US3163734A

US3163734A - Vacuum-type circuit interrupter with improved vapor-condensing shielding

Info

Publication number: US3163734A
Application number: US168959A
Authority: US
Inventors: Thomas H Lee
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-01-26
Filing date: 1962-01-26
Publication date: 1964-12-29
Anticipated expiration: 1981-12-29
Also published as: DE1203349B; GB1028791A

Description

Dec. 29, 1964 T. H. LEE VACUUM-TYPE CIRCUIT INTERRUPTER WITH IMPROVED VAPOR-CONDENSING SHIELDING Filed Jan. 26, 1962 OPT/MUM TiNCK/V568 /N INCHES TH/NLESS STEEL 2 Sheets-Sheet l ATTOR/VEX Dec. 29, 1964 T. H. LEE 3,163,734

VACUUM-TYPE CIRCUIT xNTERRuPTER WITH IMPRovED vAPoR-coNnEwsING SHIELDING Filed Jan. 2e, 1962 2 sheets-sheet 2 K /N VEA/Tof?.-

z a6 THOMAS H. LEE,

BY ATTORNEX United States Patent O M VACUUllvl-'lmiflll CIRQUE INTERRUPTER Willi-I MPRVED VAPORCNDENSING SHLELDN; Thomas H. Lec, Media, Pa., assigner to General Electric Company, a corporation of New York Filed Jan. 26, 1962, Ser. No. 168,959 Claims. (Cl. 20S-144) This invention relates to a vacuunntype circuit interrupter and, more particularly, relates to improved shielding means for condensing the vapors generated by arcing between the contacts ofthe interruptor.

The usual vacuum-type circuit interruptor comprises a pair of separable contacts disposed within a vacuum chamber. Circuit interruption is initiated by separating these contacts to establish ya gap across which `an are is formed. The arc vaporizes some of the contact material to create a local atmosphere through which current flows until about the time a natu-ral current zero is reached. After the current zero point has been reached, the usual recovery voltage transient begins building up across the gap :between the contacts. lf the circuit is to be interrupted at this cur-rent zero, the gap must have suiiicient dielectric strength to withstand this recovery voltage transient without breakdown. Whether or not the gap will have this much dielectric strength depends to a large degree upon the extent to which the vapors generated by `arching have been removed lfrom the gap by the time the gap is stressed by the .recovery voltage transient.

I have found that for certain classes of vacuum interrupters, an impor-tant determinant of whether the gap will be free of vapors during this critical interavl is the construction of the usual shield which surrounds the arcing gap. For low current interiupters, this shield construction is not particularly important. But for high current interrupters, eg., those that are used for interm rupting more than 10,000 amperes R.M.S., the shield construction is most important. In this regard, a high current arc imparts large amounts of energy to the shield, and this high energy input can raise the temperature of the shield to such `a level that its ability to condense the arc-generated vapors will be seriously impaired. If this happens, I have ound, 'an excessive number of particles will rebound from the shield into the high stress region between the contacts, and a dielectric breakdown can occur.

For low current interrupters, other factors are usually present to limit the .amount of current that can be interrupted to a maximum value which is below the value at which the penformance ot the shield becomes important. For example, with the low current interrupters that have refractory metal contacts, eg., tungsten contacts, there is typically so much t-hermionic emission from the ccntacts, even at 4,000 or 5,000 amperes, that the interrupter i cannot successfully interrupt higher currents. In such interrupters, using a rshield constructed in accordance with my invention, as compare-d to certain prior shields, would not appreciably increase the current-interrupting ability of the interrupter. 'llhermionic emission from the contacts rather than shield construction is the principal limiting facto-r in this particular type of interruptor.

Another limiting factor in the interrupting performance of certain vacuum switches is gas-emission yfrom the contacts during arcing. tremely high degree of rfreedom from sorbed gases and contaminants which decompose under the iniiuence of the arc into permanent gases, there will be gas particles in the gap at the critical instant of interruption 'to impair the interrupting ability of the interruptor. It these gas ticles are present in significant quantities, constructing the Unless the contacts have an ex- Patented Dec. 29, 1964 ICC shield in accordance with the present invention will not appreciably increase the current interrupting capacity of the interruptor. The limit of interrupting capacity in such a device will be determned by the gas content of the contacts rather than by the pet0rmance oi the shield. l, therefore, am concerned with a vacuum interruptor that has non-refractory metal contacts that have an extremely high degree of freedom from sorbed gases and other gasforming contaminants. As a measure of this freedom from sorbed gases and contaminants, the contacts ot my interrupter can be deeply eroded by arcing in a two liter chamber evacuated to .105 mm. of mercury without significantly increasing the pressure present wit-hin the chamber 1go of a second after arcing. It is with an interrupter having contacts or this type that the shield construction becomes an important determinant of the amount of current that can be interrupted.

That the interupting capacity of such interrupters is limited to an important extent by the manner in which the metal shield is constructed has not been fully appreciated heretofore. Perhaps one factor underlying this lack or" understanding has Ibeen a failure to 4fully apperciate the high levels to which the energy input into the shield can rise. In this connection7 there have been statements in f' the literature that the arc Voltage of a vacuum arc varies very little with current. Across a gap between the electrodcs of a given material, it has been stated that the are voltage increases only very slightly as the current incre-ases. Despite these statements, however, I have found that for the high currents I am concerned with, the arc voltages developed with a given electrode material are much higher than those previously reported. For example, for copper contacts interrupting 30,000amperes peak arcing current, peak `arc voltages of volts and even higher have been developed. A previously published iigure for the arc voltage of a copper gap is 20.5 volts. Since the power input into the shield is generally proportional to the yarc voltage, it will be apparent that for currents in the range of 30,000 anrperes interrupted with copper contacts, 4the peak power input into the shield is approximately six or even more times as high as would be expected from previously reported arc voltage figures.

This unexpectedly high power input has been responsible for many of the problems heretofore encountered in attempting to interrupt high currents with a vacuum in terrupter. Previously proposed shields have been heated to such an extent by this high power input that particles have rebounded therefrom at an excessive rate that has caused a dielectric breakdown.

An object of the present invention is to reduce the changes for a dielectric breakdown resulting from arcgenerated lparticles rebounding from the metal shield of a high current vacuum interruptor.

ln carrying out my invention in one form, l provide an interrupter that is rated for interrupting commercial power frequency alternating currents in excessot' t0,000 amperes Riv/LS. symmetrical. Under the most severe interrupting conditions, this current may be almost fully offset so 'that the actual kRMS'. value is ialmost twice the symmetrical value. The interruptor is rated to interrupt such currents within at least one electrical cycle of arcing current. This interrupter includes non-refractory metal contacts that are free or" sorbed gases and other contaminants to lsuch an extent that if deeply eroded by arcing in a two liter chamber evacuated to a pressure of itl-5 millimeters of mercury, the pressure in the chamber one thirtieth ot' a second after arcing will. not be above its initial value. metallic shield that is of such a size that the average power input into the portion of the shield immediately about the contacts during the interruption of maximum Surrounding these contacts is a tubular rated current is less than 100 kilowatts per square inch of surface area. The material of the shield is of such a character that the square root of its thermal diifusivity divided by its thermal conductivity is less than .085, where the diifusivity is expressed in inches square per second and the thermal conductivity in watts per inch C. The thickness of the shield in inches is at least as great as about 1A times the square root of the thermal diffusivity of the material.

Examples of shields that can meet these latter two requirements are a shield of copper having a thickness at least as great as about .10 inch, a shield of aluminum having a thickness at least as great as about .09 inch, and a shield of silver having a thickness at least as great as about .125 inch.

1n carrying out my invention in another form, I surround the arcing gap with a tubular metal shield having corrugations formed on its inner surface in the region of the arcing gap. These corrugations reduced the power input per unit of area into a shield of a given diameter by providing an increased effective area exposed to the arcing products as compared to a smooth surface shield of the same diameter. All straight line paths between the arcing gap and the insulation of the interrupter are intercepted either by the shield or other metallic structure.

in still another form, 1 construct the shield from at least two tubular structures, one of which is disposed within the other. The inner structure is perforated so as to allow some of the arcing products to penetrate this inner structure and condense on the next tubular struc ture. The two tubular structures are electrically connected together so that the region between the two tubular structures is free from electrical stress. In such a stress free region, the arcing particles cannot initiate a dielectric breakdown. This means that particles rebounding from the outer tubular structure cannot initiate a breakdown unless they penetrate the inner tubular structure in a backward direction. Before such backward penetration can occur, however, a large portion of the rebounding particles are condensed on the inner tubular structure, thus lessening the likelihood of a dielectric breakdown.

For a better understanding of my invention reference may be had to` the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view through a vacuum interrupter embodying one form of my invention;

FIG. 2 is a perspective View of a contact forming a part of the interrupter of FIG. 1;

FIG. 3 is a graphical representation of certain characteristics of shields of various materials;

FIG. 4 illustrates a modified type of shield for inclusion in an interrupter corresponding to that shown in FG. l;

FIG. 5 illustrates another modified form of shield;

FiG. 6 illustrates still another modified form of shield.

FlG. 7 is an enlarged detadled View of a portion of the shield of FIG. 6.

FIG. 8 illustrates still another modified form of shield.

Referring now to the interrupter of FIG. 1, there is shown a highly evacuated envelope 1G comprising a casing il of a suitable insulating material and a pair of metallic end caps 12 and 13 closing off the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to render the envelope vacuum tight. The normal pressure within the envelope l@ under static conditions is lower than lO-lv mm. of mercury, so that a reasonable assurance is had that the mean free path for electrons will be longer than the potential breakdown paths in the envelope.

Located within the envelope is a pair of

separable contacts

17 and 18, shown in their engaged or closed-circuit position; The upper contact 1'7 is a stationary conv tact suitably attached toa conductive rod 17a, which at its upper end is united to the upper end cap 12. The lower contact 15 is a movable .contact joined to a conductive operating rod lea which is suitably mounted for vertical movement. Downward motion of the contact 18 separates rthe contacts and opens the interrupter, whereas return movement of contact 18 reengages the contacts and thus closes the interrnpter. The space between the contacts is referred to hereinafter as the arcing gap. A typical arcing gap length when the contacts are fully open is about 1/2 inch. The operating rod 18a projects through an opening in the lower end cap 13 and a ilexible metallic bellows 2@ provides a seal about the rod 18a to allow for vertical movement of the rod without irnpairing the vacuum inside the envelope lil. As shown in FIG. l, the bellows Ztl is secured in sealed relationship at its respective opposite ends to the operating rod 13a and the lower end cap 13.

The are that is established across the arcing gap upon contact-separation vaporizes some of the contact material and creates forces which drive the vapors outward in all directions from the arcing region via generally straight line paths. The internal insulating surfaces of the casing 11 are protected from the condensation of arc-generated metal vapors thereon by means of a tubular metallic shield l5 suitably supported on the casing 11 and preferably isolated from both end caps 12 and 13. This shield acts to intercept arc-generated metallic vapors before they can reach the casing 1l. To reduce the chances for vapor bypassing the shield, the shield extends along a major portion of the length of the casing 11 and is of a generally conical configuration at its axially-opposed ends. To further reduce the chances for vapor bypassing the shield 15, a pair of end shields 16 and 16a are provided at opposite ends of the central shield to increase the tortuousness of the path leading from the arcing region to the insulating housing 11. These end shields generally correspond to those disclosed and claimed in Patent No. 2,892,9l2-Greenwood et al., assigned to the assignee of the present invention. All straight line paths from; the arcing region to the insulation of the housing which are not intercepted by the end shields 16 or 15a are intercepted by the central shield 15. Thus, metal parts are disposed between all solid insulation in the interrupter and the arcing region. This is quite important because bombardment of the insulation by the hot arcing products can result in gases being liberated from the insulation that could seriously impair the interrupting ability of the interrupter. In addition to protecting the insulation of the casing 11 from exposure to the arcing products, the shield performs still another important function which will be described in greater detail hereinafter.

All of the internal parts of the interrupter are substantially free of surface contaminants. These clean surfaces are obtained by suitably processing the interrupter, as by baking it out during its evacuation. In addition, the contacts 17 and ld are effectively freed of gases absorbed internally ofthe contact body so as to preclude evolution of these gases during high current arcing. In the particular interrupter that I am here concerned with, the contacts have such a high degree of freedom from sorbed gases and other contaminants which decompose in the presence of an are to form permanent gases that the contacts could be deeply eroded by arcing in a chamber of two liters volume evacuated to 10-5 of mercury without significantly increasing the pressure prevailing in the chamber 1/30 of a second after arcing above its initial value. This high degree of freedom from sorbed gases Vand contaminants is preferably obtained by relying upon the zone refining techniques disclosed and claimed in application S.N. 146,245 Hebb, led October 19, 1961, and assigned to the assignee of the present invention.

Although this invention is not limited to any particular contact configuration, l prefer to use the contact configuration disclosed and claimed in U5. Patent 2,949,520-d Schneider, assigned to the assignee of the present invention. Accordingly, each contact is of a dislt shape and has one of its major surfaces facing the other contact. The central region of each Contact is formed with a recess 29 in this major surface, and an annular contactmaking area 30 surrounds this recess. These annular contact-making areas 30 abut against each other when the contacts are in their closed position of FIG. l, and are of such a diameter that the current flowing through the closed contacts follows a loop-shaped path L, as is indicated by the dot-dash lines of FIG. l. This loopshaped path has a magnetic effect which tends in a Well-known manner to lengthen the loop. As a result when the contacts are separated to form an arc between the areas 30, the magnetic effect of the loop will impel the are radially outward.

As the arc terminals move toward the outer periphery of the discs 17 and i8, the arc is subjected to a circumferentially-acting magnetic force that tends to cause the arc to move circumferentially about the central axes of the discs. This circumferentially-acting magnetic force is preferably produced by series of slots 32 provided in the discs and extending from the outer periphery of the discs radially-inward by generally spiral paths, as is shown in FIG. 2. These slots 32 correspond to similarly designated slots in the aforementionad Schneider patent and, thus, force the current flowing to or from an arc terminal located at substantially any angular point on the peripheral region of the disk to follow a path that has a net component extending generally tangentially with repect to the periphery in the vicinity of the arc. This tangential configuration will be apparent from the path L shown in FlG. 2 leading from rod 13o to the terminal.

of an arc 38 on theouter periphery of contact 1S. This tangential configuration of the current path causes the magnetic loop L to develop a net tangential force component, which tends to drive the arc in a circumferential direction about the contact. In certain cases, the arc may divide into a plurality of parallel arcs, and these parallel arcs move rapidly about the contact surfaces in a manner similar to that described hereinabove.

Heretofore, it has not been fully appreciated that the shield 15 often plays an important role in limiting the amount of current that can be handled by an interrupter of the particular class I am concerned with. In regard, if the shield becomes excessively heated during interruption, its ability to condense the arc-generated vapors will be seriously impaired. This, I have found, can result in a sufficient number of metallic particles rebounding from the shield into the high stress region between the contacts l? and i8 to cause a dielectric breakdown.

For low values of current, there are few such rebounds and thus little likelihood of a breakdown resulting from such rebounds. But, when the amount of current being interrupted is higher than about 10,000 amperes RMS. this problem becomes much more acute, and it is an object of the present invention to provide a solution to it.

Perhaps one reason that this phenomena in a vacuum interruptor has gone largely unrecognized is that there has been little appreciation of the high levels to which the energy input into the shield may rise. In this connection, the prior art has treated the arc voltage for a given contact material as an y,approximately constant value which increases only very slightly as the current increases. From this, one would expect the power input to increase merely in proportion to the amount of current being interrupted. But l have found that such an assumption is highly inaccurate at the higher values of current that I am concerned with. For such currents, i.e., in excess of 10,000 aniperes KMS., l have consistently recorded peak arcing voltages many times higher than those previously published for the contact material in question. For example, at 18,000 amperes peak arcing current, peak arc voltages of between 90 and llO amperes have been observed for copper contacts, as compared to a Value of 20.5 volts appearing in the prior art as a characteristic arc voltage for copper contacts. This would result in a power input at the peak current of about 4 or 5 times as high as would be expected from the prior art figure of 20.5 volts. At peak arcing currents of 30,000 amperes, arcing voltages of 120 to 170 volts have been observed for copper contacts. For higher currents, even higher arcing voltages have been observed.

To reduce the chances for a breakdown being produced by such rebound of particles from the shield during high current interruption, I make the diameter of the shield 15 large enough to limit the power input into the critical region of the shield aligned with the arcing gap to a value of less than kilowatts per square inch of shield surface. The material of the shield is also important, and in this respect, I prefer to use a material which is of such a character that the square root of its thermal diffusivity (k) divided by its thermal conductivity (K) is less than .085, where the ditfusivity is expressed in inches square per second and the conductivity in watts per inch-degree centigrade, all at normal room temperature, i.e., about 25 C. The thickness of the shield is also of importance and, expressed in inches, is at least as great as about 1%: times the square root of the thermal diifusivity at normal room temperature, i.e. 1iV/c. Examples of shields that can meet these latter two requirements are a shield of copper having a thickness at least as great as about .l0 inch, a shield of aluminum having a thickness at least as great as about .09 inch, a shield of silver having a thickness at least as great as about .125 inch, and a shield of molybdenum having a thickness at least as great as about .07 inch.

By meeting these requirements for shield geometry, material, and thickness, I can limit the temperature rise of the exposed shield surface to a lower level than is obtainable with previously proposed shields used in a high current interrupter under corresponding voltage and current conditions. As pointed out hereinabove, this reduction in temperature is important because it leads to an improved ability to condense the arc-generated vapors. This ability to condense the vapors striking a surface can be expressed mathematically in terms of an accommodation coefficient. The accommodation coefficient is the ratio of the number of atoms condensing on a substrate to the total number striking it. This accommodation coeliicient depends upon the composition of the vapor particles striking the substrate and also upon the temperature of the substrate. Although the composition of the substrate is an important determinant of its temperature, this composition does not significantly influence the accommodation coeiiiceint for any given temperature.

The shield thickness is important because the thickness is one determinant of the rate at which heat can be conducted away from the exposed inner surface of the shield. As thickness increases, the rate of heat conduction increases. But there is an optimum thickness for a given material, and increases in thickness beyond this optimum value have little effect on the surface temperature. This optimum thickness is determined by the thermal diifusivity of the shield material. The approximate optimum thicknesses for nickel, aluminum, copper, silver, molybdenum and a nickel-chromium-iron alloy sold under the trademark Inconel X are plotted on the curve of FIG. 3. Inconel X is a heatresisting alloy commonly used in vacuum work and having a composition of about 73% nickel, 15% chromium, 7% iron, 2.5% titanium, 1% columbiurn, .5% manganese, .4% silicon, .7% aluminum, and .04% carbon.

Assuming that the shield thickness is at least as great as the optimum thickness, then the temperature rise for a given material will be generally proportional to the square root of the ditfusivity divided by the thermal conductivity, i.e., V'. For nickel, this ratio is about .101, where k is in inches square per second and K is in watts per inchdegrees centigrade. For copper the ratio is about .042. For silver, the ratio is about .048, for aluminum, about .066; and for molybdenum, about .077. For stainless steel, the ratio is about .188, and for Inconel X, the ratio is about .234. This means that for a nickel shield of at least the optimum thickness, the temperature rise relative to that for a copper shield of optimum thickness will be .101/ .042 or about 2.4 times as high as that for the copper shield. For a stainless steel shield of at least the optimum thickness, the temperature rise will be .18S/.042 or about 4.5 times as high as for a copper shield of optimum thickness. It will be apparent from the graph of FIG. 3 that the use of copper for the shield will result in the lowest temperature rise of those materials plotted, and it is therefore a preferred material. Other materials, however, may be used if they are characterized by a Vm less than .085. Although not as effective as these materials in maintaining a low shieldftemperature, nickel is considerably better than stainless steel and lnconel X, and in its broader aspects the present invention is intended to comprehend nickel as a shield material, providing the shield conforms to the other requirements set forth hereinabove.

In a vacuum interruptor of the design shown in FIG. l having copper contacts that are fully opened to a l/z inch gap during interruption, the approximate average power input into the shield in the immediate region of the arcing gap is0.7 11/a where la is the R.M.S. value of the arcing current, Va is the R.M.S. value of the arc voltage, and .7 represents the portion of the total arc energy that is imparted to the shield in the immediate region of the arcing gap. The remainder of the arc energy is dissipated by heat conduction through the electrodes and by heat transfer to other parts of the interrupter. The critical region of the shield is a restricted portion or band of about 2 inches in width extending about the inner periphery of the shield in alignment with the arcing gap. Since the gap is assumed to be one-half inch in length, this band extends for about three-fourth inch on each side of' the gap. The entire periphery of the shield/can be used in these calculations since the arc or arcs rotate at high speed about the longitudinal axes of the contacts I7, I8, as described hereinabove to distribute the energy about'this entire band. If the surface area covered by this band be designated A, then the approximate power input into this critical region of the shield is: 0.7 Vala/A. Assuming that an interruptor or" the design shown in FIG. l is rated to interrupt a maximum available current of 18,000 symmetrical amperes R.M.S. with any degree of asymmetry', then in order for the average power-input to be held below 100 lov/in?, the shield area for a 2 inch band should be 26 square inches. This means the shield diameter should be greater than 4 inches. In the above example, it was assumed that under the most severe interrupting conditions, the current would be fully asymmetrical so that the actual RMS. value of the current during the first current loop would be almost twice 18,000 amperes. It was further assumed that arcing current persisted until the second current zero under these most severe conditions.

As an illustration of the extent to which the interrupting ability `of a vacuum circuit interruptor can be improved by using a shield constructed in accordance with the present invention, I have been able to interrupt 19,000 symmetrical amperes R.M.S. current at 15.5 kv. with an interrupter constructed as shown in FIG. 1 having a copper shield of a thickness slightly exceeding the critical value. In an interrupter identical in all essential respects except having a shield of nickel instead of copper, the best performance that could be obtained was the interruption of 14,000 symmetrical amperes R.M.S. current at 15.5 kv. This indicates that the copper shield can tolerate roughly 2 times as much power input as the nickel shield.

In the above discussion, emphasis has been placed on preventing particles of contact vapor from rebounding from the shield into the electrically stressed regions of the interrupter. It is equally important that the stressed regions of the interrupter be maintained essentially free from particles derived Afrom any other source during buildup of the recovery voltage transient. The shield can be a source of such particles if it is not specially procesed before interrupter operation. In this connection, it is not enough merely to remove contaminants from the shield surface. Gases absorbed internally of the shield material tend to be released under the short-duration high temperature conditions accompanying interruption. To minimize the chances that any significant quantity of such gases will be released, I bake the shield prior to its assembly in the interruptor for a prolonged period at temperatures just below its melting point. These temperatures are much higher than the temperature at which the entire interru ter assembly is baked-out. For example, a copper shield I bake at about l000 C. as compared to a temperature of about 400 C. for bake-out of the interrupter assembly. One indication of the degree to which the shield is free of sorbed gases is that insufficient gases are released from the shield even by the temperatures accompanying the interruptionof maximum rated current to raise the pressure within the interrupter above 10-4 mm. of mercury lo of a second after interruption.

FIG. 4 illustrates one way of lreducing the power input per unit of area into the shield without increasing the overall dimensions of the shield. Here the shield I5 is provided with annular corrugations 50, which preferably extend about the entire inner periphery Vof the shield I5. These corrugations make available more area in the critical region adjacent the arcing gap as compared to the area present with the smooth-surface shield of FIG. 1.

. Distributing the average power input over this larger available area, of course, reduces the power input per unit of shield surface area.

Another way of reducing the power input per unit of area into the shield is illustrated in FIG. 5. Here the shield is constructed from an outer imperforate metallic tube 65 and a pair of tubular metallic screens 60, @l disposed in generally concentric relationship within the outer tube 65. The screens are mechanically and electrically joined together and to the outer tube 65 by means of suitable metallic discs 66 and 67 attached to the screens and the tube 65 at the axially opposed ends of the screens. The screens are formed from a heavy metallic wire, preferablyof copper. The heat transferred to a given screen depends upon the number of arc-generated particles that condense thereon. Those particles that pass through the openings in a given screen contribute little or no energy input to that particular screen. Those particles that miss the first screen 60 will have an opportunity to condense on the second screen 61, or if they do not condense there, on the imperforate outer member 65. Thus, the arcing energy is distributed between a series of concentric tubular members instead of being borne entirely by a single tubular member. If the wire of the screens is heavy enough, the temperature rise of the screens can be limited to the desired level.

Another advantage of using the screen-type shields is that those particles that bounce ofil the second screen 6l or the tube 65 are in a region that -is essentially free from electric field since the screens and the tube are all electrically connected together and are at the same potential. Thus, so long as a particle is behind the rst screen 60, it is in a region free of electric stress. Accordingly, as long as these rebounding particles do not penetrate the first screen in a backward direction, they will be unable to trigger a dielectric breakdown across the stressed regions of the interrupter. Since the screens 60 and 6I can capture a large percentage of these rebounding particles before they can penetrate the innermost screen 60, the likelihood of a breakdown being initiated by these particles is materially reduced.

Although I have illustrated the inner members 60 and 6l as being of a wire-screen construction, it should be apparent that these members can alternatively be of some other perforated construction. For example, they can be formed from sheet metal tubes containing a large number of closely-spaced holes corresponding to the openings in the screen. It should also be understood that fewer or more perforated members than are shown at 6d and 61 can be used, depending upon the amount of arcing energy that must be handled by a given interrupter. Also, it should be apparent that the outer tube 65 can be provided with corrugations on its inner surface to lower the power input into this surface, as described in connection with FIG. 4.

In FIG. 6 still another modied form of shield is shown. Here, the shield comprises a tubular outer member Si) and a plurality of inner members 82 welded or otherwise secured at their outer periphery to the tubular member dit. Each of the inner members 82 is an annular disc projecting for a short distance radially inward Ifrom the outer member dd. The inner edges of the plates S2 are preferably rounded to prevent undue concentrations of electric stress at this point. The members S2 are disposed in spaced-apart relationship to each other along the longitudinal axis of the tubular outer member Sil. All of the members 8d and S2 of which the shield is constructed are of metal and are at the same voltage since they are electrically connected together by the weld joints which secure the discs d2 to the tube 8d. Accordingly, the spaces d3 between adjacent discs are essentially free of electric stress.

During interruption, particles from the arcing region enter the spaces 83. ln the region that is axiallyeligned with the arcing gap, most of these particles travel entirely through the spaces 83 and strike the inner surface 86 of the tube Sil, where they are condensed and adhere. Some of the particles will, however, bounce oft of this inner surface 8o. Such rebounding particles are unable to trigger a dielectric breakdown so long as they remain in the spaces 83 since these spaces are essentially free of electrical stress, as pointed out hereinabove. The plates 82 serve the highly desirable function of condensing a large percentage of these rebounding particles before they can leave the spaces 83, thus reducing the chances for a dielectric breakdown.

The arc-generated particles appear to follow straight line paths in traveling outwardly from the arcing region, but those particles that rebound from a shield surface 86 may travel at almost any angle to the surface following the rebound. In other words, the angle of incidence may be widely dierent from the angle of reilection. Thus, in 7, a particle that enters the space 83 along a path S7 perpendicular to the inner surface `86 of the tube 8d may bounce oif this inner surface 86 at almost any `angle f to the surface within the 180 degree range bounded by the surface. All those rebounding particles that follow any path other than one within the angle D of FlG. 7 will strike the surface of one of the plates 82 and will thus have another opportunity for condensing and adhering.

The tubular member 8d, though it is illustrated in FIG. 6 as having a smooth internal surface, can be provided with a corrugated inner surface to lower the power input per unit of area in those interrupters that are to handle particularly heavy currents. Another modication that can be made in the shield of FIG. 6 is to form the inner members 82 with a frusto-conical configuration instead ot with the planar disc conguration shown. Such a modiiication is illustrated in FIG. 8. Each of the frustoconical members S?. overlaps the adjacent member 82 immediately thereabove. The space between the adiacent members is substantially free of electric stress since the outer periphery of each member 82 is electricallyconnected to the tubular metallic member 80. A significant percentage of the particles bouncing oil the platesk 82 will bounce into this stress-free region and be captured on the metallic surfaces bounding the stress-free region.

Although the mod' red shields of FIGS. 4, 5, and 6 will perform their vapor-condensing function with improved etliciency if the previouslydescribed limits of power input,

lili material, and thickness are observed, it is to be understood that the invention, in certain of its broader aspects, is intended to comprehend interrup'ters with these modie tied shields even though such limits are not observed. It is important, however, that even the modied interrupters be free from features which limit their interrupting ability to a level below that at which particle-rebound from the shield becomes a significant limiting factor in the interrupting process. For example, the arcing contacts should not be of a refractory metal or contain appreciable amounts of gas or gas-producing contaminants; the shield should not emit signicant amounts of gas even under the most severe interrupting conditions; and the insulation or" the switch should not be exposed to bombardment by vapor particles following a single straight line path from the arcing region. If any of these latter features are present, the improved vaponcondensing ability of the shield is `of little or no signiticance since the interrupting limit is usually reached before significant benefits can be obtained from the vaponcondensing ability of the shield.

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

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

l. In a vacuum type circuit interrupter rated to interrupt commercial power frequency currents in excess of 10,000 symmetrical available KMS. amperes with any degree of asymmetry within one cycle `of arcing time:

(a) a highly evacuated envelope,

(b) a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal that is free of sorbed gases and contaminants to such an eX- tent that if the contacts are deeply eroded by arcing in a two liter chamber evacuated to 10-5 mm. of mercury, the pressure in the chamber is no higher than its initial level 1/{30 of a second after arcing,

(c) a metallic shield surrounding said arcing gap and spaced therefrom, said shield having a restricted portion of its surface area aligned with said arcing gap and extending beyond said gap for 3A inch on either side of said gap,

Virl) said shield being sufficiently spaced from said gap to limit the average power input into said restricted y portion of the shield surface to kilowa'tts per square inch during the most severe interruptions within the interrupting rating of said interrupter;

te) the metal of said restricted portion of the shield being of such a character that Vm is less than .985, where k is the thermal ditusivity of the metal in inches square per second and K is the thermal con ductivity in watts per inch-degrees centigrade, both at lnormal ro'om temperature, the thickness of said restricted portion of the shield being at least as great as about lu/ inches.

2. The interruptor of claim l in which said shield is free of sorbed gases and other contzuninants to such an extent that the yquantity of gases released from said shield in response to the temperatures accompanying said most severe interrupting conditions is so low that the pressure in said envelope is below l0*4 mm. of mercury 1/{0 second after interruption, and in which all those straight line paths leading from the region of said arcing gap to any solid insulation in the interruptor that are not intercepted by other metal parts are intercepted by said metal shield.

3. The interruptor of claim 1 in which the thickness of said restricted portion of the shield is about 1A \/F/\ inches.

4. In a vacuum type circuit interrupter rated to interrupt commercial power frequency currents in excess of 10,000 symmetrical available R.M.S. amperes with any degree of asymmetry within one cycle of arcing time:

(a) a highly evacuated envelope,

(b) a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal that is free of sorbed gases and contaminants to such an eX- tent that if the contacts are deeply eroded by arcing in a two liter chamber evacuated to 10-5 mm. of mercury, the pressure in the chamber is no higher than its initial level lf3() of a second after arcing,

(d) said shield being suiiiciently spaced from said gap to limit the average power input into said restricted portion of the shield surface to 100 kilowatts per square inch during the most severe interruptions 'within the interrupting rating of said interrupter,

(e) the metal of said restricted portion of the shield being copper, and the thickness of said restricted portion of the shield being at least as great as about .l inch.

5. A vacuum type circuit interrupter comprising:

(a) a highly evacuated envelope;

(b) a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal and substantially free of sorbed gases and contaminants which decompose in the presence of an arc to produce permanent gases;

(c) a shield surrounding said arcing gap and spaced therefrom for condensing vapors emitted from said gap during arcing;

(d) said shield comprising a first tubular member of metal directly exposed to said vapors and bei-ng of a perforated construction that permits some of said particles to penetrate said first tubular member, additional tubular metallic structure surrounding said iirst tubular member for condensing vapor particles that pentrate said first tubular member, and means for electrically connecting said tubular members together so that they are both normally at the same potential;

(e) all of the arcing within said interrupter normally being coniined to a location disposed radially inward of said irst tubular member and separated from said `lirst tubular member by an evacuated space.

6. The interiupter of claim 5 in which said first tubular member is of a screen-type construction with openings that allow some of the vapor particles to pass therethrough and to condense on said additional tubular structure, said first tubular memberbeing spaced radiallyinward from said additional tubular structure.

7. The interrupter of claim 5 in which said additional tubular structure is suiiiciently imperforate to prevent vapor particles from passing completelytherethrough in a radially-outward direction.

8. The interruptor of claim 5 in which said perforated tubular member: Y

(a) is of a metal having a \/k/K less than .11, where k is the thermal diflusivity in inches square per second and K is the thermal conductivity in watts per inch C., both at normal room temperature, and

(b) has a thickness in the region of said arcing gap at least as great as about 1n/ k.

9. The interruptor of claim 8 in which the metal of said perforated member has a \/k/K less than .085.

10. The interrupter of claim 1 in which said shield has a corrugated inner surface in the region of said gap to increase the effective area available for vapor condensation as compared to a similar shield with a smooth surface.

11. A vacuum type circuit interrupter comprising a highly evacuated envelope, a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal and substantially free of sorbed gases and contaminants which decompose in the presence of an arc to produce permanent gases, a metallic shield surrounding said arcing gap and spaced therefrom for condensing vapors emitted from said gap during arcing, said shield having a corrugated inner surface in the region of said gap to increase the effective area available for vapor condensation as compared to a similar shield with a smooth inner surface, al1 those straight line paths leading from the region of said arcing gap to any solid insulation in the interrupter that are not intercepted by other metal parts being intercepted by said shield, the space between said arc gap and the corrugated inner surface of said shield being unobstructed and free of solid material capable of intercepting vapors traveling from said arcing gap to said corrugated inner surface.

12. The interrupter of claim 1 in combination with metallic structure disposed radially-inward of said inner surface and containing openings extending generally radially therethrough to expose said inner surface to vapor particles from said arcing gap, said metallic structure being electrically connected with the remainder of said metallic shield and providing condensation surfaces for vapor particles rebounding from said inner suface.

13. A vacuum type circuit interruptor comprising a highly evacuated envelope, a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal and substantially free of sorbed gases and contaminants which decompose in the presence of an are to produce permanent gases, a metallic shield electrically isolated from both of said contacts, said shield surrounding said arcing gap and spaced therefrom for condensing vapors emitted from said gap during arcing, said shield comprising a tubular metallic outer member and metallic inner structure disposed radially-inwardly of said outer member and containing openings extending therethrough to expose the inner surface of said outer member to vapor particles from said gap, said tubular outer member being sufiiciently imperforate to prevent vapor particles from passing completely therethrough in a radially outward direction, means for electrically connecting said inner metallic structure to said outer tubular member so as to produce a region substantially free of electric stress immediately adjacent the inner surface or" said outer member, all those straight line paths leading from the region of said arcing gap to any solid insulation in the interrupter that are not intercepted by other metal parts being intercepted by said metallic shield all of the arcing within said interrupter normally being confined to a location disposed radially inward of said inner metallic structure and separated from said inner metallic structure by an evacuated space.

14. The interrupter of claim 13 in which said inner structure comprises a plurality of annular metallic plates projecting radially inwardly from said outer tubular member, said metallic plates being electrically connected to said outer tubular member and deiining regions therebetween substantially free of electric stress. y

15. in a vacuum type circuit interrupter rated to interrupt commercial power frequency currents in excess of 10,000 symmetrical available R.M.S. amperes with any degree of asymmetry within one cycle of arcing time:

(a) a highly evacuated envelope;

(b) a pair of relatively movable contacts that are separable to establish an arcing gap therebetween, said contacts being of a non-refractory metal that is free of sorbed gases and contaminants to such an extent that if the contacts are deeply eroded by arcing in a two liter chamber evacuated to 10-5 mm. of inercury, the pressure in the chamber is no higher than its initial level 1/@0 of a second after arcing;

(c) a metallic shield surrounding said arcing gap and spaced therefrom, said shield having a restricted portion of its surface area aligned with said arcing gap and extending beyond said gap for 3/r inch on either side of said gap;

(d) said shield being suticiently spaced from said gap to limit the average power input into said restricted portion of the shield surface to 100 kilowatts per square inch during the most severe interruptions within the interrupting rating of said interrupter;

(e) the metal of said restricted portion of the shield being of such a character that \//c/K is less than .11, Where k is the thermal ditfusivity of the metal in inches square per second and K is the thermal conductivity in watts per inch degrees centigrade, both at normal room temperature,

(f) the thickness of said restricted portion of the shield being at least as great as about 1A \/k inches;

(g) said shield being 'free of sorbed gases and other contaminants to such an extent that the quantity of gas released from said shield in response to the ternperatures accompanying said most severe interrupting conditions is so low that the pressure in said envelope is below 101 Inni. of mercury j/,0 second after interruption;

(h) all those straight line paths leading from the region of said arcing gap to any solid insulation in said interrupter that are not intercepted by other metal parts being intercepted by said metal shield.

References Cited by the Examiner UNITED STATES PATENTS 1,819,154 8/31 Eschholz 200--144 2,740,915 4/56 Jennings 20G-144 2,794,885 6/57 Jennings 200-144 2,863,027 12/58 Jennings 200-144 2,892,912 6/59 Greenwood et al. 200-144 2,897,322 7/59 Reece 200-144 2,900,476 8/59 Reece 200-144 2,975,256 3/61 Lee et al. 200-144 2,976,382 3/61 Lee et al. 200-144 3,014,107 12/61 Cobine et al. 200-144 FOREIGN PATENTS 571,959 1/58 Italy.

OTHER REFERENCES Publication: Reece, The Vacuum Switch and its Application to Power Switching, Journal of the Institution 25 of Electrical Engineer (British); May 1959, vol. 5; pp.

BERNARD A. GILHEANY, Primary Examiner.

ROBERT K. SCHAEFER, Examiner.

Claims

1. IN A VACUUM TYPE CIRCUIT INTERRUPTER RATED TO INTERRUPT COMMERCIAL POWER FREQUENCY CURRENTS IN EXCESS OF 10,000 SYMMETRICAL AVAILABLE R.M.S. AMPERES WITH ANY DEGREE OF ASYMMETRY WITHIN ONE CYCLE OF ARCING TIME: (A) A HIGHLY EVACUTED ENVELOPE, (B) A PAIR OF RELATIVELY MOVABLE CONTACTS THAT ARE SEPARABLE TO ESTABLISH AN ARCING GAP THEREBETWEEN, SAID CONTACTS BEING OF A NON-REFRACTORY METAL THAT IS FREE OF SORBED GASES AND CONTAMINANTS TO SUCH AN EXTENT THAT IF THE CONTACTS ARE DEEPLY ERODED BY ARCING IN A TWO LITER CHAMBER EVACUTED TO 10-5 MM. OF MERCURY, THE PRESSURE IN THE CHAMBER IS NO HIGHER THAN ITS INITIAL LEVEL 1/30 OF A SECOND AFTER ARCING, (C) A METALLIC SHIELD SURROUNDING SAID ARCING GAP AND SPACED THEREFROM, SAID SHIELD HAVING A RESTRICTED PORTION OF ITS SURFACE AREA ALIGNED WITH SAID ARCING GAP AND EXTENDING BEYOND SAID GAP FOR 3/4 INCH ON EITHER SIDE OF SAID GAP, (D) SAID SHIELD BEING SUFFICIENTLY SPACED FROM SAID GAP TO LIMIT THE AVERAGE POWER INPUT INTO SAID RESTRICTED PORTION OF THE SHIELD SURFACE TO 100 KILOWATTS PER SQUARE INCH DURING THE MOST SEVERE INTERRUPTIONS WITHIN THE INTERRUPTING RATING OF SAID INTERRUPTER; (E) THE METAL OF SAID RESTRICTED PORTION OF THE SHIELD BEING OF SUCH A CHARACTER THAT $K/5 IS LESS THAN .085, WHERE K IS THE THERMAL DIFFUSIVITY OF THE METAL IN INCHES SQUARE PER SECOND AND K IS THE THERMAL CONDUCTIVITY IN WATTS PER INCH-DEGREES CENTIGRADE, BOTH AT NORMAL ROOM TEMPERATURE, THE THICKNESS OF SAID RESTRICTED PORTION OF THE SHIELD BEING AT LEAST AS GREAT AS ABOUT 1/4$K INCHES.