GB2136644A

GB2136644A - Composite fusible element

Info

Publication number: GB2136644A
Application number: GB08406788A
Authority: GB
Inventors: Terence Victor Mcghie; Derek Mawson
Original assignee: Dorman Smith Fuses Ltd
Current assignee: Dorman Smith Fuses Ltd
Priority date: 1983-03-15
Filing date: 1984-03-15
Publication date: 1984-09-19
Also published as: GB8406788D0; GB2136644B

Abstract

A fusible element 1 comprises a composite strip having end actions 2,3, of copper or an alloy, and an intermediate bridging section 4 of silver or other metal or metal alloy, having a melting point approximating to that of the end sections. The fusible element 1 is adapted at at least one position in the bridging section and in at least one end section for fusion upon the passage of a short circuit current. A coating or body 5 is of a relatively low melting point alloy is immediately adjacent the bridging section and on the occurrence of a sustained overload, will alloy eutectically with the metal or metal alloy of the bridging section. <IMAGE>

Description

SPECIFICATION An improved fusible element This invention relates to fusible elements for use in cartridge fuses of the kind comprising a substantially rigid tube of electrically insulating material fitted at each of its ends with a terminal of metal or metal alloy and containing at least one fusible element extending between and electrically connected to the terminals, the tube usually, but not necessarily, being filled with an arc-quenching material, such as sand.

In cartridge fuses of this kind, it has been the general practice to employ fusible elements made wholly of pure silver. However, on occasions when the price of pure silver rises to an unacceptable level, it has been proposed to use fusible elements made in part of copper. One such alternative form of fusible element that has been proposed comprises end sections of copper and, between and joining these copper end sections, a bridging section of pure silver or of other metal or metal alloy differing from, and having a relatively high melting point approximating to that of, copper.It has been the practice to provide, in one or each end section, a position or positions at which the end section is locally flattened, is reduced in cross-section or has a hole therethrough and where fusion will occur upon passage of a short circuit current, and to provide, on the bridging section, a coating or body of a metal or metal alloy which has a low melting point relative to the melting point of pure silver or copper and which will alloy eutectically with the underlying silver upon fusion caused buy a sustained overload. Usually the coating or body on the bridging section is made of solder, tin and/or indium. The heat which occurs on sustained overload results in an eutectic effect assuring fusion at a predetermined temperature, thereby providing for a relaible overload protection.

We have found that where the end sections of the fusible element are made of copper, there is a substantial risk that, when heating occurs on sustained overload, cyclic overloading or during repeated transient overloads or pulses of current, the solder, tin, indium or other relatively low melting point metal on the bridging section of the fusible element tends to disperse over the surface of an adjoining copper end section, and sometimes over said position or positions in the end section, to cause such consequential erratic operation of the fusible element that in many instances a cartridge fuse incorporating such a fusible element will fail to operate in accordance with its designed characteristics.In some cases, such a fusible element may melt at the bridging section and in others a similarfusible element with the same dsigned characteristics may melt where the solder or other relatively low melting point metal has migrated over a copper end section and either at one of said positions or elsewhere.

It is an object of the present invention to provide an improved fusible element which is comparable in its operation to a fusible element made wholly of pure silver and which provides for reliable overload protection.

According to the invention, the improved fusible element comprises a composite strip comprising end sections of copper, a copper-based alloy or other metal or metal alloy of high melting point and, inter-connecting said end sections, a bridging section of silver or other metal or metal alloy differing from and having a melting point approximating to that of the metal or metal alloy of the end sections, at least one end section and the bridging section each being adapted at at least one position for fusion upon passage of short circuit current, and a coating or body of a relatively low melting point metal or metal alloy which is immediately adjacent the bridging section and which will alloy eutectically with the metal or metal alloy of the bridging section upon fusion arising from passage of a sustained overload.

When a cartridge fuse incorporating any fusible element is subjected to passage of its normal rated current, or a sustained over-current or overload, a temperature gradient is established along the axial length of the element such that an intermediate region of the element is hotter than the end regions of the element. This temperature gradient occurs mainly because of the difference in the heat transfer properties of the intermediate region, where the element is surrounded by sand, and of the end regions which are electrically connected to metal end terminals, but it is also influenced by the fact that the power developed by the element is greater at the positions for fusion upon passage of a short circuit current than at the end regions of the element.

By virtue of the facts that the bridging section of the improved fusible element is adapted at at least one position for fusion upon passage of a short circuit current and that the coating or body of a relatively low melting point metal or metal alloy is immediately adjacent the bridging section in the event of a sustained overload or over-current the low melting point metal or metal alloy is encouraged to migrate on to orto remain on the bridging section because the bridging section will be at a higher temperature than the part of the end section underlying or adjacent to the low melting point metal or metal alloy. As a result, the improved fusible element provides overload protection that is substantially more predictable and reliable than other fusible elements not made wholly of pure silver hitherto proposed and used.

Under short circuit conditions, the fusible element will melt at one or more than one of the positions at which it is adpated for fusion upon passage of a short circuit current, whether such a position is in an end section or the bridging section.

The coating or body of relatively low melting point metal or metal alloy preferably at least partially overlies a part of an end-section immediately adjacent the bridging section. The bridging section is preferably so positioned between the end sections that one end section is substantially greater in length than the other, e.g. over twice as long, and in this case the coating or body of relatively low melting point metal or metal alloy preferably at least partially overlies a part of the shorter end section immediately adjacent the bridging section. The joints between the bridging section and the end sections may be effected by cold bonding, by the formation of or introduction of eutectic, or by welding, rivetting or other mechanical means.

The part of the end section immediately adjacent the bridging section, or the part of the bridging section and the immediately adjacent part of the end section or the bridging section, may be shaped to form a transversely extending trough which holds the body of relatively low melting point metal or metal alloy.

At each position on an end section and on the bridging section where it is adapted for fusion upon passage of a short circuit current, the end section or bridging section may be reduced in transverse cross-section and/or may have at least one hole therethrough and/or may be locally flattened to create a relatively high resistance zone where fusion will commence upon occurrence of a short circuit current It will be appreciated that the transverse crosssection of and/orthe size of a hole in and/or the local thickness of the bridging section in relation to that or those of or in one or each end section at the position adapted for fusion upon passage of a short circuit current may be so arranged that, in order to compensate-under short circuit conditions for the different electrical and physical properties of the metal or metal alloys of the bridging and end sections, it may be necessary for the cross-sectional area of the or each such position in the bridging section to be different from that of the or each such position in the or each end section. The crosssectional areas of these positions in the bridging and end sections may also differ from one another in order to take account of the difference in Metcalf effect activity between the metals or metal alloys of the bridging and end sections.

Preferably, the end sections are made of copper but, in some instances, they may be made of a copper alloy such as brass, aluminium or an aluminium-based alloy, iron or an iron-based alloy, or zinc or zinc-based alloy. Where the end sections are made of copper, preferably the bridging section is made of pure silver or a silver-based alloy.

Preferred relatively low melting point metals and metal alloys include solder, tin, a tin/lead alloy, a tin/lead/cadmium alloy and indium.

The invention is further illustrated, by a description by way of example by a description of a preferred fusible element with reference to the accompanying diagrammatic drawings, in which Figure lisa plan view of the preferred fusible element drawn on an enlarged scaie; Figure 2 shows three alternative arrangements for providing positions of reduced cross-section in a fusible element such as that shown in Figure 1, and Figure 3 shows fragmental plan views of four alternative forms of the preferred fusible element.

Referring to Figure 1, the preferred fusible element 1 comprises a composite strip, having end sections 2,3 made of copper, where end section 3 is approximately twice the length of end section 2, a bridging section 4 of pure silver interconnecting the two end sections, and a coating of solder 5 overlying an area of the shorter end section 2 immediately adjacent to the bridging section. Spaced apart along the length of the fusible element 1 are several points 6 where the fusible element is adapted to create a zone of relatively high resistance and where fusion will commence on the occurrence of a short circuit current or a sustained overload. The positions 6 are each constituted by a reduction in transverse crosssection.

The fusible element 1 is used in a cartridge fuse of the type comprising a rigid hollow tube and fitted at each end with a metal terminal assembly, and a fusible element housed in the tube and extending between and electrically connected to the terminal assemblies, said tube being filled with sand to act as an arc quenching material.

Upon the occurrence of a sustained overload, due to the difference in heat transfer properties between the fusible element 1 and the surrounding materials a temperature gradient is established along the axial length of the fusible element 1 such that the intermediate region of the fusible element is generally at a higher temperature than the end regions.

Due to the higher resistance induced at the positions of reduced cross-sectional area 6, each position will be at a higher temperature than the metal of the surrounding area. The temperature gradient causes the coating of solder 5 to migrate towards the position of reduced cross-sectionai area 6 in the bridging section 4. When an overload is sustained, the solder and metal around said position will alloy eutectically, finally leading to failure of the fusible element at this position.

If the fusible element 1 is subjected to a short circuit current, failure may occur by melting of the fusible element at any of the position 6 throughout its length.

Alternative arrangements for providing positions of reduced cross-section which may be employed in this form of fusible element are shown in Figure 2.

Figure 2a shows part of an element where the reduction is achieved by a combination of a hole 7 and punched endentations 8 in the side edges.

However, each of the above arrangements may be used singularly as shown in Figures 2b and 2c.

In Figure 3a, a minor portion of the coating 5 overlies a part of the bridging section 4, in Figure 3b approximately half of the coating 5 overlies a part of the bridging section 4 and in Figure 3c the whole of the coating 5 overlies a part of the bridging section 4.

Claims

1. Afusible element comprising a composite strip having end sections of copper, a copper based alloy or other metal or metal alloy of high melting point, inter-connecting said end sections, a bridging section of silver or other metal or metal alloy differing from and having a melting point approximating to that of the metal or metal alloy of the end sections, at least one end section and the bridging section each being adapted at at least one position for fusion upon passage of short cirucit current, and a coating or body of a relatively low melting point metal or metal alloy which is immediately adjacent the bridging section and which will alloy eutectically with the metal or metal alloy of the bridging section upon fusion arising from passage of a sustained overload.

2. A fusible element as claimed in Claim 1, wherein the coating or body of relatively low melting point metal or metal alloy at least partially overlies a part of an end section immediately adjacent the bridging section.

3. A fusible element as claimed in Claim 1 or 2 wherein one end section is substantially greater in length than the other and the coating or body of relatively low melting point metal or metal alloy at least partially overlies a part of the shorter end section immediately adjacent the bridging section.

4. A fusible element as claimed in Claim 1 or 2, wherein the whole of the coating or body of relatively low melting point metal or metal alloy overlies the bridging section.

5. A fusible element as claimed in any one of the preceding Claims, wherein the part of the end section immediately adjacent the bridging section, or the part of the bridging section and the immediately adjacent part of the end section, or the bridging section, is shaped to form a transversely extending trough which holds the body of relatively low melting point metal or metal alloy.

6. A fusible element as claimed in any one of the preceding Claims, wherein the relatively low melting point metal or metal alloy is solder, tin, a tin-lead alloy, a tin-lead-cadmium alloy or indium.

7. A fusible element as claimed in any one of the preceding Claims, wherein said end section and bridging section are each reduced in transverse cross-sectional or has at least one hole therethrough and/or is locally flattened to create said position or positions for fusion.

8. A fusible element substantially as hereinbefore described with reference to and as shown in Figures 1,2 or 3 of the accompanying drawings.

9. A cartridge fuse encorporating a fusible element as claimed in any one of the preceding Claims.