US3265837A

US3265837A - Electric circuit breaker provide with an improved terminal strap for mounting a bimetal element

Info

Publication number: US3265837A
Application number: US405758A
Authority: US
Inventors: Keith W Klein
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1964-10-22
Filing date: 1964-10-22
Publication date: 1966-08-09
Anticipated expiration: 1983-08-09
Also published as: ES318419A1; DE1513166A1; GB1088698A

Description

Aug. 9, 1966 K. w. KLEIN 3,265,837

ELECTRIC CIRCUIT BREAKER PROVIDED WITH AN IMPROVED TERMINAL STRAP FOR MOUNTING A BIMETAL ELEMENT Filed 001;. 22, 1964 INVENTOR.

A TTORNE Y United States Patent ()flice 3,265,837 Patented August 9, 1966 3,265,837 ELECTRIC CIRCUIT BREAKER PRGVIDED WITH AN IMPROVED TERMENAL STRAP FOR MOUNT- ING A BIMETAL ELEMENT Keith W. Klein, Sinisbury, Conn, assignor to General Electric Company, a corporation of New York Filed Oct. 22, 19M, Ser. No. 405,758 1 Claim. ((31. 200-116) My invention relates to electric circuit breakers, and more particularly to electric circuit breakers of the type including a terminal member having a first portion adapted to receive an external electrical conductor for connection therewith, and a second portion supporting a current responsive element, such as an elongated bimetallic strip.

In electric circuit breakers of the type including thermal current responsive means for causing automatic opening of the circuit breaker, it is common practice to support the current responsive means on the inner end of a terminal strip member, the outer end of which also serves to make connection to an external conduct-or or wire of the circuit to be protected. Calibration of the circuit breaker is accomplished by adjusting the position of the bimetallic strip. This is done by adjusting a screw which acts on the inner end of the terminal strap and bends the end wi respect to the fixed portion of the strap.

The labor involved in the calibration of such circuit breakers at the time they are manufactured constitutes a significant part of the cost of manufacturing such circuit breakers. Prior circuit breaker designs of the type described have exhibited certain disadvantageous characteristics as regards the calibration process. Thus, for example, after such circuit breakers have been calibrated it is sometimes found that the adjustment or calibration of the breaker changes after a few hours or days. Such calibration drift is believed to be due primarily to the mounting of the aforesaid terminal strap. For example, such terminal straps are ordinarily anchored to the insulating casing of the circuit breaker by means of a clamping screw which stresses the insulating material, ordinarily in compression, as well as stressing the material of the screw itself. In particular, the insulating material directly under the head of the screw is placed under extremely high compressive force such, for example, as 5,000 pounds per square inch. Although generally accepted standard grades of insulating material, such as a general purpose phenolic molding compound, are used for such casings, it is often found that because of the high pressures exerted by such a mounting screw, the material changes its dimensions with the lapse of time. Since the adjustment of the calibration of the circuit breaker is accomplished by bend ing a portion of the terminal strap, the change in dimension of the casing material at the point of the mounting screw causes a change in the stresses in the terminal strap, with a corresponding slight change in the position of the adjusted end.

A second major problem presented by circuit breakers of the prior art in this connection has to do with changes of such calibration after the circuit breaker has been put into use. In the process of connecting an electrical conductor to the outer end of the terminal strap which supports the bimetallic strip or other current responsive member, it has been found that pressures exerted on the clamping screw and on the end of the terminal strap during the process of connecting the conductor may bodily move or bend the terminal strap, causing changes in the position of the inner end and therefore destroying the accuracy of calibration. A second source of difiiculty is that after a conductor has been connected to the outer end of the terminal strap, forces exerted on the conductor, such as by bending it to force it into position in an enclosing box along with other wires, etc., substantial forces are also exerted on the terminal strap which may cause slight changes in its position and therefore destroy the accuracy of calibration.

It is an object of the present invention to provide an electric circuit breaker of the type described which includes a terminal strap supporting a current responsive element on its inner end and having wire connecting means at its outer end which shall maintain the calibration as set at the time of manufacture and which shall not be adversely affected by minor changes in the dimensions of the insulating casing or by forces exerted on the wire connecting means.

It is an object of the invention to provide an electric circuit breaker of the type described including common means for supporting a current responsive member and a wire connecting member which shall not be adversely affected by-minor changes in the dimensions of the insulating casing of the circuit breaker.

It is another object of the invention to provide an electric circuit breaker of the type described including means supporting a current responsive member and a wire connecting means by means of which adjustment of the current responsive means shall not be readily affected by forces exerted on the wires connecting means.

In accordance with the invention, an electric circuit breaker is provided including an insulating casing and an elongated terminal strap supported in the insulating casing. An elongated bimetallic strip is rigidly mounted on the inner end of the terminal strap. The intermediate portion of the terminal strap is fixedly anchored to the casing by suitable means such as by a screw. The outer end of the terminal strap carries a wire clamping means. The extreme outer end of the terminal strap is bent upwardly and includes a pair of outwardly-directed lug portions which are received in corresponding pockets or recesses of the insulating casing, thereby securely supporting the outer end of the terminal strap. In addition, the terminal strap is strengthened adjacent the mounting hole at the intermediate portion such as by means of a strengthening rib, and the strap is also weakened at each side of'the central mounting portion, that is between the mounting portion and the end portion which supports the bimetallic strip and also between the mounting portion and the end portion which carries the wire connecting terminal. By this means, stresses which originate in any of the three major areas of the terminal strap are, in eflect, isolated, and prevented from affecting the other portions, and particularly the portion which supports the bimetallic strip.

The invention will be more fully understood from the following detailed description, and its scope will be pointed out in the appended claim.

In the drawing,

FIGURE 1 is a side elevation view of an electric circuit breaker incorporating the invention, one-half of the insulating enclosing case being omitted to show the parts;

FIGURE 2 is an end elevation view of the circuit breaker of FIGURE 1, the enclosing casing being shown complete;

FIGURE 3 is an exploded perspective view of portions of the load terminal supporting strap and wire connecting means;

FIGURE 4 is a perspective view of the wire connecting member of the circuit breaker of FIGURE 1, the clamping screw being omitted;

FIGURE 5 is an end elevation view of the wire connecting means of the circuit breaker of FIGURE 1 shown with a stranded conductor in clamped condition therein; and

FIGURE 6 is a fragmentary side elevation view of a portion of a circuit breaker incorporating another embodiment of the invention.

In the drawing, the invention is shown in FIGURE 1 as incorporated in an electric circuit breaker including an insulating casing 10, comprising a generally tray-shaped body portion A and a generally planar cover 10B (see FIGURE 2).

As shown in FIGURE 1, the parts of the circuit breaker are retained in generally conforming recesses in the body portion 10A, and include a line terminal socket member 11 having an integral extension 11A supporting a relatively stationary contact 12. A movable contact 13 is mounted on a contact arm 14 which in turn is pivotally supported at 15 on an insulating handle member 16.

The handle member 16, which is pivotally supported in the insulating casing at 17, may be manually rotated about its pivotal support to move the pivot point 15 of the contact arm 14 back and forth across the line of action of a tension type spring 18. The spring 18 has one end connected to the contact arm 14 and the other end connected to an intermediate point of a releasable support member 19. This causes the contact arm 14 to move between open circuit position as shown in full lines in FIGURE 1 and closed circuit position as shown in dotted lines in FIGURE 1, with a snap action. The tension spring 18 exerts a force downwardly as viewed in FIGURE 1 on the intermediate portion of the releasable member 19, tending to rotate it clockwise about its pivotal support 20 in the insulating casing. The releasable member 19 is normally prevented from rotating clockwise, however, by engagement of a latchable portion 21 thereof with a latch projection 22 carried by a combined armature-latch member 23.

The armature-latch member 23 is rotated about its pivotal support 24 in the insulating casing by the movable end portion 25 of a current responsive member comprising an elongated bimetallic strip 26. The armature-latch member 23 is biased toward latching position at all times by means of a compression spring 27 acting between the end portion of the member 23 and the insulating casing, as shown in FIGURE 1. The contact arm 14 is connected to the movable end 25 of the bimetallic strip 26 by means of a flexible conductor or braid 28.

On the occurrence of extremely high overloads or short-circuit currents, the magnetic piece 29 carried by an intermediate portion of the bimetallic strip- 26 is energized by the flux associated with current passing through the bimetallic strip and causes the pole portions thereof to attract the armature-latch member 23 to cause tripping. The strip 26 is supported in the casing 10' by having one end thereof rigidly attached by suitable means such as by welding to a first end 30 of an elongated terminal strap 31. The terminal strap 31 is anchored to a wall portion 10C of the casing body 10A by means of a screw 32. The outer end of the strap 31 carries a generally tubular wire connecting member 34 having a clamping screw 35 threadedly engaged therein.

The terminal strap 31 includes an end portion 37 which is turned up at right angles to the adjacent portion of the strap and which includes outwardly directed l'ug portions 38 which are received in correspondingly shaped pockets or recess 39 (see FIGURE 3) in the body 10A and the cover 10B of the insulating casing 10 respectively.

The lower end of screw 35 bears on the outer end portion of strap 31. As the screw 35 is turned inwardly, the tubular member 34 rides upwardly with respect to the strap 31, guided in the recess 34A and clamps a conductor between the bottom wall of the tubular member and the under surface of the strap 31, as shown particularly in FIGURE 5. In addition to its function of more rigidly supporting the end of the strap 31, the bent-up end portion 37 serves to prevent the insertion of a conductor on the wrong side (i.e. at the top) of the strap 31.

The tubular member 34 is constructed as shown particularly'at FIGURES 4 and 5 and comprising a single piece wrapped around to form a tubular body, with the Current responsive element mounting and operation As previously noted, the bimetallic strip 26 is rigidly attached to the end 30 of the terminal strap 31 by suitable means such as by welding. Passage of current through the bimetallic strip 26 causes it to warp so as to move the movable end thereof to the right as viewed in FIGURE 1, which in turn engages the return bent portion 23A of the armature-latch member 23 and rotates the armature-latch member 23 about its pivotal support 24 and disengages the latch projection 22 from the end 21 of the releasable member 19. The calibration or adjustment of the circuit breaker which determines the amount of current required to cause release of the latch 22 or tripping is determined by the positioning of the bimetallic strip 26.

In accordance with the invention, the terminal strap 31 which supports the bimetallic strip 26 is constructed and mounted in such a way as to greatly reduce the likelihood of any changes of position of the bimetallic strip occurring such as to affect the calibration of the circuit breaker after initial adjustments. For this purpose, the strap 31 is provided with a rectangular hole or cut-out 45 adjacent the end 30. See FIGURE 3. In addition, a second rectangular hole or cut-out 46 is provided between the intermediate portion 42 of the strap 31 and the end portion 43, as shown in FIGURE 3. In addition, the central poriton 42 is strengthened by the provision of an embossed rib 47 therein extending through the portion thereof in which the tapped hole 48 is provided for the screw 32.

The cut-out portion 45, together with the reinforcing embossed rib 47 assures that the bending which occurs when the calibrating screw 40 is turned inwardly against the surface 30A of the end 30, takes place at a predetermined, localized and controllable area of the strap 31. In particular, this structure assures that such bending will not take place adjacent the hole 48 in the intermediate portion 4-2 of the terminal strap. Bending adjacent the hole 48 of the central portion 42 would be undesirable for tfo reasons: (1) the amount of such bending would be difficult to predict and control, and (2) any bending in this area would set up stresses which would be relieved to some extent should there be any loosening of the screw 32, or change in the dimensions of the portion of the casing clamped between the strap 31 and the head of the screw 32.

The cut-out portion 46 similarly assures that forces exerted on the outer end of the strap 31 will not be transmitted to the rest of the strap.

As previously mentioned, the likelihood that forces exerted on the screw 35 in clamping a conductor in place, or forces exerted by such a conductor itself when bending it, etc., is greatly reduced by the bent-up portion 37 having lugs 38 which are received in pockets 3 9. This firmly supports the outermost end of the strap 31 and supports it directly adjacent the part on which most force is likely to be exerted.

If desired, the strap 31 may be weakened for the. purposes indicated by reducing its thickness at the bend area, such as by provision of a notch as shown at 131A in the embodiment of FIGURE 6. This has the advantage of reducing the stiffness of the strap at this point to a greater degree for a given reduction of cross-sectional area than the construction of FIGURE 1. This is because a reduction of thickness reducesthe moment of inertia shown in FIGURES 1-3 is preferred, however, in the application illustrated since it is easier to control to accurate dimensions.

-While the invention has been shown in only one embodiment, it will be readily appreciated that many modifications thereof may readily be made. It is intended, therefore, by the appended claim, 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:

A current responsive device comprising:

(a) -an insulating casing,

('b) an actuating member movably mounted in said insulating casing,

(c) a supporting member fixedly mounted in said insulating casing, said supporting member comprising an elongated strip of metallic material having a first end portion extending within said insulating casing and a second end portion accessible from outside of said insulating casing for connection to an external conductor, a

(d) means fixedly mounting an intermediate portion of said metallic strip on said insulating casing,

(e) an elongated strip of bimetallic material having one end thereof fixedly mounted on said first end portion of said metallic strip within said insulating casing and having its other end in engagement with said actuating member,

( f) a wire connecting member carried by said second end portion of said metallic strip for connecting said metallic strip to an external conductor,

(g) said metallic strip including reinforcing means at said intenmediate portion resisting bending movement of said intermediate portion,

(1]) adjustable positioning means acting between said first end portion of said metallic strip and saidcasing, said positioning means acting on said first end portion of said metallic strip to bend said metallic strip between said first end portion and said intermediate portion to adjustably position said bimetallic strip in said casing,

(i) said cross-section of said metallic strip being reduced between said first end portion and said intermediate portion whereby positioning of said bimetallic strip in said casing is accomplished substantially entirely by bending of said metallic strip between said first end portion and said intermediate portion,

(j) means weakening said metallic strip between said second end portion and said intermediate portion whereby forces exerted on said second end of said metallic strip cause bending of said metallic strip substantially exclusively at said portion between said second end portion and said intermediate portion,

(it) said second end portion of said metallic strip including a terminal portion bent at right angles to the main portion of said end portion, said bent portion including a pair of outwardly directed lugs received in corresponding recesses in said insulating casing and supporting said metallic strip at its outermost end portion.

References Cited by the Examiner UNITED STATES PATENTS 2,328,458 8/ 1943 Jackson et al. 20088 2,908,782 10/ 1959 Kiesel et al 20088 3,073,926 1/1963 Ellsworth et a1 20088 BERNARD A. G ILHEANY, Primary Examiner.