Method For Manufacturing Electric Fuses

Abstract

Method for manufacturing a fuse comprising, prior to fixing the responsive element of the fuse to the two strips for connecting the element to the electric circuit, fixing the two strips at a fixed distance apart in a case of insulating material. The responsive element is thereafter soldered to the two strips and the interior of the case is closed off and filled with powdered material. The case is then finally closed. Various embodiments of the fuse obtained are disclosed.

Description

The present invention relates to the manufacture of electric fuses having a high cut-off or break power of the type comprising a sensitive or responsive element in the form of wires or a metal strip embedded in a powdered mass, the assembly being enclosed in a housing of insulating material. The sensitive element is electrically connected to two blades or strips projecting from the body of the fuse and serving to place the assembly on a base or wiring panel or the like provided with tongs which ensure the connection with the circuit controlled by the fuse.

Up to the present time, difficulties have been met with in the manufacture of these fuses owing to the very poor stiffness of the sensitive elements which are not only very thin but provided with calibration orifices to afford a carefully predetermined cut-off value. These orifices result in zones of smaller cross section which still further increase the risk of damage to the sensitive elements by the deformation which could easily occur during handling in the course of manufacture and particularly when the elements are fixed to the strips which have a mass distinctly greater than the elements.

The object of the invention is to avoid all these risks of damage to the sensitive or responsive elements.

The invention provides a method for manufacturing electric fuses comprising, prior to fixing the responsive element of the fuse to the strips which serve to connect it to the exterior electric circuit to be protected, placing said strips in a case of insulating material so as to impart to the strips a fixed spacing therebetween in said shell, soldering or otherwise welding said responsive element to said strips, closing off the interior of said case, filling said case with a powdered material and closing said case.

Owing to these features of the invention, the responsive element is mounted on the strips only after they have been given a fixed distance apart and prevented from moving relative to each other, so that deformations of the responsive element are avoided.

According to one embodiment of the invention, the closing operations comprise moulding under high pressure an insulating material which covers at least a part of the case by injecting in the mould employed an amount of insulating material whose volume is slightly greater than the space available in the mould after introduction of the case.

This particular feature of the invention enables a very high compression of the powdered mass to be achieved by a small inward deformation of at least a part of said case, this compression being essential in order to avoid formation of arcs in the fuse upon cutoff.

Another object of the invention is to provide a fuse obtained by the method defined hereinbefore, said fuse comprising a case and two strips adapted to connect the fuse to an exterior circuit, a responsive element located inside said case and fixed to said strips, a powdered material filling said case, means for holding said strips at a fixed distance apart during all the manufacturing stages of said fuse, and means for closing said case .

Further features and advantages of the invention will be apparent from the ensuing description with reference to the accompanying drawings.

In the drawings:

FIG. 1 is an exploded perspective view of two half-cases provided with strips and a sheet element constituting the sensitive or responsive element, the assembly constituting an intermediate product of the method according to the invention;

FIG. 2 is a perspective view of one of the half-cases shown in FIG. 1, the strips and responsive element having been removed;

FIG. 3 is an exploded and perspective view of one of the strips of the fuse with its strip support;

FIG. 4 shows a closing stage of the method according to the invention;

FIG. 5 is a perspective view of the finished fuse;

FIG. 6 is a perspective view of a modification of the strip support;

FIGS. 7-10 are sectional views and FIG. 11 a perspective view of intermediate products obtained after various manufacturing stages of a modification of the method according to the invention, and

FIG. 12 is a perspective view of a modification of the fuse according to the invention.

According to the embodiment shown in FIGS. 1-5, two identical half-cases 1 are first manufactured, these half-cases constituting after assembly a rectangular-sided housing having rounded corners. The half-cases are of an insulating material, such as a thermosetting plastics material. Their transverse or end walls are combined with integral flanges 2 each of which forms a cavity 3 in which can be closely embedded a support 4 for fixing a blade or strip 5 to the case.

FIG. 3 shows a blade or strip 5 and its support 4. It can be seen that the strip has the shape of a blade slotted at 6 for mechanical and electrical connection to the electric system of which the fuse subsequently forms part. The opposite end of the strip has two lateral recesses 7.

The strip support 4 preferably of metal comprises a body 8 in one piece with a roughly T-shaped tab 9. One of the branches 10 of the T-shaped tab defines with an edge of the body 8 a recess for receiving the strip 5, the distance between the bottoms of the recess 7 being adjusted to the distance between the edge of the branch 10 and said edge of the body 8. The strip is thus maintained in lateral position. It is maintained in transverse position owing to the cooperation therewith of notches 11 respectively provided in the lateral walls of each half-case 1 and in the flanges 2.

Each strip support 4 is provided with four small projecting portions 12 each of which terminates in two fastening points 13. The projecting portions are respectively embedded in aligned rectangular slots 14 provided in the bottom of each cavity 3.

The strip support and the strip itself are of metal, such as copper, covered with a layer of tin or other metal.

The component parts described up till now having been manufactured in the conventional manner, they are assembled by introducing the strips in their support 4 and embedding thereafter the latter in their cavities 3 provided for this purpose in the half-cases 1.

A number of half-cases being thus provided with strips, a sensitive or responsive element 15 is then fixed in each of them by soldering or welding the ends of the element to the inner ends of the strips at 16 as shown in FIG. 1.

The strips 5 are prevented from moving longitudinally relative to each other, since they are trapped by their support 4 in the half-case. Thus, there is absolutely no risk of damage to the responsive element 15 by bending during handling and the precision of its calibration is in no way affected. A half-case with its strips and its responsive element can therefore be easily stored and handled with no risk.

After the soldering operation, the assemblies thus obtained are enclosed in the identical half-cases the centering of which is facilitated owing to provision of tabs 17 on the longitudinal walls of each half-case.

The assembled cases 10 (FIG. 4) are thereafter rendered rigid by deforming the fastening points 13.

The following operation of the manufacturing method comprises filling the assembled case 18 with a powdered material, for example fine sand, adapted to stifle the arcs which might occur when the fuse operates and the responsive element is severed by the Joule effect. The filling can be carried out through an aperture 19 previously provided in each half-case when moulding the latter. The sand is tamped by subjecting the case to vibrations.

FIG. 4 shows the following stage in the method according to the invention. In this stage, the finished case 18 filled with sand is placed in a mould 20 in which are provided passages 21 for the strips 5. The case is suitably centered in the mould 20 owing to a series of studs 22 and ribs 23 (FIGS. 1 and 4) which are moulded on each half-case 1.

An appropriate material, such as a thermoplastic or thermosetting material, is introduced under high pressure in the mould 20. This material forms, after stripping from the mould, an outer case 24 which covers the case 18. This moulding serves two purposes :

1. The outer case 24 renders the fuse tamperproof and improves its rigidity.

2. Owing to the high moulding pressure, the inner case is compressed and even slightly deformed and this results in an additional tamping of the powdered material inside the fuse. The pressure results in the introduction of a volume of material slightly greater than the space available before application of pressure in the mould 20.

FIG. 5 shows a finished fuse obtained by the method described hereinbefore. It can be seen that the fuse is provided with a conventional indicator or telltale element 25 which indicates when the fuse has cut-off the circuit and is inserted between the tabs 26 forming part of the strip supports 4. The fuse can also be provided with a plate showing its cut-off value or some other indication.

FIG. 6 shows another embodiment of a strip support 4.sup.a. The body 8.sup.a of this support is in one piece with two parallel tabs 9.sup.a which define an oblong opening 27 in which the strip is lodged after a slight deformation of one of the tabs 9.sup.a, as indicated by the dotted line 28 in FIG. 6. Similar to the support 4 shown in FIG. 3, the support 4.sup.a has four projecting portions 12.sup.a and a tab 26.sup.a provided for fixing an indicator or telltale element 25 which indicates when the fuse has cut-off the circuit.

In the modification of the method according to the invention shown in FIGS. 7-11, the starting component is a case 29 having the shape of an open box or housing in which are introduced subsequent to the moulding thereof, two strips 30 and two supports 31. The upper edges of the walls of the case are provided with edge portions 32 having a dove-tail cross section moulded thereon.

With the strips 30 thus trapped in the case 29, a sensitive or responsive element 33 is soldered to their inner ends (FIG. 9). It will be observed that the responsive element therefore cannot be damaged by bending after assembly with the strips.

With the case filled with a powdered material and closed with a thin plug or screen 34, it is introduced in a mould in which a cover 35 is formed. The latter is rigidly secured to the case, after stripping from the mould, owing to the dove-tail edge portions 32. The entire moulding is carried out under pressure, the excess material introduced in the mould resulting in a tamping of the powdered material consequent to a slight deformation of the screen 34.

FIG. 11 shows the finished fuse. It can be seen that an indicator or telltale element 36 can be engaged in a groove 37 provided in the case 29. The telltale element is clamped between two tabs 38 respectively forming part of the strip supports 31.

A modification of the fuse in which the final moulding has not been carried out is shown in FIG. 12. In this fuse, the case 39 comprises an edge portion having a dove-tail section only on the edge portions of two opposite walls and a cover 40 is slidable on these edge portions. Tapped holes 41 enable the cover 40 to be secured to the case 39 by means of screws (not shown). If desired, this fuse can be re-armed after having performed its cutting-off function by replacing the melted responsive element by another element and filling the case once again with a powdered material.

Although specific embodiments of the invention have been described, many modifications and changes may be made therein without departing from the scope of the invention.

Claims

We claim:

1. A method for manufacturing electric fuses comprising a responsive element connected to strips which serve to connect the responsive element to an exterior electric circuit to be protected, said method comprising mounting said strips on responsive opposite walls of an open case of insulating material so that the strips are held in position, each strip extending on both sides of its respective wall in a direction substantially perpendicular to the latter, whereby the spacing between the strips inside said case is fixed, then metallurgically bonding said responsive element to respective parts of said strips inside said case, placing a cover on said case, said cover having a filling aperture, filling the interior of said case with a powdered material through said aperture and blocking said cover on said case while stopping up said aperture.

2. A method for manufacturing electric fuses comprising a responsive element connected to strips which serve to connect the responsive element to an exterior electric circuit to be protected, said method comprising mounting said strips on respective opposite walls of an open case of insulating material so that the strips are held in position, each strip extending on both sides of its respective wall in a direction substantially perpendicular to the latter, whereby the spacing between the strips inside said case is fixed, then soldering or otherwise welding said responsive element to respective parts of said strips inside said case, placing a cover on said case, said cover having a filling aperture, filling the interior of said case with a powdered material through said aperture and blocking said cover on said case while stopping up said aperture by moulding under high pressure in a mould an insulating material which covers said case and said cover, said moulding including injecting under high pressure in said mould an amount of insulating material whose volume is slightly greater than the space available in said mould after introduction of said case and said cover therein, so as to deform said cover and said case and thereby compressing said powdered material.

3. A method as claimed in claim 1, said case and said cover being of a thermosetting plastic material.

4. A method as claimed in claim 2, wherein said insulating material introduced in said mould is a thermoplastic material.

5. A method as claimed in claim 1, wherein said powdered material is fine sand.

6. A method for manufacturing electric fuses comprising a responsive element connected to strips which serve to connect the responsive element to an exterior electric circuit to be protected, said method comprising moulding tow identical shells of insulating material forming respectively an open case and a cover for said case, providing a hole in the shell forming said cover, mounting said strips on respective opposite walls of said shell forming said case so that the strips are held in position, each strip extending on both sides of its respective wall in a direction substantially perpendicular to the latter, whereby the spacing between the strips inside said shell forming said case is fixed, then soldering or otherwise welding said responsive element to respective parts of said strips inside said case, applying said shell forming said cover onto said shell forming said case, filling the interior of said shells with a powdered material through said hole and blocking said shells one on the other by moulding a layer of insulating material around the whole of said shells.

7. A method for manufacturing electric fuses comprising a responsive element connected to strips which serve to connect the responsive element to an exterior electric circuit to be protected, said method comprising moulding an open box of insulating material and fixing said strips in opposite walls of said box in the course of said moulding, each strip extending through its corresponding wall in a direction substantially perpendicular to the latter so that said strips have a fixed spacing therebetween, then metallurgically bonding said responsive element to the parts of said strips inside said box, filling the interior of said box with powdered material, closing off said interior with a yieldable closing member and moulding in a mould a cover on said box over said closing member by injecting in said mould moulding material under pressure.

8. A method as claimed in claim 7, wherein said box comprises walls and edge portions on said walls, said edge portions having a dove-tail cross section and being embedded in said cover during moulding of the latter.

9. A method for manufacturing electric fuses comprising a responsive element connected to strips which serve to connect the responsive element to an exterior electric circuit to be protected, said method comprising moulding an open box of insulating material and fixing said strips in opposite walls of said box in the course of said moulding, each strip extending through its corresponding wall in a direction substantially perpendicular to the latter so that said strips have a fixed spacing therebetween, then soldering or otherwise welding said responsive element to the parts of said strips inside said box, filling the interior of said box with powdered material, and closing said box by sliding a cover on dove-tail section edge portions provided on two opposite walls of said box.