EP0526050A2

EP0526050A2 - Fuse

Info

Publication number: EP0526050A2
Application number: EP92306533A
Authority: EP
Inventors: Toshiyuki c/o DAITO COMMUN. APPAR.CO. LTD Hanada
Original assignee: Daito Communication Apparatus Co Ltd; Daito Tsushinki KK
Current assignee: Daito Communication Apparatus Co Ltd; Daito Tsushinki KK
Priority date: 1991-07-29
Filing date: 1992-07-16
Publication date: 1993-02-03
Also published as: JPH0536344A; EP0526050A3

Abstract

Compact fuse has a fusible metal wire (12) extending between a pair of metal terminals (11). The wire (12) and the portion of the terminals (11) to which the wire (12) is connected are enclosed in an envelope (13) made of insulating material. The ends of the terminals (11) protrude outside the envelope (13). A barrier may be provided by filling the envelope with cellular resin (15) which covers the wire (12) and creates many isolated spaces that dissipate the thermal energy generated when the wire melts from overcurrent, thereby preventing damage to the envelope.

Description

This invention relates to a fuse used to protect electronic parts and equipment from an overcurrent.
A fuse generally performs its function of protecting circuit components from the overcurrent by breaking the circuit when a fuse element, generally metal, is heated by joule heat to its melting point.
When the fuse element melts, the metal vaporizes, causing the pressure within an envelope containing the fuse element, generally a wire, to increase. This increase of internal pressure can substantially damage the fuse.
The protecting role of a fuse is completed when the smallest part at the center of the fusible wire, the part where heat dissipation is most difficult, melts. However, the current that flows into the metal wire is large. Thus the entire wire melts, often vaporizing instantly.
Hence fusion caused by the overcurrent instantly generates a large amount of thermal energy, thereby increasing the pressure within the envelope containing the fuse element. Such an increase of internal pressure places a mechanical load on the envelope. In the worst case, the increase of internal pressure causes damage to or breakage of the envelope containing the fuse element.
To ensure reliable functioning, some conventional fuses provide extra space within the envelope containing the fuse element and position terminals connected to the fuse element outside the envelope. The dimensions of the extra space are chosen in relation to the amount of metal in the fuse element.
The increasing miniaturization of electronic circuitry makes strong demands for compact fuses. It is difficult to make a fuse compact with the configuration just described, which ensures the circuit-breaking function of a fuse and prevents damage to the envelope containing the fuse element merely by providing extra space therein.
To overcome this problem, fuses are known that cover the fusible metal wires with flexible synthetic resin, thereby substituting the resin for the extra space. Such a fuse is disclosed, for example, in japanese Utility Model Publication No. 38988/1983. However, covering fusible metal wires with flexible synthetic resin presents a problem. Such a configuration cannot ensure sufficient circuit-breaking function while simultaneously resisting the mechanical load from the envelope containing the fuse element.
The present invention has been devised to solve this problem.
An object of the present invention is to provide a compact fuse capable of reliably dissipating thermal energy generated by an overcurrent.
An further object of the present invention is to provide a compact fuse that overcomes the drawbacks of the prior art.
Still a further object of the present invention is to provide a compact fuse capable of reliably dissipating thermal energy generated by an overcurrent before it damages the envelope containing the fuse element.
Still a further object of the present invention is to provide a compact fuse that utilizes the space in the envelope containing the fuse element more effectively than the prior art, thereby improving the circuit-breaking function of the fuse.
Briefly stated, the present invention provides an improved compact fuse that has a fusible metal wire extending between a pair of metal terminals. The wire and the portion of the terminals to which the wire is connected are enclosed in an envelope made of insulation material. The ends of the terminals protrude outside the envelope. Filling the envelope with cellular resin covers the wire and creates many isolated spaces that dissipate the thermal energy generated when the wire melts from overcurrent, thereby preventing damage to the envelope.
According to an embodiment of the invention, a fuse comprises: means for breaking an electric circuit in response to an overcurrent; envelope means enclosing the means for breaking and defining a substantially closed space therebetween; and a cellular resin substantially filling the substantially closed space.
According to a feature of the invention, a fuse comprises: a metal wire effective for breaking an electric circuit in response to an overcurrent; a pair of metal terminals; the metal wire extending between the pair of metal terminals; an envelope made of insulation material; and the envelope forming a substantially enclosed space between itself and a cellular resin covering the metal wire and substantially filling the substantially closed space.
According to another feature of the invention a fuse comprises: means for breaking an electric circuit in response to an overcurrent; first envelope means enclosing the means for breaking and defining a first substantially closed space therebetween; and second envelope means enclosing the first envelope means and defining a second substantially closed space therebetween.
According to still another feature of the invention, a fuse comprises: a metal wire; the metal wire being fusible at a high current therethrough for breaking an electric circuit; an envelope enclosing the metal wire; the envelope defining a substantially closed space between itself and the metal wire; and means for dividing the substantially closed space into a plurality of substantially closed spaces, whereby products generated by fusing the metal wire remain inside the envelope.
In summary, according to a further embodiment, the fuse of the present invention has a second space formed inside the envelope containing the fuse element. The second space is made by enclosing the fusible metal wire contained in the envelope with a box-shaped enclosure frame. Even if the enclosure frame is cracked or broken by the increase of pressure that results from fusion of the fuse element, thermal energy, which may otherwise damage the envelope, is consumed, thereby reducing the applied load.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawing in which like reference numerals designate the same elements.
Fig. 1 illustrates how a fuse works according to a first embodiment of the present invention.
Fig. 2(a) is an exploded front view of a fuse according to the embodiment of the present invention shown in Fig. 1.
Fig. 2(b) is an exploded side view of a fuse according to the embodiment of the present invention shown in Fig. 1.
Fig. 3 is a vertical sectional view without a lid of a fuse according to the embodiment of the present invention shown in Fig. 1.
Fig. 4 is a vertical sectional view of with a lid of a fuse according to the embodiment of the present invention shown in Fig. 1.
Fig. 5 is a vertical sectional view of a conventional fuse.
Fig. 6 is a vertical sectional view of another conventional fuse.
Fig. 7 illustrates how a conventional fuse works.
Fig. 8 illustrates a fuse according to a second embodiment of the present invention
Referring to Fig. 1, a fuse 14 has a fusible metal wire 12 extending between a pair of metal terminals 11, 11. Wire 12 and portions of metal terminals 11, 11 are contained in an envelope 13. The ends of terminals 11, 11 protrude from envelope 13. Cellular resin 15 covers wire 12 and the portions of terminals 11, 11 inside envelope 13. When an overcurrent passes through a circuit of which fuse 14 is an element, wire 12 is heated to melting, thereby breaking the circuit.
Referring to Figs. 2(a), 2(b), and 3, a fusible metal wire 20 extends between a pair of metal terminals 21, 21. Wire 20 and portions of metal terminals 21, 21 are contained in an envelope 22 made of insulating material, such as, for example, polyether sulfone. The ends of terminals 21, 21 protrude from opening 23 of envelope 22. Cellular resin 24, such as, for example, KE521 (A · B) manufactured by Shinetsu Chemical Co., Ltd., is poured from opening 23 into envelope 22 to cover wire 20 and the portions of terminals 21, 21 inside envelope 22.
After envelope 22 is filled with cellular resin 24, opening 23 may be covered by lid 25, which may be formed from polyethylene terephthalate.
Since the space surrounding wire 20 is filled with cellular resin 24, many air-filled, isolated spaces are formed around wire 20 by cellular resin 24. When wire 20 melts, the pressure in these isolated spaces increases, dissipating thermal energy. Therefore, even if part of the material forming the isolated spaces is damaged, any energy sufficient to damage envelope 22 is dissipated, thereby reducing the mechanical load applied to envelope 22 Thus reliable circuit-breaking is ensured without the possibility of damage to envelope 22.
Referring to Figs. 4 - 6, tests were carried out on samples representing an embodiment of the present invention to compare their efficacy with conventional fuses. Sample A had the configuration of the embodiment of the present invention shown in Fig. 4, whose description is the same as that in Figs. 2 and 3 and is therefore omitted here.
Sample B had the conventional configuration shown in Fig. 5. A fusible metal wire 32 connected to metal terminals 31 at both ends is contained in an envelope 30. A space 33 is provided around wire 32.
Sample C had the conventional configuration shown in Fig. 6. A fusible metal wire 37 connected to metal terminals 36 at both ends is contained in envelope 31. Wire 37 is covered with a flexible resin 38 (Silicone Varnish KR-2038 manufactured by Shinetsu Chemical Co., Ltd.).
A circuit-breaker test was conducted by running 50 Amperes of current at 130 V DC for 1.3 mm/second through five examples each of Samples A, B, and C. The results of the test are shown in Table 1 below.
In Table 1 "○" indicates where the current was broken without any problem. "×" indicates where, when the current was broken, abnormalities developed, such as cracking and breaking of the envelopes.
All the Sample A fuses successfully broke the current without any problem, thereby confirming the superior performance of fuses according to the present invention. All of the Sample B fuses, which have a space around the metal wires, failed to extinguish an arc between the terminals, which arc damaged their envelopes. Three out of the five Sample C fuses, whose metal wires are covered with flexible resin, presented abnormalities such as cracks in and breakage of their envelopes.
Although the present embodiment has a configuration in which a fusible metal wire is covered by a cellular resin, a combination of a cellular resin and a flexible resin may be used instead. Improving the circuit-breaking function of a fuse without damaging its envelope is also possible by means of a cellular resin in which a filler, such as an arc-extinguishing agent, is dispersed.
Furthermore, the method of the present invention is not limited to the above embodiment where a cellular resin is poured into a box-shaped envelope. Fusible metal wires may be covered with cellular resin by casting, or envelopes may be produced by casting or by transfer molding under low pressure.
By covering fusible metal wires with cellular resin, the present invention makes it possible to improve the circuit-breaking function of fuses with a simple structure, while preventing cracks or breakages of envelopes when the pressure inside the envelopes increases as a result of melting or vaporization of the metal wires.
Referring to Fig. 7, in order to ensure reliable breaker function, some conventional fuses provide an envelope 2 with a space 3 therein, the dimension of space 3 being determined with respect to the amount of metal used in a metal wire 1 that serves as the fuse element. A pair of terminals 4, 4 connected to both ends of wire 1 are placed with as much of their body as possible out of envelope 2.
Referring to Fig. 8, in a second embodiment of the present invention, a second space 6 is formed inside envelope 2 by enclosing wire 1 within envelope 2 in a box-shaped enclosure frame 5. Even if frame 5 is cracked or broken by the increase of pressure in second space 6 when wire 1 fuses, enough thermal energy that might otherwise damage envelope 2 is dissipated to reduce the load applied to envelope 2 below the damage point.

Claims

A fuse comprising: means for breaking an electric circuit in response to an overcurrent, an envelope, (2, 13, 22) enclosing a space around the breaking means, characterised in that a barrier means is provided in the space between the breaking means to divide said space into a plurality of substantially closed spaces whereby products generated by fusing the breaking means are confined within the envelope (2, 13, 22).
A fuse according to claim 1 wherein the means for breaking is a metal wire (1, 12, 20).
A fuse according to claim 1 or claim 2 wherein the envelope (2, 13, 22) is made of insulating material.
A fuse according to claim 2 or claim 3 wherein the metal wire (1, 12, 20) extends between a pair of terminals (11).
A fuse according to any one of the preceding claims wherein the barrier is provided by a cellular resin (15) substantially filling the space.
A fuse according to claim 5 wherein the cellular resin (15) is a flexible plastics material.
A fuse according to any of claims 1 to 4 wherein the barrier comprises a second envelope (5) spaced from the first envelope (2) and the breaking means.
A fuse according to claim 7 wherein both envelopes (2, 5) are made of insulating material.