ELECTRICAL SWITCHING DEVICE

Abstract: Electrical switching device with a contact arm (1) which is fixed on a shaft (2) in the form of a thick-walled tube and is turnable between a closed position and an open position of the switching device under the influence of a torsion rod spring (4). At one of its ends the rod spring (4) is fixedly connected to the contact arm (1) and at its other end to a percussion hammer (8). In the closed position the tensioned spring (4) is arrested by means of a latch (9). When releasing the latch (9) for opening of the switching device, the spring energy is transformed into rotational kinetic energy in the hammer (8) and is transferred to the shaft (2) by impact of the hammer (8) against a projection (10) on the shaft (2). In this way the contacts (1,7) of the switching device immediately attain a great r elative separation velocity.

Description: The invention relates to an electrical switching device according to the precharacterising part of Claim 1. The switching device may, for example, be a current-limiting circuit-breaker or a current limiter arranged in series with a circuit-breaker, for example of the kind described in the EP-A-0 212 661. It is previously known to use torsion rod springs as energy reservoir in operating devices for circuit-breakers. In prior art designs of this kind the springs are connected to the movable contact of the respective circuit-breaker via links or gear wheels (see e.g. DE-C-673 315 and DE-C-29 07 714). One advantage with springs of this kind is that they enable a rapid energy output since the mass of a torsion rod is concentrated near the axis of rotation, which resides in a small moment of inertia of the rod. However, the fixed connection between the spring and the breaker contact in the prior art designs, in particular in breakers with butt contacts (i.e. contacts in which the direction of the contact movement is substantially perpendicular to the contact surface), results in a low velocity of contact separation during the first part of the contact opening phase. This renders the movement of the arc away from the contact unit and along the runner rails difficult, since a considerable amount of arcing gases containing metal vapour, etc., are generated at the contact unit before the contact gap reaches the minimum size (about 1mm) necessary to enable a movement of the arc at all. A further drawback in prior art designs is that the torsion rod spring has to be dimensioned to enable storage of a relatively large amount of energy, since the rod spring during the opening movement is counteracted by a contact pressure spring or some other contact pressure generating member. In order to enable a switching device to efficiently limiting and possibly breaking short-circuit currents, it is necessary for the contact system of the device to be able to be opened in a very short time (smaller than 1 ms) and that the arc thus arising be rapidly removed from the contact surfaces, which requires the buildup of a contact separation velocity of about 6 m/s within 0.1 ms from the starting moment. The invention aims at providing an electrical switching device of the above-mentioned kind that in addition to being capable of carrying high operating currents, meets the afore-mentioned requirements. To achieve this aim the invention suggests an electrical switching device according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. By using one and the same torsion rod spring to serve both as contact pressure spring and opening spring and by transferring the energy for the opening to the movable contact arm by a blow of a hammer fixed to the torsion rod spring, several advantages are attained: By the blow of the hammer, the movable contact will immediately attain a great contact separating velocity and the arc is able rapidly to travel off the contact surfaces and onto runner rails arranged adjacent to the contact unit. Since, in addition, the contact force automatically disappears during the opening, the electrical switching device according to the invention requires a small stored spring energy (only 2-3 J at a contact force of 1 kN). In addition, the invention enables a simple design with few and robust parts, which ensures high reliability. According to a further development of the invention, the device is supplemented with at least one rod made of a high-magnetostrictive material which is adapted, in case of a short-circuit current, to influence the contact arm such as to bring about contact opening even before the impact from the percussion hammer occurs. With this solution an ultrarapid contact system can be achieved, in which the opening time, including a 1 mm contact gap when the arc leaves the contact, is less than 0.2 ms. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 a schematic perspective view of a first embodiment of a contact system for a current limiter constructed according to the invention, Figure 2 in the same way as Figure 1 a second embodiment of such a contact system, Figure 3 the width of the contact gap as a function of time after a short-circuit current has been detected in a contact system according to Figure 2. The switching device shown in Figure 1 comprises a contact arm 1 which is fixedly attached to a shaft 2 in the form of a thick-walled tube. The shaft 2 is journalled in a throughhole in a fixed contact block 3. The contact arm 1, the shaft 2 and the contact block 3 are made of an electrically conducting material, for example copper. The shaft 2 with the contact arm 1 is turnable from an on-position to an off-position of the switching device under the influence of a torsion rod spring 4. In the on-position the contact arm 1 is pressed via contact elements 5,6 against a fixed contact carrier 7, thus enabling a current I to flow from the contact carrier 7 to the contact block 3 according to the arrows shown in Figure 1 or vice versa. The torsion rod spring 4 is fixedly connected at one end to the contact arm 1 and at the other end to a percussion hammer 8. When the free end of the hammer 8 is subjected to a force F₁, the rod spring 4 becomes elastically twisted and the contact pressure arises. In the on-position the hammer 8 and the spring 4, tensioned in this position, are arrested by a latch 9. When a short-circuit occurs in the circuit into which the device is connected, the latch 9 is immediately withdrawn from the free end of the hammer 8, for example by action of an electro-dynamic force originating from a capacitor discharge current. When the free end of the hammer 8 is released, the energy stored in the torsion rod spring 4 is transformed into rotational kinetic energy in the hammer 8 and the torsion rod spring, which is indicated in Figure 1 by the arrow v and the instantaneous position 8′ of the hammer 8 shown in dash-dotted lines. After a certain acceleration distance, the hammer 8 hits a projection 10 arranged on the free end of the thick-walled shaft 2, whereby part of the energy is transferred to the shaft 2. How large this part of the energy will be, depends on the relationship between the mass moments of inertia of the parts 4,8 on the one hand, and 1,2 on the other hand. By the impact of the hammer 8 against the projection 10, the contact element 5 immediately attains a great separation velocity and the arc can be rapidly moved from the contact elements and travel out onto the runner rails, for example as shown in the above-mentioned EP-A-0 212 661. An important advantage with the design according to the invention is that a great contact force can be attained by simple means, for example 2 kN in a device with a rated current of 1 kA, and despite this, a very rapid contact opening is obtained. This is due, among other things, to the fact that the contact force is reduced during the movement of the hammer 8 and is practically zero when the impact against the projection 10 occurs. In that way only relatively little energy is needed for the opening of the device. Furthermore, since as opening spring there is used a torsion rod, the mass of which is located near the axis of rotation of the rod and therefore has a small moment of inertia and a high resonance frequency (e.g. of the order of magnitude of 1kHz for the spring 4 together with the hammer 8), the time taken for the movement of the hammer 8 from the point where it is released from the latch 9 to the point where it hits the projection 10 will be very short (e.g. 0.2 ms). The contact elements 5,6 may suitably be made of silver or a silver alloy, which together with the great contact force of 1kN or above results in a relatively small transition resistance of the contact unit. Since in the closed position the same great contact force prevails between the shaft 2 and the contact block 3, where the contact surfaces are relatively large, the transition resistance at this location will be even smaller than between the contact elements 5,6. To avoid damage by burning on bearing surfaces from small arcs between the shaft 2 and the contact block 3 when the contact force disappears in connection with the contact opening, the contact arm 1 may be connected to the block 3 via a flexible conductor (not shown in the Figure). Since this conductor only need to carry current during the time from the contact opening until the arc has been extinguished or travelled out onto runner rails, which have been arranged near the contacts, this conductor may be relatively thin. With the design shown in Figure 1, an opening time, including a 1 mm contact gap when the arc leaves the contact unit, of 0.4-0.5 ms can be achieved. Figure 2 shows an improved design, in which the opening time can be reduced to less than 0.2 ms. In this design the embodiment according to Figure 1 is supplemented with a bracket 11 which is fixedly connected to the shaft 2. In diagonally opposite corners of the bracket 11, two rods 12 of high-magnetostrictive material carrying excitation windings 13 are arranged in such a way that the extension of the rods 12 leads to a rotary motion of the bracket 11 and hence to a contact opening. The material in the rods 12 may be an alloy between the metals terbium and iron, for example of the kind described in US-A-4,308,474. With such material, an extension of the order of magnitude of 1 per mille may be obtained by a magnetic field. As counter support against the extension of the rods 12 a mass (counter weight) 14 is arranged at the outer end of each rod 12. The mass 14 is pressed against the end of the rod 12 by means of a weak force F₂, preferably produced by an appropriate spring. Figure 3 shows a curve of the contact gap d versus the time t for the switching device according to Figure 2 during the first part of an opening movement. At the time t = 0, an overcurrent is detected and a current surge is sent through the excitation windings 13. In this way the desired rapid extension of the rods 12 is obtained, and even after a time of about 0.2 ms a contact gap of 1 mm has been attained, which is sufficient for the arc to be able to move from the contact unit. At approximately the same time (i.e. at t = 0.2 ms), the hammer 8 is released from the latch 9, and after an additional 0.25 ms (i.e. at t = 0.45 ms), the hammer 8 strikes against the projection 10. The mechanism described with reference to Figure 1 then completes the contact opening to full insulation distance in a time of less than 3 ms. The invention is not limited to the embodiments shown but can be materialized in several different ways within the scope of the claims. For example, the device can be constructed with two or more parallel contact arms 1, allowing an increase of the rated current of the device. The contact arms 1 need not, of course, be arranged at one end of the tubular shaft 2 but may, for example, be located in the middle of the shaft 2, one end of the shaft 2 being formed with the projection 10 and the other end being fixedly connected to the torsion spring 4 arranged in the shaft 2.

Claims: Electric switching device comprising a contact arm (1) which is fixed on a shaft (2) mounted in a fixed contact block (3) and is turnable between a closed position and an open position of the switching device under the influence of a torsion rod spring (4), one end of which is fixedly connected to the contact arm (1) or the shaft (2) and the other end of which cooperates with a latching device (9) which is so constructed that in the closed position it arrests the torsion rod spring (4) in tensioned condition, while said contact arm (1) is pressed against a fixed counter contact carrier (7) for closing a current path through the contact arm (1), characterized in that said other end of the torsion rod spring (4) is fixedly connected to a percussion hammer (8), which is so designed that, upon opening of the switching device after releasing of the latching device (9), it strikes against a projection (10) fixedly arranged on the contact arm (1) or its shaft (2) to bring about rapid contact opening. Switching device according to Claim 1, characterized in that the shaft (2) consists of a tube surrounding the torsion rod spring (4). Switching device according to Claim 2, characterized in that the shaft (2) is provided at one end with said contact arm (1) and at the other end with said projection (10). Switching device according to Claim 2, characterized in that the contact arm (1) is arranged in the mid-section, preferably in the middle, of the shaft (2), which at one end is provided with said projection (10) and at the other end is fixedly connected to the torsion rod spring (4). Switching device according to any of the preceding Claims, characterized in that the shaft (2) and the contact block (3) are made of a material with good electrical conductivity and are included in said current path. Switching device according to any of the preceding Claims, characterized in that it comprises at least one rod (12) made of magnetostrictive material which is adapted, in the case of a short-circuit current, to influence the contact arm (1) such as to bring about contact opening prior to the percussion hammer (8) striking against the projection (10). Switching device according to Claim 6, characterized in that a bracket (11) is fixed on the tubular shaft (2) and that the magnetostrictive rod (12) is arranged in such a way in relation to the bracket (11) that an extension of the rod (12) causes a rotary motion of the bracket. Switching device according to Claim 7, characterized in that as counter support against the extension of the rod (12) there is arranged a mass (14) which, with the aid of a resilient member, is pressed against that end of the rod (12) facing away from the bracket (11). Switching device according to any of the preceding Claims, characterized in that the contact arm (1) and the contact block (3) are connected via a flexible conductor. Switching device according to any of the preceding Claims, characterized in that runner rails are arranged adjacent to the contact means of the switching device in such a way that the arc produced upon contact opening is rapidly moved away from the contact under the influence by the magnetic field generated by the current.

Inventors: VALDEMARSSON, STEFAN
Applicants: ASEA AB
Application year: 1987
Main classification: H01H33
IPC codes: H01H3