US2084495A - Fuse - Google Patents

Description

S. I. LINDELL June 22, 1937.

FUSE

4 sheets-sheet? Filed Nov. 6, 1955 VII/l orneu s June 22, 1937. s. 1. LINDELL ,4

FUSE Filed Nov. e, 1935 4 Sheets-Sheet 5 Patented June 22, 1937 UNITED STATES PATENT "OFFICE- FUSE Sigurd 1. Lindell, Chicago, n1., assignor m Schweitzer & Conrad, Inc., Chicago, lll., a corporation of Delaware Application November 6, 1935, Serial No. 48,551

22 Claims. (01. 200423) which would result in a current flow of sufficient ,magnitude to develop excessive temperatures, and impair the winding insulation, involving the possibility of complete destruction of the transformer. To cope with this situation the fuses must be capable of blowing at very, low current values. It is also required that the fuses be capable of blowing and clearing the circuit promptly in case of primary short circuit, to protect the system to the fullest extent against the disturbances that would result from prolonged At the same time the heavy short circuits. time-current characteristics of the fusible release element must provide sufficient time-lag to prevent the blowing of the fuse by transient overcurrents, such as energizing transients or switching surges.

By utilizing a compact mechanical linkage for providing high mechanical advantage, it is possible to use a fusible release element of small cross-section and low minimum fusing current, I

yet being capable of supporting the appreciable mechanical force required to propel the moving terminal in order effectively to bring the are ex- 1 tinguishing medium into play when a short circult current of considerable magnitude is to be interrupted at high voltage. A fuse of this type is disclosed in the Ramsey Patent No. 1,907,581, granted May 9, 1933. This construction is der pendent upon the time-current characteristics of a strand of high tensile strength, high melting point, corrosion-resistant material, such as- Nichrome V or Chromel A, consisting of an alloy of nickel\ and chromium. It is recognized that the use of a high melting point, corrosion-resistant material is desirable because it provides a dependable, per-' manent time-current characteristic, its physical properties remaining essentially permanent, and. 55 its. high melting point rendering it practically independent of ordinary variations in terminal and ambient temperatures. However, in the use of a fusible element of this type, there is a very definite limit, beyond which the ability of a fuse of a certain minimum fusing current to withstand transients cannot be increased by the usual means, although this. would be very desirable. This need for increased time-lag is particularly apparent where fuses are applied to protect potential transformers of a voltage rating of 132KV, or higher, the usual transformer design permitting very high transient currents to pass through the primary fuses, partly due to the capacity current flow between the end turns of the primary winding and ground, when the transformer is subjected to steep wavefront voltage surges.

It is the primary object of the present invention to provide a construction of the fusible link in a high tension fuse of the character set forth in the aforesaid patent, which construction will improve the time-current characteristic, that is, increase the ability of the fuse to stand transient surges without increasing the minimum fusing current, or, vice versa, to provide a fuse of lower minimum fusing current without reducing its ability to withstand transientsyet retaining all the desirable properties of the high melting point, corrosion-resistant fuse link material.

The manner in which I accomplish this will be apparent from the detailed explanation of my invention appearing below. Suffice it at this point to state that an essential principle involved is the control of the ambient temperature of the part of the link which is intended to part under the influence of excessive current flow and mechanical stress imposed thereupon. This control of the ambient temperature exercises a desired control upon the parting of the severable portion within the portion of the timecurrent range corresponding to low current long time operation, without adversely affecting the high currentshort time operation.

The normal load current, in installations of the type under consideration, constitutes only a small fraction of the minimum fusing current which must be provided in'the fuse element characteristic to give positive protection in case of secondary faults. Therefore, the normal operating temperature of the fusible section is only slightly above the ambient temperature and in discussing the time-current characteristics it may be assumed that the link will be blown starting at ambient temperature.

Consider first the case of a fusible element not subjected to mechanical stress, in which case some part of the fusible section must be brought to the melting point to start the circuit interruption. We find that the time-current curve of a plain wire supported in air between two terminals of relatively large heat capacity and good conductivity depends on the physical properties of the material employed and also on its physical dimensions, that is, on its cross-section and free length between the terminals.

That portion of the time-current characteristics which determines the ability of the fuse to withstand transients of high magnitude but of very short duration, corresponds to blowing currents greatly in excess of the minimum fusing current, and consequently very high current densities are involved. At these high current densities the fusible section is brought to the melting point so fast that the amount of heat lost to the terminals and to the ambient medium is negligible compared to the heat retained in the fusible section. The time required to blow the link by such heavy currents is essentially independent of the ambient medium and the free length between'the terminals, and is determined by the material and cross-section employed.

When a link is blown by currents equal to or slightly above the minimum fusing current, the temperature rise at the mid-point of the fusible section depends upon the rate at which heat is lost to the surroundings or is conducted to the terminals. With short free lengths, the latter factor is of considerable importance in determining the minimum fusing current or the time required to blow a link at currents slightly above the minimum fusing current. The minimum fusing current may, therefore, be altered by changing the free length between the terminals.

The reduction in minimum fusing current obtainable with a certain material and cross-section by increasing the free length can only be carried to the extent where the heat flow from the mid-point to the terminals becomes insignificant as compared to the heat loss from the same point radially to the circumambient medium through radiation, convection and conduction. There is therefore a definite limit to which this factor can be carried.

When a fusiblesection is stressed mechanically, the link will not reach the melting point but rupture at some lower temperature value. However, the same general conditions of heat flow that apply to the unstressed link determine the shape of the time-current curve produced by the link that is mechanically stressed, as in the particular type of high tension fuse under consideration.

In the case of very small diameters, and using materials that are poor thermal and electrical conductors, the amount of heat lost radially is of relatively great importance even at short free lengths, since the ratio between the radiating surface and the cross-section is large, and the heat conduction to the terminals is restricted by the thermal resistance of the wire. In such cases no further change in the time-current curve shape is possible by increasing the free length beyond a comparatively short distance.

In Figure 10 of the drawings hereto appended I have shown a test apparatus for determining the minimum fusing current of "Nichrome V wire of .005" diameter supporting a tension stress produced by a weight of 2%; oz'. of various free .lengths between terminals. The results of this experiment are plotted in Figure 11, where the ordinates are current in amperes and the abscissae are free length between terminals in inches. This graph shows that increasing the free length between terminals beyond will produce only a negligible further reduction in minimum fusing current.

A specific object of this invention is to provide a reduction in minimum fusing current beyond the limits stated above, or, in other Words, to produce a time-current curve of relatively greater time-lag. l

It has been shown that a condition is encountered where the minimum fusing current is determined principally by the rate at which heat is lost radially from the mid-pointof the fusible section to the circumambient medium. Considering this fact, my invention brings about the desired modification of the time-current curve by the introduction of means which are capable of effectively limiting said radial loss from the surface of the fusible section.

The rate at which heat is lost -to the surrounding atmosphere depends upon the temperature difference. I have introduced means of raising the ambient temperature to such an extent when currents equal to, or slightly above, the minimum fusing current are flowing through the assembly, that the loss of heat to the ambient medium has been greatly restricted, with the result that a much higher temperature is produced in the fusible section by any particular current.

These means do not appreciably alter the blow ing of the fusible link at high current densities because during very short time intervals no appreciable" amount of heat can escape from the interior of the fusible cross-section, since the conduction of heat to the surface is a comparatively slow process. Consequently, areduction in the minimum fusing current has been obtained without reducing the ability of the assembly to withstand surges or, vice versa, means have been provided to increase the relative timelag of a fusible link having a certain minimum fusing current.

Referring again to the curve (Figure 11) it will be apparent that the heating units employed to raise the ambient temperature do not influence the peak temperature in the fusible section by restricting the heat flow to the terminals, since the free length is already sufficient to render the temperature at the mid-point of said section independent of the lower temperatures existing at the endpoint of the fusible section.

. It should also be noted that in all the various designs that will be illustrated, it is intended that rupture should always occur in the fusible section proper because the highest temperature will always occur there and coincide with the reduced rupturing temperature produced by the mechanical stress. This insures a positive, predetermined release of the fuse mechanism at all currents equal to or greater than the minimum fusing current.

The temperature of the heating coils such as areemployed in an embodiment of my invention hereafter described could conceivably be made to exceed the temperature in the fusible section if the wire in the coils were made smaller than the cross-section of the fusible section proper. However, this would defeat the object of this invention, since at high fault currents the coil would fuse before the fusible section proper, reducing the ability of the assembly to withstand surges.

I am aware that heretofore attempts have been made to develop fuses in which a high resistance section in thermal conductive relation to a section of low melting point soft metal such as solder is provided. In such constructions the ambient temperature produced by uncontrollable factors such as weather conditions alone, is so close to the melting or yielding point "that the time current curve varies appreciably with the ambient temperature. But my present invention is not concerned with structures of that type. In accordance with my invention the heat which causes the link to part is generated within that portion of the link which parts, and the additional heat evolved irranotherpart of the link controls the ambient temperature dfthe portion which parts. Preferably, I employ a material such as a nickel-chromium alloy which is highly resistant to corrosion, of great tensile strength, and a high melting point or parting point under a suitable mechanical stress. It presents a high ratio of resistance compared to tensile strength. It is also of relatively low thermal conductivity. This fuse is preferably employed in conjunction with a mechanical advantage device, so that mechanical stresses may be coordinated with the characteristics of this material. teristics permit of a-high ratio of surface to cross section for currents of the order with which the present invention is particularly concerned. Hence at elevated temperatures," radiation from the central portion of the wire, where parting tends to occur, is important in controlling temperature for low current-long time operation. The control of the ambient temperature by the surrounding portion of the same conductor is therefore effective in securing the desired result of lowering the minimum fusing current for a given set of conditions.

While my invention is particularly applicable to a material of the above composition or characteristics, it is to be understood that the invention is not to be limited to the preferred material,-but is capable of producing an improvement in the characteristics of a fuse made of any suitable or desired material. While I describe the invention with reference to the highest present development in'low current fuses, it is to be understood that the invention maybe practiced in less advantageous'embodiments, and in' a wide variety of forms and arrangements.

Now in order to acquaint those skilled in the art with the manner of constructing and operating a device embodying my invention, I shall describe, in conjunction with the accompanying drawings, a specific embodiment thereof.

In the drawings:

Figure 1 is a longitudinal vertical section through a high tension, liquid filled fuse embodying my invention;

Figure 2 is a fragmentary side elevation, on an enlarged scale, with parts shown in section illustrating the stationary and movable terminals. and the fuse controlled linkage between them;

Figure 3 is a view similar to Figure 2, showing I the operation which occurs when the fusible link melts;

Figure 4 is a section taken on the line 4-4 of Figure 2, showing the construction of the thermal shield of the fusible link;'

Figure 5 is a cross section on an enlarged scale taken on line 55 of Figure 2;

' Figure 6 is a side view similar to'Figure 2, showing a' modified form of fusible link;

Figure '7 is a side elevational view similar to These charac- Figure 6 showing another form of the fusible link;

Figure 8 isa side elevational view, illustrating another modification of the fusible link;

Figure 9 is a chart, or graph showing the timecurrent characteristic curves for purposes of explanation;

Figure 10 is an explanatory diagram of an apparatus for determining the operating characteristics of a test piece of fusible conductor;

Figure 11 is a graph of the characteristics of a test piece as determined by the apparatus of Figure 10; r

Figure 12 is an explanatory diagram of apparatus for determining the operating characteristics'of a test piece of fuse conductor under specific conditions approximating its use in prior fuses} Figure 1 3 is a similar diagram of apparatus for de'terminingrthe operating characteristics of a a device constructed in accordance with my invention; and v Figure 14 is a tinie current curve of the characteristics of the two links shown in Figures 12 and 13.

For the purpose of explaining the influence of length of the fuse strand upon the minimum fusing current, I show in diagram Figure 10 and graph Figure 11, experimental apparatus and a graph plotted from data secured by readings on the apparatus of Figure 10.

Figure 10 shows a pair of power conductors G and H between which is connected 2. length of fuse strand I. The upper terminal J supports the upper end of the fuse strand I and is fixed to the bus bar or conductor G. The lower end of thefuse strand I is connected toa movable terminal K and it is connected by a flexible conductor N to the other conductor or bus bar H. A

weight M (in this case 2 oz.) corresponding to known length L and passing current in increasing quantity through, the same until the strand melts, the relation of fusing current to free length of strand in inches can be determined, and this relation is shown in the graph P of Figure 11, in which the abscissae is plotted in terms of free length in inches, and the ordinates in minimum fusing current. In this curve, and in the experiments to which it relates, time does not appear as a measured factor, since the approximate minimum fusing current, defined as that steady state current which would fuse the element in infinite time, is determined by a gradual approach in small increments of current.

The graph P shows that for the specific size.

v wire employed, namely, Nichrome V of a diameter of .005" suspended vertically in air and subject to a definite tension, increasing the length of the strand I beyond about produced negligible change in the minimum fusing current.

Referring now to Figure 1, I shall describe the general features of the preferred form of fuse in which my invention may be embodied. While I have shown a liquid filled fuse, the liquid supplying the arc extinguishing medium for producing an endwise Jblast through the arc, and -constituting also a dielectric medium for disconnection purposes, it is to be understood that this is by way of illustration and explanation and not of limitation, as other forms of fuses employing, for example, solid arc extinguishing material or other media may be employed to utilize my invention.

The fuse shown'comprises a glass sleeve I, upon the lower end of which is cemented, as by a suitable alloy or other cement, a cap or lower ferrule 2 closed at the lower end and having a boss 3 in which there is anchored a spring anchor 4 by means of a threaded stud 5. At the upper end of the sleevel the upper ferrule 6 is likewise cemented, as by an alloy or other cement. The interior of the sleeve-like ferrule 6 has an inwardly extending flange I which is threaded to receive the barrier plate 8. This barrier plate is a disc with a central hole surrounded by a flange 9 for receiving the arcing or blast tube l0.

This arcing tube is releasably secured in the barrier plate 8 as by a pair of matching grooves and an eccentric spring ring I2 which holds the tube Ill in the hollow boss or flange 9 against a predetermined force which may shear or break the ring I2 and expel the tube I6 in order to relieve internal pressure in the sleeve I and to save the same from breakage.

Above the flange 'I there is provided a counterbore I3 providing a ledge upon which rests the terminal plate I4 which consists of a flanged copper plate with a plurality of peripheral slots lengthwise of the flange to provide a series of spring fingers which engage the cylindrical surface of the counterbore I3 and rest upon the ledge on top of the flange I. The outer end of the ferrule 6 is closed by means of a pressure releasable cap I5 which is cemented to the end of the ferrule 6 in known manner to provide a seal which is fluid-tight and resists the action of the arc extinguishing liquid from the inside and atmospheric influences from the outside. The constructlon and cementing of this cap is known.

The terminal disc I4 has a central perforation through which extends a threaded copper stud I6 shouldered beneath the plate I4 and engaged by the nut I! to hold the-stud I6 firmly, electrically and mechanically, to the terminal plate I4. The terminal plate is connected through the stud I6 to one end of the fusible link designated as a whole by the reference numeral I8. The opposite end of the link is connected electrically and mechanically to a mechanical advantage device designated as a whole by the reference numeral I9. These will be described more in detail. The mechanical advantage device I9 comprises a series of levers 22, 23, 24, and 25, all of the second class, and connected in series between the fusible link I8 and the slotted copper tension link 26. The slotted link 26 comprises a strip of copper slotted at its upper end to embrace the levers 22 to 25 within the upper end of the slot. The link 26 is slotted at its lower end or may have one continuous slot to receive the cross pin 21 mounted transversely in the copper arcing tip 28. This arcing tip 28 has a longitudinal bore which .is intersected by the pin 21, the bore being large enough to receive freelythe lower end of the slotted link 26. The pin 21 is provided at'its central portion with a rounded annular groove 29, tending thereby to centralize the link 26- so 'as to provide an axial pull on the spring 30. The arcing tip 28 is threaded into a threaded recess in the upper rod-like terminal 32, which terminal projects up into the lower end of the arcing tube ID. The arcing tube I0 is preferably made or other suitable insulating material. The plate 8 may also be made of metal.

The rod-like movable terminal 32 has a central bore in which there is threaded the expanding screw 33 having a conical tip cooperating with radially seated pins 34 having their ends rounded and projected outwardly by the spreading screw 33 into a groove formed in the liquid director 35. The liquid director 35 may be made of suitable insulating material such as Bakelized fiber or the like, and preferably it surrounds the lower end of the arcing tube ID.

The length of the arcing tube In may, if desired, be increased, in which event the liquid director is disposed lower on the rod-like terminal 32, which may also be increased in length.

The lower end of the rod-like terminal 32 has a central threaded counterbore which receives the threaded stud 36 of a connector 31 for connecting to the said rod-like terminal 32 the flexible copper stranded cable 38 which forms a low resistance shunt for the spring 30.

The lower end of the flexible cable 38 is connected to the lower spring anchor member 4 either by a coupling such as 31 or by providing a socket in the spring anchor 4 which may be collapsed or pinched mechanically upon the lower end of said cable 38.

- The spring 30 has its upper end turns threaded into a groove formed exteriorly of the lower end of the rod-like terminal 32 and the lower end turns of the spring 30 are threaded into a groove formed on the exterior surface of the lower spring anchor member 4.

While certain of these details constitute novel features in the construction of a fuse, it is to be understood that for the purpose of embodying my present claimed invention, these details may be widely varied.

Referring now to the construction shown in Figures 2, 3, 4, and 5, the fusible link I8 and its connections will now be described.

The copper stud l6 has an annular flange forming a shoulder 39 for engaging the bottom surface of the terminal plate I4, and below this flange 39 there is formed an integral transverse tongue 46 with parallel flat sides against which are clamped the two insulating plates 42 and 43 (see Figure 5). These plates are preferably of Bakelized fiber or Bakelized cloth. They are secured at their upper ends to the said tongue 40 by a transverse rivet 44. This rivet clamps under its head the connecting plate and the rivet is expanded into a hole in the tongue 40 to provide electrical conductivity, the rivet being made of copper. The stationary terminal 46 which constitutes the upper anchorage for the fusible link, comprises a rectangular shank 41 clamped between the plates 42 and 43 and held in place by the two copper rivets 48 and 49, the latter of which holds under its head the lower end of the copper strip 45 to provide an electrical connection between the said shank 41 and the tongue 40 of the stud member I 6. The stationary terminal member 46 has a head extending out to the left of the plates 42 and 43, as viewed in Figures 2 and 5. This head is drilled and counterbored. to provide a tubular socket 50 (see Figure 4) which receives the insulating sleeve 52 of lava or like refractory insulation. The outer end of the sleeve 52 is slotted as shown at 53, to permit the upper end of the strain wire 54 to be led from inside the bore of the sleeve 52 to the outside thereof, where this wire is wrapped in closely disposed coils 55 and the end of the wire, as indicated at 56, is then wrapped in a slot 51 formed in the head of the member 46 and there securely held, mechanically and electrically, by battering over the slot upon the end of the wire. The size of these parts in practice may be realized from the fact that one commercial form of the device here described employs a fuse wire 54 of a diameter of .005 inch. The lava sleeve 52 in. such case may be of a diameter of inch, and of a total length of inch. These dimensions are not intended to be limiting, but to illustrate the size of a suitable commercial embodiment.

The lava sleeve 52 is held rigidly in its socket by the slot 58 and rivet 59. The socket is preferably formed at an angle such that the pull of the wire 54 is directly in line axially with the sleeve 52. I

The lower end of the wire 54 is passed through a slot 60 (see Figure 2) which slot is pinched upon the wire to hold it. The free end of the wire is passed aroundseveral corners as by being laced through adjacent openings as shown in the end of the lever 25 in Figure 2. This anchorage should be made mechanically and electrically solid and also should be a good thermal connection.

The levers 22, 23, and 24 are sustained entirely between the plates 42 and 43 by means of pivot pins which are set in recessed holes drilled from the inner surfaces of the plates 42 and 43 so that these pivot pins may not become displaced.

The lower lever 25 preferably has a through pin the ends of which are pinched to expand the same, as appears at 63 in Figure 5.

When the fuse link l8 parts, as will be described later,the levers 22 to 25, inclusive, swing about their pivot pins to release the link 26, as

will be apparent from Figure 3. The link 26,

losing support from the levers, allows the rodlike terminal 32 to be pulled down by the spring 30 to lengthen the are which is produced at the end of the link 26 and to produce the are extinguishing action-which will be described later.

The wire 54 is preferably a nickel-chromium alloy purchased on the market under the trade name of Nichrome V or Chromel A. It is of a very high melting point as compared with ordinary fuse links of zinc, tin, lead, copper, etc.

It has a very high tensile strength and retains that strength even under elevated temperature.

.It is also highly resistant to corrosion.

The said nickel-chromium alloy of which the wire 54 is formed does not lose its mechanical strength at temperatures as high as red heat and likewise it is not subject to-corrosion or oxidation at temperatures as high as red heat.

The surface of the fuse wire 54 is preferably oxidized except at the ends, where electrical and mechanical connections are 'made to the same, and the turns of the coiled part 54 are preferably in contact with each other, but due to the insulating character of the oxide and the small potential difference between coils, the passage of current takes place in series through the turns.

The 'fuse thus far described is intended for the protection of potential transformers and for like service where a very small flow of current constitutes the normal load, and where the fuse is to be rated at /2 ampere, by way of example. The spring 30, in order to provide a satisfactory motion of the movable terminal and liquid director, provides a relatively heavy pull, which may be of th order of from 6 to 15 pounds or more. The mechanical advantage device It reduces this only a fraction of the temperature created by currents capable of blowing the fuse.

j When a current equal to or slightly in excess of the minimum fusing current is caused to flow through the element assembly the heating coils concentrate a considerable wattage per unit axial length and, although the exterior surface of the coils is free to radiate heat to its surrounding, they tend to attain a temperature rise that approachesthe temperature of the straight portion of the strand 54 within the tube 52.

The coils are wound upon the core 52 and arein thermal contact therewith. Therefore the rising temperature of the coils will produce a flow of heat towards the inner surface of the lava core 52, which at this stage also absorbs part of the heat generated in the straight portion of the strand 54 within the tube 52. Some of this heat is transmitted longitudinally through the tube 52 to the supporting bracket 46. The ends ofthe straight strand 54 are cooled by heat flow to the anchor points, while the midsection, particularly inside of the tube, is climbing in temperature.

The temperature rise of this section is enhanced and accelerated-by the increasing temperature of the inner surface of tube 52, brought about primarily through the concentrated heat generation at its outer surface. Since the radial loss of heat from the straight portion of strand 54 to the inner surface of the tube is a function of the temperature difference between said two bodies, eventually a temperature is reached by the strand 54, inside of the tube, at which rupture occurs.

Now it may be seen that the current value at which this rupture occurs is less than thevalue required if the exposed strand 54 alone formed the fusible link. I have conceived therefore that a heavier strand of wire may be employed for a fusible link of a given minimum fusing current because, by the coils 55, I am able to control the ambient temperature about the strand 54.

The above mode of operation prevails in actual installations, for example, where the fuses are applied on the primary side of a potential transformer if a secondary short circuit or other high impedance insulation fault occurs.

On operation at very high current densities, such as may correspond to primary short circuits in a potential transformer installation, the development of heat in the strand 54 is so great and so rapid that there is practically no exchange of heat between the various parts of the element assembly. Therefore, the effect of the coil 55 and the core 52 is wholly negligible. The fusi'ng'or rupturing of the strand 54 under these conditions depends essentially upon the material from which the strand is made and upon its cross-section.

The structure as a whole then provides a time-- current characteristic of relatively greater time with high voltage transformers when the same are first connected or energized, the current may reach instantaneous values of a high order. To such surges or momentary overloads the fuse should not respond, but fuses of the prior art have been incapable, in many applications, of standing up under them, or else they were deficient on low currentlong time operation.

By my construction, with a desired low currentlong time performance, it is possible. to provide a fuse that will stand three times the short time current, or, stated in terms of time, to increase the time of fusing for a given high value nine times that of prior fuses employing high melting-point fusible links.

The increase of strand diameter made possible by the invention is an important feature, particularly in regard to corrosion which may occur. Also, it reduces corona; also, mechanical strength is increased.

Referring now to the curve of Figure 9, it may be seen that, assuming within a. given period of time-for example, four minutes-an open strand of wire such as 54 will melt at a given flow of currentfor example, ampere. The characteristics of such an open strand may be plotted to form the graph or curve A. At the point B the said strand will melt at ampere applied for four minutes. If, however, the structure of my invention, such as illustrated in Figure 4,

be utilized, the tendency for the wire to run a higher temperature and hence melt at lower current for four minutes'fiow will cause blowing at the point C on the dotted line graph E.

The graph E at the low current-long time end is considerably lower than that of the graph A. Now, if it is desired to have the fuse link of my invention blow at point B, a larger wire for the strand 54-55 may be substituted and then the blowing of such a strand may be brought about at the point B, and it will then have the characteristics of the graph curve F. Considering, therefore, the graph E, it may be stated that the present invention permits the blowing of a certain size strand at a lower current value, or, considering the graph F, it may be said that my invention permits a larger size strand to be employed with consequent improvement on the high current-short time end.

Referring now to Figures 12, i3, and 14, I have shown in Figures 12 and 13 apparatus similar to that shown in Figure 10 for determining the time-current characteristics of a given size of fuse strand under the conditions of the prior art as shown in Figure 12 and under the condition of my invention as illustrated in Figure 13, and from the results of experiments with these, the graph of Figure 14 has been plotted. This graph is not intended to be an accurate exposition of the quantities involved, but does portray tendencies by the shape of the curves.

Figure 12 is substantially identical with the apparatus shown in Figure 10 but in this construction the free length of the strand I is fixed at and the readings of the observed phenomena are with respect to time required for a given value of current to cause parting of the strand I. The apparatus of Figure 13 shows the fusible link corresponding to the link l8 heretofore described, the strand 54 being attached to the lower movable terminal K as in Figure 12, the weight M pulling upon the movable terminal K as in Figure 12 and the upper end of the link l8 being connected to the stationary terminal J. A copper supporting plate 0 corresponding to the bracket 46 supports the lower end of the lava sleeve 52, simulating the illustration of Figure 4. From the data obtained by tests of the free fuse length of the graph or curve Q has been plotted, and from tests of the apparatus shown in. Figure 13 the graph R has been plotted.

Now it will be observed that on the high current end the two graphs coincide, but at values around one (1) second the two curves diverge sharply, the curve R dropping far below the graph Q. Now, by increasing the size of the strand or wire used in the apparatus of Figure 13, it is possible to secure a curve the lower end of which will be tangent to the curve Q and the upper end of which will fall above the common portion S of the two curves Q and R. In other words, by increasing the diameter of the wire it is possible to produce a curve which would be substantially parallel to the curve SR and tangent at the lower end of the portion R to the lower end of the portion Q so that the low current operation would be identical with the low current end of the curve Q, but the high current end of such curve for larger size wire would fall above the common portion S to produce, as I have indicated, a fusible element capable of sustaining very high current densities of three times the value of a standard fuse of the prior art, or, in terms of time, capable of carrying the same current nine times as long as a fuse of the prior art.

It will be observed that the curve R has a hump therein substantially at the intermediate or divergent portion. This hump is caused by the lava sleeve 52 which, at intermediate values, delays the rise of temperature on the inner strand 54, both by failure to transmit instantly the heat of the coil 55 to the inner surface of the core and also because of its thermal capacity to absorb heat given off by the strand 54. In the form of my invention shown in Figure 8, which I shall describe hereafter, the hump to which I have referred in the curve R does not appear, inasmuch as there is no intervening medium between the surrounding coils and the central strand.

The embodiment shown in Figure '7 produces a curve similar to the curve B shown in Figure 14. The curve exhibited by the embodiment shown in Figure 6 is substantially the same as that of the embodiment of Figure 8. The lava sleeves employed in Figures '4 and 7 are utilized more as a mechanical expedient than for their thermal properties.

In Figure 7 I have shown a modified form of the link H8. The lower end of the strand 54, which is adapted to be severed under overload, is passed through one bore 65 of a parallel bore lava sleeve 66 and is then passed over an insulating pin 61 and through the other bore 68. The wire is then wrapped upon the outside surface of the lava sleeve 66 and the end 69 is anchored to the conducting terminal attached to the stud iii. In this case it will be seen that the tension is concentrated upon the free strand 54 between the lower lever 25 and the pin 61, which pin acts as a snubbing pin to relieve the remainder of the wire of the mechanical stress to which the straight portion of the strand 54 is subjected.

The theory and general mode of operation of the device shown in Figure 7 is not substantiall different from that shown in Figures 2 to 5. In

each case the straight strand 5'! tends to rupture 75 within the thermal shield, that is, within the bore of the lava sleeve such as 52 or 66, as illustrated, for example, in Figure 3. Some of the heat from the lava tube 66 tends to travel towards the copper tongue or block 40 against which the upper end of the sleeve 66 abuts. This construction relieves the mechanical necessity of clamping the sleeve in place, since the tension of the strand over the pin 61 tends to hold the lava sleeve in the desiredposition.

In Figure 6 I have shown a modified form of a fusible link 18 in which there is employed a length of nickel-chromium wire 12 corresponding to the straight portion 54 of the previous link.

The lower end of the wire 12 is securely anchored in the end of the lower lever 25. The upper end of the strand [2 is, in this case, passed through a metal fitting 16, the bore of which fitting is pinched upon the upper end of the wire 12 to provide mechanical and electrical connection. The fitting 16 has a socket for receiving a sleeve 1'! of a refractory conducting material such as Carborundum or the like. This sleeve is resistant to the flow of current and develops heat by the passage of current in series therethrough. The strand l2 loosely passes through the bore in the sleeve 11 and the flow of current is therefore through thesleeve TI and the strand 12, in series. The heat flow from the lower end of the strand is through copper lever 25. Heat developed in the strand 12 finds difficulty, however, in flowing through the upper terminal 46 because of the development of heat in the sleeve ll. Some of the heat flows in the sleeve 11 towards the lower end thereof to the terminal at 4B. As a result, the temperature of the strand i2 is at the maximum within the upper portion of the sleeve 'l'l where the strand parts when the fuse operates to interrupt the circuit.

While in the form shown in Figure 6 no insulating sleeve is inserted between the strand I2 and the concentric heater portion 11, the same general mode of operationand the same general results are secured. That is to say, the strand 12, if of a given. size wire, will melt on lower current on the long time-low current end, or, if the wire be increased to a larger size, the strand will part atthe desired time-current value in the low current end, with improved operation on the short timehigh current end.

. In Figure 8 I have shown a further modifica. tion, in which the strand 18 extends through a coiled portion 18a, which isan integral continuation of the strand 18, this coiled portion being the thermal barrier heretofore referred to.

The link 18 of Figure 8 consists of a continuous strand of wire which is bent back into a loop, as indicated at I9, coiled about itself as indicated at 18a, and then the end thereof passed throughthe loop!!! to provide the anchor end which is laid in a slot in the outer surface of the tongue-like extension 40 of the stud memher i B. The slot is collapsed upon the upper end 80 to form a good mechanical and thermal connection and to serve as an electrical coupling. The ends of the nickel-chromium wire are bright, but the intermediate portion, including insulation to cause the current to flow through the turns of theheating coil portion 18a. In.

practice, the tendency is to sever the strand 18 within the coil 18a substantially at the,point indicated by the endqof the lead line of the reference numeral 18a. l

In considering the action of this specific embodiment it will be understood that the strand l8 and the end 80 tend to-transmit heat by conduction to the terminal members 40 and 25, depending upon temperature of they wire, its thermal conductivity, and its length. Also, to some extent, there. is convection and radiation, but radiation is not. a large factor until a considerable difference of temperature is attained between the hot body and the cold body to which it may radiate heat. The conduction of heat from the coiled portion 18a to the lower terminal 25 is somewhat less than that which occurs through the loop 19 and upper end 80 to the upper terminal 40 because of the spacing indicated. This is an incident to the specific structure shown and may optionally be modified.

While I have shown in each of the above cm bodiments a heating portion which is directly in series relation between the terminals, it is to be understood that the two parts of the link, namely, the heater and the severable strand, may be otherwise arranged than the particular embodiment shown.

It is to be observedthat while the heat which caused parting of the strand is generated by the strand itself, it may be possible, in an embodiment such, for example, as Figure 6, to employ a material of a. different specific resistance from that of the strand itself to control the ambient temperature either higher or lower than would be done by coiling the strand upon itself.

In the device of my invention the radiation of heat radially from the inner strand such as 54 is restricted or limited primarily by the lack of difference in temperature; I do not try to keep the heat from being conducted radially through interposition of a poor conductor, but erect a barrier of substantially the same temperature so that the temperature gradient permitting escape of heat radially has been greatly reduced.

, v .I do not intend to be .limited to the particular embodiment of fuse mechanism herein shown,

although the particular link force multiplying which is of such minute dimensions, to be resistant to chemicals which might produce a slight degree of corrosion, for even a small degree of corrosion would seriously change the cross-set:- tion of a strand of the small dimensions herein disclosed. The liquid fuse shown in Figure 1 preferably carries the filling of liquid ata level approximately the lower end of the arcing tube It. This liquid may be a substitution product, such as a halide derivative of a hydrocarbon, many of which are now known and disclosed in patents granted to my assignee:

I claim: v

1. A fuse link comprising asingle continuous wire, one part ofthe wire being wrapped about another part, to control the radiation of heat from the inner part whereby to lower the minimum current at which the wire will rupture.

2. In combination, a fuse link comprising a single continuous wire ofhigh melting point metal, one part of which wire is wrapped about another part, and a pair of terminals between which the wrapped part of the wire is supported in tension.-

' 3. In combination, a pair of separable terminals, means tending to separate said terminals,

a strand of high tensile strength metal holding said terminalsv against separation and being connected electrically in series with said terminals, and means in series with said strand and being heated by the same current which flows through said strand for controlling the ambient temper ature of a part of said strand to cause parting of the strand upon the passage of a current in excess of a predetermined minimum value for more than a predetermined minimum time.

4. In combination with separable terminals, a fusible link connected between said terminals and comprising a part which is adapted to be severed, means acting upon one of said terminals for tensioning said part of the link, another part of said link surrounding said first part and being independently heated by the current flowing through said link, said latter part serving as a radiation shield for said first part, whereby heat generated by said first part is radiated less freely than is heat generated by said second part, the part of the link which is tensioned having a melting point at a temperature above red heat and being adapted to be severed by flow of current therethrough above a certain minimum value.

5. A fusible link for high tension fuses of small load current rating, comprising a strand of metal which does not lose its strength at a temperature substantially corresponding to red heat, said strand being adapted to be melted upon the attainment of a predetermined temperature by flow of current 'therethrough in excess of a predetermined minimum value and a mass of conductive material connected in series with said strand and serving as a thermal barrier for a portion of said strand -to control the ambient temperature of the said portion.

4 6. In a high tension fuse of small load current rating, the combination of a fusible link comprising a strand of metal which does not lose its strength at a temperature substantially corresponding to red heat, said strand being adapted to be melted and severed upon the attainment of a predetermined temperature by the flow of current in excess of a predetermined value therethrough, means for placing said strand under a predeterminedtension, and a mass of current conducting material surrounding a portion of said strand and serving to control the ambient temperature of said portion.

'7. A fusible link for high tension fuses of small load current rating, comprising a strand of nickel-:chromium alloy,.a mass of material resistant to corrosive influences at elevated temperatures surrounding said strand, said mass of material being. connected in series with said strand and being heated by the same flow of current which heats said strard and serving as a barrier for radiation of heat from said surrounded portion whereby to cause said portion to attain a higher temperature within a given time for a given flow of current, said strand being adapted to part upon the flow therethrough of a predetermined current for a predetermined time. I

8. In a high tension fuse of small load rating, a pair of spring separable terminals, a strand of metal which maintains its strength and resists oxidation at substantially red heat, said strand being connected electrically between said terminals and stressed in tension, and a coil of metal connected in series with said strand and surrounding a portion of said strand and serving as a radiation shield for said portion, said strand being adapted to be softened and severed by the flow of current therethrough in excess of a predetermined minimum value for a predetermined period of time.

-9.' In a high tension fuse of small load rating, a pair of spring separable terminals, a strand of metal connected electrically between said terminals and stressed in tension, and a tubular resistance conductor connected in series with said strand and surrounding a portion of the same and serving as a radiation shield for said portion, the metal of the strand maintaining its strength against separation and its resistance to oxidation at temperatures substantially as high as red heat.

10. The combination with the link of claim 5 with spring separable terminals and wherein said mass of material is held in compression by tension in said strand.

11. The combination of claim 8 wherein the coil is an integral continuation of said strand.

12. The combination of claim 8 with a refractory insulating sleeve between said coil and said strand.

13. In a fuse of the class described, a fusible link of low current rating comprising a pair of concentric portions electrically connected in series, both portions being resistant to corrosion and of high melting point, said outer concentric portion being free to lose heat and preventing by its temperature free radiation of heat from said inner portion, the inner portion being adapted to operate at higher temperatures than the outer concentric portion on prolonged overloads.

14. In a fuse of the class described, a pair of metal fuse terminals, spring means tending to separate them, a link supported on said terminals and comprising two concentric conducting portions, the inner portion being maintained under tension by said spring means, the outer portion being maintained in compression by said spring means the inner portion being severed upon the occurrence of excessive current flow through the link.

15. In a blast type high tension fuse, a pair of terminals, a fusible link electrically connecting said terminals, spring means tending to separate said terminals, a mechanical advantage device by which said fuse link in tension restrains said spring from separating said terminals, and an arc extinguishing material for quenching the are formed upon melting of the link, said link comprising a strand of nickel-chromium alloy wire in tension, and a concentric conductor forming a thermal radiation shield and sur rounding a portion of said strand and being traversed by current flowing between said terminals.

16. In combination, a conductive bracket having a slot, a movable terminal member having a slot, a high tensile strength high melting point wire having its free ends gripped by collapse of the slots thereupon, and a portion of the wire wound about another portion of the wire intermediate said free ends to provide a thermal radiation shield.

17. In combination, a pair of relatively massive metallic terminals of good heat conductivity, spring means for separating them, a fusible link of relatively short length having free ends anchored to said terminals, said link comprising a fine high tensile strength strand electrically connected in series between said terminals and mechanically andthermally connected to one of' them, and a concentric thermal radiation shield about said strand free on its exterior surface to radiate heat and connected in series be I tween said terminals.

- thereof fastened to said first terminal, and a metal terminal for the other end of the strand. 19. A iu'se link suitable for high voltage low current "circuits such as a high voltage potential transformer comprising a small diameter wire of high tensile strength and of a high'melting point and substantially free of corrosion or oxidation at temperatures at which its tensile strength is reduced, means for loading the wire with a definite load in order to sever the wire when its tensile strength is reduced by the passage of a predetermined minimum current for a prede termined time, and a heating element in series with said wire and surrounding a part of the wire and being so proportioned with respect to current flow that when the said predetermined minimum current flows through the wire and the element 'for said predetermined time the tensile strength oi the wire will be reduced and the wire will be severed before the element is fused.

20. A fuse link suitable for high potential low current circuits comprising a fine strand of metal which weakens at a temperature above red heat and which is substantially iree of oxidation at the temperature at which it weakens, and a heater surrounding the strand and being connected electrically in series with the strand, and a pair of metal terminals between which the link is supported in substantially free space and to which said link is electrically connected, and means for separating the terminals and severing the strand when current flow weakens-the strand. 21. A fusible link comprising a strand having ends, and having its intermediate portion coiled about itself and mechanically capable of resisting tension, said strand being of such ratio of length withrespect to cross section and heat conductivity that the current value at which the part of the strand within the coil melts is substantially independent or the temperature of the ends of v the strand.

22. In a high tension fuse, a fusible link of low current capacity, terminals adapted to be separated by spring tension, a mechanical advantage device to assist the fusible element in restraining the spring tension, and a heating-element surrounding a portion of the fusible element between the terminals and electrically in series with the fusible element, to assist the i'usible element in arriving at fusing temperatures at low rates of current flow.

SIGURD I. LINDELL.

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