US3171922A

US3171922A - Circuit breaker with releasable cam type mechanism

Info

Publication number: US3171922A
Application number: US241304A
Authority: US
Inventors: Kenneth J Stokes
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-11-30
Filing date: 1962-11-30
Publication date: 1965-03-02
Anticipated expiration: 1982-03-02
Also published as: NL301190A; GB1002965A; ES293799A1; LU44823A1; DE1463253A1; BE640606A

Description

K. J. STOKES March 2, 1965 CIRCUIT BREAKER WITH RELEASABLE CAM TYPE MECHANISM Filed Nov. 30, 1962 3 Sheets-Sheet 1 INVENTOR. BY KENNETH uisroxss ATTORNEY March 2, 1965 K. J. STOKES 1,

CIRCUIT BREAKER WITH RELEASABLE CAM TYPE MECHANISM Filed Nov. 30, 1962 5 Sheets-Sheet 2 II 'III/IIII/IIII.

mmvron 4 KENNETH d. Sroxes A TTORNE Y March 2, 1965 K. J. STOKES 3,171,922

. CIRCUIT BREAKER WITH RELEASABLE CAM TYPE MECHANISM Filed Nov. so, 1962 s Sheets-Sheet z INVENTOR. KENNETH J. STOKES BY FIG. 6 -Q A TTORNEY United States Patent 3,171,922 CIRCUTT BREAKER WITH RELEASABLE CAM TYPE MECHANHSM Kenneth J. Stokes, Wethersfield, Conn., assignor to General Electric Company, a corporation of New York Filed Nov. 30, 1962, Ser. No. 241,304 6 Claims. (Cl. 200-116) My invention rel-ates to electric circuit breakers, and

more particlularly to electric circuit breakers of the type .including at least one movable contact and over-center spring operating means for manually and automatically operating the movable contact between open and closed circuit positions.

.In accordance with the prior art, a particular type of operating mechanism has been widely used for manually and automatically operating movable contacts between open and closed circuit positions in electric circuit breakers of medium and high capacity, such for example as 100 to 1200 amperes. This type of mechanism incorporates a pivotally supported contact arm and means for operating the contact arm between open and closed circuit positions, comprising a pair of toggle links interconnected between the movable contact arm and a releasable support member. The releasable support member is normally held in a position such that when the toggle links are in straightened condition, the movable contact arm is held in closed circuit position and when the toggle links are in collapsed condition, the contact arm is in an open circuit position. Upon release of the support member or cradle, the action of the over-center spring is such as to cause opening movement of the contact arm even through the operating handle is manually held in the closed or on position.

An electric circuit breaker of the type described is shown and described, for example, in Patent No. 2,921,- 169, Judd et al., issued January 12, 1960 and assigned to the same assignee as the present invention.

With the greatly increasing utilization of electric power, there has been for some years a continuing need for increasing the normal current carrying capacity of such circuit breakers and also for increasing their short circuit interrupting capacity, all without substantially increasing the overall size of the devices. The normal current carrying capacity of such devices, however, is largely a factor of the amount of contact pressure which can be maintained on the contacts while in their closed position. Also, the short circuit interrupting capacity of such devices is substantially affected by their speed of automatic opening. The amount of contact pressure which can be supplied, however, is largely determined by the kind of operating springs which can be included, and this in turn is limited by the amount of space available for such springs. Speed of contact opening, likewise, depends partly on the nature of the operating springs which can be provided, and also on the rate of possible movement of the component parts of the mechanism. Thus, for example, a finite amount of time is required for each part of such a mechanism to move, and therefore, a limit is reached as to possible speed of movement, regardless of strength of operating springs.

It is an object of the present invention to provide an electric circuit breaker including at least one movable contact and operating means for moving the contact between open and closed circuit positions which mechanism is extremely fast acting during automatic opening.

It is another object of the invention to provide an electric circuit breaker of the type described including an operating mechanism which makes possible the use of operating springs of higher strength then heretofore possible.

It is a further object of the invention to provide an 3,171,922 Patented Mar. 2, 1965 electric circuit breaker of the type described including an operating mechanism which is simple and less expensive than comparable prior art operating mechanisms.

It is another object of the invention to provide an electric circuit breaker including an operating mechanism which is easier to assemble than prior art mechanisms.

In accordance with the invention in one form, an electric circuit breaker is provided of the type including a pivotally supported movable contact arm having an operating link pivotally connected thereto. A releasable support member is also provided, which is normally latched and which is releasable upon the occurrence of predetermined current conditions. The releasable member is provided with an arcuate cam surface, and the link connected to the movable contact arm is provided with a cam follower, preferably in the form of a roller disposed and arranged to travel along the aforesaid cam surface. One or more tension springs are also provided connected between the cam follower or roller and the movable portion of a manually operable member. The manually operable member is carried on a handle support which is pivotally supported on a fixed pivot in the enclosing casing.

Other objects of the invention will in part be pointed out, and will in part become apparent from the following detailed description, and the scope of the invention will be pointed out in the appended claims.

In the drawings,

FIGURE 1 is a side elevation view of an electric circuit breaker incorporating the invention, the side wall being broken away, and the parts being shown in the normal open circuit positions;

FIGURE 2 is a side elevation view of the operating mechanism portion of the circuit breaker of FIGURE 1, the parts being shown in the normal closed or on position;

FIGURE 3 is a view similar to FIGURE 2, the parts being shown in the automatically opened or tripped condition;

FEGURE 4 is a fragmentary sectional view taken substantially on the line 44 of FIGURE 2;

FIGURE 5 is an exploded perspective view of a portion of the operating mechanism of FIGURE 1, and

FIGURE 6 is a side elevation view of another embodiment of the invention.

Referring to the drawings, the invention is shown as incorporated in an electric circuit breaker comprising an insulating casing including a base 19 and a cover 11. The casing is generally rectangular and includes three line terminals, not shown, at one end, and three load terminals, not shown, at the other end. Electrical continuity between each pair of corresponding line and load terminals is controlled by a pair of contacts including a relatively movable contact 12 and a relatively stationary contact 13, only one pair shown. The current path provided between each pair of line and load terminals is referred to conventionally as a pole, and the circuit breaker illustrated is therefore ia three-pole circuit breaker.

Each of the movable contacts 12 is carried by a movable contact support 14, which is pivotally connected to a generally channel-shaped contact arm 15 by a pivot pin 16. The forward end of the contact support 14 has a post 14A extending through an opening in the contact arm 15. A compression spring 14A is positioned on the post 14A between the support 14 and the contact arm 15. The contact support 14 has a flexible conductor 14C connected thereto adjacent the pivot pin 16 by suitable means, such as by brazing. The contact arm 15 for the central pole space is illustrated in FIGURE 1, and is pivotally supported on pivot pin 17 which in turn is supported between two spaced-apart side plate portions 18 of a stationary frame assembly. The contact arm 15 3 is generally channel-shaped throughout its major portion, and the

pins

16 and 17 pass through depending side portions of the channel-shaped portion.

The contact arm for each pair of line and load terminals is anchored by suitable fastening means to a contact cross-arm 21. Movement of the center contact arm 15 about its pivot point 17 in the frame 18 therefore carries with it the two outside contact arms, not shown.

For the purpose of operating the movable contact arms 15, together with their associated contact support members 14 and movable contacts 12 between open and closed circuit positions manually, and to open circuit position automatically, operating mechanism is provided at the center pole, including a pair of operating links 24 which extend through slots 15A in the contact arm 15 and are pivotally connected to the contact arm 15 by means of pivot pin 16. The pin 16 is trapped in position by the side plates 18 as illustrated in FIGURE 4. At their upper ends, the links 24 are interconnected by a pin 25 which carries a generally spool-shaped roller 26 generally centrally thereof. The links 24 are retained on the pin 25 by suitable means such as by snap-rings 27 engaging in grooves 28 of the pin 25.

A releasable cam member 30 is also provided which is rigidly attached or may be made integral with a supporting shaft 31 which is pivotally supported in bearings or holes in the upper portion of the slide plates 18. The releasable cam member 30 also includes a latch portion 32 at the end thereof opposite the shaft 31. The latch 32 is normally engaged and releasably held by a latching or restraining member 33, which is controlled by suitable current-responsive means, to be described. The releasable cam member 30 also includes an elongated convex arcuate cam surface 34, on which the roller 26 is adapted to ride. The roller 26 also includes a pair of flange portions 26A disposed and arranged to maintain the roller 26 in proper relation to the cam surface 34 of the releasable member 30.

The latch member 33 is supported on a pin 45 pivotally journalled in a bracket 46 which is rigidly attached to the insulating casing 47 of a trip unit 48. The bracket 46 also carries a pivot pin 49 which supports an intermediate latch member 50. The latch member 50 carries a latch roller 51 at an intermediate point, and a latch surface 52 at an end point thereof. The latch surface 52 is releasably engaged and held by a latch member 53 carried by a trip bar 54 pivotally supported at 55 in the trip unit casing. The trip bar 54 is biased counterclockwise as viewed against a suitable stop means, not shown, by compression spring 56, and carries a calibrating screw 57. The latch member 33 is biased counterclockwise, as viewed, by a torsion type spring 58, which also serves to bias the intermediate latch member counterclockwise as viewed.

In operation, it will be observed that since the latch portion 32 of the releasable cam member 30 is biased upwardly in FIGURE 1, it exerts a clockwise bias on the restraining latch member 33. The latch member 33, in turn, engages roller 51 carried by the intermediate latch member 50, and thereby biases the intermediate latch member 50 clockwise about its pivot 49, urging the latch member 52 into engagement with the latch member 53 carried by the trip bar 54. Under normal operating conditions, the parts remain in this condition, the member 32 being held by the latch mechanism described. Upon rotation of the trip bar 54 clockwise as viewed, however, the latch member 53 is withdrawn from the latch 52, permitting the intermediate latch member 50 to rotate clockwise, and permitting the latch member 33 to also rotate clockwise, releasing the latch portion 32'of the cam member 30.

For the purpose of rotating the trip bar 54 so as to cause release as described, upon the occurrence of predetermined current conditions, a bimetallic strip 60 is provided. The bimetallic strip 60 is rigidly supported on a conducting strap 61 which extends between the terminal 62 and the flexible conductor 14C, being connected to the conductor 14C by means of the bolt 63. The conductor 61 has a constricted cross-section at the point where the bimetallic strip 613 is mounted thereon. The passage of current therethrough therefore causes heating of the conductor 61, and such heat is transmitted to the bimetallic strip 6t) by conduction, causing the bimetallic strip to warp so as to move the free end thereof to the right as viewed to engage the calibrating screw 57 and rotate the trip bar 54 as previously described.

A generally U-shaped handle support member 35 is provided, which is pivotally supported at its end portions on support pins 36 carried by the side plates 18 respectively (see FIG. 4). The bight portion of the handle support member 35 includes a downwardly bent portion 37 which is adapted to engage a portion 38 of the releasable cam member 30, during resetting of the mechanism, in a manner to be described. A manually engageable handle member 39 is also provided mounted on the handle support member 35 and including a manually engageable part extending outwardly through an opening 46 in the casing cover 11, and an enlarged skirt portion having closely fitting engagement with the bight portion of the U-shaped handle support 35. Suitable fastening means, such as screws, not shown, are also preferably utilized to attach the insulating handle member 39 to the handle support member 35.

A pair of tension type operating springs 40 are also provided, one on each side of the releasable cam member 30. The springs 40 are connected at one end, by means of retaining pins 41, to the handle support member 35, and are connected at the other end to the pin 25, on opposite sides of the cam follower roller 26, respectively. The operating springs 40 bias the cam follower roller 26 in the general direction of the center of the curvature of the cam surface 34.

The cam surface 34 is designed so that in the off position of the operating handle as shown in FIGURE 1, the tension springs 40 act on the roller 26, in such a way as to have a resultant force which urges the cam follower roller 26 to the right as viewed in FIGURE 1, thereby retaining the contact arm 15 and the associated contact arms of the other poles carried by the contact cross-arm 21, in the off or open-circuit position.

Conversely, when the operating handle is in the on position and the cam member 33 is in latched position, as shown in FIGURE 2, the tension springs 40 act on the roller 26 in such a way as to have a resultant force which urges the cam follower roller 26 to the left as viewed in FIGURE 1, thereby retaining the contact arm 15 in the on or closed-circuit position.

In operation, the manually engageable handle member is moved from the off position as shown in FIGURE 1 to the on position as shown in FIGURE 2. As the handle member 39 is moved toward the left, or toward the on position, there is no motion of the roller 26 along the cam surface 34 until the angle of the springs 40 with respect to the cam surface 34 is such as to cause such movement. This occurs when the handle 39 has travelled part way toward the full on position. At that time the roller 26 moves along the cam surface 34 quickly with a snap-action, and the parts then move to the condition shown in FIGURE 2. In this position, travel of the roller 26 to the left and the links 24 to the left is limited by the limitation to the travel of the contact arm 15 presented by the compression spring 14A and, in addition, by the engagement of the link 26 with the portion of the bight of the contact arm 15 which forms one end of each of the elongated slots or apertures 15A through which the links 24 extend. Movement of the handle 39 back toward off position reverses the action such described. During on to off operation also, there is no motion of the roller 26 along the came surface 34 until the line 1) of action of the springs 46) reaches a predetermined intermediate point, thereby storing energy in the spring which is suddenly released when the line of action of the spring reaches a certain angle. The parts thereupon return to the condition shown in FIGURE 1. Motion of the contact arms '15 in the open-circuit direction is limited by suitable means, such as by engagement of the contact arm with the shaft 31 or the portionof the releasable cam member 39 adjacent the shaft 31.

Tripping operation When the parts in the condition illustrated in FIGURE 2, releasing movement of the restraining latch member 33 release the right hand end or latch portion 32 of the releasable cam member 30. The effect of the tension springs 40 acting on the pin and through the pin 25 on the cam member is to exert a force thereon in an upwardly direction, tending to rotate the cam member 30 counter clockwise at all times. When the latch portion 32 of the cam member 30 is released, therefore, the member 30 retates counterclockwise to the positioniillustrated in FIG- URE 3, regardless of whether the handle 39 is held in the on position.

When the parts are in the condition shown in FIGURE 3, the links 24 are in locked relation withrespect to the arm 15, being in engagement with the portion 15B forming one end of the aperture 15A through which the links 24 extend respectively. The stop for automatic opening motion of the system comprises the engagement of the contact arm 15 with the shaft 31 or with the portion of the releasable member 32, closely adjacent the shaft 31. In the position shown in FIGURE 3, therefore, there is no appreciable force exerted on the releasable member 30, :and it is essentially free-floating between the roller 26 and the handle support member 35. For example, if the handle support member 35 is held in the on position, as shown in dotted lines in FIGURE 3, the releasable cam member 30 will remain in the position shown due to the action of gravity thereon. If the handle member 39 is not held in the on position when tripping occurs, the handle will be moved to an intermediate or trip indicating position as indicated in FIGURE 3. This action occurs because the roller 26 carries thepin 25 to the right as viewed, so that the line of action of the springs lies to the right of the pivotal support 36 of the handle support member 35. The pull of the springs 40, thereafter, on the handle support member 35, is therefore such as to tend to cause clockwise rotation of the handle support member 35. The handle support member 35 therefore moves to the right as viewed in FIGURE 3 until the depending portion 37 engages the portion 38 of the cam member 30. The handle therefore remains in an intermediate or trip indicating position following automatic opening.

In order to reset and reclose the circuit :breaker, the handle 39 is moved to the full off position, that is, to the extreme clockwise position, as shown in FIGURE 1. This causes the handle support member .35 to move the releasable member 30 clockwise by reason of the engagement of part 37 of the handle support 35 with portion 38 of the member 30, until the latch portion 32 is once again engaged under and held by the retaining member 33. The handle may thereafter be moved back toward on position, to reclose the circuit breaker in the manner previously described.

It will be observed that, as compared to prior art circuit I breaker mechanisms of the toggle type which include an upper pair of toggle links and a lower pairof toggle links, interconnected by a knee pivot (see for example the aforementioned Patent 2,921,169), the mechanism of the present invention has several important advantages. By reference to FIGURE 4, it will be observed that more room is afforded for the operating springs 40. These springs must of necessity be positioned between the releasable member 30 and the sides of the U-shaped handle support member 35. .In accordance with this invention, since there are no toggle :links which must also extend within this space, this full space is available for :the operating springs, and the diameter of the springs may therefore be increased. This in .turn permits the use of springs having a higher .force without unduly increasing the stresses exerted on the spring.

Ithas also been found that the mechanism of the present invention is more easily assembled than the type which incorporates a toggle-type construction. This is not only because these .parts may be omitted without the necessity of substituting other parts therefore, but also because in accordance with the present invention, .the movable contact ar-m assembly is no longer positively connected to the releasable :member. The fact that this connection therefore need not be made, simplifies the assembly. In addition, since the contact arm is not connected to the releasable member, it becomes easier-to assemble the operating springs 40 between the pin 25 and the handle support member 35.

A further unexpected advantage has been found by tests to be afforded by the novel mechanism of the present invention. It has been discovered that the speed of contact opening under short circuit conditions is higher with the present mechanism than with a mechanism of the type including a link-connected contact arm and releasable member. It is believed that this increased speed of opening is due to the fact that there are fewer moving parts and friction producing bearing surfaces. Such increased speed of opening is especially important when interrupting direct current, since the amount of power generated in the arc, which must be dissipated during the arc interruption process is directly proportional to the length of'time that the arc is in existence.

In FIGURE 6, there is shown another embodiment of the invention, in which a releasable member is provided which includes a stop projection integral with the releasable cam member 130. The other par-ts of the mechanism are otherwise similar to the form previously described. The projection 145 acts as a stop for the travel of the Links 24 in the contact closing direction.

Since the releasable member 130 is an accurately-made piece, manufactured by either stamping or by forging and machining, the exact location of the stop 145 is readily assured by this construction and can be more accurately controlled than when a portion of the contact arm 15 is used as such a stop as shown in the previous form. Operation of this form of the invention is otherwise similar to that previously described.

While the invention has been shown and described in only two specific embodiments, it will be readily appreciated that many modifications thereof may readily be made by those skilled in the art, and it is therefore intended by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

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

1. An electric circuit breaker comprising:

(a) an insulating enclosure,

(b) at least one pair of relatively movable contacts supportedin said insulating enclosure,

(c) a releasable cam member pivotally supported in said insulating casing and including a convex cam surface and a latch portion,

(d) said cam member being movable from a normal latched position to a released position.

(e) current responsive means carried by said enclosure and normally engaging said latch portion of said cam member and restraining said cam member in said latched position, said current responsive means being operable in response to predetermined current conditions through said contactsto release. said cam member,

(f) a cam follower member supported in said enclosure and movable along said cam surface between open and closed circuit positions,

(g) means connecting said cam follower member and said contacts for movement of said contacts between open and closed circuit positions in response to movement of said cam follower member between said open and closed circuit positions respectively while said cam member is restrained in said latched position,

(It) a manually operable member supported in said enclosure and movable between open and closed circuit positions,

(i) operating spring means connected between a point of said manually operable member and a point of said cam follower member and moving said cam follower member between open and closed circuit positions in response to movement of said manually operable member between open and closed circuit positions,

(j) said operating spring means acting on said cam follower member when said cam member is in said latched position and said cam [follower member is in said open circuit position to urge said cam follower member in a direction to move said relatively movable contacts toward open circuit position, and said operating spring means acting on said cam follower member when said cam member is in said latched position and said cam follower member is in said closed circuit position to urge said cam follower member in a direction to cause closing of said relatively movable contacts,

(k) said operating spring means acting on said cam member when said cam member is released by said current responsive means to rotate said cam member so as to shift said cam surface with respect to said cam follower and with respect to the line of action of said spring means to cause said spring means to move said cam follower along said cam surface in a direction to cause opening of said relatively movable contacts regardless of whether said manually operable member is forcibly restrained in said closed circuit position, and

(I) said operating spring means biasing said cam follower member toward said point of connection of said operating spring means to said manually operable member at all times.

2. An electric circuit breaker as set forth in claim 1 wherein operating spring means comprises at least one tension type spring having one end thereof connected to said point of said manually operable member and having the other end thereof connected to said point of said cam follower member.

3. An electric circuit breaker as set forth in claim 1 wherein said operating spring means biases said cam follower member in the general direction of the center of curvature of said convex cam surface.

4. An electric circuit breaker as set forth in claim 1 wherein said point of connection of said operating spring means to said manually operable member and said point of connection or" said operating spring means to said cam follower member lie on opposite sides of said cam sur face, and said operating spring means comprises at least one tension type operating spring means connected between said points.

.5. An electric circuit breaker comprising:

(a) a generally rectangular insulating enclosure having opposed side and top and bottom walls,

(b) at least one pair of relatively movable contacts supported in said enclosure, 1 (c) at least one elongated contact arm pivotally supported in said enclosure for movement toward and away from said bottom wall and operably connected liO operate said movable contacts between closed and (open circuit positions respectively, 5 ,releasabie cam member pivotally supported in 8 said enclosure between said contact arm and said top wall and having a cam surface and a latch portion,

(e) a manually operable handle member pivotally supported in said insulating casing for movement about an axis extending perpendicular to said side walls and including a portion projecting through said top wall, said handle being movable between on and off positions,

(f) a cam follower member supported in contact with said cam surface of said releasable cam member and movable therealong between on and off positions,

(g) at least one connecting link connecting said cam follower member .to said elongated contact arm,

(11) at least one tension type operating spring connecting said manually operable member to said cam follower member, said tension spring urging said handle member toward said back wall and urging said cam follower member upwardly toward said top wall into engagement with said cam surface, whereby to urge said cam member for rotation about its pivotal support upwardly toward said top surface,

(i) current responsive means normally releasably latching said latch portion of said cam member against movement toward said top wall,

(i) said operating spring moving overcenter with re-' spect to said axis of said handle as said cam follower moves from said off to said on position and vice versa,

(k) said cam member rotating about its pivotal support in a direction to move said latch portion toward said top wall of said encloure upon release of said cam member by said cu-nrent responsive means, when said cam follower is in said on position, whereby to cause said operating spring to move said cam follower member along said cam surface in a direction to move said contact arm toward said open circuit position irrespective of the maintenance of said handle in said on position.

6. An electric circuit breaker comprising:

(a) an insulating enclosure,

(b) at least one pair of relatively movable contacts supported in said insulating enclosure,

(0) a releasable cam member pivotally supported in said insulating enclosure and including a convex cam surface and a latch portion,

(d) said cam member being movable from a normal latched position to a released position,

(e) current responsive means carried by said enclosure and normally engaging said latch portion of said cam member and restraining said cam member in said latched position, said current responsive means being operable in response to predetermined current conditions through said contacts to release said cam member,

(I) a cam follower member supported in said enclosure and movable along said cam surface between open and closed circuit positions,

(g) means connecting said cam follower member and said contacts for movement of said contacts between open and closed circuit positions in response to movement of said cam follower member between said open and closed and circuit positions respectively while said cam member is restrained in said latched position,

(h) a manually operable member supported in said enclosure and movable between open and closed circuit positions,

(i) a pair of tension type operating springs each connected between said manually operable member and said cam [follower member on opposite sides of said releasable cam member, said operating springs moving said cam follower member between open and closed circuit positions in response to movement of said manually operable member between open and closed circuit positions,

(i) said operating springs acting on said cam follower member when said cam member is in said latched position and said cam follower member is in said open circuit position to urge said cam follower member in a direction to move said relatively movable contacts toward open circuit position, and said operating springs acting on said cam follower member when said cam member is in said latched position and said cam follower member is in said closed circuit position to urge said cam follower member in a direciton to cause closing of said relatively movable contacts,

(k) said operating springs acting on said cam member when said cam member is released by said current responsive means to rotate said cam member so as to shift said cam surface with respect to said cam follower and with respect to the line of action of said springs to cause said springs to move said cam 1501- 20 lower along said cam surface in a direction to cause 10 opening of said relatively movable contacts regardless of whether said manually operable member is forcibly restrained in said closed circuit position, and (I) said operating springs biasing said cam follower member toward said manually operable member at all times.

References Cited by the Examiner UNITED STATES PATENTS 2,124,413 7/38 Frank et a1. 200-116 2,174,232 9/39 Baker 200116 2,214,695 9/40 Jennings 200116 2,287,875 6/42 Grober 2001l6 2,613,296 10/52 Wood 200--116 2,892,127 6/59 Leonard 20088 2,921,169 1/60 Judd et a1. 200153 2,955,177 10/ 60 Brackett 200116 3,003,046 10/61 De Torre 200116 BERNARD A. GILHEANY, Primary Examiner.

Claims

1. AN ELECTRIC CIRCUIT BREAKER COMPRISING: (A) AN INSULATING ENCLOSURE, (B) AT LEAST ONE PAIR OF RELATIVELY MOVABLE CONTACTS SUPPORTED IN SAID INSULATING ENCLOSURE, (C) A RELEASABLE CAM MEMBER PIVOTALLY SUPPORTED IN SAID INSULATING CASING AND INCLUDING A CONVEX CAM SURFACE AND A LATCH PORTION, (D) SAID CAM MEMBER BEING MOVABLE FROM A NORMAL LATCHED POSITION TO A RELEASED POSITION. (E) CURRENT RESPONSIVE MEANS CARRIED BY SAID ENCLOSURE AND NORMALLY ENGAGING SAID LATCH PORTION OF SAID CAM MEMBER AND RESTRAINING SAID CAM MEMBER IN SAID LATCHED POSITION, SAID CURRENT RESPONSIVE MEANS BEING OPERABLE IN RESPONSE TO PREDETERMINED CURRENT CONDITIONS THROUGH SAID CONTACTS TO RELEASE SAID CAM MEMBER, (F) A CAM FOLLOWER MEMBER SUPPORTED IN SAID ENCLOSURE AND MOVABLE ALONG SAID CAM SURFACE BETWEEN OPEN AND CLOSED CIRCUIT POSITIONS, (G) MEANS CONNECTING SAID CAM FOLLOWER MEMBER AND SAID CONTACTS FOR MOVEMENT OF SAID CONTACTS BETWEEN OPEN AND CLOSED CIRCUIT POSITIONS IN RESPONSE TO MOVEMENT OF SAID CAM FOLLOWER MEMBER BETWEEN SAID OPEN AND CLOSED CIRCUIT POSITIONS RESPECTIVELY WHILE SAID CAM MEMBER IS RESTRAINED IN SAID LATCHED POSITION, (H) A MANUALLY OPERABLE MEMBER SUPPORTED IN SAID ENCLOSURE AND MOVABLE BETWEEN OPEN AND CLOSED CIRCUIT POSITIONS, (I) OPERATING SPRING MEANS CONNECTED BETWEEN A POINT OF SAID MANUALLY OPERABLE MEMBER AND A POINT OF SAID CAM FOLLOWER MEMBER AND MOVING SAID CAM FOLLOWER MEMBER BETWEEN OPEN AND CLOSED CIRCUIT POSITIONS IN RESPONSE TO MOVEMENT OF SAID MANUALLY OPERABLE MEMBER BETWEEN OPEN AND CLOSED CIRCUIT POSITIONS, (J) SAID OPERATING SPRING MEANS ACTING ON SAID CAM FOLLOWER MEMBER WHEN SAID CAM MEMBER IS IN SAID LATCHED POSITION AND SAID CAM FOLLOWER MEMBER IS IN SAID OPEN CIRCUIT POSITION TO URGE SAID CAM FOLLOWER MEMBER IN A DIRECTION TO MOVE SAID CAM FOLMOVABLE CONTACTS TOWARD OPEN CIRCUIT POSITION, AND SAID OPERATING SPRING MEANS ACTING ON SAID CAM FOLLOWER MEMBER WHEN SAID CAM MEMBER IS IN SAID LATCHED POSITION AND SAID CAM FOLLOWER MEMBER IS IN SAID CLOSED CIRCUIT POSITION TO URGE SAID CAM FOLLOWER MEMBER IN A DIRECTION TO CAUSE CLOSING OF SAID RELATIVELY MOVABLE CONTACTS, (K) SAID OPERATING SPRING MEANS ACTING ON SAID CAM MEMBER WHEN SAID CAM MEMBER IS RELEASED BY SAID CURRENT RESPONSIVE MEANS TO ROTATE SAID CAM MEMBER SO AS TO SHIFT SAID CAM SURFACE WITH RESPECT TO SAID CAM FOLLOWER AND WITH RESPECT TO THE LINE OF ACTION OF SAID SPRING MEANS TO CAUSE SAID SRPING MEANS TO MOVE SAID CAM FOLLOWER ALONG SAID CAM SURFACE IN A DIRECTION TO CAUSE OPENING OF SAID RELATIVE MOVABLE CONTACTS REGARDLESS OF WHETHER SAID MANUALLY OPERABLE MEMBER IS FORCIBLY RESTRAINED IN SAID CLOSED CIRCUIT POSITION, AND (L) SAID OPERATING SPRING MEANS BIASING SAID CAM FOLLOWER MEMBER TOWARD SAID POINT OF CONNECTION OF SAID OPERATING SPRING MEANS TO SAID MANUALLY OPERABLE MEMBER AT ALL TIMES.