 2,988,620, issued June 13, 1961, to better adapt such structures for use in motor circuits, and like circuits, calling for fuses capable of carrying particularly high inrush currents, such as motor starting currents, without blowing.

Still another object of this invention is to improve the fuse structures disclosed in my copending patent application Ser. No. 106,994, filed March 21, 1961, for Blade- Type Electric Fuses by imparting to these structures increased time-lags, or increased delay times, while taking full advantage of the high interrupting capacity characteristics of these fuse structures.

Other objects of the invention` and advantages thereof will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the invention reference may be had to the following description thereof taken in connection with the accompanying drawings, in which:

FIG. l is mainly a longitudinal section of an electric fuse embodying this invention showing some parts thereof in side elevation rather than in section;

FIG. 2 is a section along 2*2 of FIG. l;

FIG. 3 is a section along 3 3 of FIG. l;

FlG. 4 is mainly a longitudinal section of another electric fuse embodying this invention showing some parts thereof in side elevation rather than in section;

FIG. 5 is a section along 5 5 of FlG. 4;

FIG. 6 is a section along 6-6 of FIG. 4;

FIG. 7 shows time-current curves explaining the theory underlying the present invention;

FIGS. 8er-8d are diagrammatic representations of fuses embodying this invention further explaining the design principles underlying the same;

FIGS. 9a and 9b are temperature-time curves illustrating the mode of operation of fuses embodying the present invention; and

FIG. 10 shows time-current curves.

Referring now to the drawings, and more particularly to FIGS. 1-3 thereof, the fuse structure shown therein comprises a tubular casing 1 of insulating material closed on both ends thereof by terminal elements in form of caps 2 mounted on'the outside of casing 1. Each cap 2 is provided with a rectangular cut-out 3 and washers 4 having rectangular cut-outs in registry with the rectangular cut-outs of caps 2 are interposed between caps 2 and the axially outer ends or rims of casing 1. A pair of blade contacts 5 having a predetermined width project from the outside of casing 1 transversely through cut-outs 3 in caps 2 and the registering cut-outs in washers 4 into the inside ICC of casing 1. Thus the pair of blade contacts 5 extends in a direction longitudinally of casing 1. The width of blade contacts 5 is but slightly less than the inner diameter of casing 1. Pins 6 project transversely through casing 1 into bores 7 in blade-contacts 5, thus maintaining blade contacts 5 properly aligned with the casing or fuse tube 1. A pair of multiperforated fuse links L1, L2 in ribbon form having substantially the same width as blade contacts 5, or having a width which is but slightly less than the width of blade contacts 5, interconnect conductively the axially inner ends of blade contacts 5. Each fuse link L1, L2 is made of a metal having a relatively high conductivity and a relatively high fusing point, preferably copper or silver, and each fuse link L1, L2 is provided with an overlay 8 of a low-fusing-point link-severing metal extending transversely across the entire width of the respective fuse link. Overlays 8 are preferably made of tin. The method of making such overlays and the exact configuration thereof has been described in considerable detail in my aforementioned U.S. Patent 2,988,620, and reference may be had to that patent for information on such details. Each fuse link L1, L2 has live transverse lines 9 of circular perforations and overlays 8 extend. parallel to these lines 9 of perforations. The perforations of each line 9 define points of reduced cross-section, or necks, and overlays 8 jut into the gaps formed between contiguous perforations but do not extend, or do not extend significantly, beyond the points of reduced cross-section, or necks, defined by contiguous perforations. The fuse structure shown in FIGS. 1 3 further comprises an additional pair of multiperforated fuse link means L3, LA, in ribbon-form conductively interconnecting blade contacts 5, i.e. shunting fuse link means L1, L2. Fuse link means L2, L4 are arranged between the radially outer surfaces of fuse link means L1, L2 and the inner surface of casing 1. Fuse link means L1, L2, L3, L., are substantially identical. All of four fuse link means L1, L2, L3, L4 are perforated in the same fashion, and all of the aforementioned four fuse link means are provided with the same low-fusing-point link-severing metal overlays 8 extending transversely across the entire width of the respective fuse link means. All four fuse link means L1, L2, L3, L., are held in position, and conductively connected to blade contact 5, by means of clamping plates 10 and rivets 10a projecting across fuse links L1, L2, L3, L4, clamping plates 10 and blade contacts 5. Fuse link means L1, L2, L3, L4 are submersed in a pulverulent arc-quenching ller 11 having a relatively small thermal conductivity. The thermal conductivity of the filler 11 inside of casing 1 should be considerably less than that of quartz sand and the amount of gas evolved from filler 11 when subjected to the heat of an electric arc should be relatively limited to prevent bursting of casing 1, or to allow casing 1 to be made of an insulating material having a relatively limited bursting strength. Gypsum is a iiller material complying with these two requirements.

The circular perforations in fuse links L1, L2, L3, L4 define five transverse lines 9 of perforations and six lines 12 of perforations extending in a direction longitudinally of casing 1 and longitudinally of fuse links L1, L2, L3, L4. There are seven perforations in each transverse line 9 and five perforations in each longitudinal line 12. The reasons underlying these numerical relations are set forth in considerable detail in my aforementioned patent application Ser. No. 106,994. The lower the circuit voltage and the higher the current rating, the larger the ratio of perforations per transverse line 9 to the number of perforations per longitudinal line 12. The particular numbers and arrangement of perforations shown in FIGS. 1-3, inclusive, have been found to be desirable for fuses having a voltage rating of 600 volts and current ratings of several hundred amps. and casings of standard sizes. Fuses having smaller voltage ratings, eg. 250 volts, require fuse links having a verse center line 9 of perforations.

relatively smaller number of transverse lines 9 of perforations and hence a relatively smaller number of perforations per longitudinal line 12.

As clearly shown in FIG. 2 each of links L1, L2 includes two portions arranged in planes enclosing an obtuse angle and defining an edge at the intersection of said planes located in the center region of casing 1. In a similar fashion each of links L5, L4 includes two portions arranged in planes enclosing an obtuse angle and defining an edge at the intersection of said planes located in the center region of casing 1. The aforementioned edges of links L1, L2, L5, L4 are located in the plane marked 3 3 in FIG. l.

Referring now to FlGS. 46, inclusive, numeral 1 has been applied to indicate a casing of insulating material closed on both ends thereof by terminal caps 2'. Washers 4 of asbestos or like material are interposed between the axially outer ends or rims of casing 1 and caps 2. A pair of blade contacts 5 project from the outside of casing 1 through rectangular openings 3 in terminal caps or terminal elements 2' into the inside of casing 1 in the same fashion as described more in detail in connection with FIGS. 1-3. Pins 6 project transversely through casing 1 and blade contacts 5 maintaining the latter in proper alignment. Pins 6 are preferably formed of a resilient material such as steel which is spirally wound as more fully disclosed in my copending patent application Ser. No. 4,417; filed January 25, 1960, for Fuse Structures, now U.S. Patent 3,007,020, issued October 3l, 1961. A pair of multiperforated fuse links interconnect conductively the axially inner ends of blade contacts 5. The aforementioned fuse links or fusible elements are substantially channel-shaped, each including a web portion L5 having substantially the same width as that of blade contacts 5 and having axially outer ends 13 overlapping the axially inner ends of blade contacts 5 and sandwiching the latter therebetween. Each of the channel-shaped fuse links or fusible elements conductively interconnecting blade contacts 5 further includes a pair of integral flange portions L5 each enclosing an angle of less than 90 degrees with the aforementioned web portions L5. Web portions L5 and flange portions L5 are made preferably of an integral sheet of a metal having a relatively high conductivity and a relatively high fusing point such as copper or silver. Each fuse link L5, L5 is provided with an overlay 8 of a low-fusing-point link-severing metal, preferably tin. Each overlay 8 extends transversely across the web portion L5 and the two flange portions L5 of each of the two fue sliuks L5, L5. Each fuse link L5, L5 is provided with five transverse lines 9 of circular perforations and with a considerably larger number of lines l2 of circular perforations extending in a direction longitudinally of casing 1 and longitudinally of fuse links L5, L5. Overlays 8 extend parallel to the transverse center lines 9' of perforations and jut into the spaces defined by contiguous perforations of said lines without projecting signicantly beyond the lines of narrowest crosssections of links L5, L5dened by their respective trans- Overlays 8 extend over the entire width of fuse links L5, L5, i.e. they are coextensive with the web portions L5 and with the two flange portions L5 of each fuse link L5, L5. i Casing 1 is filled with a pulverulent arc-quenching Vfiller 11 indicated only in portions of FIGS.V 4-6 and broken away, or deleted, in other portions thereof. Filler 1' has a smaller thermal conductivity than quartz sand and evolves but relatively limited amounts of gas when subjected to the heat of an electric arc. Gypsum complies fairly well with these requirements. Y Calcium carbonatefor chalk would likewise be satisfactory as far as thermal conductivity requirements are concerned but tends to evolve too much gas under the heat of electric arcs and is, therefore, notrquite as satisfactory as gypsum powder. Y i

The flange portions L5 form thermalshields precluding Y heat generated in web portions L5 from freely reaching cassubstantial percentage of the electric current carried by blade contacts 5. |Thus flange portions L5 have substantially the same functions as the radially outer fuse links L5 and L4 in the structure of FIGS. 1-3 consisting in keeping the heat generated close to the axis of the casing inside the casing-and thus having a derating action to be explained below more in detailand in shunting the currentpath delined by the radially inner portions of the composite link structure. As a result of these features the temperature of the radially inner portions of the composite link structures will be relatively high and their conductivity will be relatively low in comparison to the temperature and conductivity of the radially outer portions of the composite link structures. At the occurrence of relatively small overloads of inadmissible duration the link-severing overlays fuse initially at the radially inner portions L5 of the composite link structure and sever these portions by a metallurgical reaction. This greatly increases the current density and heating action of the radially outer portions of the composite link structure, causing the same to be severed by a metallurgical reaction rapidly following that causing the radially inner portions of the composite link structure to be severed.

In the structure of FIGS. 5 and 6 the fuse links L5, L5 are conductively connected to blade contacts 5 by other means than those applied in the structure of FIGS. 1 3. According to FIGS. 5 and 6 each web portion L5 has axially outer extension tabs bent twice degrees. Numeral 13 has been applied to indicate the axially outer ends ofthe aforementioned extension tabs which sandwich the axially inner ends of blade contacts 5. These tab ends 13 are silver brazed to blade contacts 5', preferably in the fashion more fully disclosed in the copending patent application of Paul C. Hitchcock, Ser. No. 79,269, filed March 21, 1961, for Knife Blade Type Fuses and Method for Manufacturing the Same.

lt will be apparent from FIGS. 4 6, inclusive, that the total width of each fuse linrk L5, L5, i.e. the aggregate width of their web portions L5 and of their two flange portions L5 exceeds considerably the inner diameter of casing 1. The aggregate width of each channel-shaped fuse link L5, L5 exceeds the diameter of the chamber defined by casing 1 but is less than the circumference of said chamber.` This Width feature is conducive to increasing delay times on occurrence of large protracted overloads if .combined with the other features which are characteristic of the structure of FIGS. 4-6. The effect or" the width of a fuse link of copper or silver and that of a link overlay of a link-destroying low-fusingpoint metal on time lag has been set forth in detail in my aforementioned U.S. Patent 2,988,620; and reference may be had to that patent for additional information on this particular aspect of the structure of FIGS. 446. As indicated above the lateral or flange portions L5 situated to both sidesA of, and partly overlapping, the webV or center portions L5 of links L5, L5 form thermal shields substantially limiting the loss of heat from the web portions or center portions L5. The resulting derating effect will be discused below more in detail. One of the two bendsrof 90 degrees in the axially outer ends of web portions or center portions L5 increases the spacing thereof beyond the thickness of blade contacts 5'. This, in turn, increases the mass of pulverulent arc-quenching filler l1 accommodated between the planes of link portions L5 to the extent required for achieving a'satisfactory interruption of faulted circuits over-a.wide range of fault currents. As is readily apparent from FflGS. 4-6 link portions L5 are arrangedin planes which are parallel to, and spaced from, the general plane defined by blade contacts Y5. It will be further. notedfromFIGS. 4-6 that the portions L5 and L5 of each link L5, L5 define a passageway extending in a direction longitudinally of casing The'two passagewaysV bounded by links L5, L5 are open at the axially outer ends thereof, thus making it possible for the products of arcing formed vincident to interruption of a faulted circuit to diffuse readily throughout the entire space bounded by casing 1.

Referring now to FG. 7, character A has been applied to indicate the time-current curve of a fuse structure substantially identical to that shown in FIGS. 1 and 2, except for the fact that links L3 and L4 have been omitted and links L1 and L2 made thicker so as to result in the same current rating as the structure of FIGS. l and 2, with all its four links L1, L2, L3, L4, i.e. a current rating of 400 amps. The time-current curve B of FIG. 7 refers to the structure of FIGS. 1 and 2 with all of its four links L1, L2, L3, L4 in place, these links having such dimensions as to result in a current rating of 400 amps. It is apparent that the time-lag in the case of B is much larger than in the case of A. Reference character A has been applied to indicate the timecurrent curve of a fuse having the same characteristics as the fuse having the time current curve A but having a smaller current rating. Time current curve B' is related to time current curve B in the same way that A is related to A. A and A are parallel and B and B are parallel. For a better understanding of the striking difference between characteristics A and B the differential equation governing the rise of temperature in a structure will be derived and brieily discussed below.

The time constant T of a structure is the ratio of its heat absorbing capacity C (in terms of watt sec./ deg. C) to its heat dissipating ability A (in terms of watt sec./ deg. C). Hence Heat dissipation increases as the temperature r increases, i.e. heat dissipation is equal to the product A-f. Let Q be the heat generated per unit of time, i.e. per second (expressed in terms of watts), then the rise in temperature depends on Q-AT. The rate of rise in temperature T: (ser.)

dr r

is inversely proportional to the heat absorbing capacity C of the system. Hence d1 Q--T 2 dt C if the thermal insulation of the system under consideration were perfect, i.e. if the heat dissipating ability A Were zero, this would result in a constant rate of rise in temperature, i.e. the temperature would rise linearly with time. The larger the heat Q generated per unit of time and the smaller the heat absorbing capacity, the steeper the rise in temperature.

When the heat Q generated per unit of time is equal to the heat dissipation A, i.e. if

temperature Tm. This end temperature is defined by the equation deg. C. (4:)

tbl@

i.e. the end temperature rm would instantly be reached.

6 The solution of diterential Equation 2 depends upon the limit conditions under consideration. If the initial temperature 70:0, the solution of Equation 2 is If the initial temperature To dilfers from zero, the solution of Equation 2 is t t T=Tm(1-e T +Trre T deg. C.

The first term in the above equation can be represented by a curve rising `from Zero to the constant end temperature Irm and the second term in the above equation can be represented by a curve decaying from the initial temperature To to zero. The algebraic sum of both curves describes the actual rise in temperature.

Combining Equations l, 4 and 5 yields The degree of time-lag of any particular fuse is generally expressed by its time-current curve. As mentioned above, in FlG. 7 reference character A has been applied to indicate the time current curve of a given fuse. The time current vcurve A is parallel to time current curve A but displaced toward the left. This means that curve A refers to a fuse having a smaller current rating than the fuse to which curve A refers and that both fuses under consideration have the same characteristics as far as time-lag is concerned.

A rigorous comparison of the operating characteristics of fuses requires a rigorous definition of the term rated current or current rating. Most fuse standards do not contain a rigorous definition of this term. The Standard Pub. No. FU l-1959 of the National Electrical Manufacturers Association bases current rating on the ability of a particular piece of equipment to meet certain temperature rise and opening time specifications. Most other standards establish similar criteria for current rating. Adoption of these criteria yields a relatively large family of time current curves, or a relatively wide band of such curves, each having the same current rating.

It is possible to more rigirously define current rating by resorting to the concept of the fusing factor. The fusing factor is the ratio of minimum fusing current to current rating. For the purpose of further analysis it will be assumed that only fuses having the same minimum fusing current have the same current rating or, in other Words, that equality of fusing factor is a prerequisite of equality of current rating. Thus the fuses whose time-current curves are designated in FIG. 7 by the reference characters A and B are fuses having the same current rating, curve A referring to a relatively fast fuse and curve B to a fuse having a relatively long timelag.

Achieving time-lag in a fuse may be visualized as taking such measures or steps as to achieve a conversion from characteristic A to characteristic B. Comparing characteristics A and B more closely it is apparent that curve B may be conceived as being derived from curve A by shifting each point of the upper portion of curve A farther to the left than each point of the lower portion thereof. If the fuse whose behavior is represented by curve A is subjected to a relatively small current i1, its fusing time is t1 and if the same fuse is subjected to a relatively high current i2 its fusing time is t2. The fuse to which characteristic B refers blows within the time t1 if subjected to a current il and this fuse blows within the time t2 if subjected to a current i2. Current il is nl times smaller than current i1 and current i2' is n2 times smaller than current i2. It is apparent from FIG. 7 that deg. C.

In other words, the fuse to which time-current curve B2 refers may be conceived as having been derived from the fuse to which curve A refers by measures involving a higher decrease of current carrying capacity, or a higher derating, in the range of relatively low currents than in the range of relatively high currents. To achieve such a derating ratio as little heat as possible should be dissipated in the range of relatively low currents, thus effecting a drastic decrease of the minimum fusing current, and as much heat as possible should be dissipated in the range of relatively high currents to delay blowing as much as possible in that current range. The first mentioned object can be achieved by measures tending to minimize heat dissipation from the fusible element, or fuse link, in the range of relatively low currents and Vrelatively long fusing times. To achieve the second mentioned object it is necessary to adopt measures tending to maximize heat dissipation from the fusible element, or fuse link, in the range of relatively high currents and relatively long fusing times. The rst mentioned object can be achieved by structural means involving the fuse structure in its entirety--irrespective of where such structural means are located within the fuse structure-tending to minimize over relatively long periods of time the dissipation of the heat generated by the fusible elements within the fuse structure to points outside of the fuse structure. Such means must furthermore not stand in the way of a relatively effective dissipation .of heat from the fuse link, if heat generation occurs therein at a relatively high rate. The second mentioned object, i.e. to delay blowing in the range of relatively high currents, can be achieved by structural means maximizing heat dissipation from the points whose temperature controls the initiation of the interrupting process. Such means must be situated immediately adjacent to such points in order to minimize the times involved in heat transfer away from such points. It will be apparent that both conditions for achieving time-lag are contradictory, to a certain extent. These apparently contradictory conditions can readily be reconciled by stating that it does Vnot matter whether a predetermined measure effecting a derating in the ,range ofV relatively low currents and relatively long blowing times causes also a derating in the range. of relatively high currents Yand relatively short blowing times as long as the derating factor nl for relatively low currents and relatively long blowing times is considerably larger than the derating factor n2 for relatively high currents and relatively short blowing times. It does not matter particularly whether a predetermined measure for derating in the range of relatively high currents and relatively short blowing times is relatively ineffective in achieving heat dissipation for that particular range as long as that particular measure does not tend to unduly increase heat dissipation in the range of relatively small currents and correspondingly long blowing times. These general statements will become even more apparent from what follows: Y

In FIG. 8a the dash-and-dot line encompassing the entire fuse structure indicates the portion thereof involved in a predetermined rate of heat dissipation, or heat transfer,

dQ 71T ywhen the fuse structure is carrying its minimum fusing current. As the current carried by the fuse is increased, the portion of the fuse structure involved in the above predetermined rate of heat dissipation, r heat transfe matically indicated by the dash-and-dot lines of FIGS. 8b and 8c. When the current carried by the fuse is further Vincreased. to a fairly high multiple ofthe rated Ycurrent the heat transfer occurs butjatthe points of the fuse structure immediately adjacent the transverse lines of perforations 9 and 9 (see FIGS. l-6) as indicated in FIG. 8d. At all points of the fuse structure lying outside the small zones which are indicated by dash-and-dot lines in FIG, 8d the rate of heat transfer prior to blowing of the fuse is less than the above referred-to predetermined value of yet immediately adjacent to the points indicated by dashand-dot lines the aforementioned predetermined rate of heat transfer may be far exceeded.

For a better understanding of the mechanism of timedelay or time-lag it is useful to plot the rise in temperature of the point of initial break formation against time. If initial break formation is caused by a metallurgical reaction between a high fusing point base metal such as, for instance, silver or copper, and a low fusing point overlay metal such as, for instance, tin, the process of interruption will be initiated when the latter metal reaches the fusing point thereof. In order to simplify the present analysis it will be assumed that the time when the low fusing point metal reaches the fusing point thereof is the same as the time when the fuse blows. Actually there is a time interval between the time when the low fusing point metal reaches its fusing temperature and the time when the fuse blows and interrupts the overloaded circuit. In drawing FIGS. 9a and 9b the above interval between reaching the fusing point of the overlay metal and interruption of the circuit has been neglected. The exponential curves A shown in FIGS. 9a and 9b refer to the same fuse structure whose time-current curve has been designated bythe letter A in FIG. 7, andthe exponential curves B shown in FIGS. 9a and 9b refer to the same fuse structure whose time-current curve has been designated by the letter B in FIG. 7.l These fuse structures will hereinafter simply be identified by the letters A and B, respectively. In drawing FIG. 9a it has been assumed that fuse A is carrying the relatively high current i2 and fuse B is carrying an even higher 1'2". The tin overlay of both fuses reaches the fusing point of tin at the time t2 andthe rise in temperature of the tinroverlay of both fuses A and B is described by the saine exponential curve. Supposing now that fuses A and B are caused to carry the current i1 and il, respectively, which are considerably smaller than the currents i2, i2 as is readily apparent from FIG. 7; then the tin overlay of both fuses A and B will reach the fusing point of tin after an interval of time t1 and the rise in temperature of the overlay of both fuses A, B will again be described by the same exponential curve (FIG. 9a right). The rate of rise of the last mentioned exponential curve is not as steep as that of the first mentioned exponential curve and the time t1 for reaching the fusing temperature of tin is much longer in the last mentioned case than the corresponding time t2 in the first mentioned case.

In drawing FIG. 9b it was assumed that both fuses A and B were caused to carry the same current i2 (see also FIG. 7). Under such conditions the rise of the 'temperature of the tin overlay on fuse A is much faster than the rise in temperature of the tin overlay of fuse B and the overlay of the former reaches the fusing point of tin at the time t2, whereas the overlay of the latter reaches the fusing point of tin at the time t3 considerably after the time t2.

In the structures of FIGS. Vl-6 the rate of heat dissipation at the hottest area ofthe fuse structure--which areais coextensive with one of the boundary lines of the tin overlays-is maximized at relatively high currents by (virtue. of the relatively great width of the fuse link, i.e.

If a fuse link having either the geometry shown in FIGS. 1 to 3, or the geometry shown in FIGS. 4-6, were submersed in a pulverulent arc-quenching iiller having a heat conductivity approximately as high as that of quartz sand rather than in a relatively good thermal insulator such as gypsum, a substantial increase of the rate of heat dissipation at relatively high currents could be achieved, but such an increase would be accompanied by a substantial increase of the rate of heat dissipation in the range of relatively small currents or, in other Words, such an increase would be accompanied by an increase of the minimum fusing current. Speaking in terms of time-current characteristics (FIG. 7), increasing the width of a fuse link tends to shift the right or lower side of a time-current curve to the left, but to have relatively little efect upon the left or upper side of the time-current curve, i.e. upon the minimum fusing current. Increasing the thermal conductivity of a pulverulent arc-quenching filler tends to cause a shift of both the right side and the left side of a time-current curve to the left, the latter shift tending to exceed the former.

The overlapping of some portions of wide ribbon-type fuse link means by other portions of wide ribbon-type fuse link means tends to decrease the minimum fusing current of the fuse structure without having an equally significant effect upon the right or lower side of its timecurrent curve. Therefore such overlapping tends to result in large timedags, as evidenced by the table below and by the two time-current curves shown in FIG. 10.

540 amps. 800 amps. 1,200 amps. 1, 600 amps. 2, 000 amps 12 sec.

The above table refers to two different fuses each having the same current rating and voltage rating, namely 400 amps., 600 volts. The second column marked II indicates various fusing times at overload currents stated in the first column and refers to a fuse structure substantially identical to that shown in FIGS. 1 3 except for the omission of link means L3 and L4 and thickening of link means L1 and L2 to impart a current rating of 400 amps. to the fuse structure. In other words, the second column refers to a time-lag fuse of the kind disclosed in the aforementioned United States Patent 2,938,620.. The column marked I indicates various fusing times at overload currents stated in the first column and refers to a fuse structure identical to that shown in FIGS. 1-3, including the four superimposed fuse link means L1, L2, L3, L4. The structure of FIGS. 4-6 has substantially the same time-current characteristic as that of FIGS. l-3.

Equations

5 and 7 presuppose that the structure which is being heated is homogeneous and a fuse structure is far from being a homogeneous structure. Nevertheless it is possible and useful to compare the hehavior of a rela.- tively rapid fuse and that of a time-lag fuse by considering the above equations.

The requirement that the right or lower portion of curve A of FIG. 7 be shifted to the left as little as pos sible means in thermal terms that in spite of relatively large heat generation, Q, by virtue of the flow of relativedly large currents, z', the rise in temperature of the link-severing tin overlay should be relatively slow.

The requirement that the left or upper portion of curve A of FIG. 7 be shifted as much as possible to the left means, in thermal terms, that in spite of relatively small heat generation, Q, by virtue of the flow of relatively small currents, i, the rise in temperature of the linksevering overlay should be relatively rapid.

In the foregoing only the case of interruption of overload currents, as distinguished from interruption of major fault currents, or short-circuit currents, has been treated.

On occurrence of currents of the latter kind all points of reduced crosssection fuse in rapid sequence, virtually simultaneously. The large number of points of reduced cross-section per transverse line of perforations (9 and 9') tends to greatly decrease the current density per point of reduced crosssection and thus to result in a very high interrupting capacity.

While, in accordance with the patent statute, I have disclosed the specific details of two embodiments of the invention, it is to be understood that these details are merely illustrative and that many variations thereof may be made without departing from the spirit and scope of the inven tion. It is my desire, therefore, that the language of the accompanying claims shall be interpreted as broadly as possible and that it be limited only as required by the prior state of the art.

I claim as my invention:

1. A time-lag fuse comprising a tubular casing of insulating material, a pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing transversely through said pair of terminal elements into the inside of said casing, a pair of multiperforated composite fuse link structures of a metal having a relatively high conductivity and a relatively high fusing point, said pair of fuse link structures having axially outer ends sandwiching the axially inner ends of said pair of blade contacts, each constituent fuse link structure of said pair of fuse link structures comprising a rst portion having about the same width as said pair of blade contacts and being arranged relatively close to the longitudinal axis of said casing and each of said pair of fuse link structures further comprising a second portion arranged generally more remote from said longitudinal axis and overlapping and shunting said first portion, overlays of a low-fusingpoint-link-severing metal extending transversely across said first portion and extending transversely across said second portion of each of said pair of fuse link structures, and a pulverulent arc-quenching filler inside said casing for submersing said pair of fuse link structures, said filler having a smaller thermal conductivity than quartz sand.

2. A time-lag fuse as specified in claim l wherein said casing defines a cylindrical chamber having a predetermined diameter, and the aggregate width of said first p0rtion and of said second portion of each of said pair of fuse link structures exceeding said predetermined diameter.

3. A time-lag fuse comprising a tubular casing of insulating material, a pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing transversely through said pair of terminal elements into the inside of said casing, a pair of multiperforated fuse links in ribbon form ot a metal having a relatively high conductivity and a relatively high fusing point, said pair of fuse links having substantially the same width as said predetermined width of said pair of blade contacts and sandwiching and conductively interconnecting the axially inner ends of said pair of blade contacts, a pair of overlays of a low-fusing-point link-severing metal each extending transversely across one of said pair of fuse links, an additional pair of multiperforated fuse links in ribbon form of a metal having a relatively high conductivity and a relatively high fusing point shunting saidpair of fuse links and arranged between the radially outer surfaces of said pair of fuse links and the inner surface of said casing, each of said additional pair of fuse links being arranged at the ends thereof in abutting relation to the ends of one of said pair of fuse links and each of said additional pair of fuse links being arranged at the center region thereof in spaced relation from the center region of one of said pair of fuse links, an additional pair of overlays of a low-fusing-point link-severing metal each extending transversely across one of said additional pair of fuse links, and a pulverulent arc-quenching filler inside said casing for submersing said pair of fuse links and said addi` alaaess tional pair of fuse links, said filler having a smaller thermal conductivity than quartz sand.

4. A time-lag fuse as specified in claim 3 including a filler of gypsum.

5. A time-lag fuse comprising a tubular casing oi insulating material, a pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing through said pair of terminal elements into the inside of said casing, a pair of multiperforated copper fuse links in ribbon-form having substantially the same width as said predetermined width of said pair of blade contacts conductively interconnecting the axially inner ends of said pair of blade contacts, the perforations in each of said pair of fuse links defining a plurality of lines of perforations extending transversely across each of said pair of fuse links and further defining a plurality of lines of perforations extending in a direction longitudinally o each of said pair of fuse links, a pair of overlays of a lowfusing-point link-severing metal each extending transversely across one of said pair of fuse links, an additional pair of multiperforated copper fuse link means in ribbon-forni shunting said pair of fuse links interposed between the radially outer surfaces of said pair of fuse links and the inner surface of'said casing, the perforations in said additional pair of fuse link means dening a plurality of lines of perforations extending transversely across each of said additional pair of fuse link means and further defining a plurality of lines of perforations extenting in a direction longitudinally of each of said additional pair of fuse link means, the total number of perforations per line extending transversely across each of said pair of fuse links and across each of said additional pair of fuse link means exceeding the number of perforations per line extending in a direction longitudinally of each of said pair of fuse links and each of said pair of additional fuse link means, an additional pair of overlays of a low-fusing-point link-severing metal each extending transversely across one of said additional pair of fuse link means, and a pulverulent arc-quenching iiller inside said casing for submersing therein said pair of fuse links and said additional pair of fuse link means, said filler having a smaller thermal conductivity than quartz sand.

6. A time-lag fuse comprising a tubular casing of insulating material, a pair of terminal elements closing the ends of said casing, a pair of blade contacts having `a predetermined width projecting from the outside of said casing through said pair of terminal elements into the inside of said casing, la first pair of multiperforated fuse links in ribbon-form having substantially the same width as said predetermined width of saidpair of blade contacts sandwiching and condnctively interconnecting the axially inner ends of said pair of blade cont-acts, a lirst pair of overlays of la low-fusing-point link-severing metal each extending transversely 'across one of said first pair of fuse links, a second pair of multiperforated fuse links in ribbon-form having substantially the same iwidth as said predetermined width of said pair of blade contacts overlapping the axially `outer ends of said first pair of fuse links kand conductively interconnecting the axially inner ends of said pair of blade contacts, each of said second pair of fuse links being interposed between the radially outer surface -of one of said rlirst pair of fuse links and the inner surface of said casing, "a second pair of overlays of a low-fusing-point link-severing metal each extending transversely across one of said second pair of fuse links, and a pulverulent arc-quenching liller inside of said casing for surnbersing therein said first pair of fuse links and said second pair of fuse links, said iiller having a thermal conductivity of the same order fas Vgypsum. powder.

7. A time-lag fuse as specified in claim 6 including a filler of qypsum powder. g Y Y 8. A Vtime-lag fuse comprising a tubular casing of insulating material, a pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing through said pair of terminal elements into the inside of said casing, a rst pair of multiperforated copper fuse links in ribbon-form arranged within said casing, said first pair of fuse links having substantially the same width as said predetermined width of said pair of blade contacts and sandwiching and conductively interconnecting the axially inner ends of said pair of blade contacts, a second pair of multiperforated copper -fuse links in ribbon-form arranged within said casing, said second pair of fuse links having substantially the same width as said predetermined width of said pair of blade contacts .and sandwiching the axially outer ends of said first pair of Ifuse links and the 'axially inner ends of said pair of blade contacts, the perforations in each fuse link of said first pair of fuse links and the perforations in each fuse link of said second pair of fuse links defining lines of perforations extending transversely across each `fuse link of said first pair of fuse links and each fuse l-ink of said second pair of fuse links and said perforations further defining lines of perforations extending in a direction longitudinally of each fuse link of said iirst pair of fuse links and of each fuse link of said second pair of fuse links, the number of perforations in each said lines extending transvensely exceeding the number of perforations in each of said lines extending longitudinally, overlays of a link-severing metal having a relatively low fus-ing point extending transversely across each fuse link of said first pair of `fuse links and each fuse link of said second pair `of fuse links, and a pulverulent arcqnenching yfiller inside of said casing for submersing therein said first pair of fuse links and said second pair of fuse links, said filler having a smaller thermal conductivity than quartz sand.

9. A time-lag fuse comprising a tubular casing of insulating material, ya pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing through said pair of terminal elements into the inside of said casing, a pair of multi-perforated fuse links of a high conductivity high-fusing-point metal conductively interconnecting the axially inner ends of said pair of blade contacts, each of said pair of fuse links being substantially channel-shaped and each of said pair of `fuse links including `a web portion having substantially the same width as said predetermined :width of said pair of blade contacts and having axially outer ends overlapping the axially inner ends of said pair of blade contacts, eachV of said pair of fuse links further including a pair `of integral flange portions each enclosing an angle of less than degrees with said web portion thereof, a pair' of overlays of a lowfusing-point link-severing metal each extending transversely yacross said-web portion'and said pair of ilange portions of each of said pair of fuse links, and a pulverulent arc-quenching iiller inside said casing lfor submersing therein said pair of fuse links, said iiller having a smaller thermal conductivity than quartz sand. i

10..A time-lag fuse comrising `a tubular casing of insnlating material, la pair of terminal elements closing the ends of said casing, a pair of blade contacts having a predetermined width projecting from the outside of said casing through said pair of terminal elements into the inside of said casing, a pair of multi-perforated fuse links of a metal having a high conductivity and a high fusing point conductively interconnecting the axially inner ends of said pair o-f blade contacts, each of said pair of fuse links being substantially channel-shaped and each of said pair of fuse links includ-ing a web portion having substantially the same width as said predetermined width of said pair of blade contacts-and having `axially outer ends overlapping the axially inner ends of said pair of blade contacts, each of said pair of fuse links further including a pair of integral flange portions ,enclosing'an angle of less than 90 degrees with said web portion thereof, the perforations in each of said pair Vof fuse `links defining a first plurality of lines of perforations Vextending transversely lacross said web portion and said flange poptions of each of said pair of fuse links and sai-d perforations further defining -a second plurality of lines of perforations extending in a direction longitudinally of said lweb portion and said flange portions of each of said pair of fuse links, the number of perforations in each line of said first plurality of lines of per-forations exceeding the number of perforations in each line of said second plurality of lines of perforations, overlays of a link-severing metal having a relatively low fusing point extending transversely across said Web portion and said flange portion of each of said pair of fuse links, and a pulverulent arc-quenching filler inside said casing for submersing therein said pair of fuse links, said filler having a smaller thermal conductivity than quartz sand.

ll. An electric time-lag fuse comprising a tubular casing of insulating material; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively Wide lateral surfaces situated inside of said casing; a first pair of relatively Wide ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said first pair of ribbons having axially outer ends conductivel'y attached to one of said pair of lateral surfaces of one of said pair of blade contacts; an additional pair of relatively wide ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said additional pair of ribbons forming a shunt across one of said first pair of ribbons; each of said first pair of ribbons and each of said additional pair of ribbons being provided with a plurality of transverse lines of perforations each forming a zone of reduced cross-section; each of said additional pair of ribbons being arranged to form a thermal shield limiting heat generated in said first pair of ribbons from reaching said casing and being dissipated by said casing; each of said first pair of ribbons and each of said additional pair of ribbons being provided immediately adjacent to one of said plurality of transverse lines of perforations with a transverse overlay of a metal having a relatively loW fusing point; and a pulverulent arc-quenching filler inside said casing in intimate engagement with said first pair of ribbons and with said additional pair of ribbons, said filler having a smaller thermal conductivity than a filler of quartz sand.

l2. An electric time-lag fuse comprising a tubular casing of insulating material; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively wide lateral surfaces situated inside of said casing; a first pair of relatively Wide ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said pair of ribbons having axially outer ends conductively attached to one of said pair of lateral surfaces of one of said pair of blade contacts, each of said first pair of ribbons including two portions arranged in planes enclosing an obtuse angle and defining an edge at the intersection of said planes located in the center region of said casing; an additional pair of relatively wide ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said additional pair of ribbons forming a shunt across one of said first pair of ribbons, each of said additional pair of ribbons including two portions arranged in planes intersection of said planes located in the center region of said casing; each of said first pair of ribbons and each of said additional pair of ribbons being provided with a plurality of transverse lines of perforations each forming a zone of reduced cross-section; each of said additional pair of ribbons being arranged to form a thermal shield limiting the heat generated in said first pair of ribbons from reaching said casing and being dissipated by said casing; each of said first pair of ribbons and each of said additional pair of ribbons being provided immediately adjacent to one of said plurality of transverse lines of perforations with a transverse overlay of a metal having a relatively low fusing point, and a pulverulent arcquenching filler inside said casing in itimate engagement with said first pair of ribbons and with said additional pair of ribbons, said filler having a thermal conductivity of the same order as gypsum powder.

13. An electric time-lag fuse comprising a tubular casing of insulating material having a predetermined diameter; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively wide lateral surfaces situated inside said casing; a pair of ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said pair of ribbons having axially outer ends conductively attached to one of said pair of lateral surfaces of one of said pair of blade contacts; each of said pair of ribbons having a width exceeding said predetermined diameter of said casing; each of said pair of ribbons including a center portion arranged parallel to the general plane defined by said pair of blade contacts and each of said pair of ribbons including a pair of lateral portions bent to partially overlap said center portion and to form thermal shields substantially limiting the loss of heat from said center portion; each of said pair of ribbons being provided with a plurality of transverse lines of perforations each forming a zone of reduced cross-section and each extending transversely across said center portion and transversely across said lateral portions of each of said pair of ribbons; each of said pair of ribbons being provided immediately adjacent to one of said plurality of transverse lines of perforations with a transverse overlay of a metal having a relatively low fusing point, said overlay extending transversely across said center portion and transversely across said lateral portions of each of said pair of ribbons, and a pulverulent arc-quenching filler inside said casing for submersing said pair of ribbons, said filler having a smaller thermal conductivity than quartz sand.

14. An electric time-lag fuse comprising a tubular casing of insulating material defming a chamber having a predetermined diameter and a predetermined circumference; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively wide lateral surfaces situated inside said casing; a pair of ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said pair of ribbons having axially outer ends conductively attached to one of said pair of lateral surfaces of one of said pair of blade contacts, each of said pair of ribbons having a Width intermediate said predetermined diameter of said chamber and said predetermined circumference of said chamber, each of said pair of ribbons including a center portion arranged parallel to the general plane defined by said pair of blade contacts and each of said pair of ribbons including a pair of lateral portions bent to partially overlap said center portion and to form thermal shields substantially limiting the loss of heat from said center portion; each of said pair of ribbons being provided with a plurality of transverse lines of perforations each forming a zone of reduced cross-section and each extending transversely across said center portion and transversely across said lateral portions of each of said pair of ribbons; each of said pair of ribbons being provided immediately adjacent to one of said plurality of transverse lines of perforations with a transverse overlay of a metal having a relatively low fusing point, said overlay extending transversely across said center portion and transversely across i 5 said lateral portions ot each of said pair of ribbons; and a pulverulent arc-quenching lilrer inside said casing for submersing said pair of ribbons, said filler having smaller thermal conductivity than quartz sand.

15. An electric time-lag fuse comprising a tubular casing of insulating material; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively wide lateral surfaces situated inside said casing; a pair of relatively wide ribbons of a metal having a relatively high fusing point arranged in said casing in a direction longitudinally thereof, each of said pair of ribbons having axially outer ends conductively attached to one of said pair of lateral surfaces of one or" said pair of blade contacts; each of said pair of ribbons including a center portion parallel to the general plane defined by said pair of blade contacts and each of said pair of ribbons including a pair of lateral portions enclosing acute angles with said center portion and forming thermal shields substantially limiting the loss of heat from said center portion; each of said pair or" ribbons being provided with a plurality of transverse lines of perforations each forming a zone of reduced cross-section and each extending transversely across center portion and transversely across said lateral portions of each of said pair of ribbons; and each of said pair of ribbons being provided immediately adjacent to one of said plurality of transverse lines of perforations with a transverse overlay of a metal having a relatively :low fusing point, said overlay extending transversely across said center portion and transversely across said lateral portions of each of said pair of ribbons, and a pulverulent arc-qnench ing ller of gypsum inside said casing for submersing said pair of ribbons. i

16. An electric time-lag fuse comprising a tubular casing ot insulating material deiining a chamber having a predetermined diameter and a predetermined circumference; a pair of terminal caps each closing one end of said casing; a pair of aligned spaced blade contacts each projecting transversely through one of said pair of caps into said casing and each having a pair of relatively wide lateral surfaces situated inside said casing; a pair of copper ribbons arranged in said casing in a direction longitudinally thereof, each of said pair of ribbons having axially outer ends conductively attached to one of said pair of lateral surfaces of one of said pair of blade contacts, each of said pair ol ribbons having a Width intermediate said predetermined diameter of said chamber and said predetermined circumference of said chamber; each of said pair Vof ribbons including a center portion arranged parallel to the general plane defined by said pair of blade contacts and each of said pair of ribbons including a pair of lateral portions bent to partially overlap said center portion and to form thermal shields substantially limiting the loss of heat from 'said center portion; each oi said pair of ribbons being provided with a pluralityof tra sverse lines oi perforations each forming alone of reduced cross-section and each extending transversely across said center portion and transversely across said lateral portions of each of said pair or" ribbons; each of said pair of ribbons being provided immediately adjacent to one of said plurmity of transverse lines of perforations with a transverse overlay of a metal having a relatively low fusing point, said overlay extending transversely across said center portion and transversely across said lateral portions of each of said pair of ribbons; and a pulverulent arc-quenching lil-ler inside said casing for submersing said pair of ribbons, said filler having a smaller thermal conductivity than quartz sand.

17. A time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal caps closing the ends of said casing;

(c) a pair of blade contacts each projecting from the outside of said casing transversely through one of said pair of caps into the inside of said casing;

(d) a irst pair of multiperforated ribbon fuse links of a metal having a relatively high conductivity and a relatively high fusing point each supporting a transverse overlay of a relatively low-fusing-point linksevering metal arranged inside of said casing, said first pair of fuse links having axially outer ends sandwiching the axially inner ends of said pair of blade contacts;

(e) an additional pair of multiperforated ribbon fuse link means of a metal having a relatively high fusing point each supporting a transverse overlay of a relatively lcw-fusing-point linlosevering metal arranged inside or" said casing, each of said additional pair of fuse link means shunting at least in part the current path of one of said first pair of fuse links including the region of said overlay thereof, overlapping at least in part one of said first pair of fuse links and being arranged at least in part closer to the inner surface of said casing than said rst pair of fuse links; and

(f) a pulverulent arc-quenching liller inside said casing enveloping said first pair of fuse links and said overlay on each of said first pair of fuse links and enveloping said additional pair of fuse link means and said overlay on each of said additional pair of fuse ink means, said filler having a substantially smaller thermal conductivity than quartz sand.

References Cited in the tile of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 3,123,693

Marchl 3 1964 Frederick J'. Kozacka It is hereby certified that error appears in the above numbered patent requiring correctionand that the said Letters Patent should read as corrected below.

Column 13, line 67, after "planes" insert enclosing an obtuse angle and defining an edge at the n Signed and sealed this 28th dey of July l94 (SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ESTON G; JOHNSON Attesting Officer UNITED STATES PATENT oEEICE CERTIFICATE 0F CORRECTION Patent No, 3, 123,693 March 3 1964 Frederick J. Kozacka It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 13, line 67, after "planes" insert enclosing an obtuse angle and defining an edge at the No Signed and sealed this 28th day of July l964 (SEAL) Attest:

ESTON G. JOHNSON EDWARD J. BRENNER Attestng Officer Commissioner of Patents

Claims

1. A TIME-LAG FUSE COMPRISING A TUBULAR CASING OF INSULATING MATERIAL, A PAIR OF TERMINAL ELEMENTS CLOSING THE ENDS OF SAID CASING, A PAIR OF BLADE CONTACTS HAVING A PREDETERMINED WIDTH PROJECTING FROM THE OUTSIDE OF SAID CASING TRANSVERSELY THROUGH SAID PAIR OF TERMINAL ELEMENTS INTO THE INSIDE OF SAID CASING, A PAIR OF MULTIPERFORATED COMPOSITE FUSE LINK STRUCTURES OF A METAL HAVING A RELATIVELY HIGH CONDUCTIVITY AND A RELATIVELY HIGH FUSING POINT, SAID PAIR OF FUSE LINK STRUCTURES HAVING AXIALLY OUTER ENDS SANDWICHING THE AXIALLY INNER ENDS OF SAID PAIR OF BLADE CONTACTS, EACH CONSTITUENT FUSE LINK STRUCTURE OF SAID PAIR OF FUSE LINK STRUCTURES COMPRISING A FIRST PORTION HAVING ABOUT THE SAME WIDTH AS SAID PAIR OF BLADE CONTACTS AND BEING ARRANGED RELATIVELY CLOSE TO THE LONGITUDINAL AXIS OF SAID CASING AND EACH OF SAID PAIR OF FUSE LINK STRUCTURES FURTHER COMPRISING A SECOND PORTION ARRANGED GENERALLY MORE REMOTE FROM SAID LONGITUDINAL AXIS AND OVERLAPPING AND SHUNTING SAID FIRST PORTION, OVERLAYS OF A LOW-FUSINGPOINT-LINK-SEVERING METAL EXTENDING TRANSVERSELY ACROSS SAID FIRST PORTION AND EXTENDING TRANSVERSELY ACROSS SAID SECOND PORTION OF EACH OF SAID PAIR OF FUSE LINK STRUCTURES, AND A PULVERULENT ARC-QUENCHING FILLER INSIDE SAID CASING FOR SUBMERSING SAID PAIR OF FUSE LINK STRUCTURES, SAID FILLER HAVING A SMALLER THERMAL CONDUCTIVITY THAN QUARTZ SAND.