US2167665A - Circuit breaker - Google Patents

Description

- Aug. 1, 1939.

G. A. MATTHEWS CIRCUIT BREAKER Filed Nv. 5, 19:54

3 Sheets-Sheet 1 a 1, 93 G. A. MATTHEWS 2,167,665

CIRCUIT BREAKER 7 Filed Nov. 5, 19 34 3 Sheets-Sheet 2 J? 65 3i 6 "Q; I b 6 1 5 'l 156 lll 5i V 36 W O I l 7 1, 1939? cs. A. MATTHEWS 2,167,665

- CIRCUIT BREAKER:

- Fiied Nov. 5. 1954 s Sheets-Sheet s M gg.

Patented Aug. 1, 193 9 PATENT OFFICE 2,167,865 omcm'r BREAKER George A. Matthews, Detroit, Mien, miraclto The Detroit Edison Company, Detroit, Mich; a corporation of New York Application November 5, 1934, Serial No. 751,511 13 Claims. (Cl. 200-149) 10 carrying an arc is conducting because it is ionized;

that is, because a considerable number of normally electrically neutralized molecules are broken up into electrically charged parts called ions. If the-arc is to be extinguished and, if the gas is to ll recover its normal insulating state, these ions must disappear. Left to itself, the ions, in the ionized gas disappear spontaneously by direct v recombination. This process is relatively slow and'is of little practical use in the interruption 7 20 of an alternating current circuit.

It has been found in this connection that a very I effective means for increasing the rate of deionization, and hence the more rapid extinguishment of the arc, is to bring the surfaces of Solids 25 close to the arc in a confined space so that ions may collect thereon andv recombine there.-

Moreover, there seems to be a great difference p in the effect of these surfaces, depending upon whether the surfaces are refractory, or whether 30 under the heat of the arc, the surfaces are decomposed giving ofilarge quantities of gas. In

the latter case, under proper conditions much higher voltage and higher power circuits can be interrupted. The increased effectiveness of de- 35 composing walls is believed to be due to the rapid evolution of gas; and the turbulence introduced into the confined are 'by the rapid intermixing of freshly generated un-ionized gas is believed to be an extremely effective means for causing rapid 40 deionization at current zero.

Tests have shown that, if horn fiber or other organic matter is placed adjacent the are, it will be decomposed; giving ofi relatively large percentages of hydrogen and carbon monoxide, and a 5 small percentage of hydrocarbons, which .are particularly eflicacious in the rapid deionization of the arc and hence its rapid extinguishment.

A great many electrical and mechanical devices have been tried with more or less success to em- 50 body the foregoing principles in circuit breakers for interrupting circuits under load. In the main, these devices have been too costly to permit being used in any but the most expensive installations.

With the foregoing in mind, the present inven- 55 tion seeks to provide in a circuit breaker an economical construction embodying the above principies, and which is so arranged as to not only perform the function of a circuit breaker to open a circuit under load, but may also be used as a disconnecting switch to effect a visual gap in the 5 circuit.

A further object of the invention resides in the provision of an improved circuit breaker of the dry-type, thereby lessening explosive and fire hazards to life and property. I

It is a further object of the invention to provide a circuit breaker or simple and relatively inexpensive construction and at the same time a circuit breaker having excellent arc extinguishing characteristics. Y

A further object of the invention is to provide a circuit breaker having parts which are arranged and operative in such a; manner as to enable the production of a circuit breaker of mate- I rially smaller physical size forv a given capacity, than has heretofore been possible.

It is also an object of the hereindescribed invention to provide a circuit breaker of such construction and arrangement of parts as to enable its being embodied into a plurality of arrangements, such for example, as a single or multiple break circuit breaker, and also in various combinations with fuses, etc.

In accordance with the general features of the present invention, it is proposed to provide an elongated tube of organic material such as horn fiber within which there is mounted a. contact rod of such size as to provide a space between the exterior surface of the rod and the interior surface of the tube. Contacts are respectively associated with the contact rod and the tube so that when the rod is moved axially relative to the tube, the contacts may be moved into or out of engagement. There is also provided an extension to the contact rod, this extension being similarly formed of organic'material such as'horn fiber. The extension is of such lengthv that when the rod is moved in a direction to separate the contacts, the extension serves as a follower to maintain a constant predetermined cross-sectional area in the spacewithin the tube betweenthe follower and the inner wall of the tube. The contacts just mentioned are mounted adjacent oneend ofthe fiber tube so that the resistance to the movement of the arc gases formed by the separation of the contacts would be greater in one direction than it would be 'in the other direction as they move through the space between the tube and the rod or extension thereon. Due to the fact that there is less resistance to movement of the arc gases toward the follower or extension of the rod, the arc will move in that direction and in so doing will be in contact with the organic material of the tube and also the organic material of the follower or rod extension. Under the heat of the arc, the fiber or other organic material will be decomposed and emit un-ionized gases which will act to assist deionization of the arc and hence its extinguishment. The turbulence caused by the rapid generation of these gases in the confined space also contributes to this result.

As an additional feature of the invention, the rod extension may be extended into a closed container or chamber in communication with the associated end of the fiber tube. Due to the movement of the rod extension, in opening the contacts, the extension acts as a piston and tends to create decreasedpressurewithin the chamber, this decreased pressure operating to draw the are through the space between the rod extension and the fiber tube, in a direction toward the. chamher. This chamber also serves as a relief for the expansion of the gases which have been emitted irom the decomposition, of the material in the tube and contact rod extension.

Other objects and features of this invention will more fully appear from the following detail description taken in connection with the accompanying drawings, which illustrate several embodiments thereof, and in which:

Figure 1 is an enlarged fragmentary vertical sectional view showing the contacts and associated parts of a circuit breaker embodying the principles of the present invention;

Figure 2 is an enlarged transverse fragmentary sectional view of the same, taken substantially on line IIII of Figure 1;

Figure 3 is an enlarged fragmentary vertical sectional view showing the construction of one of 'the terminals of a single break circuit breaker embodying the present invention;

Figure 4 is a view in elevation showing a modifled arrangement 01' the present invention, wherein the circuit breaker is mounted in combination with a fuse;

Figure 5 is a side view of the same;

Figure 6 is another modified arrangement showing a single break circuit breaker of the hereindescribed type mounted so as to form a hinged switch blade which may be opened to effect a visible break in the circuit;

Figure 7 is an enlarged fragmentary detail plan view showing the upper contact of the switch blade;

Figure 8 is another modified arrangement wherein the present invention is embodied in a double break circuit breaker;

Figure 9 is an alternative arrangement of Figure 8, to efiect a visible break in the circuit;

Figure 10 is a detail view showing the contact rod utilized in connection with the circuit breaker of Figures 8 and 9; and

Figure 11 is a fragmentary view partly in crosssection showing a modified lower end for the contact rod to facilitate improved operation of the circuit breaker.

As shown on the drawings:

Referring to the drawings, a preferred form of the present invention is shown in Figures 1, 2 and 3 as a single break circuit breaker. This form of the invention comprises in general a contact assembly, generally indicated at A for initially interrupting the power circuit; and a terminal construction as generally indicated at B for completing the interruption of the electrical circuit after the load interrupting contacts have opened. v

Thecontact assembly A comprises a casting I! having one end of a cylindrical'tube ii of organic material such as horn fiber supported therein. At one side, the casting I5 is shaped to define a chamber H which houses a'portion of the contact mechanism. One wall of the chamber I1 is in the form of aplate or cover "Which may be secured in position in any appropriate manner, but is preferably removably secured to enable access to'the contact mechanism. This wall is centrally provided with a screw stud I! having a shank portion 20 which serves as a guide for a contact casting 2| having a stem portion 22 which is centrally apertured at 23 to receive the shank 20 therein and permit axial movement of the contact casting on the shank.

The contact casting 2! is mechanically and electrically connected to the wall I. by a pinrality of flexible spring connectors 24 of approximately horseshoe shape. The inner face of the contact casting has a cylindrical'extenslon 25, preferably of circular cross-section, axially alined with the stem 22 and slidably received in a lateral opening 26 in the wall of the tube It. A contact member 21 of any appropriate material is secured, by soldering or the like, to the inner end of the extension 25 and is normally pressed towards the center of the tube 16 by a spring 24 that surrounds the stem 22 and bears against the wall I! and the outer face of the contact casting 2|.

Diametrically opposite the opening 2.,is a similar opening 29a in the tube I for receiving a contact member 30 which is axially movable relative to the opening 29a by an adjusting screw 3| to which it is secured and which is in threaded engagement with the casting ll. I

Within the tube l6, there is disposed a rod 32 of conducting material. At the upper end of this rod is an axially extending bolt 33 threaded at both, ends, one end of this bolt being secured in the upper end of rod 32 and the other end oi this bolt extending into and threadedly engaging an elongated member 34 of fibrous or other organic material, which forms an extension for the rod 32. "Clampingly secured between the extension 34 and the rod 32 by means of the bolt 33 is an annular contact member 35 which may also be soldered to the rod 32 in order to reduce the electrical resistance of the joint. It will be noted that the rod 32, contact member I! and the extension 34 are of the same diameter so as to virtually form a continuous rod, .and that the diameter is such that a limited clearance or annular shaped space is secured between the outer surface of the rod assembly and the inner surface of the tube Hi.

The extension 34 is of such length that when the contact 35 is engaged by the contacts 21 and 30. the extension extends beyond the upper end of the tube ii. The upper end of this extension may be of frusto-conical shape as shown in Figure 1. It will be noted that the end faces of the contact members 21 and 3| are not arcuate to conform to the cylindrical surface of the movable concentrically surrounding the extension 24 tend to straighten out with a temperature rice. I I

and forming an extension of the tube I is an exteriorly threaded tubular member 35 of metallic construction which is secured into the casting l5 as by welding at 31 and provided with a plurality of slots 36a at its outer, end. The member 36 is closed by means of a cap 33 so as to form a' chamber 39 which is in communication with the clearance space between the rod assembly and the interior surface of the tube IS. The cap 38 is of larger diameter than the tube 36 and at its lower end is in threaded engagement with the exteriorly threaded surface of the tube .36, this arrangement enabling variation of the size of the chamber 39 in a given size unit to meet different operating conditions. While it has been found'that the ratio of the total volume of chamber 39 to the volume displaced by the rod 34, when the breaker is in the fully closed position, may be varied without marked effect on the performance of the breaker, it is preferable to use a ratio lying between 2 to 1 and 3 to 1 for breakers that are to be used on a circuit carryf ing up to approximately 1200 amperes at 5000 1 small size willquickly extinguish the arc.

volts. The metallic wall of this chamber will, by quickly condensing the gases, serve to dissipate the heat of the arc.

In the arrangement described above, it will be apparent that in order to open the contacts, the rod assembly may be moved downwardly and that during this movement the extension 34 serves as a follower to maintain the arcing space, that is, the space between .the rod assembly and the interior surface of the tube H5 at a constant value. Likewise, as the extension 34 moves out of the chamber 39, there is a decrease in the pressure within this chamber which acts to draw the are caused by the breaking of the contacts into the arcing space where it is confined between surfaces of fibrous or other organic material. Although I have disclosed this means of causing the arc to travel upwardly relative to the tube I6 from the contacts, the arc would travel in this direction even though there were no decrease of pressure in the chamber 39 as would be the case when the tubular member is open or not provided with a cap 33. This movement of the arc would take place for the reason that the length of the arcing space belowthe contacts is of greater extent than the arcing space above the contacts, in which case the latter would offer less resistance to the arc and the arc would naturally take this course.

As indicated above, the invention provides a dry type of circuit breaker which may be used in place of oil switcher and circuit breakers to open transmission lines carrying heavy currents at high voltages. It is impossible to prevent arcing when contacts are separated to interrupt current flow of the order of hundreds of amperes in, for example, a 5000 volt transmission line, butcircult breakers of the described .costruction and of The surfaces of the tube and follower which are subjected to the arc will be decomposed and emit gases which are effective in setting up a violent turbulence and in causing deionization of the arc and hence its extin'guishment.

Immediately below the contact 21, there is pro- 'vided in the wall of the tube IS a passageway M which connects the arcing space with the chamber 11. Where metal contacts are used, any

globules of metal which are formed when the contacts are separated under load, will be driven through this passageway into the otherwise closed. chamber 11 where they may solidify without interfering with the movement of the rod assembly.

Line connections, it will be obvious, may be made to any portion of the casting 15. In the present instance I have shown the plate 18 being extended to form a terminal generally indicated at 4|. The electrical circuit would therefore be from the terminal 4| to the plate 18, the flexible connector 24 to the contact casting 2|, and also through the adjusting screw 31 to contact and thence from the contacts 21 and 30 to contact 35 and the rod 32.

The load breaking contact unit is easily embodied in a variety of arrangements. In the preferred and'more simple arrangement of the invention, the circuit is completed, as shown in Figure 3, through the terminal B. This terminal comprises a cap 42 which is threadedly secured to the lower end of tube l6 and carries a lug 43 to facilitate making a line connection thereto. This cap is centrally apertured as shown at 44 so as to enable the rod 32 to extend therethrough. Spring contacts 45 are secured at one end in the cap 42 and are so arranged that the free ends thereof frictionally engage the rod 32 to thereby complete the electrical circuit from the rod 32 to the cap 42 and lug 43, which is connected to the line.

The rod 32 may be actuated by anyappropriate power means or, as disclosed herein, the rod may be fitted at its lower end with a casting 46 having a portion defining .an opening 41 for receiving an operating hook, whereby the rod may be manually operated. The casting 46 is also shaped at its upper end to form a socket 48 to receive the free ends of the contact springs 45. The sides of this socket closely engage the sides of the springs 45 and serve to press them firmly against the contact rod 32, thus preventingarcing. Moreover, the contact springs due to their abutment with the bottom of the socket 43 also serve as a stop to limit the upward'movement of the rod assembly. If desired, a shield 49 may be disposed around the contact springs and supported from the cap 42 in any appropriate manner.

fuse 50 of the removable hinged type. The lower 3 end of the fuse element is provided with a casting 51 which forms a terminal for engaging with contact clips 52. This terminal also forms one of the'hinge members and is provided with trunnions 53 which are adapted to be removably supported in spaced bracket members 54-54. For effecting a removable support for the fuse, the brackets are respectively provided with openings 55 which are in communication with a U-shaped slot so that the trunnions 53 may be inserted in the openings 55 and moved into the U-shaped slot.

The upper end of the fuse carries a terminal 56 which is adapted to engage spring clips 51 sup- 90 having one end fixedly secured by a nut andbolt connection 9| to the insulator. This bolt and nut connection also secures a terminal lug front of the casting 63, whereby the operator. 4c

that the trunnions 53 62 by means of which aline connection may be made to the fuse element.

For disengaging the latch spring 69 so that the fuse maybe swung out of contact with the spring contact clip 51, there is provided a casting 63 which is pivoted at and adapted to be engaged by an operating hook. This casting has an arm 65 which extends beneath the latch spring so that when the casting is engaged by the operating hook and a downward force is exerted, the arm'65will raise the latch spring 60 so as to release the fuse and enable the operator to swing it out of contact with the spring clip 51. If desired, the fuse, after it has beenopened, may be totally. disengaged by moving it upwardly so are released from the bracket 54. 1

The lower terminal of the fuse is electrically connected through a strip of current conducting material 66 to the casting l5 of the circuit breaker so that the complete circuit through the device would be from lug 62 through contacts 51, the fuse, contacts 52, connector 66, casting l5, contacts 2'! and 30, contacts 35, and thence through rod 32 (Figure 1), spring contacts 45 to lug 43.

In the foregoing arrangement, it would be very undesirable to open the fuse until after the circuit breaker contacts have been actuated to open the load circuit. In order to prevent improper sequence of operation, the circuit breaker and fuse are interlocked so as to make it compulsory to first open the circuit breaker before it is possible to open the fuse.

This is accomplished by mounting a barrier or shield 67 for pivotal movement about a pivot member 68 supported at the upper end of the circuit breaker on tube 38. This barrier is so disposed that when in lowered position it lies in cannot insert the hook therein to disengage the latch 80 and thereby enable the fuse to be swung to open position.

It will be observed that the upper end of the tube 36 is, in this instance, left open so as to enable the extension 34 of the rod assembly to project outwardly thereof when the circuit breaker is closed. In this design the tube 36 is an integral extension of the tube 56, and in order to produce, in this open ended design, somewhat the same gas confining effect as is produced in the closed-end design, the clearance between the rod 3% and tube 36 is made as small as the clearance between the rod 32 and the tube iii. For automatically moving the barrier to effective and ineffective positions in response to the closing and opening of the circuit breaker, an arm 69 is secured to the barrier so as to extend at substantially right angles thereto and is so disposed as to be engaged by the extension 34 when the contacts of the circuit breaker are closed. Engagemerit-of this arm by the extension forces the bar-' rier to assume effective position to prevent opening of the fuse by the operator. Assuming that the circuit breaker is opened by pulling down on the rod assembly, the extension 34 is, moved downwardly and enables the rotation of the arm 69 and barrier 61 in a counter-clockwise directionunder the influence of a spring 10 to the position shown in dotted lines in Figure 4 In this position of the barrier or shield, the fuse may be swung to open position. 7

Another 'modification of the preferred-form of the hereindescribed invention is disclosed in Figure 6, wherein the circuit breaker disclosed 15 in Figures 1,2 and 3 is hingedly mounted for operation as a switch blade so as to form a. visible break in the circuit. modification, an interlocking arrangement is provided which prevents opening the blade until the circuit breaker has been opened. Corresponding parts have been indicated by like numerals as heretofore referred to in the description. The circuit breaker is hinged at its lower 'end by providing a pivot H on the cap 42, this cap being identical with the cap shown in Figure 3, except for the addition of the pivot. The entire assembly is made removable by supporting the pivot ends in aligned slots Ha-Ha of a bracket 12 which is secured in the usual manner to an insulating support 73. The bracket 12 also serves as an electrical connection from the cap 42 to the associated circuit connection lead. The cast- Moreover in this ing l5 instead of being provided with a terminal I the rod assembly, when swinging, the circuit 0 breaker about the pivot H to disengage the contacts i4 and it. On the other hand, it will be seen that when the rod assembly is pulled downwardly to open the circuit breaker and is entirely removed, the circuit breaker is then free to be swung to form a visible break in the circuit. This movement is accomplished by inserting a switch hook in an opening 19 which may be provided in the casting 55. Q

In Figures 8 and 9, the invention is shown as being embodied in a modified form covering a double break circuit breaker. In this arrangement, two of the load breaking contact assemblies A-A are utilized. The construction of each of the contact assemblies is the same as in the single break circuit breaker and each functions in the same manner as previously described in connection with the description of the preferred form of the invention. The only difference is that in this arrangement, the rod as disclosed in Figure 10 is of fibrous material which is made up of an end section as, a central section BI and an upper end section 82 which are separated by contact elements 83 and 84. The sections and contacts are assembled to form in effect a single rod, a. bolt 85 extending through the central section and threadedly engaging at one end contact 84 and section 82 and at its other end contact 83 and section 80. It will therefore be seen that this rod also performs the additional function of forming an electrical connection between the contacts Hand 84. The electrical circuit in this contact assembly to contact 84, throughthe rod 85, contact 83, and thence to the casting l5 of the lowermost contact assembly A.

As shown in Figure 9 this modification may alternatively arranged to form a visible break case would be from the casting I5 of the upper by the simple expedient of forming an opening I or in other words discontinuing a portio'rTof'tli tube l5 lying between the upper and lower contact unit. In the alternative arrangement, when the rod is entirely withdrawn, a visible break is farmed between the contact units.

:In Figure 11, an alternative arrangement is shown for the member to be engaged by the switch hook in opening the breaker. In this arrangement instead of having a rigid connection with the rod 32, the sleeve 86 is provided within which there is reciprocably mounted a pull member 81 which is arranged at its lower end with an opening 88 for engagement by a switch hook. The upper end of the member 81 is connected 'to one end of a coil spring 89 which is anchored at its other end to the rod 32. This arrangement,

moved with slower movement to withdraw the same from the breaker. This arrangement has been found to be much more effective than where the rod is initially moved by a direct pull with the switch hook.

From actual tests which have been made upon circuit breakers embodying the hereindescribed invention, it has been found that the best results are attained when the difference between the outside diameter of the rod 32 and the interlor diameter of the tube ii are between of an inch and a; of an inch. With the lower limit, small metal particles thrown oil from the con tact under the influence of the arc will not be suiliciently large to cause the rod to jam. Of course, most of these particles will be ejected through the opening 40, but some of them may remain in the space between the rod and the surrounding tube. If the limit is made greater than a l of an inch there will be a tendency for the ionized gases to be discharged through the lower end of the tube It.

From the foregoing description, it will be apparent that this invention provides a novel circuit breaker of the dry type, which has excellent arc extinguishing characteristics, which may also be utilized not only as a circuit breaker but as a disconnecting device; which is economical to construct and which may be embodied in a variety of combinations.

Now, it is of course to be understood that although I have described in detail several embodiments of my invention, the invention is not to be thus limited but only insofar as defined by the scope and spirit of the appended claims.

I claim as my invention:

1. A circuit breaker comprising a tubular member of organic material, a rod of similar material mounted for longitudinal movement within said member and having its surface inwardly.

spaced from the inner wall of said member to ,form an arc-extinguishing chamber, cooperative electrical contacts respectively associated with said member and rod and adapted to open and close an electrical circuit in response to the movement of said rod, and means responsive to a circuit-opening movement of said rod for decreasing pressure at one end of said tube.

2. A circuit breaker of the character described comprising a cylindrical chamber including tubular member of organic material adapted to emit an arc extinguishing medium under the influence of anelectric ate, a metallic rod mounted for longitudinal movement within said tubular member, cooperable contacts respectively. associated with said rod. and member, the

contact associated with the tubular member being located intermediate the ends of said chamber, and a smooth walled cylindrical element of material similar to that forming the tubular member extending from and movable simultaneously with the rod, said element being spaced from the inner surface of said tubular member, whereby an are formed by the separation of said contacts will impinge upon the opposed surfaces of organic material of said tubular member and rodlike element. v l

3. A circuit interrupting device of the character described comprising relatively movable contacts, means contiguous said contacts defining an arcing space of constant volume and adapted under the influence of an arc to emit an arc extinguishing medium, means forming a closed expansion chamber for said medium in communication with said arcing space, and means whereby the volume of said chamber may be varied to adopt the device for different. conditions of operation.

4. A circuit breaker comprising a pair of contacts, means for eil'ecting relative movement of said contacts to engage and alternatively to separate the same, means forming an annular arc extinguishing chamber which extends in opposite directions from said contacts and retains a substantially constant cross-section during relative movement of said contacts, said chamber metal member in axial alinement, arod of insulating material mounted for reciprocating movement in said tubular members, contact means carried by said member of insulating material at the end thereof adjacent the metal member, and contact means on and spaced from the ends of said rod, said rod having a continuous cylindrical outer surface of less diameter than the bore of .said tubular members to provide an .arc extinguishing chamber of annular cross-section.

6. A circuit breaker as claimed in claim 5, wherein said rod has a diameter of the order of 3 2 inch less than the bore of the tubular member.

7. A circuit breaker as claimed in claim 5, wherein said first contact means comprises a metallic casing into which one end of said tubular member is secured, a contact movably supported by said casing and extending into a transverse bore in said tubular member, and spring means pressing said contact towards the interior of the center of said tubular member.

8. In a circuit breaker, a tube of insulating material, alined transverse openings through said tube, contacts mounted in said openings and positioned with respect to the portion oi the contact that is engaged by said contact memher. i i

10. A circuit breaker comprising a tubular member of insulating material, contact means extending into the bore of said member at a region spaced from the ends oi said member, a rod of insulating material extending into said tubular member, contact means on said rod for cooperation with the contact means of said tubular member, said rodbeing of less diameter than the bore of said tubular member and including an extension movable into transverse alinement with the contact means of said tubular member when the rod is displaced to separate the re-.

spective contact means, and a vent openingilatorally through the wall of said tubular member adjacent the contact means thereof, said vent opening being positioned at that side of said contact means towards which the contact means of said rod is moved to separate the respective contact means.

-11. A circuit breaker comprising relatively separable contacts, wall-forming means contiguous said contacts defining an arcing space of constant cross-section extending in opposite di= rections from said contacts when engaged and during separation thereof, and means for positively reducing the pressure at one end of said space upon the separation of said contacts to elongate the resultant arc and extend the same into said space, said wall-forming means comprising material adapted when subjected to an arc to evolve an arc extinguishing medium.

12. A circuit breaker comprising a tubular member of insulating material capable of vaporizing to produce an arc-extinguishing medium,

terminal contact means spaced from the ends of mg section having a smooth cylindrical wall spaced from the inner wall of said member by an annular gap having a radial length of the order of a inch. t

13. A circuit breaker as claimed in claim 12, 1

wherein said tubularmember has a restricted opening extending laterally through the wall thereof adjacent said terminal contact means and at the side thereof where an arc is formed when the electrical connection is broken.

GEORGE A. BEAmi-IEVVS.

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