CIRCUIT BREAKER WITH BLOW-OPEN CONTACT ARM

Abstract: The invention relates to a circuit breaker having a movable contact arm pivotally mounted on a contact arm carrier and adapted to be electrodynamically blown open upon the occurrence of a high-level overcurrent. The contact arm (23) has associated therewith a contact pressure spring (61) having a force arm (67) which acts upon a tail portion of the contact arm to normally hold the latter in a position constraining it to move as one with the contact arm carrier (43), and to provide contact pressure when the contacts (17, 19) are closed. As the contact arm is blown open, the force arm (67) of the spring slides toward and at least to the contact arm pivot (25), whereby the contact-­ pressure providing spring force on the contact arm is reduced essentially to zero.

Description: This invention relates generally to electric circuit breakers and, more particularly, to a circuit breaker having a movable contact arm which is adapted to be electrodynamically "blown open" upon the occurrence of an overcurrent exceeding a certain magnitude. So-called "blow-open contacts" are employed, especially with high-performance molded-case circuit breakers, in order to permit high-level overcurrents, such as short-circuit currents, to be effectively limited by enabling the contacts to separate more rapidly under such conditions than could ordinarily be achieved alone through operation of the circuit breaker trip means and operating mechanism responding to the abnormal current condition. The more rapid contact separation typically is made possible by enabling the contact arms to move to their contact open positions independently and ahead of their supporting structure under the action of magnetic blow-off or repulsion forces resulting from current flow therethrough. These forces tending to open the contacts must overcome the forces of springs usually employed to provide contact pressure when the contacts are closed. Since ordinarily the force of contact-pressure springs increases and the magnetic repulsion force acting upon a contact arm decreases with increasing contact separation, the contact springs tend to militate against rapid contact separation under the action of magnetic repulsion or blow-off forces, for which reason it is desirable to at least minimize the contact-pressure generating spring force as soon as possible after the incipience of a high-level overcurrent condition. A scheme seeking to accomplish this is disclosed in US-A-4,480,242 showing a circuit breaker with a contact arm which adjacent one end thereof carries a contact, at its opposite end is pivotally supported on a contact carrier, and at an intermediate portion thereof is shaped to provide cam surfaces which coact with a torsion spring adapted to apply to the contact arm a contact-pressure generating torque which is relatively high when the contact arm is in its contact closed position and diminishes as the contact arm is driven toward its contact open position. DE-A-8 428 351 discloses an arrangement in which a torsion spring has its force arm engaging a notch in the tail portion of the contact arm, with the torsion spring being arranged behind the contact arm so as to be able to pivot about its stationary end and so to follow the movement of the contact arm. This, however, renders the whole structure bulky and voluminous what is contrary to the general requirement to make the circuit breaker device as compact as possible. EP-A-0145990, which has a priority date of 19-12-83 and is published on 26-06-85, discloses an arrangement in which a pin on the tail portion of the contact arm moves in the direction of the fulcrum of the torsion spring , thereby decreasing the spring force applied by the spring arm against the pin. The present invention has for its principle object to achieve the above mentioned desired result in a relatively simple manner allowing for a compact construction. With this object in view, the present invention resides in a circuit breaker as defined in claim 1 and, with further specific features, as defined in subclaims 2 to 4. With this arrangement, as the point of spring action on the contact arm moves toward the pivot of the latter, the force or lever arm of the contact pressure spring decreases and becomes zero as said point of spring action reaches the contact arm pivot. By preferably providing for the force arm of the spring to slide beyond the pivot of the contact arm during independent contact separating movement of the latter, the spring will become a contact opening spring as soon as its point of action on the contact arm passes to the side of the contact arm pivot which is nearer the contact-carrying end of the contact arm. It will be appreciated that the arrangement according to the invention, in which the contact pressure spring normally acts against the tail portion of the contact arm located on the side of the contact arm pivot opposite from the contact-carrying end of the contact arm, achieves minimization of the contact pressure without requiring the use of a specially and intricately shaped contact arm or contact-arm carrier and also without a moving spring, and permits provision to be readily made for a directional reversal of the spring force into a contact opening force to occur during independent contact separating movement of the contact arm. A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a vertical sectional view through the center pole of a multi-pole circuit breaker embodying the invention; Fig. 2 is a partial view of the contact structure of the circuit breaker, showing the movable contact arm both in the "blown open" position (solid lines) and the contact closed position (phantom lines) thereof; and Fig. 3 is a view similar to Fig. 2 but showing the movable contact arm together with its contact arm carrier in the normal contact open position. Referring to Fig. 1 of the drawings, the circuit breaker illustrated therein and generally designated with reference numeral 5 includes a housing, or molded case, formed of a suitable insulating material, such as an epoxy, and consisting of a base 7 and a cover 11 suitably secured, such as screwed, to the base. The circuit breaker has a line terminal 13 and a load terminal 15 in each pole unit, and it includes a circuit breaker mechanism 9 supported within the insulating housing. The circuit breaker mechanism 9 includes a stationary contact 17 and a movable contact 19. The stationary contact 17 is disposed on a conductor 21 extending and connected to the load terminal 15, and the movable contact 19 is disposed on a movable contact arm 23 pivotally mounted, as at 25, on a contact arm carrier 43 secured to a cross-bar 47 which extends through all pole units of the circuit breaker and is pivotally supported in a support frame comprising side plates, such as plate 41, the frame being stationarily supported in the insulating housing. The circuit breaker mechanism 9 includes further an operating mechanism 27 for pivotally moving the cross-bar 47 together with the contact arm carriers 43 thereon between contact open and contact closed positions. The operating mechanism 27 may be of the kind described in US-A-3,600,539 or 4,114,005 in that it comprises a toggle linkage formed of toggle links 29,31, and overcenter springs 33 operatively connected between the knee of the toggle linkage and the bight of a generally U-shaped switch arm 37 which has a manually operable handle 35 affixed to its bight and is pivotally supported, at the free ends 39 of its legs, in generally V-shaped recesses formed in the side plates 41. As well known in the art, manual movement of the handle 35 between ON (as shown) and OFF positions causes the toggle linkage 29,31 either to be straightened and thereby to close the contacts or to collapse and thereby to open the contacts, depending upon which of its two positions the handle is being moved to. When the circuit breaker contacts are closed, a complete circuit in each pole unit of the circuit breaker extends from the line terminal 13 through fixed conductors 49 and 51, a flexible conductor or shunt 53, the movable contact arm 23, the contacts 19,17, and the fixed conductor 21 to the load terminal 15. Upon the occurrence of an overcurrent up to a predetermined value in any of the pole units, trip means associated with the affected pole unit and generally designated 55 will respond to the overcurrent and cause a releasable member 57, commonly known as a cradle, of the operating mechanism 27 to be released, whereupon the latter will automatically open the contacts 17,19 in all pole units, all as well known in the art. Electric arcs drawn between the contacts 17,19 during separation thereof under load are stretched and extinguished in an arc chute 59. As mentioned hereinbefore, the movable contact arm 23 in each pole unit is mounted on the associated contact arm carrier 43 so as to be pivotable (about pivot 25) relative thereto. This enables the contact arm 23 in a known manner to be "blown" open by the magnetic blow-off force (or repulsion force, if a conductor electrically in series with the contact arm were to extend parallel thereto) typically generated upon flow of a severe fault or short-circuit current. This contact arm movement effected by the electrodynamically generated force and limited by a stop 71 in the circuit breaker housing is rapid and, as illustrated in Fig. 2, is independent of the contact arm carrier 43 which latter however will immediately follow the independently moved contact arm to the contact open position (see Fig. 3) upon release of the operating mechanism 27 by the trip means 55 responding to the overcurrent. Likewise as set forth earlier herein, it is customary to bias each movable contact arm to a normal position, relative to the contact arm carrier, so as to constrain the contact arm to move as one together with the contact arm carrier, when actuated, and to provide contact pressure when the contacts are closed. In the illustrated embodiment of the invention, the bias means provided for this purpose is a torsion spring 61 which is supported on the contact arm carrier 43 by means of a pin 63, which furthermore has a retaining arm 65 engaging a portion of the contact arm carrier, and has a force arm 67 which is slideably engaged with and acts against an end or tail portion of the contact arm 23 extending from the contact arm pivot 25 to the end of the contact arm located opposite from its contact-bearing end. As seen best from Figs. 2 and 3, the arrangement is such that in the normal biased position of the contact arm 23 relative to the contact arm carrier 43, the force arm 67 of the torsion spring 61 engages the tail portion of the contact arm 23 at a point (referred to herein also as the point of action of the spring upon the contact arm) which is spaced from the contact arm pivot 25 a distance 69 (see Fig. 2) resulting in a lever arm sufficient to maintain adequate contact pressure when the contacts are closed (phantom-line position in Fig. 2), and to force the tail portion of the contact arm 23 against a stop 45 on the contact arm carrier 43 to constrain the contact arm to move as one together with the contact arm carrier. When the contact arm carrier 43 together with the contact arm 23 thereon is in the contact closed position thereof, as shown in Fig. 1 and, in phantom, in Fig. 2, and the contact arm 23 is "blown open" due to the occurrence of a high-level overcurrent, as illustrated in Fig. 2, the tail portion of the independently moving contact arm 23 will depress the force arm 67 of the torsion spring and, in so doing, will cause it to move toward the contact arm pivot 25, as indicated at 67a in Fig. 2. Thus, the point of action of the torsion spring on the contact arm shifts to the contact arm pivot 25 so that the lever arm on the contact arm is reduced essentially to zero and the resistance (spring force times lever arm) ordinarily offered by the contact pressure spring 61 to contact separating movement of the contact arm 23 is virtually removed. It will be appreciated that if, through some simple modification of the illustrated embodiment, the force arm 67 of the torsion spring 61 is allowed to slide beyond the contact arm pivot 25, there will be a directional reversal of the spring force causing it to become a contact-separation aiding force. As the contact arm carrier 43 follows the independently moved contact arm 23 to the contact open position, as hereinbefore explained, the contact arm, being held by the stop 71, will rotate, relative to the contact arm carrier, clockwise about its pivot 25 until its tail portion is reengaged with the stop 45 on the contact arm carrier and the point of spring action on the contact arm has returned to its normal position, as shown in Fig. 3.

Claims: A circuit breaker comprising a pair of cooperating contacts (17,19), a contact-arm carrier (43) supported for pivotal movement thereof between a contact closed position and a contact open position, a contact arm (23) which carries one of said contacts (19) adjacent one end thereof and is pivotally supported on said contact arm carrier (43) in a manner permitting independent contact separating movement of the contact arm (23) under the action of an electrodynamical force generated upon flow of an overcurrent exceeding a predetermined value, and an operating mechanism (27) operatively connected to the contact-arm carrier (43) and operable to effect movement thereof to said contact closed and contact open positions, said contact arm (23) having associated therewith a spring (61) biasing the contact arm to a normal position with respect to the contact arm carrier (43) thereby to provide contact pressure when the contacts are closed, and to constrain the contact arm (23) to move as one together with the contact arm carrier (43), said spring(61) having a force arm (67) engaging the tail portion of the contact arm (23) extending between the end of the contact arm opposite the contact bearing end and the pivotal connection (25) thereof with the contact arm carrier, characterized in that said spring (61) has its force arm (67) acting upon said contact arm tail portion in sliding engagement therewith, the point of engagement on the contact arm (23) being spaced from the pivotal connection (25) of the contact arm (23) with the contact arm carrier (43) when the contact arm is in the contact closed position, and said point of engagement moving toward and at least to said pivotal connection (25) during independent contact separating movement of the contact arm (23). A circuit breaker according to claim 1, characterized in that said spring (61) is arranged such that said point of engagement of said force arm (67) of the spring (61) with said contact arm (23) slides beyond said pivotal connection (25) during said independent contact separating movement of the contact arm (23). A circuit breaker according to claim 1 and 2, characterized in that said spring (61) is a torsion spring supported on said contact arm carrier (43). A circuit breaker according to claim 1, 2 or 3, characterized by a fixed stop (71) cooperable with said contact arm (23) to limit said independent contact separating movement thereof and to hold the independently moved contact arm stationary during follow movement of the contact arm carrier (43) to said contact open position.

Inventors: MAJCHER, JOHN PETER, MCKEE, JERE LEE, SCHLOSSER, DONALD EUGENE, SMITH, WILLIAM EDWARD
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: H01H77
IPC codes: H01H77, H01H73