EP2856488B1 - Arc control device for vacuum chamber - Google Patents

Description

TECHNICAL AREA

The invention relates to a device with two movable contacts relatively to each other, in particular used for vacuum bulbs, for controlling the arc that can be formed by forcing its trajectory while diffusing it. In particular, the stackable electrodes comprise a patch coupled to a base comprising slots and arrangements.

The invention also relates to a medium voltage bulb and an electrical switchgear apparatus implementing the type of arc control developed by the contact device.

STATE OF THE ART

Medium voltage distribution electrical equipment, particularly between 12 and 72 kV, can use vacuum bulbs which must then withstand the permanent current flow, typically of the order of 1250 A to 10 kA, without undergoing excessive heating, and to cut the short-circuit currents of the order of a few thousand amperes, typically from 25 kA to 100 kA. The vacuum bulbs thus comprise two electrodes movable relative to each other, which are in contact for the passage of nominal current and separate for cutting.

The cut can cause the appearance of an electric arc that must be controlled and dissipate as quickly as possible. The arc control can thus be of axial magnetic field or AMF (" Axial Magnetic Field ") or radial or transverse magnetic field type, that is to say RMF or TMF (" Radial / Transverse Magnetic Field" ) : see figures 1 .

In an RMF or TMF type arc control, the arc 1 is concentrated, contracted, into a column which typically has a diameter of about 1 cm. Thanks to the radial magnetic field or transverse created by the passage of the current in the contacts 3, this arc 1 performs a rotational movement along the periphery of the two contacts 3 and its thermal energy is distributed over a large area. To create the magnetic field, many contact shapes 3 have been developed, in particular on the basis of the "cut" type models 3A (see FIG. DE 372 48 13 or Figure 1A ) for the RMF or "petal" type 3B (see FR 2 541 038 or Figure 1B ) for the TMF. These controls offer good breaking capacity and good performance with long arcing times (greater than 15 ms), withstanding the effect of current loops created by the connection bars of vacuum bulbs in circuit breakers. and the cells. However, the rotating arches cause excessive erosion of the contacts (as well as the closing of the space between petals 3B if necessary) and thus the electrical endurance of the device is moderate; in addition, the dielectric strength remains average, especially after fault current failures.

In an AMF control, the arc 5 is kept diffused, that is to say composed of several more or less parallel arc columns, in order to minimize the thermal energy density on the surface of the two contacts 7 through the natural zero of the current and its interruption: figure 1C . The relatively uniform distribution of the energy of the arc 5 offers a very low rate of erosion. However, if the arc 5 can be kept relatively diffuse for given rms values, in certain phases of the current wave, especially when the instantaneous current is very high and in strong asymmetries, the determined parameters do not allow to completely diffuse this arc 5, and a main column, surrounded by a halo, can be generated. As the thermal load is no longer evenly distributed, non-breaking can occur; moreover, when closing on a fault current, welds can be formed between the two surfaces of the contacts 7. Cutouts on or under the surface of the contacts to solve these problems have been proposed ( WO 2001/41173 ), which leads to a drop in dielectric performance, while not completely solving the problems. An example of axial control is also described in US 2006/124600 in fact, two identical electrodes are placed face to face.

To take advantage of both types of control, some systems have been developed combining the two actions: see for example WO 2012/038092 or US 2008/67151 who use contacts comprising a central portion of TMF type and a peripheral portion of AMF type. However, besides these contacts are expensive, the result obtained remains a compromise and retains the weak points of the two previous types. In particular, the contact erosion caused by the RMF control remains, as well as the clogging of the space between the petals. In addition, if the initial arc starts on the peripheral part, only the axial control remains, without influence of the radial control managed by the central part of the contact.

SUMMARY OF THE INVENTION

The aim of the invention is therefore to propose a mixed control of the arc generated at the cut by a new contact device, based on the fact that the force which is responsible for the diffusion of the arc is of a different nature from the force itself. conferring a rotational movement.

The invention thus relates to a device comprising two contact electrodes for in particular a medium voltage vacuum bulb. The two electrodes of the device are mirror-symmetrical with each other, and each mounted on one rod: in the closed position, one surface of each electrode is in contact with the other; in the open position, a translation along at least one of the rods has been performed, and the two surfaces are separated from each other while remaining parallel.

Each electrode comprises a contact pad associated with a base. The two pellets are superimposable at the level of the circular contact surface of the electrode. Advantageously, the pellets are in the form of solid discs, flat, and material adapted to the presence of arc, including a copper alloy.

On its surface opposite to the contact surface, the pellet is coupled to a base, preferably by brazing. The coupling surface of the base is circular, of diameter less than or equal to the diameter of the pellet; arrangements may be provided, for example a groove associated with a rim of the pellet.

The base may be in the form of disc, or cup, of conductive material, preferably copper; advantageously, its external shape does not comprise a sharp angle, with the possible exception of the coupling surface. The base can be hollowed in its center, so that the pellet is only secured to a peripheral rim; a metal reinforcement can then be put in place in the center of the hollow to strengthen the structure.

The base comprises a plurality of cuts, slots or grooves, which make it possible to determine the trajectory of the current lines flowing therein, which is the basis of the phenomenon of diffusion of the arc. In particular, the base comprises at least three, preferably five, through slots between the coupling face and the opposite face, which separate the base into quarters. The slots extend between a first peripheral end, which can open the base or not, and a second end internal to the base, towards its center; at their inner end, the slots are tangent to a circle concentric with the rod. The slots may be rectilinear or curved; preferably, all the slots are superimposable between them, and spaced from each other by a constant angle so that the quarters are identical.

To further direct the current lines, it may be advantageous to provide recesses also at the rim used for securing with the pellet. In particular, it may be advantageous for the central hollow of the base to extend at each quarter, for example to form a coupling surface comprising uniformly distributed ring sectors delimited on one side by one of the cutouts. Alternatively or in addition, the securing part of the base may be of non-circular adapted shape, for example with a central star hollow.

The invention also relates to a vacuum interrupter comprising a device as defined above associated with means for mobilizing at least one of the rods. The invention finally relates to a medium voltage switchgear apparatus in which the contact device makes it possible to separate two lines, or line parts, from an electrical network or to isolate an electrical appliance from the network, notably an alternator.

BRIEF DESCRIPTION OF THE FIGURES

• Les figures 1A, 1B et 1C, déjà décrites, illustrent le principe de fonctionnement des dispositifs de contact selon l'art antérieur.
• La figure 2A illustre le principe de fonctionnement du dispositif de contact selon un mode de réalisation de l'invention ; les figures 2B et 2C montrent un dispositif de contact selon un mode de réalisation préféré de l'invention, en éclaté et en position de montage ; la figure 2D illustre une ampoule à vide selon un mode de réalisation de l'invention.
• Les figures 3A et 3B représentent des alternatives de montage de la pastille sur un socle dans un dispositif selon l'invention.
• La figure 4 illustre l'action sur les lignes de courant d'un socle pour un dispositif selon l'invention.
• Les figures 5A, 5B et 5C montrent des alternatives de socle pour un dispositif selon l'invention.
• La figure 6 montre la dispersion dans la mesure de résistance électrique dans une ampoule commerciale munie d'un dispositif selon l'invention et d'un dispositif classique.

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention, given by way of illustration and in no way limiting, represented in the appended figures.

• The FIGS. 1A, 1B and 1C , already described, illustrate the principle of operation of the contact devices according to the prior art.
• The Figure 2A illustrates the operating principle of the contact device according to one embodiment of the invention; the Figures 2B and 2C show a contact device according to a preferred embodiment of the invention, exploded and in the mounting position; the 2D figure illustrates a vacuum interrupter according to one embodiment of the invention.
• The Figures 3A and 3B represent alternative mounting of the chip on a base in a device according to the invention.
• The figure 4 illustrates the action on the current lines of a base for a device according to the invention.
• The FIGS. 5A, 5B and 5C show alternative bases for a device according to the invention.
• The figure 6 shows the dispersion in the electrical resistance measurement in a commercial bulb provided with a device according to the invention and a conventional device.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As presented above, in the existing arc control types, the magnetic force of a radial or transverse field causes the arc to rotate but allows it to contract, while the magnetic force of the axial field allows the arc to be held. arc as diffuse as possible over a certain surface of the contacts without changing arc zone. These two options help to dissipate the energy of the arc.

• l'effet de rotation de l'arc est obtenu par la force magnétique radiale créée par le mouvement global du courant dans la structure de l'électrode du dispositif de contact ;
• l'effet de diffusion de l'arc est obtenu en forçant les lignes de courant à suivre des trajectoires définies avec une forte densité de courant lorsqu'il pénètre dans l'électrode du dispositif de contact ;
• puis une plus faible densité de courant au moment où les lignes de courant pénètrent la partie qui forme la surface du contact, pour passer à l'arc et à la deuxième électrode.

According to the invention, the energy of the arc which is formed during the separation of the contacts of the vacuum interrupter is distributed so as to be able to hold the transient recovery voltage TTR which appears between the terminals of the bulb immediately. after the extinction of the arc when the current passes through its natural zero, said distribution being performed according to another principle in the prior art, seeking the origin of one of the two forces, among the force allowing to rotate the arc and that to diffuse it, elsewhere than in the magnetic field. In particular :

• the rotation effect of the arc is obtained by the radial magnetic force created by the global movement of the current in the electrode structure of the contact device;
• the diffusion effect of the arc is obtained by forcing the current lines to follow defined paths with a high current density when it enters the electrode of the contact device;
• then a lower current density at the moment when the current lines penetrate the part that forms the surface of the contact, to pass to the arc and the second electrode.

In fact, the arc 9 is diffused as with an AMF axial arc control, but undergoes a rotational movement as in TMF / RMF, this however over the entire surface of the contacts, including the center of the latter: see Figure 2A . This type of arc control therefore offers better breaking capacity than axial control while maintaining a very low level of erosion.

In particular, the contacts between which the arc arises are formed in two parts, a support for distributing the current lines and for accelerating the rotation of the arc and then a contact surface at which the arc burns. The path of the current is defined by the form of cuts in the support, which can be straight or curved to define the spiral effect, and the fact that the two contacts are symmetrical in mirror, that is to say non-superimposable.

In particular, the diffusion of the arc formed in the support is ensured by the fact that the current lines naturally occupy all the available volume as they pass through. the base: from the center to the periphery, the current lines see the volume they cross expand, and therefore they disperse. On the anode, the same phenomenon occurs in the opposite direction: the current lines enter the anode by the widest part and are therefore scattered at the level of the arc, which gives the latter its relatively diffuse appearance. ; then the current lines move towards the center of the base where they converge, far from the arc.

Thus, as illustrated in Figures 2B and 2C , the contact device 10 comprises two electrodes 12, commonly called "contacts", mirror symmetrical with respect to each other. The two electrodes 12 are mounted on two rods 14 coupled to actuating means (not shown) to allow relative movement between the two electrodes 12, said movement being effected by translation along the rod 14. Usually, one of the rods 14 ₁ is fixedly mounted in the vacuum bulbs 16 and the other 14 ₂ is movable in translation ( 2D figure ). When the device 10 is used in a vacuum interrupter 16, it is placed within an insulating enclosure, conventionally ceramic, with often a metal screen 18, made of copper or stainless steel for example, located around the electrodes 12 which whatever their relative position.

The electrodes 12 are generally circular in order to better distribute the electric field lines; their diameter varies according to the fault current that the vacuum interrupter 16 must cut and restore, in particular between 20 mm for fault currents of less than 20 kA to more than 140 mm for fault currents of the order of 100 kA or more.

Each electrode 12 consists of a base 30 of low-resistivity material, generally copper, and a contact patch 20 forming the contact surface between the two electrodes 12. According to the invention, the patch 20, sometimes called also "contact tip", is a solid disk, conductive material conventionally used in this application, including a copper / chromium or copper / tungsten alloy; the disc 20 could also be bulging. Preferably, the contact surface 22 of the pellet 20 is flat, without presenting a particular profile, even if it would be possible to add cuts therein; alternatively, as illustrated in figure 3A , the pellet 20 'could, on its opposite side to the contact surface 22, comprise a flange 24 which allows a protection of the support 30 vis-à-vis the effects of the arc, covering the periphery. But in fact, a solid disc and flat without cutting, easy to manufacture and therefore cheap, guarantees the best dielectric performance of the vacuum bulb 16 in which the contact device 10 will be mounted.

The thickness of the tablet 20 may vary from one to a few millimeters depending on the level of fault current that the vacuum interrupter 16 has to interrupt and / or restore. The pellet 20 may be of the same size as the face of the support 30 to which it is secured. In a preferred embodiment illustrated in figure 3B the diameter of the disk 20 is greater than that of the base 30, for example of the order of its thickness, in particular of 0.5 mm, 1 mm or 5 mm; the overhangs 26 can reach several times the thickness of the pellet 20, so as to extend the arc diffusion zone.

Each pellet 20 is therefore associated with a base, or base 30, preferably by brazing. The base 30 comprises a circular coupling surface 32, superposable on the pellet 20 or of slightly smaller diameter; its general shape may be a disc, or a cup, but preferably, the base 30 has rounded edges 34 to ensure good dielectric performance. The thickness of the base 30 may be of the order of a few millimeters, up to ten, depending on the rated current that the bulb 16 must conduct permanently.

The base 30 is hollowed at its center so as to leave a rim 36 on which the wafer 30 rests. The depth of the recess 37 is a few millimeters, advantageously 2 mm, which makes it possible to minimize the electrical resistance while guaranteeing good compensation. case of crushing of the contacts during the hundreds, or even thousands, of maneuvers effected by a vacuum interrupter 16. To stabilize the assembly, particularly for large electrodes, a central reinforcement 38 may be put in place to support the pellet 20; the reinforcement 38 is preferably made of stainless steel and cylindrical; in a preferred embodiment illustrated in figure 3B it is placed in an appropriate arrangement 39 of the base 30.

The base 30 comprises cutouts 40 which force the trajectories of the current lines during their passage from one electrode 12 to another. The cuts are slots 40 passing through the base 30 between its coupling surface 32 and the opposite face, to form quarters 42 of the base 30. The slots 40 extend between a first peripheral end 44 and a second central end 46; advantageously, the slots 40 are open, that is to say that the first end 44 corresponds to the outer wall of the base 30. Alternatively, as shown in FIG. Figure 5A the slots 40 do not open, and the first ends 44 form a circle inscribed in the base 30; the circle thus formed typically has a diameter of 1 to 2 mm, or even a few millimeters, less than that of the base 30.

As illustrated in figure 4 , the direction of the current lines I depends on the orientation of the cutouts 40: to flow between the two electrodes 12, the current I must pass from the center of the base 30 at its periphery to the cathode, and conversely to the anode, in the volumes defined by the cuts 40. The slots 40 are arranged to be tangent at their second end 46 to a circle 48 centered with respect to the base 30. The angle α thus defined between the slot 40 and the circle 48 is identical preference for all the slots 40 of the base 30, but in any case, the angles α are always in the same direction, that is to say that the quarters 42 are of increasing size from the center to the periphery, the size being measured along the arc centered on the base 30 / the rod 14. Advantageously, the slots 40 are superposable and / or distributed uniformly around said circle 48, the slots 40 differing from each other only by a rotation around the center of the base 30, preferably a constant angle.

The width of the cutouts 40 is sufficient to allow the separation of the zones in which the stream lines I circulate, which confers on them their trajectories and controls their density according to whether they are near the center or at the periphery of the base 30, while remaining limited to maintain the pedestal 30 stable; preferably, the slots 40 are of the order of 1 mm wide. Likewise, at least three slots are present, but the increase in their number makes it possible to optimize the trajectories of the current lines when they pass through the base 30. To remain within economically and mechanically interesting feasibility limits, it is preferred to to set up five or six slots 40.

The slots 40 may be linear for manufacturing reasons. Alternatively, as illustrated in Figure 5B , the slots 40 'can be bent to form quarters 42' petals, helices, preferably stackable, to amplify the rotation of the diffuse arc.

To force the trajectories of the current lines I and cause the rotation and the acceleration of the arc, it is advantageous to further provide recesses 52 on the soldering flange 36: thus, as illustrated in FIG. figure 4 , the current lines I concentrate on an edge portion of the recess 50, on the flange 36. The width of the recesses 52 is adapted to the base 30 so as to ensure sufficient electrical conduction between the two parts 20, 30 of the electrode 12, while causing a rotation and a better acceleration of the arc. Preferably, the recesses 52 are identical for all the quarters 42 and represent about a quarter to half of the rim 36.

Alternatively, as schematized in Figure 5C , the rim 36 is substantially closed on its periphery, with the exception of the slots 40 opening. In this embodiment, the rotation is provided by a shape adapted to the central recess 37 ', which is no longer circular but comprises sharp angles, said angles being delimited in part by the slots 40. This alternative makes it possible to have more coupling surface 32, and to offer a path substantially equal to all current lines I.

The shape of the quarters 42, narrow at the central level and widening towards the periphery, leads to dense current lines I close to the center, with a concentration zone 54, which deviate more and more as they move towards the center. periphery to minimize the current density in a divergence zone 56 and occupy all the available volume of the quarters 42 of the base 30 within the recess 37, which optimizes the diffusion of the arc.

• l'effet de rotation de l'arc est obtenu par la force magnétique radiale créée par le mouvement global du courant dans la structure du socle 30 ;
• l'effet de diffusion de l'arc est obtenu en forçant les lignes de courant à suivre des trajectoires définies avec une forte densité de courant. Lorsque le courant quitte la tige de l'ampoule 14₁ pour la cathode, il circule du centre du socle vers sa périphérie - et traverse la zone 54 qui offre peu de matière pour les lignes de courant I ; à la périphérie du socle de la cathode, les lignes de courant I traversent un volume plus étendu de matière, et se dispersent en occupant le volume disponible avant de passer à l'arc qui s'est formé entre les deux contacts, puis au deuxième contact (anode) pour faire le trajet en sens inverse vers la tige 14₂ de l'ampoule.

As specified above, the device 10 according to the invention comprises two electrodes 12 placed face to face, with cutouts 40 in mirror symmetry, in order to obtain a radial field: the slots 40 are thus in the extension one. on the other, separated only by the pellets 20. Thus, the current lines I which circulate inside the quarters 42 of the base 30 create a magnetic field which generates a force which gives a rotational movement to the arc, unlike the arc control RMF or TMF, in which the current flows in the wafer 20 to create the magnetic field that rotates the arc. The arc meanwhile remains between the two pellets 20, diffused over the entire surface: the macroscopic trajectory of the current in the two parts of the arc control generates a magnetic force which imposes a rotational movement on the arc independently of the fact that it be broadcast. In particular :

• the effect of rotation of the arc is obtained by the radial magnetic force created by the overall movement of the current in the structure of the base 30;
• the diffusion effect of the arc is obtained by forcing the current lines to follow definite trajectories with a high current density. When the current leaves the rod of the bulb 14 ₁ for the cathode, it flows from the center of the base towards its periphery - and passes through the zone 54 which offers little material for the current lines I; at the periphery of the base of the cathode, the current lines I pass through a larger volume of material, and disperse by occupying the available volume before passing to the arc which has formed between the two contacts, then to the second contact (anode) to make the path in the opposite direction to the rod 14 ₂ of the bulb.

Several tests have been carried out. In particular, in a vacuum chamber simulating a vacuum bulb, filmed images and the measurement of its voltage (at the terminals of the two contacts in the presence of the arc) showed that the arc was indeed diffused and animated by a rotation movement.

In addition, the contact device 10 illustrated in Figure 2C has been used in place of an existing contact device in VG type Vacuum Bulbs marketed by Schneider Electric: Equal dimensions (60 mm arc control with a breaking capacity of 31.5 kA under 17 , 5 kV), the vacuum interrupter can cut fault currents up to 20% higher than the maximum currents that can be interrupted by a standard bulb. In addition, as shown in the figure 6 , the electrical resistance of the bulbs with the new arc control, enabled by the device according to the invention, is lower (An average value decreased by two in the illustrated example), that is to say that the heating of the circuit breaker poles, proportional to said electrical resistance, is limited; it is also noted that the dispersion of the measurements is lower, with in particular a standard deviation of less than 1 for an average value of the resistance of the order of 7.8 μΩ compared to a standard deviation greater than 3 for a mean value of the resistance. of the order of 15.3 μΩ.

• une répartition efficace de l'énergie thermique qui permet de satisfaire les exigences d'applications particulières telles que celles avec des temps d'arc très long, comme le zéro retardé dans les coupures d'alternateurs, certaines applications ferroviaires avec des fréquences de 16 Hz,... ;
• un bon pouvoir de coupure, identique à celui d'un contrôle d'arc de type RMF ou TMF;
• une bonne performance lors de coupures avec temps d'arc longs ;
• des performances diélectriques élevées et constantes avant et après coupure de courant de défaut ;
• une endurance électrique grâce aux surfaces de contact 22 pleines ;
• une prévention des soudures lors des fermetures, du fait que l'énergie de rotation de l'arc 9 à sa création (distance entre les deux contacts inférieure au millimètre) se trouve répartie sur une surface 22 ;
• de très bonnes performances de coupure de bancs capacitifs en raison de la bonne tenue diélectrique et de la rotation du pré-arc lors de la fermeture sur des courant de banc de capacités pouvant atteindre 20 kA ou plus;
• une faible résistance électrique ;
• une localisation contrôlée de l'arc qui reste à l'intérieur de la surface de contact 22, et ne s'accroche pas à l'écran 18 de l'ampoule à vide 16 ;
• une meilleure tenue mécanique du dispositif de contact 10 que celle des types AMF ou RMF/TMF ;
• un coût de fabrication des pastilles de contact 20 inférieur à celui utilisé pour un contrôle d'arc TMF de type pétale, et surtout d'un contrôle d'arc de type AMF.

Thanks to the new type of contact device 10 according to the invention and the arc control according to the concept of diffuse arc, uncontracted, but in rotation that it allows to apply, switchgear and vacuum interrupters 16 offer the following benefits:

• an efficient distribution of thermal energy that satisfies the requirements of particular applications such as those with very long arc times, such as delayed zero in alternator interruptions, some railway applications with frequencies of 16 Hz , ...;
• a good breaking capacity, identical to that of a RMF or TMF type arc control;
• good performance during cuts with long arc times;
• high and constant dielectric performance before and after fault current failure;
• electrical endurance through full contact surfaces 22;
• a prevention of welds during closures, because the rotational energy of the arc 9 at its creation (distance between the two contacts less than one millimeter) is distributed over a surface 22;
• very good capacitive bank breaking performance due to the good dielectric strength and the pre-arc rotation when closing on capacitor bank currents up to 20 kA or more;
• low electrical resistance;
• a controlled location of the arc which remains inside the contact surface 22, and does not cling to the screen 18 of the vacuum interrupter 16;
• a better mechanical strength of the contact device 10 than that of the AMF or RMF / TMF types;
• a cost of manufacture of the contact pads 20 less than that used for a petal type TMF arc control, and especially an AMF type arc control.

Claims (12)

1. Contact device (10) for a vacuum bulb (16) comprising two electrodes (12) each rigidly attached to a shaft (14), said shafts (14) being axially aligned with each other, each electrode (12) comprising a tablet-shaped element (20) associated with a base element (30) via a coupling surface (32), the two electrodes (12) being able to take a position in which the tablet-shaped elements (20) are in contact and a position in which they are separated from one another by a relative translation along the shafts (14), a contact device (10) in which the coupling surface (32) comprises cut-outs (40) extending between a first end (44) at the periphery of the base element (30) and a second end (46) internal to the base element (30), each cut-out (40) being tangent to a circle (48) centered on the shaft (14) at its second end (46), the cut-outs (40) passing through the thickness of the base element (30), the cut-outs (40) of a base element (30) being all in the same direction in such a manner as to form sectors (42) of an arc base element (30) broadening from the center toward the periphery, characterized in that the two electrodes (12) are a mirror image of each other, such that the cut-outs (40) are superposed in the contact device (10).
2. Contact device according to Claim 1 in which the tablet-shaped element (12) is a substantially plane full disk.
3. Contact device according to Claim 2 in which the coupling surface (32) is included in a circle described within the disk of the tablet-shaped element (12).
4. Contact device according to one of Claims 1 to 3 in which each base element (30) comprises a central hollow (37), forming a peripheral lip (36) for coupling with the tablet-shaped element (20).
5. Contact device according to Claim 4 in which each electrode (12) furthermore comprises a reinforcement (38) within the hollow (37) for supporting the tablet-shaped element (20).
6. Contact device according to either of Claims 4 and 5 in which each lip (36) comprises at least one recess (52) in the extension of the hollow (37) and opening onto the periphery.
7. Contact device according to Claim 6 in which the coupling surface (32) comprises five portions of lip ring (36) separated by five recesses (52) and bounded at one end by a cut-out (40).
8. Contact device according to one of Claims 1 to 7 in which the cut-outs (40) are slots (40) opening out at their first end (44).
9. Contact device according to one of Claims 1 to 8 in which the cut-outs (40) are identical, offset from one another by rotation around the shaft (14).
10. Contact device according to Claim 9 in which the cut-outs (40) and sectors (42) are distributed uniformly around the shaft (14).
11. Vacuum bulb (16) comprising an air-tight chamber in which is positioned a contact device (10) according to one of the preceding claims, at least one of the shafts (14) of the device being associated with means of actuation allowing the tablet-shaped elements (20) to take the two positions.
12. Switchgear unit comprising a vacuum bulb according to the preceding claim.

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