GB2087669A - Fusible element for electrical fuses - Google Patents

Abstract

In a fuse for protection of electrical appliances with low rated current capacities the fusible element is composed of a monofilament of quartz glass coated with an alloy composed of silver, copper, tin and antimony.

Description

SPECIFICATION Fusible element for electrical fuses The present invention relates to electrical fuses and in particular is related to a fusible element for an electrical fuse which is useful forthe protection of electrical appliances with low rated current capacity.

Electrical fuses having low rated current capacity are well known and have been widely used for protection of electrical appliances. Fuses having low rated current capacity of the order of 100 mAorless usually require a fusible element, preferably made of silver wire having a diameter of the order of 10,um or less. However, silver wires with such extremely small diameter are difficult to fabricate and, in addition, they lack the requisite mechanical strength and structural integrity. As a practical matter, it is difficult to fabricate silver wires having a diameter smaller than about 4510Ccm.

In order to overcome the aforementioned difficulties and permit the use of fusible elements made of silver wire with the desired small diameters, it has been suggested to use, as the fusible element, a monofilament yarn made of a plastics material such as polyacrylonitrile wherein the surface of the fila- ment is either chemically coated or is electroplated after chemical coating in order to make an electrically conductive filament for use as a fusible element. One of the drawbacks of these fusible elements is the relatively low melting point of the coating on the yarn surface which is necessarily limited by the softening point of the coated polyacrylonitrile filament yarn, i.e. 125"C., or less.

Anothertype of fuse suggested in the prior art employs an insulated film of a high molecular plastics material as a supporting member, the surface of which is coated with a suitable metal to make the fusible element. The problem with this type of fusible element is that the high molecular weight plastics support member is heat sensitive and is readily deformed by the thermal expansion caused by excessive current flow. Moreover excessive current flow through the metal coating causes itto crack, and therefore, current flow may be prematurely interrupted. Also, repeated rise and fall in temperature during the current flow adversely affects the physical properties of the plastics support material and could result in its permanent and irreversible deformation, with consequent instability of the fuse.

Accordingly, it is an object of the present invention to provide an improved fuse for use in electrical appliances having low rated current capacity.

It is another object of the present invention to provide an improved fuse having a novel and unique fusible element which exhibits excellent performance and improved fusing characteristics when used in electrical appliances having low rated current capacities as low as about 1 milliampere and as high as about 250 milliamperes.

It is still another object of this invention to provide such fuses which are more stable and more durable than comparable prior art fuses.

The invention contemplates providing a fuse of the usual type and variety which, as is well known, generally comprises a fuse cartridge capped at both ends with electrically conductive terminals and wherein the fusible element is stretched in said cartride between said conductive terminals in electrical contact therewith. The fusible element itself, which constitutes the novel and unique feature of this invention, comprises a supporting member which is a monofiliment made of quartz glass fibre, the exterior surface of which is coated with an electrically conductive alloy composed of silver, copper, tin and antimony.

According to the present invention there is provided a fusible element for use in an electrical fuses, said fusible element comprising a support member made from a monofilament yarn of glass quartz fibre and a metallic alloy coating on the exterior surface of said support member, said metallic alloy comprising silver, copper, tin and antimony.

In a preferred embodiment of the present invention, the metallic alloy coating consists of approximately from 71 to 73 percent by weight of silver, approximately from 22 to 24 percent by weight of copper, approximately from 2 to 4 percent by weight of tin and approximately from 1 to 3 percent by weight of antimony.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a series of graphs which illustrates the variations in temperature resistance of fuses employing the fusible element according to the present invention, and in particular, illustrates the advantages of including antimony in the alloy coating; Figure 2 shows two graphs which compare the temperature characteristics of two fuses; one made in accordance with the present invention, and the other made as in the prior art; and Figure 3 is still another graphical representation comparing the fusing characteristics of a fusible element made in accordance with the present invention with a typical piror art fuse.

In accordance with this invention, a unique fusible element is provided which can be incorporated into any conventional fuse. Such a fuse, as in otherfamiliarfuse constructions, comprises an insulated tube or cartridge which is capped at both ends with electrically conductive terminals (e.g. ferrules), and a fusible element stretched between said two terminals in electrical contact therewith, such as by solder.

The uniqueness ofthe invention resides in the fusible element itself which comprises a support member made of a monofilament yarn of quartz glass fibre, the exterior surface of which is coated with a uniform layer of an alloy composed of silver (Ag), copper (Cu), tin (Sn) and antimony (Sb), thereby providing an electrically conductive fusible element.

While the composition of the alloy may vary somewhat and still retain the beneficial results intended herein, suitable alloys are those which consist of from about 71 to about 73 weight percent by weight of silver, from about 22 to 24 percent by weight of copper, from about 2 to about 4 percent by weight of tin and from about 1 to about 3 percent by weight of antimony. Obviously, the exact composi tions are selected to make up 100 percent by weight of silver alloy.

An alloy of silver, copper, tin and antimony having the aforementioned composition, provides an excel lent coating for the monofilament and imparts good electrical conductivity and other desirable properties thereto.

Another unique feature ofthis invention resides in the inclusion of antimony in the alloy coating com position. The advantages of including antimony will become more apparent from the ensuring description and the accompanying graphs.

One of the advantages of the fusible element of this invention is its thermal stability which permits its repeated use over many years. This is because the alloy coating has a very high melting point (872"C) and a very high recrystallization temperature (245"C). These temperatures are considerably higher than the melting point and crystallization temperature of low melting point metals which are usually about 150"C. and 20"C., or less, respectively. Thus, with the ordinary metal, the low crystallization temperature results in adverse recrystallization effects even at ambient temperatures which, in turn, adversely affect the physical properties of the coating and hence result in undesirable changed in the fusing characteristics of the fusible element.

Since the ambient temperature affects, more or less, the physical property, of the metal used in making the fusible element, it is preferably to employ, for this purpose, metals which are less affected by ambient temperatures and have high melting temperatures. A metal with higher melting point has a more stable performance as a fuse because such metals have, roughly in proportion to their melting points, higher crystallization temperature at which the physical property of the metal begins to change, and further, because such crystallization temperatures are well above the ambient temperature. It has been found that silver or silver alloys are the most preferred metals for making the fusible element since they have a high melting point and are not affected by the environment. They also have other well known excellent characteristics.

The fusible element of this invention, with its unique alloy coating and high melting point and recrystallization temperature, is more stable and shows no adverse effects on physical properties of the fuse even when used at temperature considerably above ambient temperature, over a long period of time.

Another advantage of an embodiment of this invention is its increased stability in short term use resulting from the addition of about 2 percent by weight of antimony of the silver alloy.

A further advantage of this invention is due to the use of monofilament yarn of glass quartz fibre as the support for the alloy coating. Glass quartz fibre is highly resistant to heat flow and exhibits excellent durability over repeated use at temperatures as high as 1000"C., which is higherthan the melting point of the silver alloy coating. Quartz glass is durable even over consecutive uses at temperatures as high as 1 000 C., maintaining a considerably high viscosity of 4.5 x 107 poise even at a temperature of 1500"C.

Therefore, unlike metal-coated high molecular weight plastics support members in which the melting point of the support member is usually lower than the melting point ofthe metal coating, in the fusible element of this invention the melting point of the silver alloy is unaffected by the melting point of the support member.

Still another advantageous feature of the fusible element of this invention lies in its accuracy and highly improved fusing characteristics as shown in the following table, which shows the thermal expan- sion for quartz glass fiber at different temperature ranges.

Temperature Range, "C. Thermal Coefficient x 10-7deg-1 deg 0-30 4.2 30-100 5.3 100-500 5.8 500-900 5.0 The small thermal coefficient of quartz glass fibre is in contrast with the higher thermal coefficient for plastics materials (5-2 x 10-5 deg;") and metal (4-60 x 10-6 degF1). Thus, the so-called "Joules" heat effect presents less thermal problems in quartz glass fiber than in plastics or metals.

The advantages of the present invention will now be further illustrated by reference to the graphs shown in Figure 1-3. of the accompanying drawings.

Thus, in Figure 1, where the rate of resistance to temperature variations is plotted as ordinate against temperature (abscissa), curves 1,2,3,4 and 5 represent the resistance to temperature variations of fusible elements identical but for the amount of antimony in their respective silver=alloy coating.

The amounts of antimony in the silver alloy coatings corresponding to said curves are 1,2, 3, 5 and 0 percents by weight, respectively.

As shown in Figure 1, curves 1,2 and 3 show less temperature variations, and hence greater stability, than curves 4 and 5, indicating that the performance of the fusible elements is best when the amount of antimony in the silver alloy coating is from about 1 to about 3 percent by weight. In this range, the resistance-temperature coefficient will remain within a very narrow range at temperatures up to 1 500C.

Higher amounts of antimony result in greater temp- eratu re variations. Moreover, even greater temperature variations (and hence more instability) results when no antimony is used in the silver alloy coating, thus indicating the significance of including antimony, in the desired amounts, in the silver alloy coating.

Referring to Figure 2, where the variations in rated current value is plotted against ambient temperature, graph 6 represents the temperature characteristic of a fusible element made in accordance with the prior art wherein a plastics support material is coated with a metal, and graph 7 represents the temperature characteristics of a fusible element made according to the present invention, wherein silver alloy was coated on a monofilament of glass quartz fibre.

A rated current value of 63 mA was obtained by using a fusible element with a silver alloy coating having a thickness of 1 ,um and the quartz glass support member having a diameter of 80 ,um.

As indicated in Figure 2, at 1500C. ambient temperature, the prior art fuse is subjected to considerably greater variation in rated current value (70%) as compared to the fusible element of this invention whose rated current value varies only by about 5%.

Finally, and with reference to Figure 3, where percent rated current is plotted against fusing time, graphs 8,8' represents the fusing characteristics of a prior art fusible element (as described in connection with Figure 2) and graphs 9,9' represent the fusing characteristic of a fusible element of this invention (also as described in connection with Figure 2).

Comparison ofthesegraphs show less dispersion when using a fusible element made in accordance with the present invention as compared to the prior art type fusible element.

Thus, from the foregoing description and the accompanying drawings, it is evident that an improved fusible element is provided which may be incorporated in ordinary fuses to impart thereto excellent temperature behaviour, greater thermal stability and durability over a long period of use, and a highly improved fusing characteristic.

Claims (3)

1. A fusible element for use in an electrical fuse, said fusible element comprising a support member made from a monofilament yarn of glass quartz and a metallic alloy coating on the exterior surface of said support member, said metallic alloy comprising silver, copper, tin and antimony.

2. A fusible element as claimed in claim 1 wherein said metallic alloy coating consists of approximately from 71 to 73 percent by weight of silver, approximately from 22 to 24 percent by weight of copper, approximately from 2 to 4 percent by weight of tin and approximately from 1 to 3 percent by weight of antimony.

3. Afusible element for use in electrical fuses substantially as herein described with reference to the accompanying drawings.