CA1097708A - Tin-free current limiting fuse - Google Patents
Tin-free current limiting fuseInfo
- Publication number
- CA1097708A CA1097708A CA295,987A CA295987A CA1097708A CA 1097708 A CA1097708 A CA 1097708A CA 295987 A CA295987 A CA 295987A CA 1097708 A CA1097708 A CA 1097708A
- Authority
- CA
- Canada
- Prior art keywords
- fuse
- fuse link
- link means
- current
- disposed
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Fuses (AREA)
Abstract
39,142 TIN-FREE CURRENT LIMITING FUSE
ABSTRACT OF THE DISCLOSURE
A current limiting fuse having a number of par-allel current conducting tin-free fuse links. One of the fuse links is disposed in the centralmost part of the fuse along the longitudinal axis thereof. The other parallel fuse links are radially or circumferentially disposed around the centrally located fuse link. Ideally, all fuse links carry equal currents; however, the centralmost fuse link operates at a relatively higher temperature than those which surround it. Consequently, the central fuse link will melt first during an overload condition, transferring the over-load current to the other surrounding fuse links which quickly blow because of the incremental change in current therethrough.
ABSTRACT OF THE DISCLOSURE
A current limiting fuse having a number of par-allel current conducting tin-free fuse links. One of the fuse links is disposed in the centralmost part of the fuse along the longitudinal axis thereof. The other parallel fuse links are radially or circumferentially disposed around the centrally located fuse link. Ideally, all fuse links carry equal currents; however, the centralmost fuse link operates at a relatively higher temperature than those which surround it. Consequently, the central fuse link will melt first during an overload condition, transferring the over-load current to the other surrounding fuse links which quickly blow because of the incremental change in current therethrough.
Description
UUl~J~ u~ r. 1~ UI~
Field of the Invention:
The sub~ect matter of this invention is related generally to current limiting fuses, and more specifically to tin-free current limiting fuses.
Description of the Prior Art:
Current limiting fuses are generally designed to interrupt before the peak value of the potentially available fault current can be reached. Such fuses are typically con-structed of several notched silver elements connected in parallel circuit relationship between end ferrules and enclosed in an insulating tube filled with quartz sand. A
good current limiting fuse will dissipate a relatively large amount of heat in relationship to its physical size. Durlng ~~~-~k 39,142 77 !8 normal operation, the fuse is cooled by the surroundingenvironment such that excessive temperatures are not reached in the fuse elements. At high overcurrent values, the fuse melts so rapidly that temperatures caused by the self-heating of the fuse are not damaging. However, fuse over-heating does become a problem when relatively low overcur-rents (about 1.5 to three times the rating of the fuse) occur for a long period of time. During low overcurrent conditions, the fuse heats t~ more rapidly than it can be cooled and excessive temperatures begin to build ~ in the various parts of the fuse. In some instances, the inner wall of the insulating tube begins to char and carbonize, which is of course undesirable. When the fuse elements do melt, the insulating properties of the fuse may be so deter-iorated that the fuse will not support the open circuit voltage and will consequen1~1y fail catastrophically. A
present method of avoiding the low overcurrent problem previously described consists of fixing a small amount of tin or other similar metal to the center or hottest point of each of khe silver fuse elements. When the element reaches the melting point of tin, which is much lower than that of silver, the tin dissolves the silver through a diffusion alloying process, melting the element at that point and forcing the fuse to open. Due to the lower melting temper-ature in the tin, the fuse opens before the fuse temperature becomes excessive, which is desirable. However, under a high temperature condition over a long period of time, the presence of the tin may cause a premature opening of the fuse, resulting in unnecessary circuit shutdown. It would be desirous, therefore, to elim nate this condition by ` ` 39,142 10C~77~8 providing a fuse which is made without the use of tin or a similar material. Parallelly disposed fuse links in a current limiting fuse are known. Such fuses are described in U.S. Patent 3,571,775, issued March 23, 1971 to F. J.
Kozacka and R. A. Belcher, U.S. Patent 3,777,370, issued December 11, 1973 to T. Wakui and U.S. Patent 3,213,242, issued October 19, 1965 to F. L. Cameron. The latter patent is assigned to the assignee of the present invention.
Another patent which shows parallel displaced fuse links is 3,829,808, issued August 13, 1974 to D. D. Blewitt et al and assigned to the assignee of the present invention. U.S.
Patent 2,713,098, issued July 12, 1955 to K. W. Swain shows a fuse arrangement in which parallel fuses are disposed around a central fuse; however, in that case, it is entire ; fuses, including external casings, which are disposed in the parallel relationship, not merely a fuse link. It would be desirous, therefore, if a tin-free current limiting fuse which was reliably operable under relatively low overload currents could be provided.
SUMMARY O~` TY~ VENTION
In accordance with the invention, a fuse is taught which comprises parallel fuse elements where one of the ele-ments is disposed within the fuse in such a manner that it is forced to operate at a higher temperature than any of the other elements in the fuse for generally the same amount of fuse current. By virtue of the higher current, the hotter element will always be the first to melt in a low overcurrent condition. A fuse may be considered as being supplied by a constant current source due to the relative low resistance of the fuse. Consequently, the current which had been 10~77Q8 carried by the melted high temperature element, is now divided among the remaining elements. This of course causes the remaining elements to quickly melt because of the pre-sence of the incrementally high current in each one of them.
The preceding fusing action results in a characteristic which is similar to that of a tinned fuse element, and thus causes the fuse to clear before damaging temperatures are reached, even at relatively low overload current. In another embodiment of the invention, the preceding arrangement may be used to deliberately alter the shape of the time-current characteristic of the fuse.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had to the preferred embodiment thereof, exemplified by the following Figures in which:
Figure 1 shows a cut-away side section of a multi-element current limiting fuse;
Figure 2 shows a cross-section of the fuse of Figure 1 at the section lines II-II;
Figure 3 shows a cross-section of a fuse similar to that shown in Figure 2, but for another embodiment of the invention; and Figure 4 shows a cross-sectional view of a fuse similar to those shown in Figures 2 and 3 for still another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EM~ODIMENTS
Referring now to the drawings, and Figure 1 in particular, a current limiting fuse 10 is shown. Current limiting fuse 10 may comprise a hollow cylindrical tubular portion 12 which may be made of glass melamine or wound 10977~8 fiberglass. Disposed at either end of the tubular body 12 are electrically conducting ferrules 14 and 16. There may be disposed between the ferrule 14 and the tubular body 12 an electrically conducting end piece 18 through which fuse element links may protrude for being placed in a disposition of electrical contact with the ferrule 14. The end piece 18 also acts as a spacing member for fuse links or fuse ele-ments. Likewise, disposed between the end of the tubular body 12 and the ferrule 16 may be a similar electrically conducting end piece 20. In a preferred embodiment of the invention, the ferrules 14 and 16 are mechanically affixed to the tubular body 12 in the region 22 by way of annular depressions 24. In the nonlimiting embodiment shown in Figure 1, three fuse elements 26, 28 and 30 are shown in a vertically aligned disposition. The central fuse element 28 is effectively shielded from the inner wall of the insulat-ing barrel 12 by the other fuse elements 26 and 30. Conse-quently, heat which is generated in the fuse element 2~ due to the normal conduction of current therethrough cannot be as easily dissipated through the fuse barrel 12 as heat generated in the fuse elements 26 and 30 can be. Further-more, the presence of heat in the fuse element 26 and 30 acts to add heat to the fuse element 2~ to thus raise its temperature even though the current flowing through the fuse element 28 is generally the same as the current flowing through the fuse elements 26 and 30. Conse~uently it can be seen that the effect of the heat provided by the fuse elements 26 and 30 as well as the remote distance of fuse element 28 from the fuse barrel 12 tend to raise the temperature of the fuse element 2a above the temperature of the fuse elements 26 and 30.
33,142 10~77~8 This means that for any given amount of overload current, fuse element 28 is more likely to fuse or melt first, most likely at a notched portion 32, than fuse elements 26 and 30. When this happens, the amount of current which was carried by the strap, link or element 28 is divided among ; the remaining fuse elements, thus incrementally raising the current therein and consequently the heat thereof to a point where they quickly melt.
Referring now to Figure 2, another view of the fuse shown in Figure 1 is depicted. In this case, other fuse elements 38 and 36, which were not shown in Figure 1 for purposes of simplicity, are shown. It can be determined by examining Figure 2 that the fuse elements 26, 30, 38 and 36 cooperate to effectlvely isolate the fuse element 28 from the cooling effect of the fuse barrel 12. In addition, as was mentioned previously, the ~neat that is generated in elements 26, 30, 38 and 36 tends to become concentrated in the central region of the fuse or in the region occupied by the fuse link 28, thus tending to raise its temperature 20 above that of the other fuse ~ . It will be noted that the fuse elements 26, 30, 38 and 35 are radially oriented like the spokes of a wheel around the fuse element 28.
Referring now to Figure 3, another embodiment of the invention is shown in which fuse elements 26 ', 30 ', 36 ' ~and 38 ' are oriented circumferentially, i.e. like portions of the rim of a wheel, around the central fuse element 28, thus having an even greater tendency to shield the fuse element 28 from the cooling effect of the barrel 12 and also having an even greater tendency to concentrate heat within the fuse in the region of the fuse element 28 than was the case with regard to the embodiment of Figures 1 and 2. The embodiment 10' shown in Figure 3 may in some cases provide for even quicker fusing or melting of all of the fuse ele-ments contained therein under relatively low values of overload current.
Referring now to Figure 4, still another embodi~
ment of the invention is shown in which a fuse 40 is de-picted. Fuse element 28 is shown in its centrally located position, but multiple concentric arrangements of radially oriented fuse elements 44 and 46 are also shown. From this it can be seen that the heating effect in the center of the fuse or in the region where fuse element 2~ is located may be even greater than with the previously shown embodiments.
Furthermore, the cooling effect provided by the tube barrel 42 is even more greatly reduced for the centrally located fuse element because of its relatively larger distance therefrom and because of the disposition of multiple fuse elements 44 and 46, for example between fuse element 28 and the tube barrel 42.
It is to be understood with respect to the embodi-ments of this invention that the relative sizes or diarneters of the apparatus of the embodiments 10, 10' and 40 are not limiting. Furthermore, it is to be understood that the radial arrangement of concentric rows of fuse elememts shown in Figure 4 may be (in another embodiment of the invention) circumferentially disposed such as is shown in Figure 3. It is also to be understood that in an embodiment of the inven-tion, if desired, a tinned fuse element may be used where it is important to have an even more highly defined region at which the fuse melts during relatively ]ow overcurrent conditions. It is also to be understood, as is depicted in 10~77ยข~8 I! Figure 1, that quartz sand or the like 34 may be dis-i posed within the fuse barrel 12 (or 42) to act as a heat absorption or arc-quenching medium. It is also to be under-stood that in some of the embodiments of the invention the end pieces 18 and 20 may be non-electrically conducting.
The apparatus taught herein has many advantages.
One advantage lies in the fact that the centrally located fuse element such as 28 shown in Figure 1 has a tendency to operate at a higher temperature for a given current, and thus be more likely to melt during a relatively low over-current condition than is the case with fuses shown in the prior art. The effect of the quick melting of one fuse element tends to introduce an incrementally larger amount of electrical current to all the other fuse elements, even if the overall current carried by the fuse does not increase, thus tending to rapidly melt these elements also. The net effect of such a fuse is to pr-,vide a current limiting fuse which operates at relatively low values of overload current with a well defined melting characteristic. Another advan-tage lies in the fact that the size of the fuse elements inthe arrangements within the fuse barrel may be chosen to change the characteristic of the fuse, if desired.
Field of the Invention:
The sub~ect matter of this invention is related generally to current limiting fuses, and more specifically to tin-free current limiting fuses.
Description of the Prior Art:
Current limiting fuses are generally designed to interrupt before the peak value of the potentially available fault current can be reached. Such fuses are typically con-structed of several notched silver elements connected in parallel circuit relationship between end ferrules and enclosed in an insulating tube filled with quartz sand. A
good current limiting fuse will dissipate a relatively large amount of heat in relationship to its physical size. Durlng ~~~-~k 39,142 77 !8 normal operation, the fuse is cooled by the surroundingenvironment such that excessive temperatures are not reached in the fuse elements. At high overcurrent values, the fuse melts so rapidly that temperatures caused by the self-heating of the fuse are not damaging. However, fuse over-heating does become a problem when relatively low overcur-rents (about 1.5 to three times the rating of the fuse) occur for a long period of time. During low overcurrent conditions, the fuse heats t~ more rapidly than it can be cooled and excessive temperatures begin to build ~ in the various parts of the fuse. In some instances, the inner wall of the insulating tube begins to char and carbonize, which is of course undesirable. When the fuse elements do melt, the insulating properties of the fuse may be so deter-iorated that the fuse will not support the open circuit voltage and will consequen1~1y fail catastrophically. A
present method of avoiding the low overcurrent problem previously described consists of fixing a small amount of tin or other similar metal to the center or hottest point of each of khe silver fuse elements. When the element reaches the melting point of tin, which is much lower than that of silver, the tin dissolves the silver through a diffusion alloying process, melting the element at that point and forcing the fuse to open. Due to the lower melting temper-ature in the tin, the fuse opens before the fuse temperature becomes excessive, which is desirable. However, under a high temperature condition over a long period of time, the presence of the tin may cause a premature opening of the fuse, resulting in unnecessary circuit shutdown. It would be desirous, therefore, to elim nate this condition by ` ` 39,142 10C~77~8 providing a fuse which is made without the use of tin or a similar material. Parallelly disposed fuse links in a current limiting fuse are known. Such fuses are described in U.S. Patent 3,571,775, issued March 23, 1971 to F. J.
Kozacka and R. A. Belcher, U.S. Patent 3,777,370, issued December 11, 1973 to T. Wakui and U.S. Patent 3,213,242, issued October 19, 1965 to F. L. Cameron. The latter patent is assigned to the assignee of the present invention.
Another patent which shows parallel displaced fuse links is 3,829,808, issued August 13, 1974 to D. D. Blewitt et al and assigned to the assignee of the present invention. U.S.
Patent 2,713,098, issued July 12, 1955 to K. W. Swain shows a fuse arrangement in which parallel fuses are disposed around a central fuse; however, in that case, it is entire ; fuses, including external casings, which are disposed in the parallel relationship, not merely a fuse link. It would be desirous, therefore, if a tin-free current limiting fuse which was reliably operable under relatively low overload currents could be provided.
SUMMARY O~` TY~ VENTION
In accordance with the invention, a fuse is taught which comprises parallel fuse elements where one of the ele-ments is disposed within the fuse in such a manner that it is forced to operate at a higher temperature than any of the other elements in the fuse for generally the same amount of fuse current. By virtue of the higher current, the hotter element will always be the first to melt in a low overcurrent condition. A fuse may be considered as being supplied by a constant current source due to the relative low resistance of the fuse. Consequently, the current which had been 10~77Q8 carried by the melted high temperature element, is now divided among the remaining elements. This of course causes the remaining elements to quickly melt because of the pre-sence of the incrementally high current in each one of them.
The preceding fusing action results in a characteristic which is similar to that of a tinned fuse element, and thus causes the fuse to clear before damaging temperatures are reached, even at relatively low overload current. In another embodiment of the invention, the preceding arrangement may be used to deliberately alter the shape of the time-current characteristic of the fuse.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had to the preferred embodiment thereof, exemplified by the following Figures in which:
Figure 1 shows a cut-away side section of a multi-element current limiting fuse;
Figure 2 shows a cross-section of the fuse of Figure 1 at the section lines II-II;
Figure 3 shows a cross-section of a fuse similar to that shown in Figure 2, but for another embodiment of the invention; and Figure 4 shows a cross-sectional view of a fuse similar to those shown in Figures 2 and 3 for still another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EM~ODIMENTS
Referring now to the drawings, and Figure 1 in particular, a current limiting fuse 10 is shown. Current limiting fuse 10 may comprise a hollow cylindrical tubular portion 12 which may be made of glass melamine or wound 10977~8 fiberglass. Disposed at either end of the tubular body 12 are electrically conducting ferrules 14 and 16. There may be disposed between the ferrule 14 and the tubular body 12 an electrically conducting end piece 18 through which fuse element links may protrude for being placed in a disposition of electrical contact with the ferrule 14. The end piece 18 also acts as a spacing member for fuse links or fuse ele-ments. Likewise, disposed between the end of the tubular body 12 and the ferrule 16 may be a similar electrically conducting end piece 20. In a preferred embodiment of the invention, the ferrules 14 and 16 are mechanically affixed to the tubular body 12 in the region 22 by way of annular depressions 24. In the nonlimiting embodiment shown in Figure 1, three fuse elements 26, 28 and 30 are shown in a vertically aligned disposition. The central fuse element 28 is effectively shielded from the inner wall of the insulat-ing barrel 12 by the other fuse elements 26 and 30. Conse-quently, heat which is generated in the fuse element 2~ due to the normal conduction of current therethrough cannot be as easily dissipated through the fuse barrel 12 as heat generated in the fuse elements 26 and 30 can be. Further-more, the presence of heat in the fuse element 26 and 30 acts to add heat to the fuse element 2~ to thus raise its temperature even though the current flowing through the fuse element 28 is generally the same as the current flowing through the fuse elements 26 and 30. Conse~uently it can be seen that the effect of the heat provided by the fuse elements 26 and 30 as well as the remote distance of fuse element 28 from the fuse barrel 12 tend to raise the temperature of the fuse element 2a above the temperature of the fuse elements 26 and 30.
33,142 10~77~8 This means that for any given amount of overload current, fuse element 28 is more likely to fuse or melt first, most likely at a notched portion 32, than fuse elements 26 and 30. When this happens, the amount of current which was carried by the strap, link or element 28 is divided among ; the remaining fuse elements, thus incrementally raising the current therein and consequently the heat thereof to a point where they quickly melt.
Referring now to Figure 2, another view of the fuse shown in Figure 1 is depicted. In this case, other fuse elements 38 and 36, which were not shown in Figure 1 for purposes of simplicity, are shown. It can be determined by examining Figure 2 that the fuse elements 26, 30, 38 and 36 cooperate to effectlvely isolate the fuse element 28 from the cooling effect of the fuse barrel 12. In addition, as was mentioned previously, the ~neat that is generated in elements 26, 30, 38 and 36 tends to become concentrated in the central region of the fuse or in the region occupied by the fuse link 28, thus tending to raise its temperature 20 above that of the other fuse ~ . It will be noted that the fuse elements 26, 30, 38 and 35 are radially oriented like the spokes of a wheel around the fuse element 28.
Referring now to Figure 3, another embodiment of the invention is shown in which fuse elements 26 ', 30 ', 36 ' ~and 38 ' are oriented circumferentially, i.e. like portions of the rim of a wheel, around the central fuse element 28, thus having an even greater tendency to shield the fuse element 28 from the cooling effect of the barrel 12 and also having an even greater tendency to concentrate heat within the fuse in the region of the fuse element 28 than was the case with regard to the embodiment of Figures 1 and 2. The embodiment 10' shown in Figure 3 may in some cases provide for even quicker fusing or melting of all of the fuse ele-ments contained therein under relatively low values of overload current.
Referring now to Figure 4, still another embodi~
ment of the invention is shown in which a fuse 40 is de-picted. Fuse element 28 is shown in its centrally located position, but multiple concentric arrangements of radially oriented fuse elements 44 and 46 are also shown. From this it can be seen that the heating effect in the center of the fuse or in the region where fuse element 2~ is located may be even greater than with the previously shown embodiments.
Furthermore, the cooling effect provided by the tube barrel 42 is even more greatly reduced for the centrally located fuse element because of its relatively larger distance therefrom and because of the disposition of multiple fuse elements 44 and 46, for example between fuse element 28 and the tube barrel 42.
It is to be understood with respect to the embodi-ments of this invention that the relative sizes or diarneters of the apparatus of the embodiments 10, 10' and 40 are not limiting. Furthermore, it is to be understood that the radial arrangement of concentric rows of fuse elememts shown in Figure 4 may be (in another embodiment of the invention) circumferentially disposed such as is shown in Figure 3. It is also to be understood that in an embodiment of the inven-tion, if desired, a tinned fuse element may be used where it is important to have an even more highly defined region at which the fuse melts during relatively ]ow overcurrent conditions. It is also to be understood, as is depicted in 10~77ยข~8 I! Figure 1, that quartz sand or the like 34 may be dis-i posed within the fuse barrel 12 (or 42) to act as a heat absorption or arc-quenching medium. It is also to be under-stood that in some of the embodiments of the invention the end pieces 18 and 20 may be non-electrically conducting.
The apparatus taught herein has many advantages.
One advantage lies in the fact that the centrally located fuse element such as 28 shown in Figure 1 has a tendency to operate at a higher temperature for a given current, and thus be more likely to melt during a relatively low over-current condition than is the case with fuses shown in the prior art. The effect of the quick melting of one fuse element tends to introduce an incrementally larger amount of electrical current to all the other fuse elements, even if the overall current carried by the fuse does not increase, thus tending to rapidly melt these elements also. The net effect of such a fuse is to pr-,vide a current limiting fuse which operates at relatively low values of overload current with a well defined melting characteristic. Another advan-tage lies in the fact that the size of the fuse elements inthe arrangements within the fuse barrel may be chosen to change the characteristic of the fuse, if desired.
Claims (6)
1. A fuse, comprising:
a) hollow fuse barrel means;
b) spaced terminal means securely fastened to said barrel means and communicating with the interior portion thereof;
c) first fuse link means disposed generally in a central region of said barrel means interior portion in electrical continuity with said terminal means; and d) second fuse link means disposed in said barrel means interior portion in electrical continuity with said terminal means, said second fuse link means being disposed between the inner wall of said hollow fuse barrel means and said first fuse link means, heat produced in said second fuse link means when conducting current raising the tempera-ture of said first fuse link means to a higher value which will cause said first fuse link means to melt prior to the melting of said second fuse link means during a predetermined current overload condition when said first fuse link means and said second fuse link means generally carry the same amount of overload current.
a) hollow fuse barrel means;
b) spaced terminal means securely fastened to said barrel means and communicating with the interior portion thereof;
c) first fuse link means disposed generally in a central region of said barrel means interior portion in electrical continuity with said terminal means; and d) second fuse link means disposed in said barrel means interior portion in electrical continuity with said terminal means, said second fuse link means being disposed between the inner wall of said hollow fuse barrel means and said first fuse link means, heat produced in said second fuse link means when conducting current raising the tempera-ture of said first fuse link means to a higher value which will cause said first fuse link means to melt prior to the melting of said second fuse link means during a predetermined current overload condition when said first fuse link means and said second fuse link means generally carry the same amount of overload current.
2. The combination as claimed in claim 1 wherein a plurality of said second fuse link means are disposed in said barrel means in a disposition which tends to surround said first fuse link means.
3. The combination as claimed in claim 2 wherein said plurality of second fuse link means are radially dis-posed.
4. The combination as claimed in claim 2 wherein said plurality of second fuse link means are circumferen-tially disposed.
5. The combination as claimed in claim 3 wherein said plurality of second fuse link means are radially dis-posed in a plurality of concentric circles.
6. The combination as claimed in claim 4 wherein said plurality of second fuse link means are circumferen-tially disposed in a plurality of concentric circles.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US76959877A | 1977-02-17 | 1977-02-17 | |
| GB769,598 | 1977-02-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1097708A true CA1097708A (en) | 1981-03-17 |
Family
ID=25085934
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA295,987A Expired CA1097708A (en) | 1977-02-17 | 1978-01-31 | Tin-free current limiting fuse |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1097708A (en) |
-
1978
- 1978-01-31 CA CA295,987A patent/CA1097708A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |