GB2178913A - Electrical fuse - Google Patents

Abstract

In order to cause a fuse 80 to fail quickly when it is subject to overload, a heat insulating and/or heat reflective layer is disposed between the fusible element 86 and its surrounding housing 84. Fibreglass matting or paper may be used to form an insulating layer and a metallic coating may be used as a reflective layer. <IMAGE>

Description

SPECIFICATION Electrical fuses This invention relates to electrical fuses having a fusible element at leastpartly surrounded by a housing, such as for example a blade fuse or a cartridge fuse.

When such a fuse is subject to overload the element generates aconsiderable amount of heat which, if the overload is sustained, can damage the holder in which the fuse is inserted or an associated fuse casing, and may constitute a fire hazard. In cases where the fuse housing itself is of plastics material, the housing can be degraded or distorted making the fuse subsequently difficult to remove and having other undesirable consequences.

According to the present invention the abovedescribed effects of over-heating are eliminated or reduced by the provision of an electrical fuse comprising a fusible element, a housing at least partly surrounding the element, and a thermal shield interposed between the element and the housing. By reducing the amount of heat that can be transmitted away from the element not only are the above described ill effects of overheating overcome or reduced, but also the consequent elevation of the temperature of the element and its immediate environment promote its failure when it is carrying a current at or above its rating.

The thermal barrier may be constituted by a heatinsulating layer or a heat-reflective layer, or by a combination of both. Suitable materials for forming the insulating layer are fibreglass matting and paper, and the layer may be present as a lining or surface layer on the inner surface of the housing, or as an insert, which may be unattached. Where the housing is transparent, as for example in the case of a cartridge fuse having a cylindrical glass housing, the insert may also serve as a label, being printed on its outer surface with details of the fuse. A convenient way to form a loose insert is to roll up a small sheet of paper, printed on one surface, into a tube, although of course, lengths of preformed tube could be used.

The heat-reflective layer conveniently takes the form of a coating on the inner surface of the housing or a surface, usually the inner surface, of the heatinsulating layer. Preferablythecoating is constituted by a metallic layer which may be applied by printing on the surface of an insert or lining or by deposition, as by sputtering, on the surface of the housing. As a metallic coating is by nature electrically conductive it is desirably formed with electrical breaks constituted by regular or irregular discontinuities, for example by printing the coating as spaced rectangles or sputtering to form isolated coated areas of irregular shape.

Some embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through a blade type fuse incorporating a thermal barrier according to the invention; Figure 2 is a side elevation of the fuse of Fig. 1, partly broken away; and Figure 3 is a plan view of a sheet of heat-reflective material used for forming the thermal fuse of Figs. 1 and 2.

Figs. 1 and 2 show a blade fuse 80 of conventional type, comprising two blades 82 moulded into a housing 84 of thermoplastics material and bridged at their inner ends by a fusible element 86 which is located within a cavity 88 formed in the upper portion of the housing 84.

To protect those wall portions of the housing 84 flanking the cavity 88 from the heat radiated by the element 86 when carrying a large current, the walls are lined with portions 90 of the sheet material now to be described with reference to Fig. 3.

In Fig. 3 is shown a square piece 50 of heatreflective material comprising a backing layer of 0.005" (0.013 cm) thick white paper 52 having printed on the glossy surface thereof in metallic silver a pattern of spaced squares 54 forming a gridlike pattern of rectilinear discontinuities 56 therebetween. On the opposite surface the paper is coated with a layer of adhesive, protected by a peelable layer, enabling suitably shaped portions 90 to be cut therefrom and adhered to the walls of the cavity 88.

The portions 90 thus function as heat-reflecting shields on each side of the element 86 and delay damage to the housing 84 during stepped or continuous overload of the element 86 which can give rise to a dissipation of heat energy of a wattage which can raise the temperature of the housing locally above its thermal limit.

The reduction in the temperature that the material of the body 84 is required to withstand enables a plastics material to be used which is not only cheaper but may also be transparent, allowing the inspection hole, necessary with the opaque materials in current use, to be dispensed with.

In an unillustrated embodiment a cylindrical fuse having a ceramic or glass wall incorporates a tubular insert, formed by rolling up a piece of reflective sheet material as shown in Fig. 3, between the element and the glass wall. For use in this embodiment however the paper 52 is left uncoated with adhesive on its reverse side and printed with details of the fuse, thus forming a label, when intended for insertion in the glass walled fuse.

In other unillustrated embodiments the heatreflecting shield may take the form simply of a reflective metal layer preferably with electrical breaks coated directly onto the cavity wall of a blade fuse orthe internal surface of a glass or ceramic wall of a cylindrical fuse. Coating may be effected by sputtering or vacuum deposition. Again the shield may be constituted by a layer of heat-insulating material in the form of a coating or lining.

It has been found experimentally that when a fuse incorporating a heat shield according to the present invention is connected in series with an otherwise identical fuse without a heat shield the fuses gradually loaded with a current exceeding their rating, the fuse incorporating the thermal shield always blows first.

Claims (14)

1. An electrical fuse comprising a fusible element, a housing at least partly surrounding the element, and a thermal shield interposed between the fusible element and the housing.

2. An electrical fuse according to claim 1, in which the thermal shield includes a heat insulating layer.

3. A fuse according to claim 2, in which the heat insulating layer includes paper.

4. Afuse according to claim 2 or claim 3 in which the heat insulating layer includes fibreglass matting.

5. Afuse according to any of claims 2 to 4, in which the heat insulating layer is formed as a lining attached to the housing.

6. A fuse according to any of claims 2 to 4, in which the heat insulating layer is unattached to the housing or to the fuse.

7. Afuse according to claim 3, in which the heat insulating layer is a roll of paper which forms a loose insert surrounding the fuse.

8. A fuse according to claim 1, in which the shield includes a heat-reflective layer.

9. A fuse according to claim 8, in which the heat reflective layer is in the form of a coating applied on the inner surface of the housing.

10. A fuse according to claim 8, and any one of claims 2 to 7, in which the heating reflecting layer is in the form of a coating applied to the heat insulating layer.

11. A fuse according to any one of claims 8 to 10, in which the heat-reflective layer is metallic.

12. A fuse according to claim 11, in which the metallic layer is applied by printing.

13. A fuse according to claim 11, in which the metallic layer is applied by sputtering.

14. A fuse according to any of claims 11 to 13, in which the metallic layer is formed with electrical breaks.