The present invention relates to an end cap for a fuse. In particular, the present invention relates to an end cap for a fuse comprising an end wall having a concave reinforcement section. An end cap in accordance with the present invention may form a part of a fuse installed in a transformer or other equipment filled with hot oil.

Electrical fuses are used extensively in electrical transformers. Increasingly, electrical transformers are assembled such that both the windings of the transformer and a fuse mounted to the transformer are immersed in oil. When the transformer is energised, electrical resistance can cause significant heating of the windings and thus the oil and the fuse are also heated. Depending upon the operating conditions, a fuse mounted in such a transformer may be exposed to temperatures from -35°C to 125°C, coupled with variations in ambient pressure of plus or minus 100 to 200 mbar. An example of a fuse used in such a way comprises a hollow ceramic body which contains fuse elements, surrounded with fine grains of silica quartz. Air inevitably occupies the spaces between the grains of silica quartz and thus variations in temperature can create high variations in pressure within the fuse where the fuse is hermetically sealed. Fuses used in this situation need to be hermetically sealed to avoid the egress of air and the ingress of oil into the fuse. Further, it is essential that no water is present in the oil when the transformer is assembled and therefore the transformer winding and fuse assembly is heated to high temperatures, in excess of 125°C, before the transformer is filled with oil. To ensure that the fuse is fit for use in these conditions, it is stress tested by heating in an oven during the manufacturing process, prior to assembly into the transformer.

At various points during the manufacturing process of the fuse, it is therefore subject to significant internal pressures. These internal pressures can continue through the life of the fuse during its use. Oil submersible fuses are therefore subject to internal pressure changes and external vacuums and end caps for the fuses must resist these pressure differentials without displaying excessive deformation. In certain cases, where the end cap is not of sufficient strength, the pressure differential can cause the external end face of the end cap to bulge and become convex. This can cause potential problems in two possible ways. Firstly, the bulging of the external face may cause the overall fuse assembly to become out of the dimensional tolerances required by the manufacturer or by the customer. As such, the fuse may no longer fit into the space in which it is intended to be used. Secondly, the end face acts as a part of a sealing arrangement between a striker mechanism within the fuse and an arc-quenching silica quartz filler inside the fuse body. Significant movement of the cap away from the end of the fuse body can cause the silica quartz filler to enter the striker capsule, which will render the striker capsule inoperable. This type of failure could be catastrophic, causing the fuse not to interrupt fault conditions, i.e. the fuse will not interrupt the current when it should, which would in turn cause significant and expensive damage to the external fuse gear or to any transformer in which the fuse were mounted. Further, this type of failure could cause the fuse itself to rupture, causing further damage.

In a known fuse end cap assembly, the strength of the end wall of the fuse is increased by welding a solid washer to the outer face of the end wall. The provision of the washer can increase the strength of the end wall sufficiently to resist the pressure differentials to which the end cap will subjected without excessive deformation of the end wall. A significant amount of additional material is added to the assembly by the addition of the strengthening washer, which can have numerous drawbacks. Where the strengthening washer is made from copper, which is generally a requirement in fuse technology, the additional material required can result in a significant additional expense to the overall cost of manufacture of the fuse. The cost of manufacture is further increased by the additional welding and assembly time required to add the strengthening washer to the fuse. The weight of the fuse is also increased by the additional weight of the washer.

There therefore exists a requirement for an improved fuse end cap, having an end wall of increased strength, whilst making more efficient use of materials and manufacturing time.

According to the present invention there is provided an electrical fuse end cap, comprising an end wall having a concave reinforcement section. The provision of a concave reinforcement section results in stresses created in the end cap being more efficiently distributed through the material of the end cap. The stresses found in a flat end wall section of known fuse end caps can include significant proportion of bending stresses. Stresses in the concave section comprise a lesser bending stresses than flat end wall sections and a greater degree of compressive stresses. The compressive stresses may be more effectively resisted by the material of the fuse end cap.

The end wall may further comprise a flat section for attaching tag arrangements. The provision of a flat section can facilitate the attachment of tag arrangements having a flat attachment section to the end cap. Electrical or mechanical mounting components may be connected to the tag arrangements.

The flat section may be a substantially centrally-located section of the end wall and the concave reinforcement section may extend radially outward from the flat section. This allows tag arrangements to be connected centrally to the end cap of the fuse, while the concave reinforcement section improves the resistance to internal forces of the outer section of the end wall.

The concave section may have a parabolic profile. A parabolic profile may be efficient in redistributing stresses through the end wall and thus making the most efficient use of the material in the end wall.

The profile of the concave section may describe a smooth curve. A concave section having a smooth curve reduces any stress concentrations created at sharp changes in the profile of the reinforcement section.

The profile of the concave section may describe a part of an ellipse. The concave section having the form of a part of an ellipse, for example, can lead to an efficient distribution of stresses within the end wall to improve the efficiency of the design in resisting the stresses created by pressure inside the fuse.

The end cap may further comprise a substantially cylindrical side wall section extending from the periphery of the end wall; the substantially cylindrical side wall section having an engagement section for engaging with a fuse body. This side wall section permits the end cap to adhered to the side wall of the fuse body.

The profile of the concave section may describe a curve connecting the flat section to the side wall section. This can improve stress distribution within the end wall between the central flat section and the outer side wall section to reduce the amount of material required in the end wall to resist a given internal pressure.

The end cap may further comprise a bead section disposed between the end wall and the side wall section. The bead section allows a smooth transition from the side wall section to the concave reinforcement section and reduces stress concentrations at the transition from side wall section to reinforcement section.

The concave reinforcement section may extend from the bead section to the flat section.

The bead section may have a radius of between 1 .5 times and 10 times a wall thickness of the end cap. If the radius of the bead section is too small, then stress concentrations in this area may result. The minimum radius of the bead section is limited by the thickness of the wall of the end cap. If the radius is too large, then the bead section may deform under the stresses created by internal pressures to the fuse.

The concave section may have a wall thickness of between 0.5mm and 2mm. Material having a wall thickness of 0.71 to 0.91 mm is commonly used in hot oil fuses. The present invention may be applied to fuses with diameters in the range of 25mm to 89mm. Wall thicknesses for use with this range of diameters, and other diameters, may usefully be within the range 0.5mm to 2mm.

The interior depth of the concave section, from its innermost point to the outermost point of the bead may be between 5% and 25% of a diameter of the end cap. If the interior depth is too small, then the resistance of the end wall to internal pressures may be insufficient, if the interior depth is too great, then the side walls of the end cap may extend outside the dimensional limits of the end cap and it may further be impossible to access tag arrangements attached to the central portion of the end cap to install the fuse.

A diameter of the concave section may be between 68% and 94% of a diameter of the end cap. A minimum value for this diameter may be calculated by allowing three times a wall thickness of the end cap for the lateral measurement required for the bead section and subtracting this from the overall end cap diameter. A maximum requirement for the bead section may be 40 times a wall thickness of the end cap.

Where the profile of the concave section describes a part of an ellipse, the ellipse may have a dimension in a direction of a depth of the concave section of 10% to 50% of a diameter of the end cap. Use of dimensions within this range can allow the balance to be found between sufficient depth of the concave section to provide the best pressure distribution characteristics and the need to access the central portion of the end cap for the attachment of tagging arrangements.

The end cap may be made from a material selected from the group of copper, silver, gold, platinum and aluminium. An important function of the end cap is to conduct electricity and thus other less conductive materials may not show sufficient conductivity to perform this function.

The end cap may advantageously be made from copper, due to its high conductivity.

The present invention further provides an electrical fuse comprising a fuse body and an electrical fuse end cap according to the present invention.

The present invention further provides an electrical fuse comprising a fuse body and an electrical fuse end cap according to the present invention, wherein the engagement section is engaged with the outer surface of the fuse body.

The electrical fuse may further comprise an adhesive for adhering the end cap to the fuse body.

The present invention will now be described in detail with reference to the following figures in which:

Figure 1 shows a partial section through an example of a fuse for use with an end cap according to the present invention;

Figures 2a and 2b show examples of known fuse end caps;

Figure 3 shows an end cap having reinforcing ridges on an end wall; Figure 4 shows an alternative arrangement of reinforcing ridges on an end wall of an end cap;

Figure 5 shows an alternative end cap reinforcement arrangement;

Figure 6 shows a further alternative end cap reinforcement arrangement; Figure 7 shows a fuse end cap having a flat central section and a concave reinforcement section;

Figure 8 shows an end cap having a bead section around its periphery;

Figure 9 shows an end cap having an end wall having a concave reinforcement section; and

Figure 10 shows an end cap performance comparison graph.

Figure 1 shows a partial section through a fuse assembly 1 , the assembly comprising fuse body 1 1 which contains fusible elements 12, which are wrapped around element support 13. Support cap 14 is provided to each end of element support 13.

Silica Quartz filler 1 5 fills the space around the fusible element and the element support. End disc 16 may be provided and acts to maintain support cap 14 in place and prevent quartz filler 15 from escaping into empty space 171 in end cap 17 and empty space 181 in end cap 1 8. Inner end cap 161 is provided and forms a mechanical and electrical contact with end caps 1 7 and 18. Alternative end cap configurations may be required, depending upon what type of tag arrangement 19 and 191 will be fitted to the fuse. End cap 17 has a centrally located flat section 1 72 on to which tag arrangement 191 may be welded. Concave reinforcement section 173 is provided to improve the resistance of the end cap to internal pressures generated within the fuse 1 1 . End cap 18 is provided with flat section 182 and in this alternative arrangement, an optional flanged opening 184 is provided to receive tag arrangement 19.

Figures 2a and 2b show known end cap arrangements. In Figure 2a, end cap 21 is provided with a centrally located tag arrangement 22. End wall section 23 is flat and connected to side wall section 24 by a radius portion 25. No reinforcement is provided to end wall section 23 and where a certain pressure differential across end wall 23 is experienced, end wall 23 may bow under that pressure. This has the effect of lifting tag arrangement 22 in an upward direction as seen in the figure, which in turn has the effect of extending the overall length of the fuse and can take the fuse outside dimensional tolerances.

Figure 2b shows a known method of reinforcing an end wall of a fuse end cap. A solid washer 26 may be welded to the outside of the end wall of the end cap. Opening 27 is provided for the attachment of an appropriate tagging arrangement. For certain fuse designs, the provision of washer 26 provides sufficient strength to the end wall of the end cap to prevent the end wall from bowing and therefore maintains the overall fuse dimensions within dimensional tolerances. However, a large amount of material is required to manufacture washer 26, which can significantly increase the cost of the manufacture of the end cap.

Accordingly, alternative end cap designs have been investigated in order to improve the strength of an end wall of a fuse end cap. These alternative designs have been modelled using finite element analysis software tools to compare the relative performance of the designs under simulated internal pressure.

Figure 3 shows an end cap 3 having a side wall 31 and an end wall 32. Ridges 33 are provided a wall 32 with a view to increasing the strength of end wall 32. An arbitrary internal pressure of 6.89 bar (100 psi) and areas where a factor of safety of the design was less than 2 were highlighted. In the design shown in Figure 3, the factor of safety of the design was less than 2 in a region of end wall 32 within dashed circle 34 and also around radiussed section 35.

Figure 4 shows an alternative design of an end cap, which was modelled under the same conditions as the end cap shown in Figure 3. The provision of bead 41 at the intersection between side wall 42 and end wall 43 made a minor improvement to the strength of the end and in the areas surrounding bead section 41 , there was a slight reduction in the proportion of that area in which the factor of safety was less than 2, when compared to the design shown in Figure 3. The area of end wall 43 in which the factor of safety was less than 2 was reduced to approximately to the area shown by dashed line 44.

Figure 5 shows a further alternative design for an end cap which was analysed under the same conditions as the end cap shown in Figure 3. In addition to the features shown in Figure 4, end cap 5 comprises a further radius 51 between a central end portion 52 and recessed wall portions 53. This design showed a further improvement in strength and the area of end wall 52 in which the factor of safety was less than 2 was restricted to the area within dashed circle 54. Other areas in which the factor of safety was less than 2 were limited to the uppermost parts of bead section 55 and ridges 56. Some areas around the uppermost portion of side wall 57 also showed a factor of safety of less than 2.

Figure 6 shows an alternative design for a fuse end cap which, under finite element analysis, also showed certain areas in which stress concentrations were such that a factor of safety of 2 was not achieved.

Figure 7 shows a fuse end cap 7 having a side wall section 71 , a flat central end wall section 72 and a radiused bead section 73. A concave reinforcement section 74 is provided in the end wall. When this end cap design was modelled under the same circumstances as the end cap in Figure 3, there were no areas in which the factor of safety was calculated as being less than 2.

Figure 8 shows a further alternative end cap design, wherein the end cap 8 comprises a side wall section 81 connected to an entirely flat end wall section 82 and a radiused bead section 83 provided between side wall 81 and end wall 82. For the purposes of comparison, this end cap was also modelled under the same conditions as the end cap of Figure 3. End cap 8 showed a factor of safety of less than 2 in almost all areas of the flat end wall 82, apart from some areas approximately in the region just either side of dashed line 84 and certain areas on the uppermost edge of radiused bead section 83.

Figure 9 shows a section through an end cap 9 having a configuration similar to that shown in Figure 7. An internal flat disc 21 is shown mounted within end cap 9 and performs the same function as disc 16 described in relation to Figure 1 .

End cap 9 has an external diameter D and has a wall thickness A. An appropriate wall thickness for use with, for example, a hot oil fuse as shown in Figure 1 , is 0.5 mm to 2 mm. However, other wall thicknesses may be appropriate where diameter D is significantly larger than 100 mm. A flat section 92 is provided in the end wall to the end cap, having a diameter B. In one example, D may be 64 mm and B may be 32 mm. However, the diameter of flat section 92 may be anywhere from 10% to 50% of diameter D. A diameter of 50% of D will generally be sufficient to attach tag arrangements. If the tag arrangement is larger than this, the flat section will need to be greater than 50% of D. If the flat section is significantly larger than this, it may represent an area of weakness and potential deformation. If necessary, a part of the tag arrangement or other strengthening features such as a washer may act as a reinforcement to flat section 92. A radiused bead section 93 may be provided having a radius R. The size of the radius R may be restricted by wall thickness A. R may be between 1 .5 times wall thickness A and may usefully be up to 10 times wall thickness A.

An internal diameter C of bead section 93 will be equal to D minus 4 x R. A concave reinforcement section 94 is provided. In the particular example shown, the concave section takes the form of a part of an ellipse 95. However, other concave forms may be advantageous, such as a parabolic curve, a simple radial curve or any other smooth curved form. A depth of the ellipse in a direction parallel to the axis of the end cap, or in the direction a depth of the concave section of the end cap may usefully be in the range 10% to 50% of diameter D of the end cap. The concave section may have a depth F from the outer surface of flat section 92 to the uppermost point of bead section 93. This may advantageously be 5% to 25% of diameter D1 .

Figure 10 illustrates the results of a performance comparison carried out between an end cap according to Figure 2A and an end cap having a concave reinforcement section approximately in accordance with that shown in Figure 9. Six samples of each design were tested and average results for each set are shown. The samples were manufactured in accordance with standard manufacturing processes for the fuse shown in Figure 1 and heated to temperatures of 130 to 145°C. The deformation of the central flat section of each end cap was measured after it had been heated to the temperatures shown in Table 1 below. The results of those measurements are shown in Table 1 . As can be seen from the results in the table, the deformation displayed by the end cap having the concave reinforcement section was significantly less than that displayed by an end cap having a flat end wall section. In the graph in Figure 10, the taller set of columns, in row A of the 3-dimensional graph, represent the results for the end cap shown in Figure 2A. The results shown in row B represent the results for the end cap having the concave reinforcement section in its end wall. When the end cap of Figure 2A was subjected to temperatures of 145°C, it deformed to a sufficient degree that the overall fuse assembly could cease to be within the required dimensional tolerances, since on the particular fuse in question a tolerance on the length of the fuse was +/- 1 .2mm. The fuse may already be close to the limit of these tolerances prior to deformation, due to variations in the manufacturing processes. Tolerances may exist on the overall length of the fuse assembly, as well on the angular orientation of the tag arrangement with respect to an axis of the fuse. Deformation of the end wall may result in the fuse being outside any of these tolerances.

TABLE 1