EP0243980B1 - Contact arrangement for current limiting low-tension power circuit breakers - Google Patents

Abstract

The contact arrangement for current limiting low-tension power circuit breakers has a moving contact piece which interacts with a fixed contact piece, in which arrangement a large acceleration of the contact pieces at the start of contact opening causes a rapid increase of the arc voltage. In this way, contact opening is accelerated at the start of the contact movement and the angle between the two contact surfaces is in this way rapidly increased, in that the end sections of the contact arms carrying the contact pieces are supported on the contact arms such that they can pivot and are spring-loaded. <IMAGE>

Description

The invention relates to a contact arrangement according to the preamble of claim 1.

Current-limiting low-voltage circuit breakers have the task of generating a high arcing voltage as quickly as possible in the short-circuit head in order to achieve a good current limitation. This keeps the dynamic stress (given by the current square I²) and the thermal load (given by ∫i²dt) of the short-circuit circuit including the circuit breaker itself small. This is the only way to make such circuit breakers small and economical.

It is known to cause high arcing voltages through large contact openings because the length of the arcing has a significant influence on its voltage. There are swiveling contact arms with bridge contacts, in which the goal of a large contact opening with the technically advantageous, but expensive double interruption is connected, as described for example in a prospectus from Sace. Furthermore, fixed contacts with pivotable contact arms, preferably C-shaped, are known from the prospectus of the Westinghouse company, which strive for the same goal.

The fact that large short-circuit currents and high current steepnesses (depending on the power factor of the short-circuit circuit and the time of the short-circuit) cause the current forces at the contact point (repulsive force when the contact arms are parallel and the loop force) to be used within very short times, especially in the case of the swiveling contact arms. In fractions of a millisecond, the contact closing force exceeds and thus the contacts open dynamically long before their opening is effected by magnetic triggers via the switch lock.

In this context, reference should be made to US Pat. No. 3,534,305, which describes a current-limiting circuit breaker in which the movable contact arm is divided and the section carrying the contact piece forms an opening pivot angle against the force of a compression spring in the initial phase of the electrodynamic opening. In the event of a short circuit, this contact arrangement also leads to an initially faster opening of the contact pieces, but has the disadvantage that this opening must take place against the compression spring and that the mating contact piece does not contribute to the rapid increase in the opening angle.

In this context, reference should also be made to DE-AS 1 638 154, which describes a circuit breaker in which at least one movable contact arm is pivoted by the other contact arm under the influence of short-circuit currents and their electro-dynamic forces. One end of a tension spring which can be moved via a dead center acts on the movable contact arm, while the other end is fastened to a pivotable auxiliary lever. The contact arm also has a pin for driving the auxiliary lever, which engages in an L-shaped recess in the auxiliary lever. When a short-circuit current occurs, electro-dynamic forces occur in the two parallel contact arms, which drive the contact arms apart in opposite directions. The rotation of the contact arms in the pivot pin causes the tension springs to be brought from one over-center position to the other over-center position (snap effect) and to hold the contact arms in the open position. Although this contact arrangement also leads to a rapid opening of the contacts in the event of a short circuit, it requires a great deal of technical effort.

From DD-PS 225 237 it can be seen that for current-limiting miniature circuit breakers, the opening speed of a switch contact driven by an impact anchor is increased by the fact that the switch contact arm is rotatably mounted in the center in the region of the center of gravity, thereby reducing the moment of inertia.

The current forces that still affect acceleration at the moment of contact separation are not amplified by such magnet systems. At this point in time, however, a high acceleration is desirable with regard to a quick contact opening.

The relatively slow start of contact opening has another consequence:
Immediately after the contact separation, even with swiveling contacts, there is hardly any rotational movement; the contact surfaces move away, apparently parallel to each other and straight from each other. The plasma jets emerging from the arc base have the tendency to be perpendicular to the contact surfaces from which they emerge. The tips of the plasma rays, which generate new arc base points on the opposite contact surface and could thus initiate a base point movement, strike the counter contact surface only in a relatively narrowly limited area around the respective opposite arc base point. The base points therefore move in a very small space; a movement in the desired direction towards the drain edge or towards the drain horn cannot be achieved with small contact openings.

The object of the invention is therefore to provide a contact arrangement in which the moments of inertia of the movable contact arms are reduced using technically simple means.

This object is achieved by the features specified in the characterizing part of claim 1.
The contact arrangement according to the invention has a particular advantage of a faster rise in the arc voltage and a better current limitation. As a result, there is less short-circuit stress on the system parts to be protected and achieved by the switch itself, which also leads to a smaller switching volume.

wobei K ein neuer Proportionalitätsfaktor für die Stromquadrat-Kraft-Umrechnung  ist. D.h.: die Drehbeschleunigung verändert sich umgekehrt proportional zu den Massenträgheitsmomenten der beweglichen Kontaktteile. Je kleiner diese Massenträgheitsmomente sind, desto größer ist die Beschleunigung bei gegebenen Stromkräften. Die im Augenblick der Kontakttrennung noch wirksamen Stromkräfte können die kleineren Massenträgheitsmomente der Kontaktwippen stärker beschleunigen und somit eine sehr schnelle Anfangsöffnung bewirken.The following consideration is assumed: The above-mentioned proportional factor c contains the reciprocal of the moment of inertia ϑ. The equation for the rotational acceleration is then as follows:

$\text{ε = 1 / ϑ * K * i²}$

where K is a new proportionality factor for the current square force conversion. Ie: The rotational acceleration changes inversely proportional to the mass moments of inertia of the moving contact parts. The smaller these mass moments of inertia, the greater the acceleration at given current forces. The current forces that are still effective at the moment of the contact separation can accelerate the smaller moments of inertia of the contact rockers more strongly and thus cause a very quick opening.

Figur 1a ... 1c

eine Ausführung mit einer Wippe an den Enden des beweglichen Kontaktarmes bzw. eines c-förmigen Festkontaktes.

In the drawing, an embodiment according to the invention is shown.
It shows

Figure 1a ... 1c

a version with a rocker at the ends of the movable contact arm or a c-shaped fixed contact.

In the figures, the elements for power transmission to the rocker parts carrying the contacts, e.g. Eliminate power bolts, flex straps etc. due to the clarity.

The springs shown are compression springs, the function of which can optionally also be carried out by appropriately arranged leaf springs or torsion springs.

In the embodiment according to FIGS. 1 a - 1 c, the contact arrangement consists of a movable contact arm with the main section 5 and the contact rocker 13 and a c-shaped fixed contact arm with the main section 6 and the contact rocker 14.

The contact rockers 13, 14 carry the contact pieces 1, 2 at their outer ends 16, 17 and an expansion bearing 24 at the inner ends 18, 19 Compression springs 20, 21 loaded, which are supported on a spring bearing 22 in the movable contact arm and on a spring bearing 23 in the fixed contact arm. The inner ends 18, 19 of the contact rockers 13, 14 are electrically insulated from one another up to the respective pivot point 15 in such a way that arcing between them is prevented.

1a shows the contact arrangement in the closed normal state. The locking force acting on the main section 5 of the movable contact arm presses the contact rockers 13, 14 together on the contact pieces 1, 2 and on the expansion bearing 24, the required contact pressure being applied to the contact pieces 1, 2. The springs 20, 21 are matched to one another in such a way that the contact rockers 13, 14 are aligned with the main sections 5, 6 of the contact arms in this operating state.

If in the event of a short circuit the current forces and especially the current forces exceed the contact pressure, the contact rockers 13, 14 spread out as shown in FIG. 1b, the spring forces still supporting the opening of the outer ends 16, 17. Until the largest spreading position of the two contact rockers 13, 14 is reached, they are supported on a crowned contact point 24 and thus still contribute to increasing the opening force on the movable contact arm.

Because of the small turning radius of the outer end 16 relative to the radius of the pivotable main section 5 and the pivoting of the contact rocker 14, the contact rockers 13, 14 quickly form a relatively large angle to one another. The plasma rays emerging perpendicular to the contact surfaces of the contact pieces 1, 2 will quickly migrate with their base points on the respective counter contact piece in the direction of the desired arc movement during the early rotation. From a certain opening angle, the plasma jets no longer have a base on the respective counter-contact piece and are directed into the space that lies between the contact pieces 1, 2 and an arc quenching chamber, not shown. The plasma rays contain almost no more neutral particles, but mainly electrons and ions, i.e. charge carriers, which prepare the arc column, as it were, the way out of the contact space to the arc quenching chamber.

After the contact rockers 13, 14 have reached their end positions when opened, the contact of the contact rockers on the spreading bearing 24 ends. The main section 5 of the movable contact arm moves in a known manner at a lower speed due to the greater moment of inertia as a result of the current forces acting on it (loop force , Repulsive force with parallel guidance of the contact arms) in the opening direction and the contact rockers 13, 14 remain under the action of the compression springs 20, 21 in the spread position according to FIG. 1c. The opening of the contacts obtained at the start of the switch-off process is practically retained until it is switched on again and thus until the end of the switch-off process.

When the contact arrangement closes, the contact rockers come into contact with the expansion bearing 24 and fluctuate into their parallel position against the force of the springs 20, 21 due to the locking force acting on the main section 5. When the load increases, a corresponding contact pressure is applied to the contact pieces 1, 2.

With the arrangement described, relatively small masses are accelerated especially by the current forces and already in the initial phase of the contact opening a relatively large angle of the contact surfaces against one another, which favors the movement of the arc base points and the ionization of the space between contact pieces and arc quenching chambers by the plasma rays.

It may be necessary that the parts of the main sections 5, 6 and parts of the outer ends 16, 17 close to the fulcrum 15 are electrically insulated. Even after the spreading of the contact rockers, these parts are still a short distance apart (FIG. 1c) or even in contact (15, 18, 19 in FIG. 1b). The insulation is intended to prevent the occurrence of an arc at these points and, in FIG. 1b, the current conduction over the bellied supports of the expansion bearing 24.

Claims (6)

1. Contact arrangement for current-limiting low voltage circuit breakers with a contact stud (1) at a movable divided contact arm and a counter contact stud (2) at a fixed contact arm, wherein the parts of the movable contact arm are connected together under loading by a compression spring (20), characterised thereby that the fixed contact arm is divided and the end portions, which carry the contact studs (1, 2), of the contact arms are constructed as contact rockers (13, 14) loaded by compression springs, wherein the compression springs (20, 21) acting on the contact rockers (13, 14) assist an electrodynamic opening of the contact studs (1, 2) in the manner that the contact rockers (13, 14) form by their outer ends (16, 17) a pivot angle opening opposite to the main portions (5, 6) of the contact arms, wherein the contact studs (1, 2) are disposed at the outer ends (16, 17) of the contact rockers (13, 14) and the inner ends (18, 19) of the contact rockers (13, 14) are constructed as a spreading bearing (24) loaded by compression springs.
2. Contact arrangement according to claim 1, characterised thereby that the contact rockers (13, 14) are centrally mounted at the opening ends of the main portions (5, 6) of the contact arms.
3. Contact arrangement according to claim 1 or 2, characterised thereby that the spreading bearing (24) is formed by electrically insulating spherical seats.
4. Contact arrangement according to one of claims 1 to 3, characterised thereby that the inner ends (18, 19) of the contact rockers (13, 14) are executed to be electrically insulating up to the respective fulcrum (15).
5. Contact arrangement according to one of claims 1 to 3, characterised thereby that the contact rockers (13, 14) are executed to be electrically insulating with the exception of the region in which the contact studs (1, 2) are carried.
6. Contact arrangement according to one of claims 1 to 3 or according to claim 4 or 5, characterised thereby that those parts of the contact arms near to the fulcrum (15) are executed to be electrically insulating.

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