Abstract:
An electromechanical switch of the contactor or circuit-breaker type with adjacent contacts, which are provided on respective mobile and fixed contact parts. This arrangement defines a narrow ionization slot which is maintained at the start of contact opening in order to cause a secondary arc to fire substantially perpendicular to the contact opening direction constituting a shunting arc to eliminate the time duration of the contacts and switch legs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns an arc switching electromechanical switch of the contactor or circuit-breaker type, and, more particularly, to a switch having a fixed contact and a mobile contact respectively disposed on a fixed conductive part and a mobile conductive part. 
     2. Description of the Prior Art 
     If a contactor is connected in series with a short-circuit protection device such as a circuit-breaker, for example, on the circuit supplying power to a load, an overcurrent can occur with the contactor contacts closed or virtually closed. Even if the fault current, then flowing through the contractor, is limited by the protection device, it is nevertheless very much greater than the nominal current that the contactor interrupts under normal circumstances. If the fault current causes electrodynamic opening of the contacts, the arc then generated can be accompanied by localized liquidation of the fixed and mobile contacts at the anode and cathode ends of the arc and, if it lasts more than 1 ms to 2 ms, can cause bonding or welding of the contacts when they close again. 
     To avoid this problem, special arc contacts have been used alongside the main contacts. Another solution is the provision of means for accelerating migration of the arc towards an arc extinction chamber. 
     An object of the present invention is a simple way of preventing stagnation of the arc generated between the contacts on normal or electrodynamic openings thereof, encouraging the striking and maintaining of a secondary arc. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, said fixed and mobile contact parts have respective first and second faces parallel or substantially parallel to said opening direction and said two faces are adjacent to said contacts and define between them in a contact closed position a narrow ionization slot, the width of said slot being maintained substantially constant at the beginning of contact opening in order to cause a secondary arc to fire substantially perpendicular to said opening direction. 
     The primary arc and the secondary arc co-exist briefly and the primary arc is extinguished very quickly whereas continued presence of the secondary arc is guaranteed by a sufficient length of the ionization slot. The width of the slot is advantageously less than the critical electrodynamic contact opening distance e c , this distance being defined as that causing substantial liquidation of the contacts likely to bond or weld them when they close again. For a low-voltage switch the slot width is preferably between about 1 mm and 3 mm and the depth of the slot is determined accordingly, being preferably greater than its width. 
     The fixed ionization face is formed on the wall of a recess in a J-shape part of the fixed contact or attached thereto. A metal (for example steel) part carrying one of the ionization faces can be removably attached to the fixed or mobile contact part. 
     The following description of preferred embodiments of the invention given with reference to the drawings clearly indicates its features and advantages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a contactor with ionization means for accordance with the invention; 
     FIGS. 2 through 5 show various phases of the opening of the contacts of the FIG. 1 contactor; 
     FIG. 6 is a top view of part of the fixed contact part; 
     FIG. 7 similarly shows a variant fixed contact part; 
     FIGS. 8 through 10 show other embodiments of the ionization slot; 
     FIGS. 11 through 13 show various phases of opening of the contacts of the FIG. 11 device; and 
     FIG. 14 shows the variation of the arc switching speed as a function of the width of the ionization slot. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The contactor shown diagrammatically in FIG. 1 comprises a casing 10 and a contact bridge 11 for each pole mobile in the X direction. The contact bridge is displaced by an actuator device 12 conventionally known, in one direction by electromagnetic or manual action and in the other direction by a return spring, for example. The contact bridge or its mobile support is guided by fixed guide surfaces 14 providing a small clearance in the direction Y, perpendicular to X, and providing a small clearance for rotation about the direction X. 
     The contact bridge 11 carries near each end a mobile contact pad 15 and at each end a right-angle upstand or finger 16 having two opposite sides 17, 18 parallel to the opening direction X. 
     Each mobile contact pad 15 cooperates with a fixed contact pad 20 carried by a fixed contact support 21 connected to a connecting terminal 22. The conductive support 21 has a J-shape in this embodiment to facilitate displacement in the direction Y of the arc generated between the contacts when they open. The usual means for blowing the arc in the direction Y are shown schematically at 60 and may be any of the conventional devices. Only the righthand half-bridge is shown in full in FIG. 1. 
     The fixed contact support 21 has a recess or well 23 in which the finger 16 penetrates when the contacts are closed. The surface 17 of the finger 16 facing away from the contacts defines, with the facing surface 24 of the recess, an ionization slot 25. The width e of the slot 25 is preferably between 1 mm and 3 mm and its median plane P-P&#39; is parallel to the opening direction X. The surface 18 of the finger 16 near the contact defines, with the facing surface 26 of the recess 23, and ionization slot 25a communicating with the slot 25 and whose width e&#39; is greater than that of the slot 25 to generate a secondary arc between the finger 16 and the surface 24 to shunt the primary arc and reduce the time duration of the primary arc. The length 1 of the finger 16 is between 5 mm and 8 mm to determine a substantial depth h of slot and thereby to maintain the presence of the ionization slot for a sufficient time period. The depth h of the slot is in all cases greater than its width. It is preferable that the finger 16 and the ionization slot 25 be in the immediate proximity of the contact pads 15, 20 so that the plasma generated by the initial arc between the contacts can propagate quickly toward the slot. 
     The recess 28 can have a closed contour (FIG. 6) or an open contour (FIG. 7) and in the latter case it advantageously communicates with an interrupter chamber 28 with arc-splitter plates 29 through an opening 30 in the support 21 extending in the Y direction. 
     The finger 16 can be on a metal wear part 40 replaceably attached to the contact bridge by fixing means 41 (FIG. 8). This part can be of copper but is preferably of steel. The finger can instead be an extension of an arc horn 42 shown in dashed line in FIG. 8 so that the secondary arc fired struck in the ionization slot, can migrate in the X direction along the part 40 as far as the upper part of the horn 42. 
     FIG. 9 shows that the attached metal part 40 can be U-shape with one branch 43 fixed to the contact bridge and the other branch 44 defining firstly the face 17 of the ionization slot and secondly an arc horn 45. 
     The FIG. 10 embodiment is advantageous because of its compact overall size and comprises a flat fixed contact support 21. The mobile contact bridge 11 has an upstand 50 at each end oriented in the contact opening direction, like a conventional arc horn, but so that it has a face 51 parallel to the direction X. An interchangeable metal switching part 53 is fixed to the flat support 21 by fixing means 52; the part 53 has a J-shape, L-shape or U-shape and has one branch 54 parallel to the X direction and one branch 55 oblique or parallel to the Y direction. The branch 54 has a surface 56 parallel to X to define (with the face 51) the ionization slot of constant width e between approximately 1 mm and 3 mm and is joined directly at right angles to the fixed contact support 21. The portion of the part 53 through which the arc current flows is entirely on the opposite side of the median plane P-P&#39; of the ionization slot 25 to the contact pads. 
     The operation of the FIG. 1 device is explained below with the aid of FIGS. 2 through 5. In the contacts closed position (FIG. 2) the finger 16 penetrates the recess 23. At the start of normal or electrodynamic contact opening (FIG. 12) a primary arc A1 appears between the contact pads 15, 20. When the distance between the contacts approaches the value e a secondary arc A2 fires in the ionization slot 25 between the faces 17 and 24 and/or in the slot 25a, the secondary arc A2 extending in the Y direction. As the opening of the contacts continues, the arcs A1, A2 co-exist and then the primary arc A1 is progressively extinguished so that only the arc A2 remains (FIG. 4). The chosen values of the slot width e, finger 16 length 1 and the related slot depth h are critical to achieving this substitution quickly and reliably. Thereafter (FIG. 5), the secondary arc A2 moves to A&#39;2, leaving the ionization slot 25, and then jumps or migrates to the extinguishing plates 29 (A3). Conventional arc blow-out means 60 represented schematically in FIG. 1 help to encourage propagation of the plasma towards the slot 25 and of the pre-interruption arc A&#39;2 towards the extinction chamber 28. These means can comprise the shape of the current feeds 11, 21, U-shape magnetic circuits around the contact area, etc. 
     The operation of the device from FIG. 10 is explained with reference to FIGS. 10 through 13. 
     At the start of contact opening at nominal current or with a fault current (FIG. 11). A primary arc A1 fires between the contact pads 15, 20, and then, immediately afterwards and assisted by the diffusion of the plasma into the slot 25 of width e, a secondary arc A2 in the Y direction is struck between the vertical faces 51, 56. The arc A1 disappears but the arc A2 remains for a sufficient time period by virtue of temporary maintaining of the gap e as the travel between the contacts 15, 20 increases. The arc A2 then migrates to A&#39;2 (FIG. 12) and its end where it joins onto the branch 55 of the J-shape switching part 53 is displaced towards the free end of this branch (FIG. 13). As a result the current flows in 57 in the direction opposite to X, which favors blowing out of the arc A&#39;2. 
     The configuration of the part 53 can be modified in various ways known in themselves to accentuate this blow-out effect, for example by means of side flanges 58 (FIG. 13). The arc A&#39;2 is finally switched towards the extinguishing plates (A3). 
     FIG. 14 explains the arc switching time t for the arc A1, i.e. the time up to total disappearance of current between the pads 15 and 20, as a function of the width e of the ionization slot in millimeters, in a low-distribution voltage device. 
     In the device in accordance with the invention, apart from reduction of bonding of the contacts at the time of electrodynamic repulsion under a high current, wear of the contacts is reduced during AC4 type interruption maneuvers and contact resistance variations are reduced.