GB1604819A - Electrical safety fuses - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRICAL SAFETY FUSES (71) We, AKTIESELSKABET LAUR.

KNUDSEN NORDISK ELEKTRICITETS SELSKAB, of Haraldsgade 53, DK-2100 Copenhagen, Denmark, a Danish Corporation, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statements The invention relates to electrical safety fuses. Such a fuse may have one or more fuse elements surrounded by an arc suppression material. The arc suppression material usually consists of quartz sand (SiO2), but it is also possible to use other materials.

Attention is directed to our copending application no. 32938/80, (Serial No.

1604820) which was divided from the present application and which describes and claims an electrical safety fuse.

The term "reduction ratio" as used herein is defined to mean the ratio between the resistances per unit length at a break region of a fuse element and at regions adjacent thereto, where the break region is the portion of the fuse element designed to be fused when an overload current passes therethrough.

It is generally known to provide a mechanical reduction in then current-carrying cross section of a fuse element by means of width reduction and/or thickness reduction. Thus, the descriptions of the US Patents No.

3543209 and No. 3543210 show fuses where both principles have been applied in combination.

It is also known that, in order to obtain a fast fuse characteristic, a relatively large reduction ratio is required e.g. larger than 10:1, but this reduction ratio must be achieved in such a way that the current carrying capacity for regions of the fuse element other than the break region is maintained.

According to the invention, there is provided an electrical safety fuse comprising at least one fuse element surrounded by arc suppression material, the or each fuse element comprising an electrically insulating substrate on a surface of which is provided a first electrically conductive layer, there being provided a second electrically conductive layer which substantially covers the first layer and which is divided transversely of the direction of current flow through the fuse element into at least two spaced apart regions.

In such a fuse it is possible to obtain a reduction ratio which is many times larger (5-10) than in the technology applied so far without sacrificing the current-carrying capacity of the non-narrowed parts of the fuse element. The reason for this is partly that a very thin first layer can be used and provides the only conductive path at the or each break region and partly that the material of the first layer can have a lower electrical conductivity than the material of the second layer.

Furthermore, the or each break region is effectively cooled by the substrate, which is in intimate contact therewith, and so can be loaded with substantially higher current densities than is possible with the technology known so far.

The electrically insulating substrate may consist of two or more layers of different heat conductivity. The thermal time constant for the layer on which the electrically conducting-and thus heat generating-element is built up can be varied, and consequently it is possible to construct fuses with quite special fuse characteristics. By adapting the thickness of the various layers and their heat conductivity it is furthermore possible to achieve that the thermal time constant can be adapted to different combinations of current and time.

Preferably, a piece of material of substantially lower thermal conductivity than that of the substrate is disposed between the substrate and the first layer at the or each region which is not covered by the second layer.

Such a layer acts as a heat barrier during heavy overloads and the result of this will therefore be that the fuse will break in such cases. However, during a continuous high normal load, the heat will be conducted away through the layer, whose thickness and heat conductivity may be selected to provide desired characteristics.

The electrically conducting part of the fuse element comprises several layers which can be selected individually on the basis of knowledge of exactly the specific properties of the materials which are desirable in the individual areas of the fuse element. Also here it is possible for each individual layer not to cover the entire extent of the element.

In the actual break region, one may for instance want to use metals or alloys which have a well-defined and reasonably high electrical conductivity, but of relatively low heat conductivity. Silver and aluminium and respective alloys of each will then suitable. In the areas between the break regions and in particular in the thicker and more materialconsuming areas, more importance is attached to price, and therefore copper or aluminium may be used.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows in respective a conventional fuse element with width reduction in the place of break; Fig. 2 shows in perspective another example of a conventional fuse element; Fig. 3 shows in perspective a conventional fuse element with thickness reduction in the place of break; and Fig. W8 shows in perspective a number of embodiments of fuses according to the invention.

All of the fuse elements and fuses are shown in exaggerated thickness.

Figure 1 shows a known fuse element consisting of a metal strip 1 with notches 2 and 3 which provide a reduced width portion for defining a break region 4.

Figure 2 shows another known fuse element consisting of a metal strip 5, in which holes 6, 7, 8 and 9 have been punched out.

The portions in which the holes have been placed will be the break regions because of the reduction of the cross sectional area.

Figure 3 shows a third know fuse element consisting of a metal strip 10, which has been pressed between cylindrical Jaws, so that the thickness has been reduced in order to form a break region 11.

Figure 4 shows a fuse element constituting a preferred embodiment of the invention comprising a substrate 12 consisting of a heat conducting, electrically insulating material.

Although the fuse elements are described and shown with the substrate 12 at the bottom thereof, it is of course unimportant how the fuse is oriented. Also, although individual layers are illustrated as plane layers, the layers can of course take up many shapes. The substrate consists of an electric insulator made of arc resistant material of relatively good thermal conductivity e.g.

ceramic materials such as quartz or aluminium oxide or beryllium oxide. On the substrate 12, a first electrically conductive layer 13 is disposed for instance by means of known lamination technology, and on top of this layer 13 a further electrically conductive layer is provided in two parts 14 and 15 which are separated by a groove 16, so that a break region is formed with a thickness reduction corresponding to the fuse element shown in Figure 3.

Figures 5, 6 and 7 show other preferred embodiments, which are, in principle the same as that of Figure 4, and like parts have the same reference numbers. The relative thickness of the layers are correct and shown to the same scale in the Figures, but the vertical scale, i.e. the thickness and height of the fuse elements, is highly exaggerated for the sake of clarity. Figure 5 shows a fuse element in which a reduction ratio of 16:1 has been achieved exclusively by means of thickness reduction. The substrate 12 is a ceramic substrate consisting of, for instance aluminium oxide.

Figure 6 shows a fuse for which the same reduction ratio has been obtained by means of a combination of thickness reduction and "reduction of conductivity", i.e. by using as the layer 13 a material of higher resistivity than in the layer part 14 and 15. The substrate 12 is made of the same material as in Figure 5. The layer 13 consists of silverplatinum alloy with a resistivity of 6.4x 10-8s2cm, whereas the layer 14 and 15 consists of silver with a resistivity of 1.6 x 10-8 S2cm. The ratio of the crosssectional areas of the layer 13 and the layers 13 and 14, 15 is 1:4. Figure 7 shows a design for which all three principles of reduction have been applied, and in this way, a reduction ratio of 60:1 has been achieved, the above-mentioned ratio of cross-sectional areas being 1:4, the ratio of conductivities being 1:5, and the width reduction being 1:3, as holes 17 have been formed in the layer 13.

Figure 8 shows an embodiment with a substrate 30 on which a thin, thermally insulating layer 32 has been placed under the layer 31 at the break region 31. As in the previous figures, an electrically conductive layer in two parts 33 and 34 is provided at each side of the break region. This can consist of several layers. In conjunction with high currents, the layer 32 will delay the spreading of heat downwards into the substrate 30, which will ensure that the heat generated in the break region will cause fusing and consequently the electric circuit will be opened.

The various conductive layers may be formed by vapour deposition, sputtering, silk screen printing (sevigraphy), electroplating, chemical precipitation, or a combination thereof.

WHAT WE CLAIM IS:

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. The electrically conducting part of the fuse element comprises several layers which can be selected individually on the basis of knowledge of exactly the specific properties of the materials which are desirable in the individual areas of the fuse element. Also here it is possible for each individual layer not to cover the entire extent of the element. In the actual break region, one may for instance want to use metals or alloys which have a well-defined and reasonably high electrical conductivity, but of relatively low heat conductivity. Silver and aluminium and respective alloys of each will then suitable. In the areas between the break regions and in particular in the thicker and more materialconsuming areas, more importance is attached to price, and therefore copper or aluminium may be used. The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows in respective a conventional fuse element with width reduction in the place of break; Fig. 2 shows in perspective another example of a conventional fuse element; Fig. 3 shows in perspective a conventional fuse element with thickness reduction in the place of break; and Fig. W8 shows in perspective a number of embodiments of fuses according to the invention. All of the fuse elements and fuses are shown in exaggerated thickness. Figure 1 shows a known fuse element consisting of a metal strip 1 with notches 2 and 3 which provide a reduced width portion for defining a break region 4. Figure 2 shows another known fuse element consisting of a metal strip 5, in which holes 6, 7, 8 and 9 have been punched out. The portions in which the holes have been placed will be the break regions because of the reduction of the cross sectional area. Figure 3 shows a third know fuse element consisting of a metal strip 10, which has been pressed between cylindrical Jaws, so that the thickness has been reduced in order to form a break region 11. Figure 4 shows a fuse element constituting a preferred embodiment of the invention comprising a substrate 12 consisting of a heat conducting, electrically insulating material. Although the fuse elements are described and shown with the substrate 12 at the bottom thereof, it is of course unimportant how the fuse is oriented. Also, although individual layers are illustrated as plane layers, the layers can of course take up many shapes. The substrate consists of an electric insulator made of arc resistant material of relatively good thermal conductivity e.g. ceramic materials such as quartz or aluminium oxide or beryllium oxide. On the substrate 12, a first electrically conductive layer 13 is disposed for instance by means of known lamination technology, and on top of this layer 13 a further electrically conductive layer is provided in two parts 14 and 15 which are separated by a groove 16, so that a break region is formed with a thickness reduction corresponding to the fuse element shown in Figure 3. Figures 5, 6 and 7 show other preferred embodiments, which are, in principle the same as that of Figure 4, and like parts have the same reference numbers. The relative thickness of the layers are correct and shown to the same scale in the Figures, but the vertical scale, i.e. the thickness and height of the fuse elements, is highly exaggerated for the sake of clarity. Figure 5 shows a fuse element in which a reduction ratio of 16:1 has been achieved exclusively by means of thickness reduction. The substrate 12 is a ceramic substrate consisting of, for instance aluminium oxide. Figure 6 shows a fuse for which the same reduction ratio has been obtained by means of a combination of thickness reduction and "reduction of conductivity", i.e. by using as the layer 13 a material of higher resistivity than in the layer part 14 and 15. The substrate 12 is made of the same material as in Figure 5. The layer 13 consists of silverplatinum alloy with a resistivity of 6.4x 10-8s2cm, whereas the layer 14 and 15 consists of silver with a resistivity of 1.6 x 10-8 S2cm. The ratio of the crosssectional areas of the layer 13 and the layers 13 and 14, 15 is 1:4. Figure 7 shows a design for which all three principles of reduction have been applied, and in this way, a reduction ratio of 60:1 has been achieved, the above-mentioned ratio of cross-sectional areas being 1:4, the ratio of conductivities being 1:5, and the width reduction being 1:3, as holes 17 have been formed in the layer 13. Figure 8 shows an embodiment with a substrate 30 on which a thin, thermally insulating layer 32 has been placed under the layer 31 at the break region 31. As in the previous figures, an electrically conductive layer in two parts 33 and 34 is provided at each side of the break region. This can consist of several layers. In conjunction with high currents, the layer 32 will delay the spreading of heat downwards into the substrate 30, which will ensure that the heat generated in the break region will cause fusing and consequently the electric circuit will be opened. The various conductive layers may be formed by vapour deposition, sputtering, silk screen printing (sevigraphy), electroplating, chemical precipitation, or a combination thereof. WHAT WE CLAIM IS:

1. An electrical safety fuse comprising at least one fuse element surrounded by arc suppression material, the or each fuse element comprising an electrically insulating substrate on a surface of which is provided a first electrically conductive layer, there being provided a second electrically conductive layer which substantially covers the first layer and which is divided transversely of the direction of current flow through the fuse element into at least two spaced apart regions.

2. A safety fuse as claimed in claim 1, in which the substrate comprises a ceramic material.

3. A safety fuse as claimed in claim 1 or 2, in which the material of the second layer has a lower conductivity than the material of the first layer.

4. A safety fuse as claimed in any one of the preceding claims, in which the first layer has formed therein holes in the or each region which is not covered by the second layer.

5. A safety fuse as claimed in any one of the preceding claims, in which the second layer comprises a plurality of superimposed sub-layers.

6. A safety fuse as claimed in any one of the preceding claims, in which a piece of material of substantially lower thermal conductivity than that of the substrate is disposed between the substrate and the first layer at the or each region which is not covered by the second layer.

7. A safety fuse as claimed in any one of the preceding claims, in which the substrate comprises a plurality of layers of different thermal conductivities.

8. An electrical safety fuse substantially as hereinbefore described with reference to any one of Figures 4 to 8 of the accompanying drawings.