US3089936A

US3089936A - Contact structure for an electric circuit interrupter

Info

Publication number: US3089936A
Application number: US10160A
Authority: US
Inventors: Jr Sidney R Smith
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1960-02-23
Filing date: 1960-02-23
Publication date: 1963-05-14
Anticipated expiration: 1980-05-14
Also published as: FR1280434A

Description

y 4, 1963 s. R. SMITH, JR 3,089,936

CONTACT STRUCTURE FOR AN ELECTRIC CIRCUIT INTERRUPTER Filed Feb. 23, 1960 v Inventor:

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United States Patent ()1 3,689,936 Patented May 14, 1%63 ice spasms (IGNTAQT STRUCTURE FUR AN ELECTRTQ Cl R CUlT llN'liEilRUlTElt Sidney R. Smith, the, toclthridge, Mass, assignor to General Electric Company, a corporation of New York Filed Feb. 23, 1M0, Ser. No. llhljtl l3 Qiairns. (@l. 200-44 4) This invention relates to contact structure for an electric circuit interrupter and, more particularly, to contact structure which is especially, though not exclusively, suited for use in circuit interrupters of the vacuum type.

The usual vacuum-type circuit interrupter comprises a pair of separable contacts disposed within an evacuated chamber. Circuit interruption is initiated by separating these contacts to establish an arc. Assuming that the circuit is an alternating current circuit, the arc maintains itself until about the time a natural current Zero is reached, after which the arc is prevented from reigniting by the high dielectric strength of the vacuum.

It has been recognized heretofore that the interrupting capacity of such an interrupter can be materially increased by moving the terminals of the are at high speed along the surfaces of the contacts or adjacent structure. Such movement tends to minimize the amount of metallic vapors generated from the contacts or adjacent structure by the arc and tends also to increase the degree of diffusion of the vapors that are generated. These factors enable the vacuum to recover its dielectric strength at an increased rate after a current zero and thus render the vacuum more capable of preventing reestablishment of the arc during this critical interval.

In types of interrupters other than those of the vacuum type, high speed arc-movement also facilitates circuit interruption by cooling the arc column, decreasing contactheating and vaporization, and promoting an increased rate of dielectric recovery.

Prior schemes for moving the arc terminals over the contact suriaces have usually relied upon a magnetic field generated by one or more coils connected in circuit with the contacts. These coils must generally be rather massive and bulky and are also difiicult to free of adsorbed gases, which are distinct disadvantages, particularly for a vacuum switch construction.

Thus, an object of my invention is to provide structurally-simple, easily-manufactured, and compact means for driving an are at high speed along the surfaces of a contact or similar structure.

Another object is to incorporate such arc-driving means into the contact structure itself and in such a manner that the arc is driven in a generally circular path from the instant it is first initiated.

Still another object is to construct contact structure of this characer in such a manner that butt-type circuitclosing engagement between the two contacts is made over a relatively large area and at a plurality of different points.

In carrying out my invention in one form, I provide a pair of coacting butt contacts relatively movable from a position of engagement to a position of disengagement to establish a circuit interrupting arc therebetween. At least one of the contacts is a cup-shaped member comprising a body portion and a generally cylindrical flange projecting laterally from said body portion toward the other contact. The flange terminates at its free end in a generally annular contact-making surface that extends about the periphery of the cup-shaped member and faces the other contact. The other contact has a contactmalcing surface aligned with the contact-making surface of the first contact and butting thereagainst when the contacts are engaged. Arc-motion in a circumferential direction along the contact-making surfaces is effected by means comprising a plurality of slots formed in the flange of the cup-shaped member. Each of these slots intersects the contact-making surface of the cup-shaped member and extends from this contact-making surface toward said body portion via a pair disposed at an acute angle to said contact-making surface. These slots force any current flowing to an arc terminal located at any angular position on the contact-making surface to have a net component extending generally circumferentially with respect to the flange in the region of the arc. This net circurnferentially-extending component forms with the arc a loop circuit that has a magnetic effect that acts to drive the arc in a circumferential direction about the contact-making surface.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a sectional view of a vacuum circuit interrupter comprising contact structure embodying one form of my invention.

FIG. 2 is an en arged side-elevational view of the contact structure shown in MG. 1.

P16 2:: is a view taken along the line Za-Za of FIG. 2.

FIG. 3 illustrates a modified form of contact structure for use in an interrupter of the type shown in FIG. 1.

FIG. 4 illustrates still another modified form of contact structure for use in an interrupter of the type shown in PEG. 1.

Referring now to the interrupter of FIG. 1, there is shown a highly evacuated envelope 13 comprising a casing 11 of suitable insulating material and a pair of

metallic end caps

12 and 13 closing off the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to provide vacuum-tight joints at these points.

Located within the envelope in is a pair of separable contacts 17 and 18 shown in their engaged, or closedcircuit, position. The upper contact 17 is a stationary Contact suitably secured to a conductive rod 17a, which at its upper end is integrally united to the upper end cap 32. The lower contact 18 is a movable contact joined to a conductive operating rod 18a which is suitably mounted for vertical movement. The operating rod 18a projects through an opening in the lower end cap If and a flexible metallic bellows 2%; provides a seal about the rod 13a to allow for vertical movement of the rod without impairing the vacuum inside the envelope lit. As shown in FIG. 1, the bellows 2t} is secured by means of suitable sealed joints at its respective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 130, suitable actuating means (not shown) is provided. This actuating means is capable of driving the contact 18 downwardly out of engagement with the contact 17 so as to open the interrupter and is also capable of returning the contact in to its illustrated position so as to close the interrupter. A circuit-opening operation will soon be explained in greater detail.

Each of the contacts 27 and is is of a cup-shaped configuration and comprises a body portion 25 of disc form having at its periphery a cylindrical flange 27 laterally protecting therefrom toward the other contact. The flange 2'7 of each contact terminates at its free end in a generally annular contachmaking surface 29 that extends about the periphery of the cup-shaped member 27 and faces the other contact. These contact-making surfaces 29 on each contact butt against each other in contact-making relationship when the circuit interrupter is closed, as shown in MG. 1.

The flange 27 of each contact has a series of slots 33 provided therein to form fingers 34- located between the slots. As will be apparent from FIG. 2, in each contact these slots intersect the contact-making surface 29 and extend from said contact-making surface 29 toward the body portion 25 via a path disposed at an acute angle to the contact-making surface 29. In view of this acute angular relationship, the slots may be thought of as extending from the contact-mal ing surface 29 in a generally circumferential direction with respect to flange 27. In each contact, each of these generally circumferentially extending slots terminates only after it has extended from its mouth at the contact-making surface as to a point near the angular position of the mouth of an adjacent slot. Preferably, the adjacent slots of each contact angularly overlap each other as is shown in KG. 2. As will be apparent from FIG. 2a, each of these slots 33 extends through the entire thickness of the flange 2'7, i.e., from the outer periphery to the inner periphery of the flange. In addition, the slots 33 in both contacts extend circumferentially away from their mouth at the contact-making surface 29 in generally the same angular direction, e.g., in FIG. 2 both sets of slots 33 extend circumferentially away from their mouth at the contact-making surface 29 in a clockwise direction,

as considered from the bottom of the contact structure I Assume now that the circuit is to be interrupted and to this end, the contact 13 is driven downwardly to establish a circuit-interrupting arc. The are is initiated between the two contact-making areas 29 on the particular set of fingers which were last to part. Substantially all of the current flowing between the conductor 17a and the upper arc terminal is concentrated in the particular finger 3d of contact 17 on which the arc was initiated, and, likewise, substantially all of the current flowing between conductor 1dr: and the lower arc terminal is concentrated in the particular finger of contact 13 on which the arc was initiated. Because of the generally circumferential manner in which the slots 33 extend, the current flowing through these two fingers 3:4- is required to follow a path which has a net component extending generally circumferentially with respect to the flange 27 in the region of the arc, as is illustrated by the dotdash lines 35 of FIG. 2. As a result of this circumferentially-extending configuration of the current path, there is a circuit present which has its arms extending effectively circumferentially with respect to the arc interconnecting the arms. A loop circuit has a magnetic effect which tends in a well-known manner to lengthen the loop, and because of the circumferential disposition of the arms of the loop, this magnetic effect is a circumferentially-acting force which acts in FIG. 2 to drive the arc circumferentially to the left. When an arc terminal has moved to the left hand end of the finger 3 this circumferentially-acting force drives the arc terminal across the slot 33 to the next finger 34. The current flowing to the arc terminal is then concentrated in this next finger 34, and because of the configuration of this finger, there is a new circumferentially-acting loop circuit which continues motion of the are around the contact periphery. For each of the fingers 34, there is a net circumferential force component on the are acting in the same angular direction, and, as a result, circumferential motion of the arc continues at high speed until the arc is finally extinguished. As viewed from the bottom of FIG. 2, this circumferential arc-motion would be in a counterclockwise direction. if the arc is not extinguished before one revolution is completed, circumferential motion will continue repetitively about the contact periphery until the arc is extinguished. It will be understood that the fingers are sufiiciently close together to enable the arc to be driven across any slot which separates the fingers.

The above described movement of the arc is advantageous in that it lessens the amount of contact material that will be vaporized by the arc and also increases the degree of difiusion of the vapors that are generated. This enables the vacuum to recover its dielectric strength at an increased rate and thereby improves ability to prevent re-establishment of the are after a current zero, thus increasing the interrupting capacity of the interrupter. A particular advantage of my contact arrangement shown in FIGS. 1 and 2 is that circumferentially-acting forces are present to begin circumferential motion of the arc 'rorn the instant of its initiation. No delay is required to allow time for the arc to be driven onto the arc-rotating region of the contact.

in the embodiment of FIGS. 1 and 2, the slots are of a straight-line configuration. This straight-line configuration is advantageous in that it enables the slots to be easily and quickly produced, as by a simple sawing or milling operation. Curved slots may be used if desired, but these are generally more di ficult to form than the straight-line slots shown in FIGS. 1 and 2. Whether the slots are straight or curved, however, the cup should be slotted from its contact-making surface 29 inward, and the slot configuration should be such that the current path extending between the body portion 25 of the contact and an arc terminal located at substantially any angular point on the contact-making surface Zfi has a net component extending generally circumferentially with respect to the flange 27 in the vicinity of the arc. In addition, the slot configuration should be such that this net circumferential component extends from the arc in the same angular direction for substantially all angular positions of the are on the peripheral region of the contact, so that motion of the arc terminal is continued in a single angular direction. By the term angular direction, used hereinabove, is meant a clockwise or a counterclockwise direction relative to the central region of the contact.

In the contact structure of FIGS. 1 and 2, the circumferentially-acting force component tending to drive the are around the contact-making surface 29 exists only so long as one of the arc terminals is attached to one of the fingers 34. If both arc terminals were to be located radially inwardly of the fingers 34, then there would be no circumferentially acting force on the arc. In my contact arrangement, there is a radially-acting force component which continuously biases the arc in a radially-outward direction, thereby preventing it from wandering into a region where there is no circumferentiallyacting force component and forcing it to remain in the outer peripheral region where there is an effective circumferentially-acting force component. This radiallyoutwardly-acting force component that biases the arc toward the outer periphery of the contact structure resalts from the fact that the loop circuit exending through the contact extends radially, i.e., through the body portion 25, as well as circumferentially, as a result of the cup-shaped configuration of the contacts.

It will be apparent that the contact arrangement of the present invention requires no special coils or magnets for effecting the desired arc rotation and is therefore more compact than prior arrangements that have utilized such supplementary devices. A feature that further coutributes to the compactness of my contact arrangement is the fact that the fingers 3- which are responsible for the circumferentially-extending disposition of the current paths leading to and from the arc, extend from the body of each contact in a generally axial direction relative to the longitudinal axis of the envelope iii. By extending in an axial direction rather than in a radial direction, an envelope of considerably smaller diameter is needed for enclosing the contacts with the required radial clearance. A somewhat increased length of envelope is required, but increases in length can be eflected with con siderably more economy than increases in diameter of the envelope.

If lower speeds of are motion can be tolerated, only a single one of the contacts need be provided with areimpelling means. For example, as shown in FIG. 3, the lower contact can be formed as a planar disc abutting the contact-making surface 29 of the upper contact about its outer periphery. The lower contact, though unslotted, will still provide a radially-acting loop component which tends to hold the are on the outer peripheral region of the contact structure where the circunrfer'entially-acting force component is effective.

Another advantage of the disclosed contact arrangement is that contact is made over a relatively large area at a plurality of different points. In this regard, the fingers 34 are capable of yielding slightly in an axial direction to allow firm contact to be made at a plurality of different fingers on each contact. By making contact at a plurality of points, I am able to lessen the amount of current flowing through any single point when the contacts are closed, and this reduced current lessens the tendency for contact welding to occur at any given point during the occurrence of current surges.

Although I have shown the slots 33 as being uniformlyspaced about the periphery of the contact, it is to be understood that non-uniform peripheral spacing could instead be utilized. Non-uniform spacing between the slots would be advantageous in lessening the tendency of the contacts to bounce temporarily out of engagement in response to the impact produced by initial contactclosing engagement. This follows from the (fact that nonuniform spacing of the slots would result in fingers 34 of differing mass and hence, differing natural frequency. Because of their differing natural frequencies, there would be less tendency for any bounce that would occur to result in all of the fingers separating from the mating contact simultaneously. Some of the fingers would always tend to be in contact with the mating contact, thereby minimizing any tendency to are during this interval.

In some cases, it may be advantageous to make contact and initiate arcs on a portion of the contact structure apart from the arc-running surface. One way in which this can be accomplished is illustrated in the embodiment of FIG. 4. Here each contact is provided with a slotted flange 27 corresponding to the flange '27 of FIGS. 1 and 2, but in addition, each contact has a contact-making ring concentric .with the flange 27 and located internally thereof. The contact-making ring 40 of each contact extends alongside the slotted flange 27 from the body portion 25 to a location slightly beyond the end of flange 27 so that con tact is made at the outer surface 4 1 of the ring 4t} rather than on the flange 27. When the contacts are separated, an arc is initiated between the contact-making rings 40 and is driven radially outward onto the arc-running surfaces 29 of the flanges 27 by the radially-outwardly-acting magnetic effect resulting from the loop circuit 4-2. When the arc reaches the arc-running surface 29", it is driven in a circumferential direction in the same manner as described in connection with FIGS. 1 and 2.

A gap 44 is provided between the contact-making ring 40 and the arc-running flange 27 to insure that current flowing to and from an arc terminal on the arc-running surface 29 is forced to flow through the fingers 34 of the flange 27 rather than being allowed to follow a path through the rings 40 that would bypass the lingers 34 and impair their ability to produce arc-movement circumferentially about the surface 29;

Another advantage of the disclosed contact arrangements is that they readily lend themselves to the establishment of a symmetrical electric field in the region of the arcing gap, i.e., a field which is generally symmetrical with respect to a reference plane which bisects the arcing gap between the fully-open contacts and extends perpendicular to the longitudinal axes of the contact supporting rods. This symmetry follows from the fact that each set of the disclosed contacts themselves are of generally the same shape, particularly in FIGS. 1 and 4, and are disposed generally symmetrical with respect to the above reference plane. With regard to this general matter, it should be noted that arcing gaps in general have a lower breakdown strength when subjected to voltage of one polarity than when subjected to voltage of an opposite polarity. The more non-symmetrical is the electric field in the region of the gap, the more pronounced is this polarity effect. By providing a generally symmetrical field, I am able to minimize this polarity effect. As a result, my interrupter is not subjected to the unduly prolonged arcing that could result from low dielectric strength dur ing alternate half-cycles.

For protecting the insulation of a vacuum interrupter from the build-up of a metallic coating thereon as a result of arc-generated vapors condensing thereon, it is customary to provide a vapor-condensing shield between the arcing gap of the interrupter and the protected insulating surface. Such a shield is shown at 50 and comprises a cylindrical metallic member disposed between the contacts 17, 13 and the internal surface of insulating casing 11. This shield preferably corresponds to a similar shield shown and claimed in Patent No. 2,892,911, Crouch, assigned to the assignee of the present invention, and is therefore electrically isolated from both contacts and ground.

While I have described my invention panticul-arly with regard to a vacuum circuit interrupter, it will be apparent that it also may be advantageously embodied in other types of interrupters. For example, the disclosed contact structure could be used in the type of interrupter that has its contacts disposed in a chamber filled with an arc extinguishing medium, which may, if desired, be pressurized.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric circuit interrupter, a pair of coacting butt contacts relatively movable from a position of engagement to a position of disengagement to establish a circuitinterrupting arc therebetween, at least one of said contacts being a cup-shaped member comprising a body portion and a flange projecting later-ally from said body portion toward said other contact, said flange terminating at its free end in a generally annular contact-making surface extending about the periphery of said cup-shaped member and facing said other contact, said other contact having a contact-making surface generally aligned wtih the contactmaking surface of said first contact and butting thereagainst when said contacts are engaged, said contact-making surfaces parting last during a circuit-interrupting operation to initiate said are between said contact-making surfaces, means for producing arc-motion in a circumferen tial direction along said contact-making surfaces comprising a plurality of slots in said flange extending through the entire radial thickness of the flange, each of said slots intersecting said contact-making surface and extending from said contact-making surface toward said body portion via a path disposed at an acute angle to said contactmaking surface.

2. The interrupter of claim 1 in which said other contact is a cup-shaped member comprising a body por tion and a flange projecting laterally therefrom With the contact-making surface thereof located at the free end of said flange, the flange of said second contact being provided with a plurality of slots, each intersecting the contact-making surface of said second contact and extending from said latter contact-making surface toward the body portion of said second contact via a path disposed at an acute angle to said latter contact-making aosaase surface, the slots of both contacts extending from their intersection with the contact-making surfaces of their respective contacts in generally the same angular direction.

3. The arrangement of claim 1 in which said slots have a straight-line configuration.

4. The interrupter of claim 1 in which each of said slots has a mouth located at the intersection of said slot and said contact-mfiing surface and in which each of said slots extends from its mouth generally circumferentially of said cup-shaped contact at least to a point closely adjacent the angular position of the mouth of an adjacent slot.

5. The interrupter of claim 4 in which at least some of said slots extend into angularly overlapping relationship with their respective adjacent slots.

6. The interrupter of claim 4 in which the spacing between some of the slots is difierent from the spacing between others of the slots so that fingers of varying mass are formed between the slots.

7. In an electric circuit interrupter, a pair of coacting butt contacts relatively movable from a position of engagement to a position of disengagement to establish a circuit interrupting arc therebetween, at least one of said contacts being a cup-shaped member comprising a body portion and a flange projecting laterally from said body portion toward said other contact, said flange terminating at its free end in a generally annular contactmaking surface extending about the periphery of said cup-shaped member and facing said other contact, said other contact having a contact-making surface generally aligned with the contact-making surface of said first contact and butting thereagainst when said contacts are engaged, means for producing arc-motion in a circumferential direction along said contact-making surfaces Comprising a plurality of slots in said flange extending through the entire radial thickness of said flange, said slots intersecting said contact-making surface and extending toward said body portion, the slot configuration being such that the current path extending between said body portion and an arc terminal located at any point on said contact-making surface forms with said arc a loop circuit having a magnetic effect exerting a net component of force on the are acting generally circumferentially with respect to said flange, said net tangential force component acting in the same angular direction for substantially all angular positions of said arc on said contact-making surface and thereby causing said are to travel rapidly in one angular direction along said contact-making surface, the portions of said contact-making surface on either side of any slot therein being sufficiently close together to enable said circumferentially-acting component of force to drive said arc across said slot.

8. In an electric circuit interrupter, a pair of coacting butt contacts relatively movable from a position of engagement to a position of disengagement to establish a circuit-interrupting arc therebetween, at least one of said contacts being a cup-shaped member comprising a body portion and a flange projecting from said body portion toward said other contact, said flange terminating at its free end in a generally annular arc-running surface extending about the periphery of said cup-shaped member and facing said other Contact, an annular arc-running surface electrically connected to said other contact and generally aligned with the arc-running surface on said first contact, means for producing an arc between said arc-running surfaces when said contacts are separated, means for producing arc motion in a circumferential direction along said arc-running surfaces comprising a plurality of slots in said flange extending through the entire radial thickness of said flange, each of said slots intersecting the arc-running surface of said first contact and extending from said latter arc-running surface toward the body portion of said first contact via a path disposed at an acute angle to said latter arc-running surface.

9. The interrupter of claim 8 in which each of said slots has a mouth located at the intersection of said slot and the arc-running surface of said first contact, and in which each of said slots extends from its mouth generally circumferentially of said cup-shaped contact at least to a point closely adjacent the angular position of the mouth of an adjacent slot.

10. The interrupter of claim 8 in which said slots have a straight line configuration.

11. The arrangement of claim 8 in which said cupshaped contact includes arc-initiating means for initiating circuit interrupting arcs radially inwardly of said arcrunning surface and arc-impelling means for driving said arc radially outward onto the arc-running surface of said cup-shaped contact, said arc-impelling means comprising a radially-outwardly-acting loop circuit that includes a conductor for carrying current to and from said cupshaped contact member located radially-inwardly of said are initiating means.

12. The arrangement of claim 8 in which said cupshaped contact includes contact-making structure disposed radially inwardly of said flange and extending alongside said slots from the body portion of said contact toward said are running region, insulating means disposed between said contact-making structure and said flange for forcing current flowing to an arc terminal located on said arc-running surface to follow a path through the slotted portion of said flange, and arc-impelling means for driving arcs initiated on said contact-making structure radially outward onto said arc-running surface comprising a radially-outwardly acting loop circuit that includes a conductor for carrying current to and from said contact-making structure located radially inward of said contact-making structure.

13. In an electric circuit interrupter, a pair of co-acting contacts relatively movable from a position of engagement to a position of disengagement to establish a circuit-interrupting arc therebetween, at least one of said contacts comprising a tubular portion projecting toward said other contact, said tubular portion terminating at its free end in a generally annular arc-running surface extending about the periphery of said tubular portion and facing said other contact, an annnular arc-running surface electrically connected to said other contact and generally ali ned with the arc-running surface on said first contact, means for producing an arc between said arc-running surfaces when said contacts are separated, means for producing arc motion in a circumferential direction along said arc-running surfaces comprising a plurality of slots in said tubular portion extending through the entire radial thickness of said tubular portion, each of said slots intersecting the arc-running surface of said first contact and extending from said latter arc-running surface generally longitudinally of said tubular port-ion via a path disposed at an acute angle to said latter arc-running surface.

References Cited in the file of this patent UNITED STATES PATENTS 716,848 Read Dec. 23, 1902 2,140,378 Biermanns et al Dec. 13, 1938 2,421,267 Huber May 27, 1947 2,900,476 Reece Aug. 18, 1959 2,949,520 Schneider Aug. 16, 1960 2,976,382 Lee Mar. 21, 1961 FOREIGN PATENTS 474,185 Great Britain Oct. 27, 1937 580,720 Germany July 15, 1933 556,719 Belgium May 15, 1957 787,846 Great Britain Dec. 18, 1957

Claims

1. IN AN ELECTRIC CIRCUIT INTERRUPTER, A PAIR OF COACTING BUTT CONTACTS RELATIVELY MOVABLE FROM A POSITION OF ENGAGEMENT TO A POSITION OF DISENGAGEMENT TO ESTABLISH A CIRCUITINTERRUPTING ARC THEREBETWEEN, AT LEAST ONE OF SAID CONTACTS BEING A CUP-SHAPED MEMBER COMPRISING A BODY PORTION AND A FLANGE PROJECTING LATERALLY FROM SAID BODY PORTION TOWARD SAID OTHER CONTACT, SAID FLANGE TERMINATING AT ITS FREE END IN A GENERALLY ANNULAR CONTACT-MAKING SURFACE EXTENDING ABOUT THE PERIPHERY OF SAID CUP-SHAPED MEMBER AND FACING SAID OTHER CONTACT, SAID OTHER CONTACT HAVING A CONTACT-MAKING SURFACE GENERALLY ALIGNED WITH THE CONTACTMAKING SURFACE OF SAID FIRST CONTACT AND BUTTING THEREAGAINST WHEN SAID CONTACTS ARE ENGAGED, SAID CONTACT-MAKING SURFACES PARTING LAST DURING A CIRCUIT-INTERRUPTING OPERATION TO INITIATE SAID ARC BETWEEN SAID CONTACT-MAKING SURFACES, MEANS FOR PRODUCING ARC-MOTION IN A CIRCUMFERENTIAL DIRECTION ALONG SAID CONTACT-MAKING SURFACES COMPRISING A PLURALITY OF SLOTS IN SAID FLANGE EXTENDING THROUGH THE ENTIRE RADIAL THICKNESS OF THE FLANGE, EACH OF SAID SLOTS INTERSECTING SAID CONTACT-MAKING SURFACE AND EXTENDING FROM SAID CONTACT-MAKING SURFACE TOWARD SAID BODY PORTION VIA A PATH DISPOSED AT AN ACUTE ANGLE TO SAID CONTACTMAKING SURFACE.