Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Fusible element actuated
Patent
1982-07-20
1985-04-30
Broome, Harold
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Fusible element actuated
337273, H01H 8542
Patent
active
045147167
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a fuse comprising a metal wire capable of being destroyed by melting more especially in the case of an excess current.
Fuses are devices, used already for a very long time, for protecting against excess currents. When the installation or the apparatus to be protected is capable of withstanding an excess current for a short time, such fuses constitute a reliable protection. With the use of electronic circuits, whose components are incapable of withstanding high overcurrents, even for very short times, being more and more widespread, it has proved that such fuses are not always capable of ensuring a sufficient protection for the circuits. This is the case for example for remote data processing circuits connected to transmission lines exposed to overvoltages atmospheric in origin. The best fuses known up to date for protecting such installations are silver-wire fuses enclosed in a glass tube, the wire being possibly stretched by means of a spring so as to ensure immediate breaking of the arc which forms at the moment when the wire breaks. Now even with the best fuses known up to date, the destruction of circuit components can be noted despite the melting of the fuse. This is explained by the vaporization of the molten silver which forms a conducting plasma inside the tube, this plasma maintaining the electric arc, i.e. a high current through the fuse. Generally, this effect may be seen by the blackening of the glass tube.
To ensure blowout of the arc, the use of deflection effects related to the presence of a magnetic field has been proposed in some fuses, this magnetic field being created by permanent magnets or coils associated with the fuse. Thus complex, expensive and bulky structures have been provided in the prior art. More especially, the metal permanent magnets used in the constructions of fuses of the prior art require, on the one hand, the interpositioning of insulating walls between the magnet and the fuse and, on the other hand, the use of bulky magnets. The space occupancy of these magnets, due in particular to the fact that they are necessarily magnetized in the direction of their length, practically rules out the formation of a uniform magnetic field over the whole length of a fuse wire. The interpositioning 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 blowout.
The present invention has as its aim to ensure, by the simplest means possible, the extinction of this arc by blowout. According to one aspect of the invention, the means serving as permanent magnet also serve as mechanical support and as extinction chamber wall. According to another aspect of the invention, the means serving as permanent magnet are magnetized in the direction of their thickness so as to reduce their space occupancy. According to other aspects of the invention, the magnetic field is produced over the whole length of the fuse wire so as to increase the mechanical stress produced in the wire and ensure blowout of the arc whatever the breakage point of the wire; the magnetic field is produced over a sufficient width on each side of the fuse wire to allow a substantially elongation of the arc and acceleration of blowout.
In its simplest form, the fuse of the invention is formed by an elongated ferrite support magnetized in the direction of its thickness and on which is disposed a non-ferromagnetic conductorfuse wire between two terminals. According to the LAPLACE law, when the wire has a current passing therethrough, in one direction or in the other, it is subjected to a force perpendicular to the wire and parallel to the ferrite support plane and this force is proportional to the product of the current and of the magnetic field. It should be noted that this force acts on any moving eletric charge, i.e. also on the electric arc likely to form at the breakage point of the wire. This force not only results in magnetically blowing out the electric arc, but in accel
REFERENCES:
patent: 659671 (1900-10-01), Hewlett
patent: 685766 (1901-11-01), Jones
Broome Harold
Plottel Roland
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