This invention relates to an electrical contact assembly used for a circuit breaker of a control center.
In a known control center including circuit breakers, control units constructed by control means, such as a circuit breaker and an electromagnetic contactor, are accommodated in multistage in a frame board of the control center so that the control units can be drawn out therefrom. In the circuit breaker, a movable contact is arranged to contact with or separate from a stationary contact disposed at the rear portion of the frame board in accordance with the movement of the control unit in a direction to draw out the movable contact from the frame board.
The circuit breaker unit includes interrupting units located on a power source side at which a bus bar and a control unit are interconnected and on a load side connected to an external cable of the control unit.
FIG. 1 is a general perspective view showing a control unit located on the load side, in which reference numeral 3 designates a control unit including a distribution circuit breaker 1 and an electromagnetic contactor 2, and bar conductors 5 are used as a movable contact unit 4a on the movable side. FIG. 2 shows a frame board 6 for accommodating the control units 3 in multistage and a plurality of compartments for accommodating control units are formed by inserting plates 7 into the frame board 6 at different levels. On the side of each compartment 8 is attached a stationay contact unit 4b which is connected to or separated from the movable contact unit 4a shown in FIG. 1.
FIGS. 3a and 3b show a general structure of a stationary main circuit breaker unit, in which U-shaped contacts 11 of the stationary contact unit 4b are provided to contact with or separate from the bar conductors 5 on the movable side 4a. Each of the U-shaped contacts 11 is prepared by forming a plate conductor into a U-shape having contacting portions 11a which are formed by slightly bending the sides thereof inwardly to form a narrow opening for clamping the movable contact therebetween. Although the bar conductor 5 is caused to engage and clamped by the contacting portions 11a, the U-shaped contact 11 can be made of a resilient electrically conductive material or the bar conductor may be sandwiched by using another suitable resilient members, not shown. The U-shaped contacts 11 are supported by a holder 12 provided with recesses 13 for receiving the contacts 11.
FIG. 4 shows a cross-sectional view taken along the line IV--IV in FIG. 3b, in which the bottom of the U-shaped contact 11 is connected to one end of a connecting conductor 14 and to the other end thereof is secured a screw 15 which connects the conductor 14 to an external cable, not shown.
When the control unit 3 is inserted into the compartment 8, the bar conductor 5 is also inserted between the narrowed contacting portions 11a of the U-shaped contact. In this inserted condition, current flows through the contacts and if a large current accidentally flows in such a case as short circuit, a large electromagnetic repulsive force is generated in proportions to the current flown, thereby widely separating the opening between the contacting portions 11a thus generating electric arc which damages the U-shaped contact 11. Therefore, in order to avoid such separation phenomenon or damage described above, it is desired to preliminarily impart contact force to the contacting portions 11a to overcome the repulsive force generated at a time when large current flows. However, in a case where a U-shaped contact with relatively large contact force is used, a large operating power is required to insert the control unit 3 into and draw out it from the frame board 6. In addition, since the connecting conductor 14 is sometimes bent by strong vibration of the cable caused by the electromagnetic force due to short circuit current, it is necessary to detachably connect the conductor 14.
FIG. 5 shows another example of a prior art circuit breaker unit on the movable side, in which a U-shaped contact 32 is arranged to contact with a stationary bar conductor 31, for example, a bus bar. In this example, the U-shaped contact 32 is formed by using two resilient conductors and the opening thereof is narrowed so as to form contacting portions 32a between which the bar conductor 31 is to be inserted and clamped.
The U-shaped contacts 32 are supported by a holder 33 covered by a cover 34 and provided with recesses 35 for receiving the U-shaped contacts 32. A projection 36 is formed in the recess 35 for guiding the contact 32. FIG. 6 shows a cross-sectional view taken along the line VI--VI in FIG. 5, in which the U-shaped contact 32 is connected to an electric cable 41 by, for example, welding. When the movable contact unit is moved to engage the U-shaped contact 32 with the bar conductor 31, the projection 36 serves to guide the contact 32 so as to cause the center line thereof to coincide with that of the bar conductor 31, and the U-shaped contact 32 is separated from the bar conductor 31 by moving the contact unit leftwardly in FIG. 6.
With the example shown in FIGS. 5 and 6, the same adverse result as described with reference to the former example shown in FIGS. 1 through 4 is caused when a large current accidentally flows. In addition, regarding the latter example, it is difficult to sufficiently utilize the space in the control unit for the reason that the cable 41 connected to the contact 32 has a vertical portion 40 as shown in FIG. 6 and another member cannot be disposed thereon.