Disposable Circuit Breaker

Abstract

A circuit breaker characterized by relatively movable contacts and operating mechanism therefor together with a bimetal trip element and a latch member releasably restraining the operating mechanism and responsive to the movement of the bimetal trip element; the bimetal trip element deflecting in response to a predetermined range of overcurrent to operate the latch member and cause said member to release the operating mechanism, the bimetal strip element being distortable by thermal stresses when subjected to a current greater than the predetermined range of overcurrent, a fusible link connected in series with the normal current path and being destructible in response to current greater than said predetermined range of overcurrent, and a housing enclosing the foregoing parts which housing comprises separable housing means and destructible fastening means for holding the separable housing members together, whereby the circuit breaker is in a disposable condition upon the subjection of the breaker to a current greater than the predetermined range of overcurrent.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to circuit breakers and more particularly to circuit breakers which are disposable upon being subjected to severe or heavy overload current.

2. Description of the Prior Art

Recently, there has been a need for a small circuit breaker or switch with a higher interrupting capacity than the normal 5000 or 10,000 amperes rating. Heretofore, circuit breakers have been provided with current limiting means which upon subjection to heavy overload currents, such as severe short circuit currents, have been impaired to such an extent that they are unreliable and no longer responsive to the maximum interrupting currents. For example, a circuit breaker of the type involved includes a bimetal trip element which is responsive to currents above a certain predetermined range. When the circuit breaker responds to a slight overcurrent, it is normally reset for continuous use. However, during the occurrence of a severe fault current or short circuit current of a magnitude greater than that which the breaker is designed to accommodate, the bimetal element is stressed thermally beyond its capacity. Thereafter, the circuit breaker may be reset and used but the bimetal element is no longer effective upon the occurrence of overload currents.

Some circuit breakers have been provided with current limiting means, such as a fuse, connected in series with the bimetal element to protect the circuit breaker. U.S. Pat. No. 2,843,702, issued July 15, 1958 to W. H. Edmunds discloses a circuit breaker having a current limiting device and the disadvantage of this construction is that the current limiting device or fuse is readily replaceable by opening the housing in a conventional manner. As a result the circuit breaker is returned to normal use, because the bimetal element appears to be in operative condition. Subsequently, when a current overload occurs, the circuit breaker fails to function because unknown to the user the bimetal element was damaged.

SUMMARY OF THE INVENTION

In accordance with this invention it has been found that the foregoing problem may be overcome by providing a disposable circuit breaker having a fusible link electrically connected in series with the bimetal trip element, which fusible link is responsive to currents greater than a predetermined range of overcurrent to which the bimetal trip element responds; the operating parts of the circuit breaker including the bimetal trip element and the fusible link being enclosed within a destructible housing, whereby an attempt to replace the fusible link results in a damaged housing.

The advantage of the circuit breaker construction of this invention is that it provides a fail-safe unit for interrupting a fault in excess of the rated short circuit currents at very little, if any, extra cost and provides the protection, overload and low short circuit currents, within its interrupting capacity, and can be restored to service.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a circuit breaker, taken on the line I--I of FIG. 2, embodying the principles of the invention;

FIG. 2 is an end view of the circuit breaker showing the base and cover in tact; and

FIG. 3 is a vertical sectional view taken on the line III--III of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, the circuit breaker comprises generally an enclosing housing including an open sided base or housing member 7 and a cover plate or cooperating housing member 9, both made of molded electrically insulating material, which members are secured together by destructible fastening means such as rivets 11 (FIG. 2) extending through aligned holes 13 in the members. The circuit breaker also comprises a stationary contact 15, a movable contact 17, an operating mechanism 19, a trip device 21, and an arc extinguisher 25.

The stationary contact 15 is rigidly secured to the inner end of a conducting member 22 which as illustrated is integral with a plug-in line terminal connector 23 supported in the base 7 and in the cover plate 9 at one end of the housing. At the opposite end of the base 7 is disposed a conducting strip 29 which, at its inner end, is connected to the trip device 21 and which at its outer end is provided with a load terminal connecting means, such as a screw 31, for connecting the breaker in an electrical circuit.

The movable contact 17 is rigidly secured on the free end of a U-shaped switch arm 33 of conducting material having its legs 35 supported in recesses in the legs 37 of a U-shaped operating lever 39 of molded insulating material. The operating lever 39 is pivotally supported by trunnions 41 molded integrally with the operating lever and mounted in companion openings in the housing members 7 and 9. An overcenter spring 43 is connected under tension between the bight of the switch arm 33 and a releasable carrier member 45 pivoted on a pin 47 supported in the housing members 7 and 9.

The operating lever 39 is provided with an integral handle portion 49 which extends outwardly through an opening 51 in the top of the house 7. The operating lever 39 is also provided with an arcuate portion 53 cooperating with the base 7 and the cover plate 9 to substantially close the opening 51 is all positions of the operating lever. The switch arm 33 is electrically connected by a flexible conductor 55 of stranded wire indirectly to the trip device 21 as is more fully described below.

The switch arm 33 is operated to manually open and close the contacts by operation of the lever 39 which is effected by manipulation of the handle 49. Movement of the handle portion 49 in a clockwise direction carries the pivoted ends of the legs 35 of the switch arm 33 across to the left of the line of force of the operating spring 43 which then biases the switch member to the open position and causes movement of the switch member to the open position with a snap action.

The contacts are manually closed by reverse movement of the operating lever 39. Counterclockwise movement of the operating lever 39 from the "off" to the "on" position moves the upper pivoted ends of the legs 35 of the switch arm 33 across to the right of the line of force of the spring 43 which then acts to close the contacts with a snap action.

The circuit breaker is tripped open after a time delay in response to overload currents below a predetermined value and instantaneously in response to overload currents above the predetermined value, or in response to short circuit currents, by operation of the trip device 21. Operation of the trip device 21 causes release of the carrier member 45, whereupon the operating spring 43 moves the releasable carrier member 45 clockwise about its pivot 47. This moves the line of force of the spring 43 across to the right of the center line and pivot point of the switch arm 33, and the spring then acts to move the switch arm to the open position with a snap action. The movement of the member 45 is arrested by engagement with a projection or wall 57 molded integrally with the base 7 and cover plate 9.

The trip device 21 is generally similar to that shown in U.S. Pat. No. 3,178,535 and comprises a bimetal element 59 the upper end of which is supported on the inner end of the conducting strip 29 and the lower end of which is electrically connected to a flexible conductor 61. The trip device 21 also comprises an armature member 63 which is pivotally supported by means of lateral projections thereon which extend into opposed openings 67 (FIG. 2) in the base 7 and the cover plate 9. As shown in FIG. 1, the armature member 63 extends along the left or high expansion side of the bimetal element 59 and has a latch opening 69 therein into which the latch end 71 of the releasable carrier member 45 extends and is thereby releasably restrained in operative position. A relatively light spring 73 is compressed between a spring seat 75 in the base 7 and a spring guide 77 on the armature member above the lateral projections 65 biases the armature member 63 to the latching position. The spring 73 is closer to the lateral projections 65 than is the lower end portion of the armature member, so that a smaller magnetic force can overcome the spring 73 and trip the breaker. A stop portion 79 at the top of the armature member 63 engages the housing to fix the latching position of the armature member and the maximum latch overlap between the latch end 71 and the edge of the opening 69.

A portion 81 of the lower end of the armature member 63 is formed substantially at a right angle to the vertical portion of the member and extends to the right below the lower end of the bimetal element 59. An upturned end portion 83 extends above the lower end portion of the element 59 as shown in FIG. 1.

The trip device 21 also comprises electromagnetic means including a channel shaped magnetic member 85 which is mounted on the bimetal element 59. With normal rated current flowing through the bimetal element 59, the magnetic member 85 is not sufficiently energized to attract the armature member 63. However, where an overload current occurs above the rated current but below a predetermined value of, for instance 1000 percent of the normal rated current, the magnetic member is sufficiently energized to pull the armature member 63 toward the bimetal element, whereupon the operating mechanism functions in the previously described manner to automatically open the contacts, such as disclosed in U.S. Pat. No. 2,568,423. In order to prevent current from passing from the bimetal element 59 and the magnetic member 85, a projection 87 of insulating material is mounted on the magnetic member as shown in FIG. 1.

In accordance with this invention, the circuit breaker is provided with a fusible link generally indicated at 89 which comprises a conductor 91 such as a fuse wire of a size calibrated to withstand the rating of the circuit breaker. The conductor 91 is disposed between the flexible conductors 55 and 61, is contained within a molded wall portion 93 of the base 7, and provides a channel 95 of suitable configuration such as U-shaped. The channel is packed with sand 97 around the conductor when a current overload of sufficient magnitude occurs to "blow" the fuse. It is noted that the wall portions 93 enclosing the fusible link are molded completely around the link with only the opposite end portions of the conductor 91 extending through the molded wall portions for connection with the flexible conductors 55 and 61. Accordingly, when a current overload of sufficient magnitude destroys the fusible link 89, the link is so inaccessible that the replacement or repair of the conductor is virtually impossible and the circuit breaker is no longer usable and must be replaced.

The fusible link 89 is connected electrically in series with the normal current path between the flexible conductor 55 and the bimetal element 59. More particularly, the normal current path includes the plug-in terminal connector 23, the conducting member 22, the contacts 15 and 17, the switch arm 33, the flexible conductor 55, the fuse link 89, the flexible conductor 61, the bimetal element 59, and the conducting strip 29 on which the terminal connecting screw 31 is mounted. The rating of the fusible link 89 is such that the breaker is fast enough to protect the link up to the short circuit rating of the breaker. Beyond that point, the fusible link will interrupt the circuit. The magnetic member 85 has a low enough rating that the breaker will trip when the fuse link blows, thus giving the effect of a double break.

The presence of the fusible link 89 provides all the advantages of the breaker and a fuse. The disadvantage is the additional cost of building the fuse link and replacing the whole unit should the fuse blow. The primary reason for disposing of the circuit breaker when the fusible link is destroyed, is that when an overload current of a magnitude sufficient to blow the fusible link, enough current "leaks through" to over heat the bimetal element 59 and induce sufficient thermal stresses beyond its normal capacity and the bimetal element thereby loses its deflection characteristics. It is not readily evident if at all to the user that the bimetal element is effectively destroyed. Accordingly, when the fusible link is destroyed, the circuit breaker should be disposed of because the bimetal element 59 will no longer function to open the circuit when the low persistent overload current occurs.

Means are provided for calibrating the trip device and comprises a screw 99 threaded through a nut 101 disposed in a recess 103 in the adjacent end wall of the base. The inner end of the screw 99 engages the upper end of the conducting strip 29 on which the bimetal element 59 is mounted. An access opening 105 is provided through which the screw 99 may be rotated to adjust the trip device after which the opening 105 may be sealed to prevent tampering. Turning the screw 99 causes bending of the conductor 29 and corresponding variation of the position of the bimetal element 59 according to the direction in which the screw is rotated to thereby vary the thermal tripping point of the breaker.

Upon the occurrence of a low persistent overload current below a predetermined value of, for example, ten times normal rated current, the bimetal element 59 is heated and bends toward the right. The releasable carrier member 45 then causes movement of the switch arm 33 to the open contact position in the previously described manner.

When an overload current above a value such as, for example, 1000 percent or more of normal rated current, or a short circuit current occurs, the electromagnetic trip means is energized and the armature member 63 is attracted toward the bimetal element 59 causing instantaneous release of the carrier means 45 and opening of the contacts. This action takes place independently of the bimetal element 59 which does not have to be bent and may be relatively stiff.

The releasable means 45 is reset and relatched and the contacts closed following an automatic opening operation by first moving the handle 49 clockwise to the "off" position and then counterclockwise to the "on" position. When the handle is moved to the "off" position, the legs 37 of the operating lever 39 engage a pin 107 in the releasable carrier member 45 and moves it counterclockwise about its pivot 47 until the latch end 71 thereof is opposite the latch opening 69 in the armature member 63 at which time, the spring 73 returns the armature to latching position.

An alternate construction for the fusible link 89 is to provide a shunt wire of a specific size for each rating of circuit breakers which wire would fuse and interrupt the circuit beyond a value of current that would effectively destroy the bimetal element. This eliminates an undesirable deficiency of prior art circuit breakers. After a short circuit in excess of the latter value of current, such circuit breakers may function properly in certain respects, such as being manually operable. However, unknown to the user, the bimetal element loses its deflection characteristics or is damaged to a point where there would be no thermal tripping. Thus, the weak link fuses and opens the circuit thereby providing an additional safety feature in the present invention. More particularly, the flexible wire has a reduced cross-sectional area in a specific location to cause fusing to take place in a predetermined area.

The alternate construction comprises all of the features of the circuit breaker and the additional advantage that the circuit breaker is a fail-safe unit in the event that it is called upon to interrupt a fault in excess of rated short circuits. Although that is attained at little of any extra cost, it does not include the current limiting ability of the fuse link built into the breaker.

Accordingly, the device of the present invention satisfies certain problems of prior art circuit breakers and provides for a fusible link integrally built into a circuit breaker. Finally, the circuit breaker disclosed herein is so constructed that an attempt to repair or replace the fuse requires the destroying of the destructible fastening means which hold the housing members together.

Claims

1. A disposable circuit breaker comprising relatively movable contact means and operating mechanism therefor, a trip device comprising a bimetal trip element having one end fixedly mounted and heated in response to the current of the circuit, the trip device also comprising a latch member releasably restraining the operating mechanism and responsive to the movement of the bimetal trip element, the bimetal trip element deflecting in response to a predetermined range of overcurrent to operate the latch member and cause the latch member to release the operating mechanism, the bimetal trip element being distortable by thermal stresses when subjected to a current greater than the predetermined range of overcurrent, a fusible link connected in series with the normal current path and being destructible in response to current greater than said predetermined range of overcurrent, a housing enclosing the foregoing parts and comprising separable housing members and destructible fastening means for holding the housing members together, the housing comprising molded wall portions forming a channel in which the fusible link is disposed, and opposite end portions of the fusible link extending through said molded wall portions.

2. The disposable circuit breaker of claim 1 in which the housing members

3. The disposable circuit breaker of claim 2 in which the destructible fastening means comprise fasteners that are removable only by being

4. The disposable circuit breaker of claim 3 in which the fasteners are

5. The disposable circuit breaker of claim 1 in which the fusible link is a

6. The disposable circuit breaker of claim 1 in which the fusible link is a

7. The disposable circuit breaker of claim 1 in which the fusible link is

8. The disposable circuit breaker of claim 1 in which the channel is packed with sand around the fusible link.