US2416951A - Switching means - Google Patents

Description

March 1947- D. c. PRINCE SWITCHING MEANS Filed Sept. 26, 1942 2 Sheets-Sheet l Inventor:

e C u .m m r r IJ m W v .s a DW uu b March 4, 1947. D. c. PRlNCE SWITCHING MEANS Filed Sept. 26, 1942 2 Sheets-Sheet 2 Inventor: David CPrince,

His Attorney 9 if F: M11146 L a l I M0 w m Patented Mar. 4, 1947 2,418,951 SWITCHING MEANS David 0. Prince, Schenectady, General Electric Company,

New York N. Y., aaaignor to a corporation of Application September 26, 1942, Serial No. 459,792 8 Claims. (01. 200-88) My invention relates to switching means and specifically to an arrangement; in which in a single device are provided all the switching and protective means required for protecting high voltage apparatus.

It is an object of my invention to provide an improved switching means that is compact, electrically efficient, durable and weatherproof, and will interrupt both large and small currents satisfactorily without any external evidence of arcing.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention reference may be had to the accompanying drawing in which Fig. 1 is a curve diagram which illustrates an operating characteristic desired of the switching and protective means of my invention; Fig. 2 is a side elevational view of a switching means or device embodying my invention shown in the closed circuit position; Fig. 3 is a view similar to Fig. 2 showing the switching means in the open circuit position; Fig. 4 is a cross-sectional view of the switching means of Fig. 2; Fig. 5 is an enlarged cross-sectional view of a portion of Fig. 4 showing the switching means in the open position; Fig. 6 is a view taken on line 6-6 of Fig. 5, Fig. '7 is a view taken on line 1-1 of Fig. 5; Fig. 8 is an enlarged perspective view of the latch mechanism of the switching means illustrated in the position shown in Fig. 4, Fig. 9 is a perspective view of the bracket for supporting the latch mechanism of Fig. 8, and Fig. 10 is a perspective view of a portion of a fuse hook which may be used to replace the blown fuse device'used in my invention.

Referring now to Fig. 1 of the drawings, the preferred form of time current characteristic of switching and protective means for most general purposes is of the nature of the curve AB of Fig. 1. With a time current characteristic of this sort the circuit is cleared only after a short time delay in the event of small overloads and this time becomes shorter as the loads increase. This characteristic of a protective device is very desirable since it permits such devices to be connected in series with one another to give selective operation for faults in different locations and furthermore permits the coordination of circuit breakers, fuses, and the like in a large system. For motor operation, particularly, it is necessary to postpone the operation of a protective device such as a circuit breaker or fuse, so as not to interrupt the circuit due to the starting currents of motors which may be several times normal running currents. However, an ideal form of tripping curve or time current characteristic should have a positive current limitation at some point and an ideal form of tripping curve would be that shown by the full lines ABD in Fig. 1. This characteristic has a positive current limitation beginning at the point B. This ideal condition can be accomplished by superimposing the operating characteristic of a current limiting fuse of the type disclosed in United States Letters Patent 2,188,816, assigned to the same assignee as the present application and having the time current characteristic CBD in Fig. 1 upon some device having the time current characteristic AB. My invention is particularly concerned with a new and improved switching and protective means which operates with a time current characteristic represented by the curve ABD in Fig. l, which I accomplish by providing in a single device the features of a high interrupting capacity current limiting fuse, a switch in series with the fuse which necessarily opens when the fuse operates, and means for interrupting overloads of insufllcient magnitude to cause fuse operation. In other words, my invention is concerned with providing in a single device all the switching and protective means ordinarily required in the high voltage circuit of a moderate or low capacity transformer.

Referring now to Figs. 2, 3, and 4 of the drawings, I have illustrated a switching and protective means generally indicated at l0, which is adapted to be mounted on a suitable insulator ll only partially shown. For supporting switching means ill on insulator ll there is provided an adapter l2 suitably bolted to insulator II. To adapter l2 in turn is bolted a supporting member l3 including a clamping portion which cooperates with a cooperating clamp member ll firmly to support the switching and protective means of my invention. Supported by clamp member I4 is a terminal strip I! to which one terminal such as It of an external circuit may be connected.

The switching and protective means III includes an outer tubular housing, or casing i1 preferably formed entirely of or lined with an insulating material from which gas is evolved when subjected to the heat of an electric arc. Preferably tubular housing or casing I! which is mounted in a vertical position is made of an arcresisting material such as fiber or the like. As illustrated in the drawings tubular housing or casing I! is supported between the cooperating clamping portions of members l3 and 4.

Referring specifically to Figs. 4 and 5, the switching and protective device l of my invention includes an inner tube or movable casing of tubular form, generally indicated at l8, which is adapted to move vertically within tubular casing As illustrated in Figs. 4, 5, 6 and 7 of the drawings, inner casing 98 is constructed of a gas evolving insulating material and comprises an upper tubular member We and a lower tubular member I81), the adjacent ends of which are threadedly engaged with a conducting member is having a flange 20 which acts as the movable contact of the switching device it embodying my invention. This conducting member it] also includes an inner integral concentric cup-shaped member We supported by the webs itb best shown in Figs. and 7. When the switching device it is in the closed or current-carrying position indicated in Fig. 4 the flange 2b which will hereinafter he referred to as the movable contact is in engagement with a contact plunger 29 forming a portion of the stationary contact of the switching and protective means it of my invention. The contact plunger 28 is mounted for reciprocal movement in an extension M01 of clamp member I4 near an opening 22 in outer casing I! so that contact plunger 2| may pass through opening 22 to engage movable contact 20. In order to insure good contact engagement between contact plunger 2i and movable contact 20 a suitable biasing means such as spring 23 is provided to bias contact plunger 2| in a direction so as to tend to cause movement thereof into outer tubular casing ll through opening 22. Good electrical connection between contact plunger 2| and terminal strip i5 is insured by means of a flexible conductor 26 interconnecting these members. The electrical connection between the flange 2d and terminal strip i5 may be further improved if desired by means of an adjustable screw contact 25 threadedly mounted as indicated at 26 in support it. A suitable lock nut 21 may also be included. Contact plunger 2i biases flange 28 against screw contact 25.

It will be obvious from the description thus far that without additional means gravity will tend to cause inner casing it to move downwardly relative to outer tubular casing H to cause contact 20 and plunger contact 28 to be separated as indicated in Fig. 5. In order to hold contacts 20 and 2| in engagement as shown'in Fig. 4, it is necessary to provide suitable latching means generally indicated at 36, best shown in Fig. 8, which latching means includes a supporting bracket 3H best shown in Fig. 9. This latching means 30 will be described in greater detail hereinafter.

The upper end of outer tubular casing ll is closed by means of a suitable closure member 32 which is removably fastened to casing H by means of a threaded flange 53. A suitable rod 34 connected to end closure member 32 supports a stationary member 35 which extends a considerable distance into the upper end of inner casing l8. Rather than depend upon gravity to cause downward movement of inner casing ll relative to outer casing I8 I provide a compression spring means 36 mounted between stationary support 35 and the upper end 38a of bracket M which bracket has the lower end Mb threadedly mounted as by screws 37 (see Figs. 5 and 6) to a plurality of the webs 69b of conducting member- I9 which are provided with cooperating threaded openings 38 (see Fig. 7.) The ends talc and 36b of bracket 8| are interconnected by flanges 8|c and 3| d, best shown in Figs. 6 and 9.

The lower end of inner casing I8 is closed by means of a conducting member or contact 39 i which is threadedly engaged with tubular member l8b. Conducting member or contact 39 is electrically connected with conducting member l9 by means of a conductor 40, winding 4|, and conductor 42. The lower end of conductor 42 is clamped to contact 39 by means of a screw 43 threadedly engaging a cup-shaped supporting member and guide 44 which in turn supports a magnetic structure 45 associated with winding 4i. Magnetic structure 45 and winding 4| form a solenoid which, depending upon the energize.- tion thereof, controls the position of a movable armature 46 movable in guide 44. Normally armature 46 is in the lowermost position indicated in Figs. 4 and 5, but upon a predetermined energization of winding 4|, it is moved upwardly, striking flange 41 which acts both as a stop and a guide. Flange 4'! is suitably supported from the magnetic structure 45 as clearly shown in Fig. 5. Conductor 40 which interconnects the upper end of winding 4| with conducting member I9 is fastened to cup-shaped portion |9a of conducting member 9 by means of a flanged member 48 which acts both as a clamp for conductor 40 and also as a guide as will be brought out in greater detail hereinafter. A suitable nut 49 best shown in Figs. 5 and 7, is threadedly engaged with flanged member 48 to insure good electrical connection between conductor 40 and conducting member 09.

As was mentioned above, in order that the switching and protective means of my invention may have the time current characteristic over a portion of the range of operation thereof represented by the curve CBD of Fig. 1, I provide as an essential part thereof a current limiting fuse, generally indicated at 50, and including terminals or ferrules 5! at either end thereof. Preferably this current limiting fuse is of the known type disclosed in United States Letters Patent 2,188,816, mentioned above. In order that fuse 50 may be connected with the switching and protective means it for ready removal and replacement, the contact 39 i provided with a threaded opening 52 for engaging a threaded extension 53 which may be suitably welded or otherwise connected to the upper terminal 5| of current limiting fuse 58. To insure good contact between this upper terminal 56 of current limiting fuse 5d and contact 39, asuitable spring washer 54 may be provided if desired. With this arrangement it will be obvious that ready replacement of fuse 50 upon blowing or rupture thereof can be accomplished. The lower terminal 5i of fuse 50 may be provided with a fitting or contact device 55, best shownlin Figs. 2 to 4,

which is clamped to lower fuse terminal 5| and which makesline contact with a cooperating contact member 56 threadedly connected to the lower end of outer tubular casing I1. Cooperating contact 56 is preferably provided with a movable member 51 and spring 58 to give the desired contact wipe. Contact member 56 is electrically connected with a terminal strip 59 and a flexible conductor 60. Another terminal. such as 5|, of an external circuit may be connected with terminal strip 59 as shown. Fitting or contact device 55 is provided with a looped extension 55' which is adapted to cooperate with the fuse hook shown in Fig. 10, to be described in greater detail hereinafter.

well- Under high current conditions the .circuit through switching means It is interrupted by currentlimiting fuse 58 within a fraction of a cycle, and subsequent release of latching means 88 to permit separation of contacts 20 and 21 will not cause any arcing. On the other hand, under low current conditions which are not of sufilcient magnitude to cause operation of current limiting fuse 58, interruption of the circuit is accomplished by extinguishment of the arc drawn between contacts 28 and 2| which is confined within the annular bore between outer casing 11 and inner casing 18. The close proximity of this arc to casings l1 and I8 will cause gas to be evolved from the insulating material which will aid in the rapid extinguishment of the arc confined within the annular bore. Furthermore the close confinement of this are will cause very rapid cooling thereof, thereby aiding in the circuit interrupting process. Upon relative movement of inner tube 18 and outer tube 11 to the position indicated in Figs. 3 and 5, the current limiting fuse 58 is pushed downwardly to assume the position indicated in Fig. 3. This extended position of current limiting fuse 58 gives a clear visual. indication that switching means has operated and that resetting thereof and perhaps replacement of current limiting fuse 58 is necessary. The current limiting fuse 58 will be provided with a suitable indicator as disclosed in the above mentioned Letters Patent 2,188,816 which will immediately indicate to the operator whether or not replacement of the fuse is necessary. Current limiting fuse 58 is, of course, readily accessible so that the operator may remove it and insert a new fuse unit using a switch or fuse hook such as is shown in Fig. 10 and will be described hereinafter.

The latching means 80 supported by bracket 8| for maintaining contacts 20 and 2| in engagement, is best shown in Figs. 5, 8, and 9. This latch mechanism is supported within inner casing l8 below stationary member 35. The latch means 38 comprises a stationary U-shaped member 53 which is attached by any suitable means to stationary member 35. Bracket 31 is provided with a cooperating opening 64 in portion Ila thereof and openings 65 in portions 3Ic and 31d thereof through which U-shaped member 83 may extend when the switching means 10 is in the closed position indicated in Fig. 4. Pivotally mounted at 86 on flanges 31c and 31d of bracket 8| is a latch member 81 having a recess 81a. which is adapted to cooperate with the closed end of U-shaped member 83 when the circuit breaker is latched closed as shown in Figs. 4 and 8. If counterclockwise rotation of latch member 51 as viewed in Fig. 8 is prevented its recess 51a will engage U- haped member 63 so as to hold inner tube l8 relative to outer tube 11 in the position indicated in Fig. 4 against the pressure of spring means 85. If, on the other hand, rotation of latch member 61 in the counterclockwise direction is permitted, its U-shaped recess 81a loses engagement with U-shaped rod 63 and the inner tube 18 under the force of spring means 88 moves downwardly relative to outer tube 18 thereby permitting separation of contacts 20 and Latch 61 is prevented from rotating in the counterclockwise direction by a notch 51b in the free end thereof which is adapted to be engaged by a trigger member 88 pivotally mounted on shaft 8-8 supported by flanges 81c and 81d of bracket II. Trigger member 88 is provided with a notch 18 which when rotated to the proper position, permits release of latch 81. Normally a torsion spring 11 associated with shaft 69 rotates shaft 88 and trigger 58 to a position which counterclockwise rotation of latch member 81 is caused upon release thereof by rotation of trigger member 68 by bent rods or fingers 12 mounted on trigger member 68 which engage pins 13 on either side of latch member 61, clearly shown in Fig. 8. Torsion spring 11 causes rotation of trigger member 68 to a position which prevents disengagement of \notch 61b of latch member 61 and counterclockwise rotation of shaft 69 is necessary to release latch member 61which counterclockwise rotation of trigger member 88 must be accomplished against the force of torsion spring 11.

In order to operate trigger means 68, a suitable crank 14 is provided at one end of shaft 59 which includes a pin 15 extending through a slot 16 in flange 310 of bracket 31. This pin 15 in turn engages with a slot 11 in a lever arm 18 pivotally mounted at 18 by flanges ale and 31d of bracket 31. Sleeve spacer 19' holds lever 18 in the position indicated in Fig. 8. Normally pin 15 of crank 14 is held in the lower position of notches 16 and 11 as shown in Figs. 5 and 8 by torsion spring .11. If, however, lever 18 is caused to rotate in a clockwise direction as viewed in Fig. 8 about pivot 19, pin 15 is moved upwardly, producing counterclockwise rotation of trigger member 68 to disengage notch 81b of latch member 61 occurs with the consequent separation of contacts 28 and 21. It will be obvious that a very slight motion of trigger member 88 is all that is necessary to accomplish the release of latching means 88.

Latching means 30 may be released in either of two different ways.. The; first way which is the manual releasing method is accomplished by means of a plunger mounted for vertical movement on end closure member 32. Plunger 88 is preferably connected to manually operable lover or eye member 8| which is pivotally mounted on end closure member 82 as at 82. Eye member 81 is constructed so that it may be readily operated by pulling down thereon by means of a switch stick having a hook' which may be inserted through the opening in the eye member 81. Rod 80 is preferably adjustably secured'to eye member or lever 8| so that downward movement of rod 88 is caused by downward movement of lever 81. A suitable stop 83 is provided to limit the downward movement which stop is adjustable as shown. In order to maintain rod 81 in the upper position except when force is applied by pulling downwardly on lever 81, I provide a suitable spring means 84. Downward movement of rod 88 will cause clockwise rotation of lever 18 about pivot 19 with the resulting release of latch member 51 and consequent interruption of the circuit by separation of contacts 20 and 2 .1 of switching means 10.

The second method of releasing latch 80 is accomplished through upward movement of vertically movable armature 48 associated with the solenoid comprising winding ll and magnetic structure 45. Armature 46 is connected to a rodshaped member 85 which extends upwardly in inner casing 18 through flange 41 which acts as a guide and flange member 48 which also acts as a guide as well as through nut 49 having a central opening therein. A suitable spring means 88 which engages a nut 81 on rod 85 normally biases armature l6 and rod 85 in a downward direction.

The upper end of rod 85 is provided with an ad- Justable member 88 threadedly mounted thereon which engages directly with pin 15 of lever 14 to eil'ect a release of latch means 30. Upward motion of armature 46 and rod 85 is caused by overload current in winding 4|. This upward movement is against the action of spring means 86.

With the above described arrangement, the release of latching means 30 which is a necessary condition before relative movement between inner casing is and outer casing l1, can occur with consequent separation of contacts 20 and 2| can be accomplished by three different ways. The first condition under which release of latching means 38 might occur is under a short circuit condition which is of sufllcient magnitude to-cause blowing or rupture of current limiting fuse 50. The current is interrupted by current limiting fuse 50 since its operation is much faster than the oper ation of the mechanical means associated with switching means l0. However, prior to the time the current is interrupted by current limiting fuse 58, suiiicient flux is produced by winding 4| to cause armature 46 to move upwardly so that plate 88 engages with pin 15 to cause upward movement thereof with consequent release of latching means 30. A second way in which release of latching means 30 may be accomplished is through the occurrence of sustained overload which will not be sufficient to cause operation of current limiting fuse 50 but the current flowing in winding 4| will produce sufflcient magnetic flux to cause armature 46 to rise and trip latching means 30. The third means by which latching means 30 may be tripped is through the manual operation of plunger 80 as was described in detail above.

In order to insert a new fuse 50 upon rupture or blowing thereof, I provide a fuse or switch hook 9|) as shown in Fig. 10 having a bell mouth or recess 9| into which the looped extension 55' of member 55 is adapted to be inserted. Fuse hook 90 also includes arm 93 and a hook member 92. It will be obvious that the hook 92 can be inserted in the loop or eye of looped extension 55 as well as in the eye 8| to move the fuse unit upwardly or to pull down on eye 8|. Consequently the bell mouth 9| of fuse hook 90 will fit closely around looped extension 55' of fitting 55 by means of which the fuse 50 may readily be lifted into position and screwed or unscrewed with respect to contact 39. It should be understood that the looped extension 55 provides sufiicient clearance below lower fuse terminal 5| so that the fuse indicator of the type disclosed in the above mentioned Letters Patent 2,188,816 will have suflicient clearance to operate and furthermore, so that the condition thereof may readily be observed by the operator or attendant. I

In view of the detailed description included above the operation of the switching and pro tective means of my invention will be obvious. With t e switching cated in Fig. 4 the latching means 35 maintains the contacts 20 and 2| in engagement so that current flows through the switching device from conductor 24, contact plunger 2|, contact 20, conductor 40, winding 4|, conductor 42, contact 39, current limiting fuse 50, contact 57, contact device 55, flexible conductor 60 to terminal strip 59. If a current of suificient magnitude flows through switching means i0, current limiting fuse 50 will cause very high speed interruption thereof so that subsequent separation of contacts 20 and 2| will not impose any interrupting duty on the gap formed interruption by the fuse is rapid compared with any mechanical motion that can occur. In cases where separation of contacts 20 and 2| is initiated due to an overload which is less than that required to operate current limiting fuse 50, the duty of interrupting such an overload is imposed on the gap formed between contacts 20 and 2|. Successful interruption of such relatively low tiated manually through operation of lever B the device may be used as a. load breaking switch.

It should be understood that my invention is not limited to the specific details and arrangement thereof herein illustrated and that changes such changes and modifications as fall within the spirit and scope of my invention.

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

1. In an electric switching means, a circuit interrupting device comprising an outer casing of insulating material, an inner casing of insulating material movably mounted within said outer casing, the external transverse dimensions of said ings, a pair of relatively movable contacts respectively associated with said casings and ar ranged to draw an arc in said annular arc space upon relative separation of said contacts, means for urging relative separation of said contacts, latching means for holding said contacts in current carrying engagement against the force of mentioned means, means for reloasing said latching means in response to an abnormal current condition so that said inner casin moves downwardly relative to said outer casing, and means supported from said inner casing and adapted to extend out of the lower end of said outer casing upon downward movement of said inner casing to give a visual indication of operation of said switching means.

2. In an electric switching means, a circuit interrupting device comprising a pair of relatively movable contacts, means comprising a pair of relatively movable concentrically arranged tubular casings of insulating material the external transverse dimensions of the inner casing and the internal transverse dimensions of the outer casing separation of said contacts is confined, urging relative separation of said contacts, latching means for holding said contacts in currentcarrying engagement against the force of said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one of said concentrically arranged tubular casings moves downwardly relative to the outer casing, and means supported from said inner casing and adapted to extend out of the lower end of said outer casing upon downward movement of said inner casing to provide a visual indication of the operation of said switching means.

between these contacts since 8. In an electric switching means, a circuit int'erruptingr device comprising a Pair of relatively movable contacts, means comprising a pair of relatively movable concentrically arranged tubular casings of insulating material the external transverse dimensions of the inner casing and the internal transverse dimensions 01' the outer casing being proportioned to provide an annular arc space within which the arc drawnupon relative separation of said contacts is confined, means for urging relative separation of said contacts, latching means for holding said contacts in current carrying engagement against the force of said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one oi said concentrically arranged tubular casings moves downwardly relative to the outer casing, means supported from said inner casing and adapted to extend out of the lower end of said outer casing' upon downward movement of said inner casing toprovide a visual indication of the'operation of said switching means, said last mentioned means comprising a fusible element connected in series with said contacts to interrupt the circuit under high current conditions.

4. In an electric switching means, a circuit interrupting device comprising a pair of relatively movable contacts, means comprising a pair of relatively movable concentrically arranged tubular casings of insulating material the external transverse dimensions of the inner casing and the internal transverse dimensions of the outer casing being proportioned to provide an annular arc space within which an arc is drawn upon relative separation of said contacts, means mounted within one of said casings for urging relative separation said contacts, latching means for holding said contacts in current carrying engagement against the force of said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one of said concentrically arranged tubular casings moves downwardly relative to the outer one of said pair of casings, means supported from said inner casing so as to extend out of the lower end of said outer casing upon downward movement of said inner casing to provide upon relative separation of said contacts, means 4 supported by and mounted within said inner casing for urging relative separation of said contacts, latchin means for holding said contacts in current carrying engagement against the force of said'last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one of said concentrically arranged tubular casings moves downwardly relative to the outer casing, and means supported from said inner casing and adapted to extend out of the lower end of said outer casing upon downward movement of said inner casing to provide a visual indication of opl0 eration of said switching means, said last mentioned means comprising a fusible element connected in series with said contacts to interrupt the circuit under high current conditions.

6. In an electric switching means, a relatively low capacity circuit interrupting device compris- I ing an outer casing of insulating material, an inner casing of insulating material movably mounted within said outer casing, the external transverse dimensions or said inner casing and the internal transverse dimensions 01' said outer casing being proportioned to provide an annular arc space between the casings, a pair of relatively movable contacts respectively associated with said casings and arranged-to draw an arc in said annular arc space upon relative separation 01' said contacts, means supported by and mounted within said inner casing for urging relative separation of said contacts, latchingmeans for holding said contacts in current carrying engagement against the force of said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one of said concentrically arranged tubular casings moves downwardly relative to the outer casing, and means supported from said inner casing and adapted to extend out of the lower end or said outer casing upon downward movement of said inner casing to provide a visual indication of operation of said switching means, said last mentioned means comprising a fusible element connected in series with said contacts to interrupt the circuit under high current conditions.

7. In an electric switching means, a circuit interrupting device comprising an outer casingof insulating material, an inner casing 01' insulating material movably mounted within said outer casing, the external transverse dimensions of said inner casing and the internal transverse dimensions of said outer casing being proportioned to provide an annular arc space between the casings, a pair of relatively movable contacts respectively associated with said casings and arranged to draw an arc in said annular arc space upon relative separation of said contacts, means supported by and mounted within said inner casing for urging relative separation of said contacts, latching means for holding said contacts in current carrying engagement against the force of said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one of said concentrically arranged tubular casings moves downwardly relative to the outer casing, and means supported from said inner casing and adapted to extend out of the lower end of said outer casin upon downward movement of said inner casing to provide a visual indication of operation of said switching means, said last mentioned means comprising a readily replaceable current limiting fuse connected in series with said contacts to interrupt the circuit under high current conditions.

8. In an electric switching means, a relatively low capacity circuit interrupting device comprising an outer casing of insulating material, an inner casing of insulating material movably mounted within said outer casing, the external transverse dimensions of said inner casing and the internal transverse dimensions of said outer casing being proportioned to provide an annular arc space between the casings, a pair oi? relatively movable contacts respectively associated with said casings and arranged to draw an arc in said annular arc space upon relative separation of said contacts, means supported by and mounted within said inner casing for urging relative separation of said contacts, latching means for holding said contacts in current carrying engagement against the force 01' said last mentioned means, means for releasing said latching means in response to an abnormal current condition so that the inner one oi said concentrically arranged tubular casings moves downwardly relative to the outer casing, and means supported from said inner casing and adapted to extend out or the lower end 0! said outer casing upon downward movement of said inner casing to provide a visual indication of operation 01' said, switching means, said last mentioned means comprising a readily replaceable fuse connected in series with' said contacts to interrupt the circuit under hish current conditions.

DAVID C. PRINCE.

REFERENCES CITED The following references are of record in the 5 file of this patent:

UNITED s'ra'rns PATENTS Number Name Date 1,011,896 Atkin Dec. 19, 1911 1,130,485 Davis Mar. 2, 1915 1,678,205 Wyman July 24, 1928 1,774,417 Burnham Aug. 26, 1930 1,930,485 Murray Oct. 17, 1933 2,047,365 Fox July 14, 1936 2,102,204 Hart Dec. 14, 1937

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