US3840836A

US3840836A - Current limiting sand fuse

Info

Publication number: US3840836A
Application number: US00275180A
Authority: US
Inventors: E Link
Original assignee: RTE Corp
Current assignee: Cooper Power Systems LLC
Priority date: 1972-07-26
Filing date: 1972-07-26
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08
Also published as: DE2412688A1; DE2412688C3; BE813542A; GB1430880A; NL175768B; FR2267633A1; NL175768C; DE2412688B2; FR2267633B1; CH568651A5; NL7403511A

Abstract

A current limiting fuse having electrical terminals at each end of a tubular housing, a fusible assembly provided within said housing to interconnect the terminals. The fusible assembly including a first fusible element of high current clearing characteristic and a second fusible element of low current clearing characteristic, the first fusible element substantially surrounded by a granular dielectric material, the second fusible element being positioned within an arc extinguishing sleeve formed of a material which is deionizing under the influence of an electrical arc.

Description

United States Patent 1191 Link 1 51 Oct. 8, 1974 [52] US. Cl 337/292, 337/161, 337/276 [51] Int. Cl. H0111 85/04 [58] Field of Search 337/162, 292, 161, 276,

[56] References Cited UNITED STATES PATENTS 1,965,239 7/1934 Hill 337/291 X 2,768,264 10/1956 Jones et a]. 200/144 C 2,809,254 10/1957 Edsall 337/162 3,179,773 4/1965 Keeley 337/276 3,287,524 11/1966 Huber et a1. 337/276 X 3,294,936 12/1966 Mikulecky .1 337/297 X FOREIGN PATENTS OR APPLICATIONS 1,513,338 2/1970 Germany ..337/292 614,615 12/1948 GreatBritain ..337/292 Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell 5 7] ABSTRACT A current limiting fuse having electrical terminals at each end of a tubular housing, a fusible assembly provided within said housing to interconnect the terminals. The fusible assembly including a first fusible element of high current clearing characteristic and a second fusible element of low current clearing characteristic, the first fusible element substantially surrounded by a granular dielectricmaterial, the second fusible element being positioned within an arc extinguishing sleeve formed of a material which is deionizing under the influence of an electrical arc.

lit-Claims, 2 Drawing-Figures 1 CURRENT LIMITING SAND FUSE BACKGROUND OF THE INVENTION Current limiting fuses of the type shown in the Mikulecky US. Pat. No. 3,294,936 are designed to interrupt currents of overload proportions as well as currents of short circuit proportions. The difficulty encountered in fuses of this type is the interruption of currents of relatively small magnitude of long duration, i.e., a second or more. The are gap which forms in the small cross sectional area portions of the fusible element of this type is progressively enlarged by vaporization and the fulgurite is generally continuous. If the arcing is prolonged in interrupting fault currents of relatively small magnitude, excessive high are energy is released and intense heating of the filler adjacent to the arc region occurs before completion of the interrupting process. This intense heating and the excessive are energy tend to delay cooling of the fulgurite and to allow flow of full or leakage current through the fulgurite often resulting in a restrike and rekindling of the are after initial interruption of the fuse.

In the Mikulecky type current limiting fuse as described in US. Pat. No. 3,294,936, lead-tin allow beads are provided in intimate contact with each silver wire. The lead-tin alloy beads reduce the melting temperature of the silver wire due to the formation of an alloy on meltingof the lead-tin bead on the silver. However, it has been found that even though the fusible element fails, the fuse does not necessarily interrupt current because of the presence of an arc gap which is not long enough to obtain arc interruption. If this condition occurs for any period of time, the temperature of the fuse can rise to temperatures higher than the destruction temperatures of the surrounding environment.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view in section showingthe current limiting fuse of the invention; and

FIG. 2 is a log log time current curve of fusing elements.

DESCRIPTION OF THE INVENTION The fuse ofthe present invention as seen in FIG. 1 includes a hollow insulating casing or housing ofa predetermined length having a conductive ferrule or cap 12 mounted on each end. A fuse assembly 14 is positioned within said casing 10 and interconnects said. caps.

. In accordance with the invention, the fuse assembly 14 includes a first means 16 of high current clearing characteristic and a second means 18 having a low current clearing characteristic. In regard to the former, the first means includes an element having a high melting point temperature, such as a silver wire which melts at 980 C or copper wire which melts at 1,082C. (Silver oxidizes in air and when temperature rises to a predetermined temperature the silver oxide converts to silver.) The element 20 depending on the desired current clearing characteristic, can be in the form of a perforated or notched strip or a plain wire. The element 20 is electrically connected to the cap 12 and to the plug 21. The element 20 is embedded in a granular refractory material 15 such as sand.

The second means 18 includes a fusible element 20 and an arc extinguishing sleeve 22. The fusible element 20 is formed from a material having a low melting point temperature such as tin wire which melts at 232C. The element 20 is electrically connected to the cap 12 and to a conductive plug 21 at the other end of the sleeve 22.

The fusible element 20 is thermally insulated by means of the arc extinguishing tube or sleeve 22 which is made of a material such as silicone rubber. The inside diameter and length of the tube can be selected to provide a clearing ability for the fuse at voltages and currents compatible with the destruction temperatures of the components of the surrounding electrical appara tus. The second means 18 can be embedded in the granular refractory material in the sleeve 10. In either case the caps 12 should be sealed to the sleeve 10 to enclose said first means 16.

Referring to FIG. 2, the time current characteristic for silver and tin is shown plotted on a logarithmic scale. It will be noted that the characteristic for tin has a greater slope than the characteristic for silver and intersects the silver characteristic. It should be apparent from the diagram that the low overload current time characteristicof the tin is being used to limit or interrupt the current in the fuse and thereby prevent the temperature of the fuse from rising to destruction temperatures. We are also using the high overload or fault current short time characteristic of the silver to clear the fuse under fault current conditions.

It is generally understood that the length of the wire in sand determines thev voltage restoration capability of the fuse. In this regard, it is generally well known that if the wire is too short, the recovery voltage will maintain the element conductive and not allow complete interruption of the circuit. The phenomenon which enables the fuse to clearis the ability of the molten silver to form a fulgurite in the sand when the silver reaches the molten stage. During the formation of this fulgurite heat is absorbed at a very fast rate thus causing a resistance to current flow to build to a point where the voltage across the fuse is not sufficient to maintain a conductive circuit.

Iclaim:

l. A current limiting fuse comprising, a housing, an electrical terminal at each end of said housing, a fusible assembly having one end connected to one of said terminals and the other end connected to the other of said terminals and within said housing, said fusible assembly formed of material which is deionizing under the influence of an electrical arc and a granular dielectric material completely filling said housing and surrounding said assembly.

2. The fuse according to claim 1 wherein said are extinguishing member is in the form of a sleeve having a length substantially equal to the length of said second element whereby said second fusible element is insulated from said granular material so that its long time minimum melting current characteristic is less than said first element.

3. The fuse according to claim 2 wherein said sleeve is formed of silicone rubber.

4. A fuse according to claim 1 wherein said second fusible element comprises a metallic conductor having a long time minimum melting current characteristic less than said first fusible element.

5. The fuse according to claim 1 wherein said first fusible element is formed from one of silver and copper wire and said second fusible element is formed from tin.

6. The fuse according to claim 1 wherein said arc extinguishing sleeve comprises a silicone rubber tube.

7. A current limiting sand fuse comprising: a hollow casing, a conductive cap at each end of said casing, a

fuse assembly positioned in said casing and having one end connected to one of said caps and the other end connected to the other of said caps, said assembly including first means for providing a high current clearin g charcteristic, second means for providing a low current clearing characteristic, said first and second means being conducted in series, a granular dielectric material completely filling said casing, and an arc extinguishing sleeve enclosing the full length of said second means for thermally insulating said second means from said granular dielectric material.

8. The fuse according to claim 7 wherein said first means includes a fusible element formed from one of silver and copper wire and said second means includes a tin element.

9. A high voltage fuse of the current limiting type, a hollow insulating casing, a conductive cap on each end of said casing, a fusible assembly within said casing and connected at each end to one of said caps, said assembly including,

a main fusible element for providing high current 11. The fuse according to claim 10 wherein said l metal wire is tin and said insulating sleeve is a silicone rubber tube.

12. The fuse according to claim 9 wherein said main fusible element includes a silver wire, said secondary fusible element includes a tin wire connected in series with said silver wire and said thermally insulating member comprises a silicone rubber tube.

13. The fuse according to claim 9 wherein said main fusible element is sealed within said casing.

PM UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. I 3, 840 836 D t d October 8-, 1974 n fl Edwin A. Link It is certified that error appears in the above-identified patent and that said Letters. Patent are hereby corrected ae shown below:

Col. 2, line 4, delete "20" and substitute ---'16- C ol. 2, line 6 delete Y'ZO" and substitute ---l6--'-; C01. 2, line 8, delete "20" and substitute ---l6-- C01; 2,- line 63, after "member" insert enc1osing the full length of said second fusible element, said arc extinguishing member Signed and sealed this 4th day of February 1975.

(SEAL) Attest: McCOY M. GIBSON JR. c. MARSHALL DANN v Attesting Officer I Comnisaioner of Patents H050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,840,836 Dated October 8, 1974 n fl Eduin A. Li k- It is certified that error appears in the above-identified patent and that said Letters, Patent are hereby corrected as shown below:

Col. 2, line 4, idelete "20" and substitute -'16- C01. 2, line 6, delete Y'ZO" and substitute ---l6-- Col. 2 line 8,' delete "20" and substitute --l6- Col. 2, line 63, after "member" insert --enc1osing the full length of said secohd fusible element, said are extinguishing member.

Signed and sealed this 4th day of February 1975.

(SEAL) Attest:

McCOY M. GIBSON 'JR. b c. MARSHA-LL DANN Attesting Officer Commissioner of Patents

Claims

1. A current limiting fuse comprising, a housing, an electrical terminal at each end of said housing, a fusible assembly having one end connected to one of said terminals and the other end connected to the other of said terminals and within said housing, said fusible assembly including a first fusible element of high current clearing characteristic, a second fusible element of metallic material of low current clearing characteristic, said first and second elements being connected together to form a series circuit, and an arc extinguishing member being formed of material which is deionizing under the influence of an electrical arc and a granular dielectric material completely filling said housing and surrounding said assembly.

2. The fuse according to claim 1 wherein said arc extinguishing member is in the form of a sleeve having a length substantially equal to tHe length of said second element whereby said second fusible element is insulated from said granular material so that its long time minimum melting current characteristic is less than said first element.

7. A current limiting sand fuse comprising: a hollow casing, a conductive cap at each end of said casing, a fuse assembly positioned in said casing and having one end connected to one of said caps and the other end connected to the other of said caps, said assembly including first means for providing a high current clearing charcteristic, second means for providing a low current clearing characteristic, said first and second means being conducted in series, a granular dielectric material completely filling said casing, and an arc extinguishing sleeve enclosing the full length of said second means for thermally insulating said second means from said granular dielectric material.

9. A high voltage fuse of the current limiting type, a hollow insulating casing, a conductive cap on each end of said casing, a fusible assembly within said casing and connected at each end to one of said caps, said assembly including, a main fusible element for providing high current clearing characteristics, and a secondary fusible element connected in series with said first element for providing low current clearing characteristics, said secondary element including a low melting point metal wire and a thermally insulating sleeve enclosing the full length of said secondary element, and granular inert refractory arc quenching material within said casing embedding said main fusible element and said sleeve.

10. The fuse according to claim 9 wherein said secondary fusible element limits the long time temperature rise to a level of not more than 300* C.

11. The fuse according to claim 10 wherein said metal wire is tin and said insulating sleeve is a silicone rubber tube.