EP0027061A1 - Lightning arrester device allowing an external short-circuiting and corresponding protection assembly - Google Patents

Abstract

1. An overvoltage protection device intended to protect a circuit against the effects of an excess voltage, the said overvoltage protection device being constituted by an external casing defining an enclosure containing an inert gas and forming a first electrode of the overvoltage protection device, the said external casing enclosing at least one metal pin forming another electrode and carrying externally at least one elastic metal device having, parallel to the external casing at least one elastic strip a first zone of which is bearing on a fusible element disposed near one end of the external casing and carried by it, the said first zone of the said device being capable of coming into electric contact by its elasticity when the said fusible element has fused with the (or one of the) above mentioned metal pin (or pins) and a second zone of which, on the opposite side from the first is connected to a metal fixing ring tightly fitted on the external casing, characterised in that, [in the known way], the external casing (1, 21) is a cylinder entirely made of metal, one part of the pins forming another electrode (5, 251, 252) projecting outside from the said casing (1, 21) and being joined thereto by an insulator (6, 26) disposed annularly between casing (1, 21) and the pin (5, 251, 252) and in that, to obtain a good accuracy in the excess voltage producing the short circuit, the fusible element (8, 18, 28) is, on the one hand, constituted of a metal alloy which is an electric and thermal conductor with proper fusion and on the other hand, the end of the strip of the elastic device (7, 27) [comprises at least one elastic strip which has a portion (7b, 271b-272b) substantially parallel to the external casing (2, 21) and whose end (7c, 271c, 272c)] is folded at right angles towards the pin forming the corresponding electrode (5, 251-252).

Description

The present invention relates to the production of a gas arrester intended in particular for protecting electronic circuits against overvoltages. The invention also relates to the production of protection assemblies using such lightning arresters.

It is known that gas surge arresters, also called spark gaps, surge protectors or voltage limiters, are particularly used for the protection of telephone installations and it is necessary that surge arresters of this type, when they allow the flow of an amount of energy greater than a threshold, definitively short-circuit to ensure effective protection of the circuit to be protected; when the aforementioned energy threshold is exceeded, the energy being dissipated in the surge arrester, it overheats and its temperature can reach approximately 600 ° C.: it is therefore necessary to establish an important flow path thanks to short-circuiting the surge arrester (the resistance must then be less than lj _L ). Of course, when a surge arrester has thus been short-circuited, it is necessary, in order to restore the installation to repair, to change said surge arrester, but this replacement represents only a minor drawback in relation to the deterioration, which would be intervened, if the circuits to be protected had suffered the effect of the overvoltage, which caused the short-circuit protection device to short-circuit.

Some legislations stipulate that the short-circuiting of a surge arrester must be done inside the surge arrester: this type of short-circuiting has the advantage of not giving rise to any phenomenon outside the surge arrester but it has two major drawbacks. In fact, firstly, if the actual conditions of use come to be different from that provided for by the standards, which can be frequent, the arrester remains externally intact but internally deteriorated and it no longer corresponds to the standards imposed, so that it is likely, for example, to cause an arc to pass between its electrodes for voltages considered normal on a working telephone network, or to strike only for very high voltages and therefore no longer protect the network. There is thus a risk of having, in installations comprising surge arresters, non-standard surge arresters, which create significant operating disturbances which are difficult to identify. This is the reason why it is desired that the deterioration of a surge arrester translate into a frank short-circuiting, so that the corresponding line is easily identifiable. Secondly, when a surge arrester with internal short-circuit is damaged, its resistance drops to a low but not zero value (a few ohms for example): it follows that, if after deterioration it is crossed by a current, it constitutes a heating resistance, the thermal release of which is likely to be dangerous. In the case of three-pole surge arresters, it should be added that it is very difficult, if not impossible, to have a surge arrester in short circuit on the three electrodes (in fact, only two electrodes in three are in short circuit) .

To avoid the aforementioned drawbacks, certain legislations provide, on the contrary, for imposing an external short-circuiting of the surge arresters; in this case, the internal structure of the arrester is not modified during the passage of an overvoltage corresponding to a short-circuiting. A device of this type comprises, for example, a surge arrester having an outer casing containing two electrodes, which face each other and which are connected to two external contacts carried by the ceramic casing; this arrester is placed inside a case and one of the contacts rests on a fusible metal washer electrically connected to one of the two terminals between which the device is inserted, the other terminal being electrically connected to the second contact carried by the ceramic envelope. If the surge arrester allows an overvoltage to pass and if the energy exceeds a predetermined threshold, the fuse metal washer melts and the liquid metal spreads in the case. So that this liquid metal comes to establish, in a sure way, a short-circuit between the two contacts associated with the two electrodes of the arrester, it is compulsory, on the one hand, to have a shoemaker leaving an important passage around the arrester and, on the other hand, to impose a determined orientation on device so that, by gravity, the molten metal can come into the case to establish the desired short circuit between the two contacts of the arrester. It can therefore be seen that, in the state of the art, devices of this type are necessarily very bulky and of imposed orientation. Other surge arresters with external short-circuiting have also been proposed, but they correspond to complicated devices, the cost price of which is high.

The object of the present invention is to eliminate the drawbacks presented by lightning arresters with short-circuiting of known type. More specifically, the object of the present invention is to describe a surge arrester device with external short-circuiting, the orientation of which can be arbitrary and whose size, although slightly larger than for the surge arresters with shorting- interior circuit, remains nevertheless relatively reduced. A state-of-the-art arrester with internal short-circuiting, as described for example in French patent applications 77-08189 and 79-06738, comprises an outer casing which contains at least one metal rod forming a electrode; the outer envelope defines an enclosure, which contains an inert gas under reduced pressure; each electrode is carried by the casing and projects outside of it; the connection between the rod forming the electrode and the envelope is made in a sealed and electrically insulating manner, by means of a glass bead.

According to the invention, provision has been made to effect the external short-circuiting by means of an elastically displaceable member with respect to the external envelope. This member is supported elastically on a fusible element, which liquefies, when the energy which flows by the surge arrester exceeds a predetermined threshold; when the fusible element has melted, the aforementioned member, under the effect of the elastic force to which it is subjected, moves relative to the outer envelope and comes to ensure the desired external short-circuiting.

The device according to the invention, being a device with external short-circuiting, avoids the drawbacks inherent in surge arresters with internal short-circuiting. If the energy passing through the arrester is only slightly below the threshold causing short-circuiting, there is no modification of the arrester; the arrester of the device can therefore subsequently correctly fulfill its role, since the relative position of the electrodes in the outer envelope of the arrester has not varied. Furthermore, the device according to the invention is particularly advantageous, given that its external dimensions are only very little greater than that of an internal short-circuit arrester; moreover, the device according to the invention can be used whatever its orientation, since the molten metal of the fusible element does not ensure any contact, said fusible element playing only a role of temporary hold, which disappears when the threshold d tolerable energy is exceeded. In addition, the resistance of the arrester according to the invention after its short-circuiting is of the order of 0.1 ohm, so that the damaged arrester cannot be the seat of a dangerous release of heat.

The present invention therefore has as its object a lightning arrester device intended, in particular, to protect an electronic circuit against the effects of an overvoltage, this device comprising a lightning arrester consisting of an external envelope, which contains at least one metal rod forming an electrode, the outer envelope defining an enclosure which contains an inert gas under reduced pressure, each electrode of said arrester being carried by said outer envelope and projecting outside of said envelope, characterized in that at least one of the electrodes of the external envelope is associated with a conductive member which is elastically displaceable relative to the envelope and in electrical contact with a blade of the device, said member resting on a fusible element and being able to come elastically, when the fusible element is melted, in electrical contact with at least one electrode constituting another blade of the device.

In a preferred embodiment, the elastically displaceable conductive member is an elastic member.

Provision may be made for the fusible element to be a pellet disposed between the external envelope of the arrester of the device and the elastic member with which said said is associated. pastille; the patch can then be bonded to the outer envelope or to the corresponding elastic member. It is also possible, as a variant, for the fusible element to have the form of a ring surrounding the outer envelope; the ring can then be centered on the metal sleeve, which contains the insulator by which the electrode with which the elastic member associated with said ring is maintained is maintained. The fusible element can also be a ball, 1 mm in diameter for example, welded, wedged or wedged between the elastic member and the outer envelope of the arrester.

In this embodiment, provision can be made for the outer envelope to be associated with a single electrode; in this case, the outer casing is advantageously metallic and itself constitutes an electrode of the arrester; the elastic member then comprises a fixing ring tightened on the metallic outer casing of the arrester and an elastic blade resting on the fusible element.

In this embodiment, provision can also be made for the outer envelope to be associated with two electrodes; in this case, it is preferred that the two elastic members associated with the two electrodes of the outer envelope constitute a single piece carried by said outer envelope; the single piece constituting these two elastic members comprises a tightening ring on the outer casing, on either side of which is disposed an elastic blade bearing on a fusible element carried by the outer casing; the two elastic blades are symmetrical with respect to the fixing ring.

In a first variant of this embodiment, the outer casing is metallic and constitutes one of the poles of the arrester; in a second variant, the outer envelope is made of ceramic.

In this preferred embodiment, whether the outer envelope is associated with one or two electrodes, when the elastic member comprises an elastic blade, the latter preferably comprises a part substantially parallel to the outer envelope, the l the free end is bent at right angles to the corresponding electrode; in addition, the elastic member which is fixed to said outer casing can advantageously consist of a cut sheet and folded made of beryllium bronze or beryllium copper; the dimensioning of the elastic member is provided so that said member allows the passage of energy, which must pass through the arrester in the event of a short circuit, without any difficulty, which is generally achieved by using a blade a few millimeters in width and a few tenths of a millimeter in thickness.

It is advantageously possible to provide that the fusible element consists of a metal alloy having a melting point of between 65 ° C and 165 ° C approximately; the alloy used can be an entectic or an alloy close to an entectic to ensure a frank fusion of the fusible element; a so-called "WOOD metal" alloy can be used, having a melting point of 70 ° and consisting of 50% bismuth, 25% lead, 12.5% tin and 12.5%. cadmium; it is also possible to use an alloy known as "LICHTENBERG" which melts at approximately 100 ° C. and which consists of 50%. bismuth, 20% tin and 30% lead, or an alloy that melts at 140 ° C and which consists of 58% bismuth and 42% tin, or an alloy that melts at about 240 ° C and which consists of 95% tin and 5% antimony, all of the above percentages being percentages by weight.

When the outer casing of the arrester of the device according to the invention is a metallic casing, it is advantageously made of nickel-plated steel and the rods forming the electrodes are made of an iron-nickel-cobalt alloy known as "KOVAR" allowing the welding of the glass bead which provides the mechanical connection between the outer casing and the electrode in question.

To better understand the object of the invention, we will now describe, by way of purely illustrative and nonlimiting examples, several embodiments shown in the accompanying drawing.

• - la figure 1 représente, en coupe axiale, un dispositif selon le premier mode de réalisation de l'invention, dans lequel le parafoudre comporte une seule électrode et l'élément fusible est constitué d'une pastille métallique ;
• - la figure 2 représente, en perspective, l'organe élastique du dispositif de la figure 1 ;
• - la figure 3 représente, en coupe axiale, un autre mode de réalisation du dispositif selon le premier mode de réalisation de l'invention dans lequel l'élément fusible est constitué d'un anneau métallique, l'organe élastique et le parafoudre étant les mêmes que ceux représentés pour le dispositif de la figure 1 ;
• - la figure 4 représente, en coupe axiale, un dispositif selon l'invention comportant un parafoudre tripolaire ayant une enveloppe extérieure métallique et deux électrodes symétriques, l'élément fusible associé à chaque électrode étant une pastille métallique ;
• - la figure 5 représente, en perspective, l'organe élastique du parafoudre de la figure 4 ;

On this drawing :

• - Figure 1 shows, in axial section, a device according to the first embodiment of the invention, in which the arrester comprises a single electrode and the fusible element consists of a metal pad;
• - Figure 2 shows, in perspective, the elastic member of the device of Figure 1;
• - Figure 3 shows, in axial section, another embodiment of the device according to the first embodiment of the invention in which the fusible element consists of a metal ring, the elastic member and the arrester being the same as those shown for the device of Figure 1;
• - Figure 4 shows, in axial section, a device according to the invention comprising a three-pole arrester having a metal outer shell and two symmetrical electrodes, the fuse element associated with each electrode being a metal pad;
• - Figure 5 shows, in perspective, the elastic member of the arrester of Figure 4;

Referring to the drawing, and more particularly to FIG. 1, it can be seen that the outer envelope of the arrester of a arrester device according to the invention has been designated by 1; this outer casing consists of a metal cylinder having a bottom 1a in the central zone of which there is an outer rod 1b which constitutes the first terminal of the arrester. At its end opposite the bottom 1a, the outer casing 1 is connected by a solder 2 to a sleeve 3 made of an iron-nickel-cobalt alloy called "KOVAR". The sleeve 3 is engaged in the outer casing 1 until it bears against a shoulder 4. According to the axis of the sleeve 3 is arranged a rod 5, which projects outside the arrester and whose end 5a constitutes the second electrode of the arrester, this second electrode being opposite the bottom inside the outer casing 1. The rod 5 is connected to the sleeve 3 by a glass bead 6 and it is produced in the same metal as the sleeve 3. The glass bead 6 constitutes a plug, which isolates from the outside the zone of the envelope 1 where the electrode 5a is arranged, this zone constituting an enclosure, where there is a rare gas , such as argon, under reduced pressure; into the atmosphere of this enclosure, a material is introduced intended to reduce the starting voltage of the arrester. The arrester which has just been described is well known in the state of the art and corresponds to the embodiments of FIGS. 1 and 3.

In the embodiment of FIG. 1, we see that, on the outer casing 1, an elastic conductive member has been put in place designated by 7 as a whole. The elastic member 7 comprises a ring 7a, which elastically encloses the outer envelope and which, if desired, can be connected to it by a soldering point after its installation. The ring 7a is integral with an elastic blade 7b, the end of which which is opposite to the blade 7a, is folded at right angles to form a return 7c. The elastic member 7 is made of beryllium bronze by cutting and folding a sheet of 0.2 mm thick; in the example described, the outer casing 1 and its sleeve 3 have an axial length of approximately 15 mm, the ring 7a has an axial length of approximately 5 mm, the elastic blade 7b has a length of approximately 9 mm and the return angle 7c has a length of approximately 3 mm; the width of the blade 7b and the return angle 7c is 2 mm and the diameter of the arrester is approximately 5 mm.

Between the elastic strip 7b and the cylindrical wall of the outer casing 1 of the arrester, there is a futible element 8, which consists of a square patch with a side of 2 mm and a millimeter in thickness. This patch can be glued to the face of the elastic blade 7b, which faces the envelope 1, in order to ensure easy installation. Due to the elasticity of the blade 7b, the pad 8 is applied against the outer casing 1 and the end of the square return 7c is then, as shown in Figure 1, about 1 mm from the rod 5 If the arrester is subjected to an overvoltage, an arc is established between the electrode 5a and the outer envelope 1 of the arrester and this arc causes the outer envelope 1 to heat up. When the temperature of the outer envelope 1 rises sufficiently, the melting point of the metal is reached, of which the pad 8 is made and, in this case, the pad 8 melts and the elastic blade 7b approaches the envelope 1 due to its elasticity; in this movement, the end of the square return 7c comes into contact with the rod 5, which short-circuits the arrester.

Of course, the nature of the metal of which the pellet 8 is made is chosen, as a function of the energy threshold that the surge arrester must allow to pass without short-circuiting. We could, for example, use to make the wafer 8 an alloy melting at 100 ° C. and consisting of 50% by weight of bismuth, 20% by weight of tin and 30% by weight of lead.

FIG. 3 represents another embodiment of the arrester device according to the invention; in this embodiment, the surge arrester itself is identical to that which has been described for FIG. 1, that is to say that all the elements bearing references between 1 and 6 inclusive are the same as for the embodiment of Figure 1. The elastic member of the arrester is also the same as the elastic member 7 shown in Figures 1 and 2. The only difference comes from the particular embodiment of the fusible element, which is associated with the elastic blade 7b of the member 7. In fact, in FIG. 3, the fuse element designated by 18 is constituted by a metal ring disposed around the projecting part of the sleeve 3. The ring 18 comes to bear against the shoulder that constitutes the border of the cylindrical lateral wall of the envelope 1 and the ring 18 is held against this border by the return at right angles 7c to the elastic member 7. In other words, after having positioned the ring 18 on the sleeve 3, the elastic member 7 is put in place on the outer casing 1, positioning it so that the angle return 7c maintains the ring 18 in position. The operation of this embodiment is the same as that which has been described for the embodiment of FIG. 1, the fusion of the ring 18 causing the arrester to short-circuit by contact between the return angle 7c and rod 5.

Referring now to FIGS. 4 and 5, which illustrate another variant of the preferred embodiment of the invention, it can be seen that the outer casing of the arrester of the device according to the invention has been designated by 21; this outer envelope consists of a metal cylinder which, at each of its ends, is connected by a solder 22 to a sleeve 23 made of an iron-nickel-cobalt alloy called "KOVAR". The sleeve 23 is engaged in the outer casing 21 until it bears against a shoulder 24. According to the axis of each of the sleeves 23 is arranged a rod 251,252 which projects outside the arrester; the ends 251a, 252a of the two rods 251 and 252 respectively constitute the two electrodes inside the outer envelope 21. The two rods 251 and 252 are each connected to their sleeve 23 by a glass bead 26; they are made of the same metal as the sleeves 23. The glass bead 26 constitutes a stopper, which isolates from the outside the zone of the envelope 21, where the electrodes 251a and 252a are arranged, this zone constituting an enclosure where there is a rare gas, such as argon, under reduced pressure; into the atmosphere of this enclosure, a material is introduced intended to reduce the starting voltage of the arrester. The arrester, which has just been described, is well known in the state of the art.

In this embodiment, it can be seen that on the outer casing 21, a metal part designated by 27 as a whole has been put in place. The part 27 comprises, in its central zone, a ring 27a, which elastically encloses the outer envelope 21 and which, if desired, can be connected to it by a weld point after its installation. On either side of the ring 27a are two elastic members 271 and 272 symmetrical with respect to the ring 27a; each of these elastic members 271 and 272 consists of an elastic blade 271b and 272b respectively, each elastic blade having its end, which is opposite to the ring 27a, folded at right angles to form a return 271c and 272c respectively. Piece 27 is made of beryllium bronze by cutting and folding a sheet of 0.2 mm thick; in the example described, the outer casing 21 and its sleeves 23 have an axial length of 16 mm, the ring 27a has an axial length of approximately 5 mm, the elastic blades 271b and 272b have a length of approximately 6 mm and the square returns 271c and 272c have a length of about 3 mm; the width of the blades 271b and 272b and the corresponding square returns is 2 mm and the diameter of the arrester is about 6 mm.

Between each of the elastic blades 271b and 272b and the cylindrical wall of the outer casing 21 of the surge arrester, there is a fusible element 28, which consists of a square patch of 2 mm side and 1 mm thick. This patch can be glued to the faces of the elastic blades 271b and 272b which are opposite the envelope 21, in order to ensure easy installation. Due to elasticity blades 271b and 272b, the pellets 28 are applied against the outer casing 21 and the ends of the square returns 271c and 272c are then, as shown in Figure 4, about 1 mm from the rods 251 and 252 respectively. If one of the electrodes of the arrester is subjected to an overvoltage, an arc is established inside the arrester between this electrode and the outer casing 1 and this arc causes the outer casing 21 to heat up. When the temperature from the outer casing 21 rises sufficiently, the melting point of the metal of which the pellets 28 are made is reached; the pellets 28 melt and the elastic blades 271b and 272b approach the envelope 21 due to their elasticity; in this movement the ends of the square returns 271c and 272c come into contact with the rods 251 and 252 respectively, which simultaneously puts the two electrodes of the arrester in short circuit.

It should be noted that this simultaneous short-circuiting of the two electrodes 251 and 252 is particularly advantageous because, usually, on three-pole surge arresters having an internal short-circuiting, only the electrode which is subjected to the overvoltage short-circuits the outer envelope, the other electrode keeping a high ignition voltage; it follows that, in the prior art, the surge arrester becomes asymmetrical which is often harmful to the protected circuit. On the contrary, the three-pole arrester according to the invention, which has just been described, allows the simultaneous short-circuiting of the two electrodes with the outer casing.

Of course, the nature of the metal from which the chip 28 is made is chosen, as a function of the energy threshold that the surge arrester must allow to pass without short-circuiting. We could, for example, use to make the pellet 28 an alloy melting at 100 ° C and consisting of 50% by weight of bismuth, 20% by weight of tin and 30% by weight of lead.

It is understood that the embodiments described above are in no way limiting and may give rise to any desirable modifications, without departing from the scope of the invention.

Claims (19)

1 - Lightning arrester device intended in particular to protect an electronic circuit against the effects of an overvoltage, this device comprising a lightning arrester consisting of an external envelope, which contains at least one metal rod forming an electrode, the external envelope defining an enclosure which contains an inert gas under reduced pressure, each electrode of said arrester being carried by said outer casing and projecting outside of said casing, characterized in that at least one of the electrodes (5,251-252) of the casing outer (1, 21) is associated with a conductive member (7,27) resiliently movable relative to the envelope and in electrical contact with a pole of the device, said member resting on a fusible element (8,18,28) and being able to come elastically, when the fusible element is blown, in electrical contact with at least one electrode (5,251-252) constituting another pole of the device. 2 - Device according to claim 1, characterized in that the elastically displaceable conductive member is an elastic member (7,27). 3 - Device according to claim 2, characterized in that the fusible element is a patch (8.28) disposed between the outer casing (1.21) of the arrester of the device and the elastic member (7.27) with which is associated said tablet (8,28). 4 - Device according to claim 3, characterized in that the patch (8,28) is bonded to the outer casing or to the corresponding elastic member. 5 - Device according to claim 2, characterized in that the fusible element has the form of a ring (18) surrounding the outer casing (1). 6 - Device according to claim 5, characterized in that the ring (18) is centered on the metal sleeve (3), which contains the insulator (6) by which is held the electrode (5), with which cooperates the elastic member (7) associated with said ring (18). 7 - Device according to claim 2, characterized in that the fusible element is a ball welded or wedged between the elastic member (7) and the outer casing (1.21) of the arrester. 8 - Device according to claim 2, characterized in that the outer envelope (1) is associated with a single electrode (5). 9 - Device according to one of claims 1,2 or 8, characterized in that the outer casing (1) is metallic and itself constitutes an electrode (lb) of the arrester. 10 - Device according to claims 8 and 9 taken simultaneously, characterized in that the elastic member (7,27) comprises a fixing ring (7a, 27a) clamped on the metal outer casing (1,21) of the arrester and an elastic blade (7,271-272) resting on the fusible element (8,18,28). 11 - Device according to claim 2, characterized in that the outer casing (21) is associated with two electrodes (251,252). 12 - Device according to claim 11, characterized in that the two elastic members associated with the two electrodes (251,252) of the outer casing (21) constitute a single piece (27) carried by said outer casing. 13 - Device according to claim 12, characterized in that the single piece (27) constituting the two elastic members comprises a fixing ring (27a) clamped on the outer casing (21), on either side of which is arranged an elastic blade (271-272) resting on a fusible element (28) carried by the outer casing (21). 14 - Device according to claim 13, characterized in that the two elastic blades (271-272) are symmetrical with respect to the fixing ring (27a). 15 - Device according to claim 1, characterized in that the outer envelope is made of ceramic. 16 - Device according to one of claims 10 or 13, characterized in that each elastic blade of the elastic member (7,27) has a part (7b, 271b-272b) substantially parallel to the outer envelope (1 , 21), the free end of which (7c, 271c, 272c) is bent at right angles rection of the corresponding electrode. 17 - Device according to claim 2, characterized in that the elastic member (7.27) consists of a cut and folded sheet made of beryllium bronze or beryllium copper. 18 - Device according to one of claims 1 to 17, characterized in that the fusible element (8,18,28) consists of a metal alloy having a melting point between 65 ° C and 125 ° C . 19 - Device according to one of claims 1 to 18, characterized in that the fusible element (8,18,28) consists of an entectic alloy or close to an entectic.

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