US2481996A - Air blast circuit breaker - Google Patents

Description

Sept. 13, 1949. F. GRUNEWALD ETAL 7 2,481,996

AIR BLAST CIRCUIT BREAKER 2 Sheets$heet 1 Filed April 2, 1945 aw, sea MD W h n 0 thUa T n w AM HE f vmm r 3. Ma d in .mf w In F H b Sept. 13, 1949. U LD ET AL 2,481,996

AIR BLAST CIRCUIT BREAKER Filed April 2, 1945 2 Sheets-Sheet 2 Jo J Fig.5.

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J9 e 4/ S 4 1.4- a; d 36 0' 52 s Fig. 4. CIRCUIT Inventors:

BREAKER "I b, be C/fiCU/f Friedrich Grfinewald,

1 JI/oRroM'I/If- 86%;? Y H M Their- Attorney- Patented Sept. 13, 1949 AIR BLAST CIRCUIT BREAKER Friedrich Griinewald, Manchester, and Eric Mc- Ewan Dakin, Ashton-undersL e, England, as signors to Ferguson Pailin Limited, Manches ter, England, a British company Application April 2, 1945, Serial No. 586,216 In Great Britain April 22., 1944 '7 Claims.

Our invention relates to fluid or gas, blast electric circuit interrupters and more particularly to those of the axial blast type wherein the are drawn is formed between separable contacts, at least one of which is hollow or tubular in form for receiving all or part of the interrupting gas blast. Our invention is concerned with improvements in fluid or gas blast. circuit breakers of the type disclosed in Bartlett Patent 2,306,242, granted December 22, 1942, and assigned to the same as! lence as the present application.

Fluid 01 gas blast circuit breakers have been extensively used in recent years in order to eliminate the fire hazard which may be present when circuit breakers employing an inflammable liquid for are extinguishing purposes are used. Itv is desirable to reduce the cost and size of such fluid blast circuit breakers while at the same time increasing both their interrupting abilityand efficiency.

It is accordingly an object of our invention to provide a new and improved fluid or gas blast electric circuit breaker of the above mentioned type that is prompt, positive and efficient in its application of the interrupting gas blast to the arc.

It is another object of our invention to provide a new and improved gas blast interrupting device for high voltage application having greatly increased interrupting ability over circuit breakers of the same size and cost used heretofore.

Still another object of our invention is to provide a new and improved nozzle construction for gas blast circuit breakers whereby the gas pressure in the nozzle entrance is maintained fairly high to prevent restriking 015 the arc and wherein relatively small contact separation is required.

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

For a better understanding of our invention ref erence may be had to the accompanying drawings in which Fig. 1 is an elevational view, partlyin section, of an electric circuit interrupting device embodying our invention; Fig. 2 is an enlarged view of the interrupting contacts of the circuit breaker of Fig. 1 with the contacts shown in the open position; Fig. 3 is a view similar to Fig. 2 illustrating a modification of our invention; and Fig. 4 is a schematic representation of two interrupters connected in series to aid in understanding the modification of Fi Ref rrin now to the drawings. there is illu trated a fluid blast circuit breaker generally indicated at l', which comprises an arcing or interrupting chamber 2 defined by a tubular mem- 3 (I inSul'altin material. Within arcing chamber 2 are mounted pair of relatively separable arcing contacts 4 and 5, respectively. It will be understood by those skilled in the art that arcing contacts 4 and 5 may both be movable contacts in accordance with the teachings of the copendlng application of Boisseau and Beall, Serial No. 570,006, filed December 27, 1944, now Patent 2,444,765 of July 6, 19.48, and assigned to the same assignee as, the present application. As illustrated in Fig. 1, however, arcing contact 5 is a stationary contact supported on a conducting casting 6 and suitably fastened thereto as indicated at I. Arcing contact 4, on the other hand, is a movable contact of the, retractable type having integrally formed therewith a flange 8 which acts as a piston reciprocal in a cylinder 9 forming a part of an upper casting ID. A suitable spring [I housed within member i2 fastened to casting I!) normally biases contact 4 to the closed position indicated in Fig. 1.

The insulating housing 3 which substantially defines chamber 2 is preferably formed of a material which is able to withstand high gas pressures and great changes of temperature. As most material which can comply with these conditions is organic insulating material and unsuitable for outdoor use, we preferably enclose insulating member 3 defining arcing chamber 2 within a weather-proof insulator l3, preferably formed of porcelain and having the usual con-' figuration illustrated in the drawings. Upper casting I0 is preferably mounted on insulating members 3 and I3 and suitable sealing means indicated at l 4 may be provided to make sure that arcing chamber 2 is completely sealed from the weather. Preferably casting l0, and consequently movable contact 4', is electrically connected to a power line l5, as indicated in Fig. 1. Similarly, the lower end of arcing chamber 2 is arranged in sealing engagement with lower casting 6 which engages insulators 3 and 13'.

The arrangement discussed thus far comprises the interrupting unit of the circuit breaker l and is generally designated by the reference numeral it. This interrupting unit I6 is preferably mounted on a suitable insulating column I! which in turn is mounted on a framework l8 supported in any suitable manner. Also mounted on framework it is a source of fluid or gas under pressure comprising the tank l9. Insulating column I! contains a pipe or conduit which is connected with tank i9 through a suitable control or blast valve 2!. Pipe or conduit 20 is formed of suitable insulating material so that the framework [8 and the tank [9 may be maintained at a potential considerably lower than the potential of the power line l5. Conduit 20 leads into casting 6 which is provided with a gas passage 22 so that when blast valve 2| is opened fluid under pressure is introduced into arcing chamber 2 of interrupting unit I6,

As will be obvious to those skilled in the art and as will become more apparent as the following description proceeds, fluid blast circuit breakers of the type employing arcing contacts biased to the closed position can satisfactorily interrupt the circuit but will not maintain the circuit in the open position when fluid is no longer supplied to the arcing chamber. As a consequence thereof, it is necessary to provide an isolating switch in series with the arcing contacts 4 and 5. Accordingly, there is provided an isolating or disconnecting switch 23 pivotally mounted to casting 6 as is indicated at 24 and electrically connected to stationary contact 5. Isolating switch 23 in the closed position is adapted to engage with a stationary contact 25 supported on a suitable insulator 26 also mounted on framework !8. Stationary contact 25 is electrically connected to a power line 21.

It is desirable that means be provided to open 13? ner to open isolating switch 23 after the arc. isv

interrupted between contacts 4 and 5. A suitable ooerating mechanism and control is disclosed in copending Boisseau et al. application, Serial No. 565,834. filed November 30, 1944, now Patent 2,450.628 of Oct. 5, 1948, and assigned to the same assignee as the present application.

Our invention is particularly concerned with the interrupting unit [6 and spec fically with the design of the arcing contacts 4 and 5. In accordance with our invention at least one of the arcing contacts is hollow so that a fluid blast may flow therethrough during a circuit interrupting operation. As illustrated in Fig. 1, both of the arcing contacts 4 and 5 are tubular and consequently provided with fluid passageways 30 and 3!. respec ively. Each of the passageways 3D and 3! near the end facing the contact other than the one with which they are associated is arranged in the form of a convergent-divergent nozzle. As is best shown in Fig. 2 the con act 4 comprises a convergent nozzle portion 32, a throat por ion 33 and a divergent nozzle portion 34. Similarly contact 5 has the fluid passageway 3! ther in formed so as to provide a convergent nozz e portion 35. a throat portion 36 and a divergent nozzle portion 31. Our invention is particular y concerned with electrostatic shielding and with spe ific dimensions of the convergentdivergent nozzles of the circuit breaker and par- 4 ticularly the length L of the convergent portions 32 and of the nozzles, the diameter d of the throats 33 and 36, the cross-sectional area of each throat d a (a) the diameter D of the nozzle entrances and the nozzle entrance cross-sectional area, each of which is Another important dimension is the optimum separation e between arcing contacts 4 and 5.

The arcing contact 4 is provided with an annular arcing surface 38 which projects very slight- 1:; from the contact 4 and the sharpness of the outer edge of the projection is removed so as to avoid distortion of the electrostatic field between the contacts 4 and 5 upon separation if; or ball-shaped surface 4!.

' or fluid rushes radially between the contacts and then flows in an axial direction through passages and 3!. Preferably the passageway 3| leads into an exhaust passageway 42 formed in casting I 6 which may be provided with suitable gas cooling 7 side of their associated nozzles.

devices or flame suppressors 43. Slots 44 in movable contact ,4 permit the exhaust gases flowing in passageway 30 to escape into exhaust passageway 45 in casting l0 and thence to atmosphere through suitable gas cooling devices or flame suppressors 46. "The flow of gas into and from arcing chamber 2 is illustrated in the drawings by suitable arrows.

If desired, suitable probe or extremity electrodes 41 and 48 may be provided which are coaxially arranged with respect to arcing contacts 4 and 5 and with their ends on the diverging It has been found that during the circuit interrupting operation the fluid blast immediately causes the arc to transfer to the extremity electrodes thus extending right through the nozzles and therefore causing substantially no burning on the arcing surfaces 38 and 39 because of the early transfer to the extremity electrodes. It will be understood that if desired a suitable resistance may be connected in series with one of extremity electrodes 4'! or 48 so that when the arc transfers to the extremity electrodes the resistance is inserted in the circuit to reduce the current and thereby aid interruption.

During the circuit closing operation the circuit may be completed merely by closing isolating contacts 23. However, as is disclosed and claimed in Thumim et al. Patent 2,418,739, granted April 8, 1947, and assigned to the same assignee as the present application, it may be desirable to close the circuit at arcing contacts 4 and 5 rather supply fluid to arcing chamber 2 shortly before isolating contact 23 moves to its closed positionthercbv opening arcing contacts 4 and and completing the circuit through these contacts when blast valve 2! again closed.

It is a well known fact. that the dielectric strength. of a gas, such as air increases in ac.- cordanoe with increase in pressure in excess of normal atmospheric pressure. It: is iurthermore well known in connection with. the now of gases through nozzles that. as long as the back pressure at an orifice does not exceed 53% of the pr s re at. the entrance of the nozzle the speed. of the gas flow through the orifice or the rate of discharge thcrethrough remains practically constant at substantially the velocity of sound. It is obvious. then with, the converging'diverging nozzles of our invention with the gas pressure in the throats 3t and 35. having a speed equal to sound velocity, that in the converging Parts of the nozzles the gas pressure must rise in accordance with the increase in section toward the nozzle entrances. Because the dielectric strength of gas. varies. increasingly in. proportion, to the increase in gas. pressure, it follows that, during an interrupting blast, a restrike due to a rapid rate of rise in recovery voltage iollowing an arc. interruption will not occur in the highv pressure region directly across the separated coacting annular contact faces 38 and, 39, but will rather tend to establish a longer path of. lesser resistance, say from the throat d of. contact 4 to the throat d of contact 5 through the ionized regions of comparatively low pressure within the convergin Portions of our nozzle contac s. For this reason, a converging-diverging double nozzle arrange ment in accordance with our invention is capable of witl'istanding a higher value of recovery voltage than a comparable arrangement of double. nozzles whose throats coincide with their entrances (that is, having no convergent funnels) and in which the low pressure low resistance restrike path is correspondingly shorter and approximates, the separation distance between the nozzle contacting surfaces. Accordingly, the provision of a sumcieni axial length L for the convergent part of each of the double nozzles is an important factor in minimizing the hazard of voltage breakdown through the ionized low pressure regions within the nozzles. In support of this phenomenon, ex-

periments have shown that in a circuit breaker designed for 66 kv., an increase in L of 30 per cent increased the voltage limit of the breaker by apprcxnoately- 20 per cent with constant gas pressure. We have found that if the length L is made equal to the diameter d of the nozzle. throat, it will be adequate for the lower limits of the voltaae for which the breaker is designed. In carrying out our invention then, L should be at least equal. to. and preferably somewhat more than. the throat diameter d.

According to further aspects of our invention the. areas for both the radial and axial gas flow between and across the nozzle entrances, re spectively, should be so much larger than those oiv the throats that at the nozzl entrances the gas pressure is at. least 40% higher than in the throats. There must of course from a practical standpoint be an upper limit to the areas. of the nozzle entrances not only to keep the dimensions of the breaker as small as possible, but also to keep the air concentrated and its speed high enough to cool the arc and to remove and. main- 6 tain it away from its starting point. We have found that all practical requirements with re. gard to the current interruption can be fulfilled by fixing the upper limit for the areas of the double nozzle at their entrances in such a way that the air speed in this region does not drop under 25% of that in the throats.

In order that the pressure of the gas and consequently the dielectric strength thereof is as uniform as possible in the zone of the nozzle entrances and to reduce turbulence and contact travel to a minimum we. found it to be desirableto make the area of the imaginary cylindrical surface between. the entrance diameters of the two coasting nozzles just where the gas flow starts to turn from the radial to the axial direction and hereinafter referred to as the annular entrance area, equal to the sum of the cross sectional area of both nozzles at their entrances. For example, in the construction shown in Fig. 2 in which two identical nozzles are used, so. that the entrance diameter D- of both nozzles are equal. the annular entrance area where e is the optimum and maximum contact separation necessary to permit a sufilcient i 1- ward radial rate of flow through the annular entrance area to produce the desired predetermined rate of flow through the two nozzle entrances of diameter D. In other words the dis tance between the contacts at their point of optimum and maximum separation should be equal to half the diameter of the nozzle entrances, a being equal to as derived from the above equation. By limiting the separation distance of the. coasting nozzle r contacts to the proper value c to produce the above defined desirable relationship between the annular entrance area and the cross-sectional area of the nozzle entrances, the maximum necessary travel of the moving contact or contacts of the interrupter to effect a circuit interruption is reduced to a minimum, thereb permitting a. greater reduction in the overall axial length of the interrupter than was heretofore thought possible. In addition to making the annular entrance area equal to the. sum of the areas across the nozzle entrances, these areas also are made sufficlently larger than the areas of the throats 33 and 36 so that in the region between and across the nozzle entrances the gas pressure is at. least 40% higher than that of the throats, while the gas speed is at least 25% of that in the throats, the speed in the throats being practically constant at substantially the. velocity of sound so n as the back pressure on the nozzle is not more than 53% of the. source pressure.

It is understood that the divergent portions of the nozzles need not be strictly conical, the requirement being simply that the cross sectional. area is continually increased anywhere dowm stream beyond the throats. To avoid undue tur. bulenco it is desirable that the gas passages 30 and. 3! are streamlined in form.

The following table furnishes practicable de sign data for the hollow nozzle contacts of our circuit interrupter in accordance with the pre- 7 c'epts of our invention, the table setting forth the range of desirable values:

Where the gas or fluid supplied to the arcing chamber is supplied from one end as is indicated in Fig. 1, it may be desirable to arrange the electrostatic shields so that one of them is concave and the other is convex. This arrangement effectively serves to guide the gas in between the separating contacts changing direction by 90 in a streamlined manner, the concave electrode acting as a gas scoop. Such an arrangement is obviously unnecessary in circuit breakers wherein the gas is supplied to the arcing chamber from both ends, as in the Boisseau and Beall copending application referred to above. In Fig. 3 we have illustrated a modification of our invention in which the arcing contact is identical with the arcing contact 5 of Fig. 2 and characterized by the corresponding reference numerals. In Fig. 3 a movable contact 49 is provided, which is very similar to the movable contact 4 in Fig. 2 except that it is surrounded by a stationary electrode 59 provided with a concave electrostatic shield 5|. With this arrangement the sudden change of the air flow from the axial to the radial direction occurs with a minimum of turbulence.

For certain purposes it is desirable to arrange a capacitor element in parallel circuit relationship with the contacts of a circuit breaker. One such instance is schematically represented in Fig. 4 where two circuit breakers designated as No. 1 and No. 2 are connected in series. The circuit is connected to a power source and, as shown, a short circuit to ground has occurred. Both the circuit breakers have their contacts in the open circuit position. In Fig. 4 the designation bi applies to the capacitance between the contacts of breaker No. 1 when the breaker is open and the designation b2 applies to the capacitance between the contacts of circuit breaker No. 2 when this breaker is opened. The letter a designates the capacitance between ground and those portions of the circuit between the two pairs of contacts. Normally the capacitance represented by the letter a is substantially larger than the capacitances b1 and b2. Under the conditions depicted in Fig. 4, the capacitance across breaker No. 2 is the sum of the capacitance b2 and the capacitance a while the capacitance across breaker No. 1 is the smaller value In. Inasmuch as the voltage across both of the seriesconnected circuit breakers of Fig. 4 will be distributed across each breaker in inverse proportion to the capacitance across each individual circuit breaker, it follows that the voltage across breaker No. 2 will be a relatively small portion only of the total voltage across both the circuit breakers connected in series while circuit breaker No. 1 will be unduly stressed. This condition cannot be corrected by merely increasing the capacitance in because a ground short circuit may occur at what is indicated in Fig. 4 as the source end of the line in which case the voltage across breakers will be unduly stressed.

8 breaker No. 2' would be considerably higher than that across breaker No. 1. If capacitance bi and be are greatly increased so that either has a value substantially greater than the capacitance 'I- a, the voltage distribution will be but slightly affected by the capacitance a and will therefore tend to divide almost equally between the two circuit breakers. Because of the capacitance a and the possibility that a short circuit may occur either as shown in Fig. 4 or on the source side of the diagram of Fig. 4, it will not be possible to insure that the voltage distribution between the two circuit breakers will be perfect. By this method, however, neither of the circuit Thus, from the above discussion of Fig. 4 it will be apparent that suitable means should be provided for increasing the capacitance between the contacts of a circuit breaker when such contacts are open so that capacitance such as a will be but a small portion of the total capacitance.

One method for arranging capacitance in parallel with a circuit breaker is shown in Fig, 3. By the construction of Fig. 8 the arcing chamber itself being constructed of insulating material serves as the dielectric for capacitor elements embedded in the tubular arcing chamber 52. These capacitor elements are designated by the numeral 53. The upper, inner conducting member 53 is connected as indicated at 55 to a conducting cylinder 54 disposed within the tubular arcing chamber 52. The conducting cylinder 54 protects the insulating material 52 against detrimental effects caused by the arc and also serves as one of the capacitor elements. As shown in Fig. 3, the cylinder 54 is connected to the stationary electrode 50 by means of the bracket 56. The outer, lower one of the capacitor elements 53 is connected to the contact 5 by the conducting bracket 51. The brackets 58 insure electrical connection between electrode at and contact 49. Thus it will be seen that the structure described thus far comprises a capacitor in parallel circuits relationship with the contacts and would correspond to the capacitances b1 and be referred to above in connection with Fig. 4. With the arrangement described thus far, the voltage stress across the capacitor elements would be in a direction substantially perpendicular to the insulating tube 52 and would exist in the region between the lower end of the inner, upper conducting cylinder 53 and the upper end of the lower, outer conducting cylinder 53. In accordance with our invention and for the purpose of utilizing efiiciently a substantial amount of the dielectric material of the insulating cylinder 52 as the dielectric of the capacitance even though the ends of the tube 52 are at opposite potential, we provide the stepped intermediate conducting cylinders which are disposed between the inner, upper cylinder 53 and the outer, lower cylinder 53. In this way the voltage stress is not perpendicular with respect to the wall of the tube 52 but instead is distributed at an angle with respect to the wall of the tube 52. In this way the insulating dielectric material of the tube 52 is utilized more effectively as the dielectric of the capacitor and, furthermore, the Voltage stress along the inside surface of the tube 52 between the lowermost tip of the cylinder 54 and the connection 51, is eiiectively reduced. Optimum results are obtained by use of the capacitor construction shown in Fig. 3 when the capacitance between any two adjacent conducting cylinders enemas 53 is substantially equal to the capacitance between other adjacent cylinders 53 and when the longitudinal spacing of the various cylinders is substantially the same for all the cylinders. Obviously the cylinder 54 could be omitted in which event the upper, inner embedded tube 53 would be connected to electrode 50 by means similar to bracket 57,

In view of the detailed description included above, the operation of the circuit breaker embodying our invention will be obviousto those skilled in the art. 'When blast valve 2| is opened separation of arcing contacts 4 and 5 is caused and by the particular design of the nozzle contact set forth above, greatly in-creased interrupting capacity and eificiency are obtained relative to size and cost. The are drawn between arcing surfaces 38 and 39 has its roots first driven very quickly radially inwards until the arc is forced to move to the extremity electrodes 41 and 48 if they are provided, or in any event is extinguished at an early current zero.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects and we, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

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

1. In a, fluid blast electric circuit interrupter comprising an arcing chamber, a pair of relatively movable interrupting contacts mounted in said chamber, a source of fluid under pressure, means for supplying fluid from said source to said chamber, a fluid motor associated with one of said contacts to cause relative separation of said contacts when fluid under pressure is supplied to said chamber, each of said contacts being hollow and arranged so as to define a pair of nozzles of converging-diverging section through which fluid from said chamber may flow upon separation of said contacts, the converging part of each nozzle having an axial length at least equal to the diameter of its throat, and the area of the entrance to each nozzle being at least 1.14 times its throat area so that the gas pressure in each nozzle entrance is at least 40% higher than in each throat.

2. In a fluid blast electric circuit interrupter comprising an arcing chamber, a pair of relatively movable interrupting contacts mounted in said chamber, a source of fluid under pressure, means for supplying fluid from said source to said cham ber, a fluid motor associated with on of said contacts to cause relative separation of said contacts when fluid under pressure is supplied to said chamber, at least one of said contacts being hollow and arranged so as to define a nozzle of converging-diverging section through which fluid from said chamber may flow upon separation of said contacts, the converging part of said nozzle having an axial length at least equal to the diameter of the throat of said nozzle, and the area of the entrance to said nozzle being at least 1.14 times the throat area so that the gas pressure in said nozzle entrance is at least 40% higher than in said throat, the separation distance of said contacts in the open position being equal to substantially one-quarter the diameter of said nozzle entrances.

3. In a fluid blast electric circuit interrupter comprising an arcing chamber, a pair of relal0 tively movable interrupting contacts mounted in said chamber, a source of fluid under rcssure, means for supplying from said source to said chamber, :a fluid motor associated with one of said contact to cause relativ separation of said con} tacts when fluid under pressure is supplied to said chamber, each of said contacts being hollow and arranged so as to define a pair of nozzles of converging-diverging section through which fluid from said chamber may flow upon separation of said contacts, the converging part of each nozzie having a length at least equal to the diameter of its throat, and the area of the entrance to each nozzle at least Ll l times its throat area so that the gas pressure in said nozzle entrances is at least 40% higher than in said throat, the axial distance of said nozzle entrances from each other in the open position of said circuit interrupter being substantially equal to one-half the diameter of said nozzle entrances.

4. In a fluid blast electric circuit interrupter comprising an arcing chamber, a pair of relatively movable interrupting contacts mounted in said chamber, a source of fluid under pressure, means for supplying fluid from said source to said chamher, and means for causing relative separation of said contacts, at least one of said contacts being hollow and arranged to define a nozzle through which fluid from said chamber may flow upon separation of said contacts, the separation distance between said contacts in the open position being such that the annular entrance area is substantially equal to the cross-sectional area of the entrance to said nozzle.

5. In a fluid blast electric circuit interrupter comprising an arcing chamber, a pair of relatively movable interrupting contact mounted in said chamber, a source of fluid under pressure, means for supplying fluid from said source to said chamber, and means for causing relative separation of said contacts, each of said contacts being hollow and arranged so as to define a pair of nozzles through which fluid from said chamber may flow upon separation of said contacts, the separation distance between said contacts in the open position being such that the annular entrance area is substantially equal to the sum of the cross-sectional areas of entrance to said nozzles.

6. A fluid blast circuit interrupter comprising a tubular arcing chamber of organic insulating material, a pair of relatively movable interrupting contacts mounted in said chamber and arranged to draw an arc upon separation thereof, an outer conducting tube shorter than said tubular chamber and embedded in said tubular chamber near one end thereof, an inner conducting tube shorter than said chamber and disposed therein near the other end of said chamber, first means for connecting said outer tube to one of said contacts, second means axially disposed with respect to said chamber from said first mean for connecting said inner conducting tube to the other of said contacts, and a plurality of telescopically arranged conducting tubes interposed between said inner and said outer tubes and embedded in said arcing chamber, said plurality of tubes and said outer and inner tubes being disposed in stepped relationship axially along said chamber.

7. A fluid blast circuit interrupter comprising a substantially cylindrical arcing chamber of insulating material, a pair of relatively movable interrupting contacts mounted in said chamber and arranged to draw an arc upon separation thereof, a first relatively small diameter conducting cylinder shorter than said arcing chamber and embedded therein near one end thereof, a. second conducting cylinder shorter than said chamber and having a larger diameter than said small diameter cylinder and embedded in said cham- 'ber near the other end thereof, separate means for connecting each of said conducting cylinders to a difierent one of said contacts, and a. plurality of intermediate conducting cylinders embedded in said chamber and interposed between said conducting cylinders, said intermediate cylinders and said first and second cylinders being disposed in stepped relationship axially along said chamber.

FRIEDRICH GRfiNEWALD.

ERIC McEWAN DAKIN.

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

Number Number 12 UNITED STATES PATENTS Name Date Rump May 20, 1930 Slepian May 23, 1933 Ruppel Nov. 28, 1933 Lange Oct. 22, 1935 Clerc Nov. 10, 1936 Kesselring et a1. Jan. 12, 1937 Ruppel Oct. 18, 1938 Smith Apr. 25, 1939 Amer Nov. 1'7, 19,42 Bartlett Dec. 22, 1942 Ludwig et a1 Mar. 9, 1943 FOREIGN PATENTS Country Date Germany Dec. 14, 1934 France Sept. 4, 1933

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