WO1991000610A1

WO1991000610A1 - Contact arrangement for electric switching devices

Info

Publication number: WO1991000610A1
Application number: PCT/SE1990/000466
Authority: WO
Inventors: Assadollah Abri
Original assignee: Asea Brown Boveri Ab
Priority date: 1989-07-04
Filing date: 1990-06-29
Publication date: 1991-01-10
Also published as: SE8902417L; SE464158B; SE8902417D0

Abstract

The invention relates to a contact arrangement, intended for a current-limiting low-voltage circuit breaker, with an elongated contact carrier (1) rotatably journalled at its mid-point, the contact carrier being arranged between two U-shaped fixed contact carriers (2) arranged spaced from each other. The fixed contact carriers have a contact leg (3) which, via a mid-portion (5), is connected to a connecting leg (4). The mid-portion exhibits a rectangular opening (6), through which the movable contact carrier (1) extends. Upon a short circuit, the contacts are opened because of electrodynamic repulsion forces between the contact legs (3) and the movable contact carrier (1). This repulsion force is reduced when the contact distance increases, but the reduction is compensated by an increasing attraction force between the movable contact carrier (1) and the connecting legs (4). In this way a greater contact-opening force is obtained during the breaking operation.

Description

Contact arrangement for electric switching devices

The present invention relates to a contact arrangement, intended for electric switching devices, of the kind described in the preamble to claim 1. The invention is primarily related to contact arrangements for current- limiting circuit breakers for rated operating voltages of up to about 1000 V, but it may, in principle, be used also for other types of low-voltage switching devices.

From, for example, DE-A-2 157 927 current-limiting circuit breakers are previously known, which comprise an elongated, movable contact carrier which is rotatably jornalled at its mid-point and which at its ends supports contact elements, which cooperate with fixed contact elements supported by U- shaped fixed contact carriers arranged on opposite sides of the movable contact carrier. When a short circuit occurs in the circuit into which such a circuit breaker is connected, because of the configuration of the current paths, large electrodynamic repulsion forces occur between the movable and the fixed contact carriers, resulting in contact opening. As the contact distance increases, however, the repulsion forces will be reduced while at the same time the spring force from the contact pressure springs of the circuit breaker increases. This may lead to the movable contact carrier being slowed down and possibly reversed, resulting in contact bounce and welding.

The object of the present invention is to provide a contact arrangement of the above-mentioned kind, particularly intended for current-limiting circuit breakers, which does not suffer from the drawbacks mentioned. This is achieved according to the invention by a contact arrangement as described in the characterizing part of claim 1. In this contact arrangement the attraction force between two parallel conductors, which are traversed by current in the same direction, is utilized, namely the movable contact carrier and the connecting legs of the fixed contact carriers. This attraction force compensates the reduction of the repulsion force, occurring upon the contact separation, between the movable contact carrier and the contact legs of the fixed contact carrier. This leads to a greater contact-opening force, the contact distance thus increasing more rapidly and a higher breaking capacity being attained.

According to one of the characteristic features of the invention, the end portions of the movable contact carrier are situated in the space between the two legs of the respective fixed contact carrier. In an embodiment schematically shown in the above-mentioned DE-A-2 157 927 (Fig. 7), admittedly one of the two series-connected breaking points has a similar configuration. It is obvious, however, that in this case the basic concept of the present invention has not been realized and utilized, since the other breaking point is designed in a different way. In that way, at best only half of the reduction of the repulsion forces can be compensated. In addition, from the points of view of manufacturing and stock-keeping, the fact that the fixed contact carriers of the circuit breaker are of different designs entails considerable disadvantages.

The mid-portions of the fixed contact carriers are suitably designed with openings, through which the movable contact carrier projects. Fixed contact carriers designed in a similar way are known, per se, from DE-B-1 258 955, but in this case it is a question of a circuit breaker with a movable contact carrier which is transversely displaceable and in which the contact separation does not take place by the influence of electrodynamic repulsion forces.

The invention will be explained in greater detail by means of an embodiment with reference to the accompanying drawings, wherein Figure 1 is a perspective view of a contact arrangement according to the invention,

Figure 2 schematically shows the configuration of the current path of this contact arrangement, and

Figure 3 shows curves of the forces acting on the movable contact carrier as a function of the contact distance.

The contact arrangement shown in Figures 1 and 2 comprises an elongated, movable contact carrier 1, which at its mid¬ point is rotatably journalled about an axis 10 directed perpendicular to the longitudinal axis of the contact carrier. At its ends the contact carrier 1 supports contact elements 11, each of which cooperates with a respective one of two fixed contact elements 12 which are arranged on opposite sides of the movable contact carrier and are supported by U-shaped fixed contact carriers 2. The fixed contact carriers are identical and are provided with two legs 3, 4 interconnected by a mid-portion 5, of which one leg 3, referred to below as the contact leg, supports one of the fixed contact elements 12, and the other leg 4, referred to below as the connecting leg, is intended for connecting the electric switching device into an external current circuit. The mid-portions 5 of the fixed contact carriers exhibit rectangular openings 6, through which the movable contact carrier 1 extends. The movable contact carrier 1 is insulated from the connecting leg 4 of the respective fixed contact carrier by an insulating coating 7 provided on the inwardly-facing side of the leg, as well as from the mid- portion 5 by the necessary insulation distance to the edges of the opening 6.

In Figures 1 and 2 the current direction at a certain moment is marked by the unmarked arrows. When a short-circuit current flows through the contact arrangement, a repulsion force F_rarises between the contact legs 3 and the movable contact carrier 1, which is then rotated in the direction of the arrow A about the axis 10, and contact opening occurs. This leads to arcs B being created between the contact elements 11 and 12, which arcs, for example by magnetic influence, are urged into deionization plate packages (not shown) arranged adjacent to the respective breaking point.

When the contact distance increases, the repulsion force F_r decreases according to the formula

r x+x.

where k_£ is a first constant, ± is the instantaneous value of the current, &i. is the initial distance between the contact leg 3 and the movable contact carrier 1 and . is the distance between the contact elements 11 and 12. In Figure 3 the dashed line shows how the repulsion force F_r varies as a function of the contact distance x.

With a growing contact distance, however, the attractive force F_a between the connecting legs 4 and the movable contact carrier 1 increases according to the formula

^Fa ^{" k}a d-(x+x, )

where j^ is a second constant and d. is the distance between the contact leg 3 and the connecting leg 4. In Figure 3 the dot-dashed line shows how the attraction force F_a varies as a function of the contact distance x.

The total electrodynamic force F_t which acts on the movable contact carrier 1 is the sum of the above-mentioned repulsion and attraction forces. In Figure 3 the unbroken line shows how this force F_t varies as a function of the contact distance x. As will be seen, the force F_t, which opens the contacts, is practically constant and considerably greater than in a conventional contact arrangement, in which only the repulsion force F_r is used. This entails a faster contact separation and higher breaking capacity.

Claims

1. A contact arrangement for electric switching devices comprising an elongated electrically conducting movable contact carrier (1) , which at its mid-point is rotatably journalled about an axis (10) , directed perpendicular to the longitudinal axis of the contact carrier, and at its ends supports contact elements (11), each of which cooperates with a respective one of two fixed contact elements (12) which are arranged on opposite sides of the movable contact carrier (1) and are each supported by an electrically conducting fixed contact carrier (2), the two fixed contact carriers (3) being substantially U-shaped and being arranged with the openings of the Us facing away from each other, each one of the fixed contact carriers exhibiting two legs (3,4) interconnected by a mid-portion (5), namely a contact leg (3) , which supports one of the fixed contact elements (12), and a connecting leg (4), characterized in that the end portions of the movable contact carrier are situated in the space between the two legs (3,4) of the respective fixed contact carriers and that the connecting legs (4) lie on opposite sides of the movable contact carrier (1) .

2. A contact arrangement according to claim 1, characterized in that the fixed contact elements (12) are arranged on the inwardly-facing side of the respective contact leg (3) .

3. A contact arrangement according to claim 1 or 2, characterized in that the fixed contact elements (12) are placed diametrically with respect to the axis of rotation

(10) of the movable contact carrier.

4. A contact arrangement according to claim 1, 2 or 3, characterized in that the mid-portions (5) of the fixed contact carriers (2) exhibit openings (6) through which the movable contact carrier (1) extends.

5. A contact arrangement according to any of the preceding claims, characterized in that the connecting legs (4) of the fixed contact carriers are provided with an insulating coating (7) on their inwardly-facing sides.

6. A contact arrangement according to any of the preceding claims, characterized in that the fixed contact carriers (2) are identically alike.