US2400491A - Protector for electric circuits - Google Patents

Description

May 21, 1946 I M. F. DUERKOB 2,400,491

PROTECTOR FOR ELECTRIC CIRCUITS Filed May l5, 1943 mi@ HN l 1 INVENTOR.

Patented May 21, 1946 PROTECTOR FOR ELECTRIC CIRCUITS Manfred F. Duerkob, St. Louis, Mo., assignor to McGraw Electric Company, Elgin, Ill., a corporation of Delaware Application May 15, 1943, Serial No. 487,419

27 Claims. (Cl. 20o-123) This invention relates to improvements in protectors for electric circuits. More particularly the invention relates to improvements in enclosed protectors for electric circuits that comprise a fuse link and a thermal cut-out.

It is an object of the present invention to provide an improved enclosed protector for electric circuits that comprises a fuse link and a thermal cut-out.

Protectors for electric circuits that comprise a fuse link and a thermal cut-out are desirable because they will open the circuit before overloads can cause injury to the circuit, but will not open the circuit needlessly. Such protectors have the quick opening of the fuse link on heavy overloads as well as the desirable time lag of the thermal cut-out on lighter overloads. When, in this description, the Word circuit is used, it does not refer to the wiring alone but refers also to any electrical devices that may be a part of the circuit protected by these protectors.

Protectors that have a fuse link and a thermal cut-out all have the same fundamental purpose and object, but their form and structure may differ considerably. The difference in form and structure may be due to space limitations as well as to considerations of performance and function. Space limitations in protectors that are used in fuse clips, already installed and in use, are standardized; and often present an almost insurmountable obstacle to the making of an enclosed protector for electric circuits that comprises a fuse link and a thermal cut-out. Not only must the protector include an additional means to give the desired time lag, but it must be small enough to flt into an enclosure intended for a fuse link only, and such an enclosure is often quite small.

The problem of making enclosed protectors for automotive, aircraft and radio work is particularly difficult, because such protectors must be operable in cylindrical cartridges that may have outsidel dimensions as small as one and one quarter inches in length and one quarter of an inch in diameter. Accordingly, the dimensions of such protectors must be very, very small. From this it is clear how difficult it is to make such protectors, and in fact, the building of such protectors has heretofore been regarded as 'impossible. The present invention makes possible such a protector by utilizing a novel form and structure which uses a minimum number of parts and makes several of the parts movable. It is, therefore, an object of the present invention to provide a protector for electric circuits that is operable in small enclosures by using a limited number of parts and by making several of the parts movable.

Protectors for electric circuits that are intended for automotive and aircraft work may be, and usually are, subjected to considerable vibration. Where this is the case, the protector must be constructed to withstand considerable vibrations or it will have a relatively short effective life. The present invention provides a long lived protector for electric circuits by resiliently spacing at least some of the parts of the protector away from the inner surfaces of the enclosure. Such an arrangement is desirable because it absorbs the vibration imparted to the enclosure and minimizes the vibrating done by these parts of the protector. It is therefore an object of the present invention to provide an enclosed protector for electric circuits wherein atleast some of the parts are resiliently spaced away from the inner surfaces of the enclosure.

In the fuses heretofore used in automotive, aircraft, and radio work, the fuse links have been initially spaced away from the inner surfaces of the enclosure to insure uniformity of operation of the protector. This initial spacing is incapable of preventing subsequent contact between the link and the inner surfaces of the enclosure and particularly is this true where the fuse link is quite long, and is subjected to considerable heat. In such cases, the link may expand suiciently to cause a portion thereof to touch the inner surface of the enclosure, and this touching will disturb the performance and operation of the protector. The present invention obviates such touching and thereby insures a uniform performance for theprotector by providing a fuse link that is short and stiff and is not heated excessively. When this fuse link expands due to heating, it cannot touch the inner surfaces of the enclosure because it is so short that its expansion will be insuliicient to cause it to contact the inner surfaces of the enclosure and the link cannot get hotter than the low operating temperature of the thermal cut-out. It is, therefore, an object of the present invention to provide an enclosed protector for electric circuits in which a short stiff fuse link is used.

In protectors for electric circuits that have a fuse link and a thermal cut-out, a member may be provided that absorbs an appreciable portion of the heat generated in the protector. This member is not the only member in the protector that absorbs heat, but it is constructed and arranged to absorb an appreciable portion of the heat generated in the protector. It is desirable that this member always be spaced away from the inner surfaces of the enclosure to insure uniformity of operation of the protector. This is necessary because the absorber is movable and any contact between the enclosure and the absorber would necessarily be variable. Such variable contact would change the rate at which heat would be transferred from the absorber to the enclosure and would thus change the time lag of the device. Such changes are objectionable because they prevent uniformity of performance of the protector. It is therefore an object of the present invention to provide an enclosed protector for electric circuits wherein the heat absorber is spaced away from the inner surfaces of the enclosure at all times.

It is often desirable in protectors for electric circuits to use the heat absorbing element as van arcing contact. Where this is done, the absorber will help extinguish any arcs that may form when the protector opens the circuit. It can do so because its relatively large mass tends to cool the arc and such cooling tends to extinguish it. Rapid quenching of an arc is desirable because it tends to prevent possible injury to the protector and to nearby objects. In the present invention, a heat absorbing member is used as an arcing contact and it is also movable. 'I'his not only permits it to cool the arc, it enables it to draw out the arc and cause it to break. It is, therefore, an object of the present invention to provide an enclosed protector for electric circuits in which the heat absorbing element constitutes a movable arcing contact of the protector.

In the operation of the present invention it is desirable to retard the rise in the temperature of a mass of heat softenable material. This retardation may be attained by limiting the rate at which heat is conducted to the mass and also by absorbing heat from the mass. In the present invention. the limiting of the rate at which heat is conducted to the mass is achieved by arranging the heat generating elements of the protector to dissipate an appreciable portion of their heat by radiation; and the absorbing of the heat from the mass is achieved by placing an element that is constructed and arranged to absorb an appreciable portion of the heat generated in the protector in intimate thermal relation with the mass. Such an arrangement of parts insures positive operation of the protector, and also insures the provision of a desirable amount of time lag. It is therefore an object of the present invention to provide a protector in which a mass of heat softenable material is positioned in intimate thermal contact with a heat absorber and is heated by elements that dissipate an' appreciable portion of their heat by radiation.

In the operation of the present invention, a portion of the protector is moved to ,open the circuit. Because of the space limitations it is necessary that this movement be longitudinal of the enclosure to provide a long arcing gap. A long arcing gap is desirable because it quenches the are more rapidly than a short gap can. It is therefore an object of the present invention to provide an enclosed protector for electric circuits in which the moving part moves longitudinally of the enclosure to open the circuit.

Other objects and advantages of the invention will become apparent from an examination of the drawing and accompanying description.

' In the drawing and accompanying description,

a preferred embodiment of the invention is shown and described, but it is to be understood that the drawing and accompanying description do not limit the invention and the invention will be defined by the appended claims.

In the drawing:

Fig. 1 is a partial cross-sectional plan view of the protector provided by the invention.

Fig. 2 is a partial cross-sectional elevational view of the protector shown in Fig. l, and

Fig. 3 shows the protector after the thermal cut-out has opened the circuit.

Referring to the drawing in detail, a cylindrical enclosure is shown that consists of a tube I0 and end caps I2 and Il. The tube I0 may be made of any suitable material and made in any suitable size. In the very small sizes, for which the present invention is particularly useful, the tube Il may be made of glass, but it may also be made of any other suitable material. Glass is particularly desirable because it enables the user to determine quickly whether or not the protector has operated to open the circuit. I'he end cape I2 and Il may be made in any suitable size and are made from metal and are used to connect the protector with the circuit to be protected. In such a case, the end caps I2 and Il constitute the contact terminals of the protector. Positioned within the enclosure and secured in good thermal and electrical conducting relation with one cap I2, as by relatively high melting point alloy I6 or any other means known to those skilled in the art, is a fusible heat generating element or fuse link I8. The fuse link I8 is preferably made relatively short and stiff so it can support Itself in cantilever fashion. Similarly positioned in the enclosure ,and secured in good thermal and electrical conducting relation with end cap Il, as by relatively high melting point alloy 20 or any other means known to those skilled in the art, is a heat generating element 22 that is also relatively short and stiff and can support itself in cantilever fashion. The end of element 22 adjacent end cap I4 is provided with a hook 23 to which a spring 24 is-attached. The other end of spring 24 bears against an enlarged portion 21 at one end of a member 26. The member 26 is arranged to receive the inwardly extending ends of heat generating elements I8 and 22 and is arranged to receive the heat softenable material 2l that normally maintains the parts of the protector in electrical conducting relation between end caps I2 and I4. Element 28 is preferably made substantially U-shaped and has the general appearance of a trough. The end 21 has a transverse dimension larger than the principal transverse dimension of element 26, and the opposite end has an upstanding portion 25 that extends upwardly to a level below the level of the sides of the trough. The member 2l is preferably made of material having good thermal and electrical conductivities and one such metal is copper, but any suitable material can be used. The upstanding portion 25 of element 2l will underlie a portion of the fuse link I8 and may even touch it. This upstanding portion 25 facilitates the provision of a short current path through the solder 28 between element Il and 26. Such a short cur rent path is desirable because the conductivity of the solder 28 is not as high as the conductivity of element 26.

The spring 24 is preferably formed so it has two different diameters. The smaller diameter is less than the width of the enlarged end 21 of element 26 and the larger diameter is larger than the end 21 of element 28 but is smaller than the inner diameter of tube I6. Such an arrangement permits a locking of the spring 24 and element 28 without the use of pins or other holding means,

and permits the operation of spring 24 inside of the .tube I0.

Element 22 is angularly disposed with relation to the axis of element 26 and extends through the enlarged end 21 of that element. This element is secured to element 28 by the heat softenable material 28 and such securement normally prevents the movement of element 26 and 22 relative to each other.

Elements I8 and 22 are required to be conductors of electricity and to possess sufilcent strength and resilience to support the member 28 in cantilever fashion. In the present commercial embodiments of the invention, these elements are made of metal. However, it is the ability of elements I8 and 22 to perform as specified herein, rather than their particular composition, that is vital.

This protector is well suited to production line assembly. The trough-shaped member 28 and elements I8 and 22 are placed in a suitable lig. This jig preferably permits the element I8 to rest on and be supported by the upstanding portion of element 26 and permits element 22 to be angularly disposed relative to the axis of element 26.

Heat softenable material 28 is then placed in the trough-shaped member 28 and permitted to solidify. Such solidiflcation forms an integrated unit that is quite stiff and unyielding. The large end of spring 2l is then slipped over the end of link I8 and is moved relative to the integrated unit until the small diameter of the spring engages the enlarged end 21 ot'member 28. Continued movement of the large end of spring 24 extends the spring until the enlarged end is slipped over and held by hook 23 on the end of element 22. This makes an assembly that is unitary in character and can be secured into tube I and attached to end caps I2 and I4 by solder or any other means known to those skilled in the art. It is not essential that this manner of assembly be observed, because other methods of assembly are entirely feasible but the method stated above is very desirable.

`In the ordinary operation of the protector, current will flow through end cap I2 into link I 8. Part of the current will iiow directly through solder 28 to the other end of member 28 and part of it will flow through solder 28 into element; 28, through element 26 and back into solder 28. The current will again separate, but most 'of it will flow through element 22 and only an insignificant part will ow through spring 26 to end cap I l. The exact amount of current flowing directly through solder 28 will be determined by the conductivity of the solder 28 and the conductivity of element 26. This proportion of current is not critical and can vary considerably Without affecting the operation of the device. In the case ofthe spring 24, however, the amount of current is critical and it is necessary that the spring be substantially free froml current ow, because current flow in a spring can sometimes injure the spring to the extent that its resilient characteristics are seriously impaired. Where this occurred, the operation of the protector would be injurlously aected. Such is not possible in the present.case because the spring is considerably longer than the element 22 and has a much higher resistance, In the commercial protectors that are made in accordance with this present invention, the spring carries only three ten-thousandths of the current carried by the protector. The passage of normal current through the protector will cause elements I8 and 22 to generate some heat, but that heat will be insufficient to raise solder 28 to its melting point or to cause element I8 to fuse.

When the protector is subjected to overloads, below the short circuit range, that if unduly prolonged would be injurious, elements I8 and 22 generate additional quantities of heat, Some of this heat will be dissipated by radiation into the atmosphere in the enclosure and some of it will be conducted into solder 28. Some of the heat conducted into solder 28 will be absorbed by element 26 and some of that absorbed heat will be radiated into the atmosphere in the enclosure by element 26. If the overload continues for a long enough time, solder 28 will be heated to such an extent that it will lose its holding power; The member 28 will then be free to move out of contact with the end of element I8 and can thereby open the circuit. In the event any arcing occurs upon the opening of the circuit it will be readily extinguished by continued movement of element 26, by the burning back of the end of element I8, and by the absorption of heat therefrom by solder 28 and element 28. The solder 28 and the'element 26 are relatively massive and can absorb enough heat from the arc to appreciably shorten its duration.

As the element 26 moves relative to element 22, the end of element 22 will be moved through the open upper portion of member 28. This is desirable because it does not put an appreciable force on the solder 28 to cause an expulsion of that solder from the end of member 28 adjacent fuse link I8. Such expulsion is also resisted by the upstanding end 25 of element 26. The angular disposition of elements 22 and 28 is quite important because coaxial disposition could result in expulsion of solder 28 in a direction toward the end of fuse link I8; and such expulsion wouldl tend to close the gap rather than open it as is desired.

If the protector is subjected to overloads in the short circuit range, the element I8 will generate enough heat to cause itself to fuse and thereby open the protector. Where this occurs, burning of the link will enlarge the gap and tend to extinguish the arc. Once again the large mass of element 28 and solder 28 will help extinguish the arc by conducting heat from the end of the link or from the arc directly;

In automotive and aircraft work, protectors are often subjected to a very considerable vibration. If the parts'are inflexible and are rigidly secured to the enclosure, the vibration imparted to the enclosure will immediately be imparted to the parts of the protector without diminution. Such vibration can be serious and can cause premature and accidental opening of the protector. The arrangement shown in the drawing is very helpful in reducing the amount of vibration imparted to the heat absorbing member and the heat softenable material of the protector. Much of the vibration imparted to the enclosure will be absorbed by the resiliency of the cantilevers I8 and 22, and only a very little of it will be imparted to member 26 and material 28. This insures that the life of the protector will not be shortened by injury caused by vibration. arrangement of parts is also helpful because it thermally lnsulates link I8, heater 22, heat absorber-28 and mass 28 from the tube I0 by spacing the absorber 26 away from the enclosure dur..

This v ing the normal operation of the protector. As a resultl a uniform rating of the device is insured.

Element 26 is positioned in the longitudinal center of the enclosure and gives a ready index of the condition of the protector. This is deairable because in ordinary fuses that are enclosed in glass tubes, it is sometimes difdcult to determine visually whether or not the fuse is intact. This is largely due'to the fact that the fuse wires are generally quite small and is partially due to the i'act that the end caps cover up a portion of the tube. Positioning element 20 in the longitudinal center of the tube is helpful because it is relatively large and it moves an appreciable distance when the circuit is opened. This movement facilitates ready detection of protectors that have operated to open the circuit.

It will be noted that in this preferred form of protector, the heaters Il and 22 remain in substantially fixed position when the movable member is moved to open the circuit. 'Ihere may be an almost imperceptible straightening or flexing of heater 22 due to the releasing of the compression exerted on it by the spring 2l, but that heater may be said to remain in substantially xed D- aition.

The protector will open before the overload can injure the circuit but it will not open needlessly. In fact the protector can hold some overloads for considerable time. 'I'his is clearly shown by the following table that is a comparicon between the time lag of the simple fuse link and a protector of the same rating. The fuse link and the protector were each rated at ten amperes and were tested at loads of one hundred thirty-five, one hundred fifty, two hundred, three hundred, four hundred, five hundred, six hundred, seven hundred and eight hundred per cent of rating. No entry is made for the fuse link above two hundred per cent load because the opening times above that load were so short they could not be measured with a stop watch.

Fuse link Protector 1% l5 sec., 40 sec 120 sec., 240 sec. l u... 4sec.,3.7 sec 22.3 sec.,27.3 sec. 200%... 1sec.,l.l sec...... 15.5sec., lsec. am? 8.2 sec., 8.4 sec. 400 a.. 3.7 sec., 4 sec.

3 sec., 2.8 sec. 600 2.1 sec., 2 sec. 7 l.6sec.,1.3 sec.

. 0.8 sec., 0.6 sec,

In the construction shown in the drawing, each of the elements Il and 22 will generate and tend to conduct heat to solder 2l. The exact amount of heat generated and conducted into solder 2B may be varied for different protectors but each of the elements contributes to the heating of solder 28. This heating may be accomplished by adding heat to the solder or by preventing the escape of heat from the solder to the end caps.

In all cases, the temperature of the protector will be less than that of the fuse link it replaces.

parts perform dual functions. For instance, in the protector shown in the drawing, the element IB functions as a heater for solder 28, as a fuse link, a support for the end of element 26 and an arcing contact. The element 26 functions as a heat absorber, a movable arcing contact and a support for solder 28. Element 22 serves as a heater, a support for member 28 and a support for spring 24. This interaction of parts is desirable because it eliminates the necessity of having supporting parts that might keep the protector from being positioned and operated in a small enclosure.

In the preferred form oi' invention shown in the drawing, the elements I8 and 22 are dimensioned to give approximately the same heating effect. However, if desired, the dimensions of these elements may be changed to permit one of the elements to be the main heater. In that event, the other element will operate as a thermal resistor.

Whereas a preferred embodiment of the lnvention has been shown and described in the drawing and accompanying description, it is obvious to those skilled in the art that various changes may be made in the form bf the lnvention that do not ail'ect the scope of the invention.

What I claim is:

1. A protector for electric circuits that comprises a movable heat absorbing member, a plurality of heat generating elements, at least one oi.' which is adapted to fuse and thereby open the circuit, and a resilient member, said movable heat absorbing member comprising a substantially infusible member and heat softenable material carried Iby said substantially infuslble member, said heat softenable material normally contacting said heat generating elements, said resilient member being arranged to move said movable heat absorbing member away from said fusible heat generating element when said heat softenable material has lost its holding power.

2. A protector for electric circuits that comprises a movable member, a heat generating element that is adapted to fuse and thereby open the circuit, heat softenable material, a second heat generating element, and a resilient member, said fusible heat generating element having one of its ends directly attached to one end of said movable member by said heat softenable material, and said second heat generating element having one of its ends directly attached to the opposite end of said movable member by said heat softenable material, said resilient member being arranged to move said movable member toward the other end of said second heat generating element when the heat softenable material has lost its holding power.

3. In a protector for electric circuit, a plurality of heat generating elements, at least one of which is fusible, heat softenable material, and a movable member, said heat generating elements being arranged to resiliently support said movable member and to remain in substantially fixed position when said movable member is moved.

4. A protector for electric circuits comprising an enclosure, contact terminals, a plurality of heat generating elements, at least one of which is fusible, heat softenable material and a mov able member, said heat generating elements being arranged to resiliently space said movable member away from the inner surface of said enclosure and to remain in substantie y fixed position when said movable memberis moved.

5. A protector for electric circuits comprising a casing, la heat generating element, a movable member, heat softenabie material, yand a resilient member, said movable member having one end thereof adapted to be connected to said heat generating element, said heat softenable material normally connecting said movable member to said heat generating. element, said resilient member normally biasing said movable member for movement substantially longitudinally of said casing, said heat generating element and said movable member being angularly disposed whereby said substantially longitudinal movement of said movable member will cause relative transverse movement between an end of said heat generating element and said movable member.

6. A protector for electric circuits comprising a heat generating element, a movable member, heat softenabie material, and a resilient member, said movable member having one end thereof adapted to be connected to said heat generating element, said heat softenable material normally constituting the sole means for connecting said movable member to said heat generating element, said resilient member biasing said movable member for telescopic movement with said heat generating element when said heat softenabie material has lost its holding power.

7. A protector for electric circuits comprising a heat generating element that is fusible on predetermined cverload of the protector, a movable member, heat softenabie material, a second heat generating element, and a resilient member, said movable member and said heat softenable material constituting the sole means of connecting said heat generating elements in electrical conducting relation, said vresilient member biasing said movable member: for telescopic movement with said second heat generating element when said heat scftenable material has lost its holding power.

8. A protector for electric circuits comprising a plurality of cantilever heat generating elements at least one of which is fusible on predetermined overload of the protector, a movable member, and heat softenable material normally connecting said movable member to each of said heat generating elements, said movable member and said heat softenabie material constituting the sole means of connecting the inner ends of said heat generating elements.

9. A protector for electric circuits comprising a heat generating element that is fusible on predetermined overload of the protector, a movable member, a resilient member, a second heat generating element, and heat softenable material, said movable member and said heat softenable material constituting the sole means of connecting the inner ends of said heat generating elements, said heat generating elements and said resilient member constituting the sole support for said movable member-and said heat soitenable material, said fusible heat generating element being arranged so the inner end thereof serves as an ,arcing contact when the movable member is moved longitudinally of said second heat generating element to open the circuit.

10. A protector for electric circuits comprising a fuse link, a movable member, and heat softenabie material, said movablemember carrying said heat softenabie material, said heat softenabie material normally connecting said fuse link and said movable member, said movable member having an upstanding portion thereof underlying and extending toward a portion of said link whereby a short current path is provided through said heat softenable material between said link and said movable member.

1l. A protector for electric circuits comprising a movable member, a heat generating element and a resilient member, said resilient member having one end thereof secured to one end of said heat generating element and having its other end secured to said movable member and being arranged to bias said movable member for movement toward said end of said heat generating element.

12. A protector for electric circuits comprising contact terminals, a movable member, a heat generating element that is fusible, a second heat generating element, heat softenabie material, and a resilient member, said resilient member biasing said movable member toward one of said terminals, said second heat generating element extending between said' movable member and said one terminal and being normally secured to said movable member by said heat softenabie material, said second heat generating element being rigid enough to resist movement of said movable member toward said one terminal and thereby free said fusible heat generating element from stress caused by said resilient member.

13. A protector for electric circuits comprising an enclosure, contact terminals, a movable member, a heat generating element that is adapted to iuse and thereby open -the circuit on predetermined overload of the protector, and a second heat generating element, each of said heat generating elements having one of its ends connected to one of said contact terminals and having its other end extending inwardly of said enclosure, said inwardly extending ends of said heat generating elements being arranged to engage and resiliently support said movable member and to remain in substantially iixed position when the movable member is moved.

14. A protector for electric circuits Vcomprising contact terminals, a transparent enclosure, a plurality of heat generating elements, a relatively large movable member that is positioned substantially centrally of said enclosure and is normally held in closed circuit position by heat softenabie material and is movable to open circuit position by a resilient member when said heat softenabie material has lost its holding power, said transparent enclosure and large movable member cooperating to indicate when the movable member is moved to open the circuit, said heat generating elements remaining in substantially fixed position when said movable member is moved to open the circuit.V

15. Av protector for electric circuits comprising a, movable member, heat softenable material, a resilient member and heat generating elements, said heat softenable material normally holding said movable member in direct contact with each of said heat generating elements, said resilient member being arranged to move said movable member away from one of said heat generating elements and thereby enable lone end of said movable .member and one end of the said one heat generating element to serve as the arcing contacts of the protector when the heat softenable material has been heated to a predetermined temperature, said resilient member being attached to said movable member and extending beyond the other end of said movable member, whereby said resilient member is always spaced away from the arcing zone.

16. A protector for electric circuits comprising a fuse link, a movable member constructed and arranged to absorb an appreciable portion of the heat generated in the protector, heat softenable material carried by said movable member and a resilient member. one end of said fuse link and one end of said movable member constituting the arcing contacts of said protector, a portion of said resilient member being coextensive with a portion of said movable member, all of said coextensive portion of said resilient member being arranged to move away from said fuse link whenever the heat softenable material is heated to a predetermined temperature, said protector being arranged so arcing is restricted to said fuse link, to the movable member and to the heat softenable material carried thereby and so the resilient member is free from arcing.

17. A protector for electric circuits comprising a movable member. a plurality of heat generating elements, a resilient member and heat softenable material, said resilient member biasing said movable member for movement to open circuit position. said heat softenable material directly contacting each of said heat generating elements and said movable member, said material being adapted to permit said movable member to move to open circuit position when heated to a predetermlned temperature and to hold said movable member in said open circuit position when subsequently cooled below said temperature.

18. A protector for electric circuits comprising a movable member, a heat generating element positioned adjacent one end thereof and angularly disposed relative to said movable member, heat softenable material on said movable member, and a resilient member biasing said movable member for telescopic movement with said heat generating element, said angular disposition being arranged to obviate expulsion of said heat softenable material from the other end of said movable member.

19. A protector for electric circuits that comprises a movable member, a heat generating element that is adapted in fuse and thereby open the circuit, heat softenable material, a second "heat generating element, and a resilient member,

said movable member being arranged to carry said heat softenable material, said resilient member being arranged to move said movable member away from said fusible heat generating element and toward said second heat generating element when said heat softenable material has lost its holding power.

20. A protector for electric circuits that comprises a movable member, a plurality of heat generating elements at least one of which is adapted to fuse and thereby open the circuit on predetermined overload of the protector, heat softenable material and a resilient member, said heat generating elements being arranged so one end of each of the elements is normally held in contact with said movable member by said heat soften able material, said fusible heat generating element normally being free of stress caused by said resilient member and bein-g arranged to remain in substantially fixed position when said movable member is moved.

21. A protector for electric circuits that comprises contact terminals, an enclosure, heat softenable material, a movable member constructed and arranged to carry said heat softenable material, a resilient member, a heat generating element that is adapted to fuse and thereby open the circuit on predetermined overload of the protector, and a second heat generating element, said fusible heat generating element having one end thereof connected to one of said terminals and having its other end extending inwardly of said enclosure in cantilever fashion to overlap and support one end of said movable member,`

said second heat generating element having one end thereof connected to another of said terminais and having its other end extending inwardly of said enclosure in cantilever fashion to overlap and support the other end of said movable mem ber, said resilient member biasing said movable member for movement when the heat softenable material has been heated to a predetermined temperature.

22. A protector for electric circuits comprising a plurality of heat generating elements, a movable member, heat softenable material carried by said movable member, and a resilient member, said heat softenable material normally securing one end of said movable member to one of said heat generating elements, said resilient member biasing said movable member for telescopic movement relative to said one heat generating element, said one heat generating element and said movable member being angularly disposed relative to each other whereby relative movement therebetween will not cause an expulsion of heat softenable material from the other end of said movable member.

23. A protector for electric circuits comprising an enclosure, a plurality of heat generating elements at least one of which is adapted to fuse and thereby open the circuit, a movable member, a resilient member, and heat softenable material normally directly attaching said movable member to each of said heat generating elements. said heat generating elements and said movable member extending substantially longitudinally of said enclosure, said resilient member biasing said movable member for movement longitudinal o! said enclosure so each of the said attachments must be broken before the movable member moves.

24. A protector for electric circuits comprising a fuse link, a trough shaped movable member, and heat softenable material, said movable member carrying said heat softenable material, said heat softenable material normally connecting said fuse link and said movable member, said movable member having an upstanding portion at one end thereof adapted to act as a dam for said heat softenable material.

25. In a protector for electric circuits. an enclosure, a plurality of fixed heat generating elements, and a resilient member having a relatively large portion thereof positioned in close proximity to said enclosure to facilitate transfer of heat therebetween and thereby retard heating of said resilient member.

26. A protector for electric circuits comprising a plurality of heat generating elements at least one of which is adapted to fuse and thereby open the circuit on predetermined overload of the protector, a movable member, heat softenable material at the ends of said movable member normally contacting said heat generating elements and resilient means biasing said movable member for longitudinal movement to open the circuit at said fusible heat generating element when said heat softenable material is heated to a predetermined temperature, said protector being arranged so the fusible heat generating element is free of stress due to the resilience of said resilient member.

27. A protector for electric circuits that comprises a. movable member, a plurality o: heat generating elements, at least one of which is adapted to fuse and thereby open the circuit on predetermined overload of the protector, heat softenable material and a resilient member, said heat generating elements being arranged so one end of each of saidelements is normally held in contact with said movable member by said heat softenable material, said resilient member normally biasing said movable member for movement away y from said fusible heat generating element, said fusible heat generating element normally being free of stress caused by said resilient member.

MANFRED F. DUERKOB.

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