EP0073201B1 - Fuse - Google Patents

Description

The subject of the present invention is a fuse comprising a metal wire capable of being destroyed by fusion, in particular in the event of an overcurrent.

Fuses are overcurrent protection devices that have been used for a very long time. When the installation or device to be protected is capable of withstanding an overcurrent for a short time, such fuses constitute safe protection. With the increasingly widespread use of electronic circuits, the components of which are unable to withstand high overcurrents, even for very short periods, it has been found that such fuses are not always capable of providing sufficient protection. circuits. This is the case, for example, of data processing circuits connected to transmission lines exposed to overvoltages of atmospheric origin. The best fuses known to date for protecting such installations are silver wire fuses under a glass tube, the wire possibly being tensioned by a spring so as to ensure an immediate rupture of the arc which forms at the time of wire break. However, even with the best fuses known to date, there is, despite the fuse blowing, the destruction of circuit components. This is explained by the vaporization of the molten silver which forms a conductive plasma inside the tube, this plasma maintaining the electric arc, that is to say a large current through the fuse. Generally this effect can be seen by the blackening of the glass tube.

To ensure blowing of the arc, it has been proposed to use in certain fuses the deflection effects linked to the presence of a magnetic field, this magnetic field being created by permanent magnets or windings associated with the fuse.

Patent GB-A-619,239 describes a fuse comprising an elongated insulating support (C) and two terminals (D, E) between which is disposed a conductive wire (A) capable of being destroyed by fusion in the event of overcurrent; the fi), conductor is placed in a magnetic field created by a magnet (B), an insulating part separating the magnet (B) and the wire (A). Likewise, patent FR-A-2080321 describes a fuse circuit breaker in which the fusible wire is placed in an elongated breaking chamber near which is placed a magnet producing a magnetic field to blow out the arc caused during the opening of the fuse wire. The breaking chamber (12) is made of an insulating material whose walls separate the fusible wire (15) and the magnets (40). Similarly, in patent FR-A-1 022 741, a fusible wire (3) is separated from a magnetic circuit (42) by insulating cheeks (5a, 5b).

The permanent metallic lovers used in the embodiments of fuses of the prior art require on the one hand the interposition of insulating walls between the magnet and the fuse and on the other hand the use of bulky magnets. We thus arrived in the prior art to complex, expensive and bulky structures. The size of these magnets, due in particular to the fact that they are necessarily magnetized in the direction of their length, practically prevents the production of a uniform magnetic field over the entire length of a fusible wire. The interposition of insulating walls necessarily increases the air gap or the distance between the pole piece and the fuse, so that the magnetic field is reduced as well as the efficiency of the blowing.

The object of the present invention is to ensure, by means as simple as possible, the extinction by an effective blowing of this arc. This object is achieved by the solution claimed in claim 1. According to one aspect of the invention, the means serving as permanent magnet also serve as mechanical support and wall of extinguishing chamber. According to another aspect of the invention, the means serving as a permanent magnet are magnetized in the direction of their thickness so as to reduce their size. According to other aspects of the invention, the magnetic field is produced over the entire length of the fusible wire to increase the mechanical force produced on the wire and ensure the blowing of the arc whatever the point of breaking of the wire; the magnetic field is produced over a sufficient width on either side of the fusible wire to allow a significant elongation of the arc and to accelerate its blowing.

In its simplest embodiment, the fuse according to the invention consists of an elongated support made of ferrite magnetized in the direction of its thickness and on which a non-ferromagnetic conductive-fuse wire is disposed between two terminals. According to LAPLACE law, when the wire is traversed by a current, in one direction or the other, it is subjected to a force perpendicular to the wire and parallel to the plane of support in ferrite and this force is proportional to the product of the current and the magnetic field. It should be noted that this force acts on any moving electric charge, that is to say also on the electric arc liable to form at the point of breaking of the wire. This force not only has the effect of magnetically blowing the electric arc, but of accelerating the breaking of the wire at its weakening point by fusion and of accelerating the spacing of the strands at the breaking point, i.e. - say to reduce the time during which an arc is likely to form.

According to a practical embodiment, the fuse consists of two flexible ferrite bands, made of ferrite powder bonded by an elastomer, fixed at a short distance from each other by means of two insulating bars, the fuse wire being housed between the two ferrite bands. The quality of electrical insulation of the ferrites makes it possible to produce a very thin air gap in which the magnetic field is high. In addition, the transverse blowing effect of the arc prevents projections of conductive material on the nearby ferrite walls, projections which would tend to prolong the existence of the arc.

The part of the insulating bars located between the ferrite strips is preferably provided with teeth, in the manner of a comb, so as to form slots forming transverse cooling and extinguishing chambers in which the arc is fragmented and magnetically blown. It is also possible to connect these chambers with the outside through holes which extend them through the insulating bars. If you want to avoid metallic projections outwards through this hole, these can be covered with an adhesive tape. The use of multiple transverse cooling and extinguishing chambers in connection with the magnetic blowing allows a considerable elongation of the arc path for a given transverse dimension of the device, while the multiple cooling chambers of the prior art, not associated with magnetic blowing, allow only fractionation and limited expansion of the molten material. This produces a fast fuse with very high breaking capacity.

According to another aspect of the invention, the teeth forming the transverse walls of the cooling chambers also serve as an intermediate mechanical support for the fusible wire, the wire being pinched between the opposite teeth.

The terminals can be produced in different ways, for example by rings or by magnetic plugs pinching the ends of the wire.

• La figure 1 est une vue en perspective d'une première forme d'exécution, la plus simple, de l'invention.
• La figure 2 est une vue partielle en perspective d'une deuxième forme d'exécution, à deux bandes aimantées.
• La figure 3 représente un exemple de réalisation des bornes.
• La figure 4 représente une autre forme de réalisation des bornes.
• La figure 5 est une vue en plan d'une troisième forme d'exécution, l'une des bandes de ferrite enlevée.
• La figure 6 est une vue en perspective de cette troisième forme d'exécution.
• La figure 7 illustre un mode d'exécution des bornes dans la troisième forme d'exécution.
• La figure 8 représente une variante, avec armature, de la troisième forme d'exécution.

The accompanying drawing shows, by way of example, some embodiments of the invention.

• Figure 1 is a perspective view of a first embodiment, the simplest, of the invention.
• Figure 2 is a partial perspective view of a second embodiment, with two magnetic strips.
• FIG. 3 represents an exemplary embodiment of the terminals.
• Figure 4 shows another embodiment of the terminals.
• Figure 5 is a plan view of a third embodiment, one of the ferrite strips removed.
• Figure 6 is a perspective view of this third embodiment.
• FIG. 7 illustrates an embodiment of the terminals in the third embodiment.
• FIG. 8 represents a variant, with reinforcement, of the third embodiment.

Figure 1 simultaneously illustrates the principle of the invention and the simplest embodiment thereof. On a ferrite plate 1 magnetized in the direction of its thickness so as to have poles N on its lower face and S on its upper face, is fixed a silver wire 2 by means of two metal rings 3 and 4 which constitute the fuse terminals. The wire 2 has, for example, a diameter of 0.1 mm and a length of 30 mm. According to LAPLACE's law, when this wire is traversed by a current I, it is subjected to a force F under the effect of the magnetic field H.

According to the embodiment shown in Figure 2, the fuse consists of two flexible strips 5 and 6 made of ferrite powder bound by an elastomer, which is found on the maché under the name of PLASTOFERRITE. These strips are magnetized in the direction of their thickness and attract each other. They are fixed face to face and kept apart from one another in the thickness direction by means of two insulating bars 7 and 8 profiled in T. The strip 5 is fixed to the bars 7 and 8 by gluing, while that the strip 6 is simply magnetically retained by the strip 5 so as to remain removable. The legs 70 and 80 of the T-profiles ensure the spacing of the two magnetic strips, spacing defining the thickness of a housing 90 in which the silver wire 9 is disposed. The transverse dimension of the housing 90 is defined by the spacing of the legs 70 and 80. At its ends, the wire 9 is fixed and galvanically connected to two terminals which can be executed for example as shown in FIG. 3 in which each terminal is constituted by a metal flange 10 whose ends 10a and 10b are folded around each of the insulating bars 7 and 8, the end of the wire 9 being folded under the flange. The end of the wire 9 could also be welded to the flange. The upper magnetic strip allows easy recharging of the fuse.

In the embodiment shown in Figure 4, the ends of the wire 9 are fixed by pinching between two magnetized metal parts 11 and 12 simultaneously constituting plugs closing the ends of the fuse and contact terminals. Considering FIG. 2, it will also be possible to pinch the wire 9 between two metal plates glued respectively on the upper face of the strip 5 and on the lower face of the strip 6. In order not to weaken the magnetic field, metals will be used non ferromagnetic, preferably diamagnetic. The ferrite strips may for example have a length of 50 mm over a width of 10 mm and a thickness of 1.8 mm with an air gap of 1.5 mm.

It is possible to increase the efficiency of the magnetic blowing effect of the electric arc by using insulating bars 13 and 14 as shown in FIGS. 5 and 6. The legs of these insulating bars 13 and 14 are provided of teeth 15 and 16 engaging between the ferrite strips 6 and 7, these teeth forming two combs the teeth of which are located one opposite the other, the slots 17 and 18 formed between the teeth constituting chambers cooling and extinguishing of the arc. The legs and teeth of the bars 13 and 14 have a sufficient length so that the silver wire 9 is moreover mechanically held between these teeth, which enables it to withstand mechanical shocks more easily without risk of breaking. It has in fact been observed that the fuse wires according to the prior art have broken by simple mechanical stress at their point of attachment. It is also possible to communicate the rooms extinction with the outside by holes 19 which extend them and which further promote the blowing of the arc and the expulsion of metallic particles. If necessary, these holes can be closed by means of adhesive strips 20 and 21 shown in phantom. At its ends, the wire 9 is pinched between two wider teeth 22 and 23 of the insulating bars. The ends of the wire can be connected to terminals by pinching or welding. FIG. 7 illustrates an example of connection according to which the wire is folded in a slot 24 of one of the bars and under a metal ring 25.

It is possible to considerably increase the magnetic field, that is to say the blowing effect in the air gap, by means of a ferromagnetic armature. Such an embodiment is shown in FIG. 8, as a variant of the embodiment according to FIGS. 5 and 6, in which the armature consists of a sheet 26 of soft iron folded back in a U. With this armature it has been possible to measure a magnetic field of 1200 to 1300 Gauss against a field of 500 to 650 Gauss without armature. The forces acting in this case on wire and arc are relatively very high.

In summary, the fuse according to the invention has a very high breaking capacity and ensures almost absolute protection of delicate components by its high cutting speed, it is easy to manufacture, it can be rechargeable and even cleanable.

Claims (8)

1. A fuse comprising an elongated support and two terminals (3, 4) between which is arranged a conducting wire (9) capable of being destroyed by melting in the case of an overcurrent, the conducting wire (9) being placed in a magnetic field caused by a magnet, characterised in that the elongated support is formed at least partially of ferrite (1), magnetized in the direction of its thickness and on which is disposed the conducting wire (9) between two terminals (3, 4).

2. The fuse as claimed in claim 1, characterised in that it is formed from two ferrite strips (5, 6) assembled and spaced apart by means of two insulating material bars (7, 8).

3. The fuse as claimed in claim 2, characterised in that the ferrite strips (5, 6) are formed by flexible strips.

4. The fuse as claimed in claim 3, characterized in that one of the ferrite strips (5) is fixed by bonding to the insulating bars whereas the other (6) is held magnetically by attraction by means of the bonded strip (5).

5. The fuse as claimed in any one of claims 2 to 4, characterized in that the part (70, 80) of the insulating bars situated between the ferrite strips (5, 6) is provided with teeth (15, 16) defining gaps (17, 18) forming electric-arc extinction chambers.

6. The fuse as claimed in claim 5, characterized in that the wire is nipped between the two rows of teeth (15, 16) of the two insulating bars.

7. The fuse as claimed in claim 5, characterized in that the bottom of the gaps (17, 18) communicates with the outside through a hole (19) situated in the extension of the gap.

8. The fuse as claimed in any one of claims 2 to 7, characterized in that the outer faces of the ferrite strips (5, 6) are connected together by means of a ferromagnetic armature (26).

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