US 1920683 A
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w. s. EDsALL 1,920,683
2 Sheets-Sheet 1 Aug. l, 1933.
ELECTRICAL GENERATING AND DISTRIBUTION SYSTEM AND APPARATUS THEREFOR Filed Aug. 12. 1926 W. S. EDSALL Aug. 1, 1933.
` ELECTRICAL GENERATING AND DISTRIBUTION SYSTEM AND APPARATUS THEREFOR Filed Aug. l2; 1926 2 Sheets-Sheet 2 nm NE QA Q VZV T ve Patented Aug. 1, 1933 UNITED` STATES ELECTRICAL GENERATINGAND DISTRIBU- Y TroN SYSTEM AND APPARATUS THERE- yFoa -YWilliam S. Edsall; Brookline, Mass.; assignor to -Ccndit Electrical Manufacturing Corporation, South Boston, Mass., a Corporation of Massachusetts v AApplication August 12, 1926. Serialv No. 128,867`
21 claims,A (C1. 17a- 298) i This invention relates to electrical generating and distributing systems and apparatusand to houses containing the apparatus, u p
The re hazard in houses containing electric generating, transforming andfswitching apparatus and handlingv large amounts of power is very great. Fire frequently develops in electric generators due to overloads orliash overs. Transformers and electric switches are immersed in oil for the purpose vof providing superior insulation, and in .thecase of the circuitV breakers to facilitate the quenching ofthe circuit interrupting arc. Under certain cases, the break down of insulation in a, transformer or the opening of L) a circuit breakerunder al heavy overload subjects the enclosing casings of the apparatus to high Vor explosion pressures and `hot or burning oil may be thrown out ofl the casings and onto the adjacent apparatus or structures. Current conductors sometimes flash over due to po tential surges on them and theflash-over arc may start a conflagration. Variousmeansvhave been proposed to minimize the fire hazard and to maintain as faras possible Vthe continuity of service. To this end, vthe transformers and switches are placed in separate and isolated compartmentsalsol mechanically isolated from Yeach other and from associated apparatus. Y cooled1 yby, artincially produced blasts.- of air to keep down the temperature thereof.- Regardless of theselprecautions, much `damage has been done by generator lires and firesproduced by the expulsion of burning oil and vaporsl from transformers and switch casings. In some instances, the hot gases have been suiciently .conducting to cause the flash over of remotely disposed circuit conductors with which it came in Contact, thereby disabling theentire system.V
It is an object of this invention to provide an electric generating and distributing wherein the yessential apparatus of the system is immersed in a medium which excludes air from the apparatus and which ordinarily is incapable of supporting combustionwithinit, whereby toY prevent the possibility of disastrous lires.
A further object is to maintain theapparatus constantly immersed in an inertV atmosphere,
which atmosphere, preferably, is continually `re-` It has been proposed heretofore to' maintainV4 AThe current carrying conductors are The generators are i syst em an inert atmosphere within the casings of cer#` tain oil-immersed electrical apparatus `as in the casings of oil immersed transformers and switches, usually by providing gas tight casings and connecting the casingsvwith a source of gas stored under pressure.` Regardless of the diniculties inherent in maintaining a pressure tight casing that will not have an excessive loss of gases therefrom and regardless of the expense of constantly renewing the charges'of inert gas in the gas containers and the danger of loss of gas supply unknowingly, the arrangement is a step in the right direction, but does not provide complete protection from fire hazard. 'While the maintenance of an ninert gas above the oil in an oil immersed apparatus serves to prevent explol sions within the casing, it can not prevent a re outside of the casing. l When an electric switch,
.for instance, interrupts its circuit under heavy load, and oil is thrown from the casing into'the chamber containing it, the oil may not be burning when expelled from the switch byireason of the presence of inert gases within the casing, vyet the gasmay be at a very high temperature due to the heat of `the,circuit-interrupting arc. When the hot oil and especially the :'hotva'pors v,expelled from the switch casing comein contact with the Vair around the casing, the hot oil and vapors may burst into ilameimmediately andv the flame may do great damage.
Itis an object of this, invention to provide an inert atmosphere surroundingthe casings of apparatus so that any oil or oil vapors expelled therefrom kare `received in the inert atmosphere andso can not burn; and it is a further object to maintain the hot oil vapors and hot oil inthe inert atmosphere at least until they have cooled belowv their ignition point in air. v
It is a urtherobject of the invention to maintainan oil immersed electrical apparatus lin a compartment containing aninert gas, and to vent the casing of the electrical apparatusinto the compartment whereby not only'to surround the casing with the inert gas but to provide for the occupation of the'casing with inert gas.
.As has been previouslyset forth, one of the greatv diiiiculties experienced is the provision of a supply of relatively inexpensive inert gas. Every coal burning electric powercv generating ksystem is also a generator-,ofY abundant volumes of inert gas in that `4the products vof combustionof fuel inthe boiler furnace are inert gases, consisting mainly ofnitrogen and carbon dioxide with so little amounts of free oxygen' and carbon monoxide as to be negligible; and itis anobject of this invention to utilize the waste products of combustion which are ultimately discharged into the atmosphere, as the supply of inert gas to satisfy the requirements of this invention. The supply of waste products of combustion is ample for the requirements of this invention and the supply is continuous for so long as power is being generated and therefore for so long as protection is needed for the essential apparatus of the system. In those systems and apparatus wherein the waste products or" combustion resulting from the generation of steam for power purposes is not available, I propose as a further object of the invention to generate my supply of inert gas by an especially provided fuel burning furnace and I may or may not, as the case may be, utilize the heat resulting from the burning of fuel for any particular purpose.
As has previously been stated, dynamo electric machines, as generators, are artiiicially cooled by circulating air through the generator casings. It is a further object of this invention to circulate a cool inert gaseous fluid through the generator casing and to utilize the inert gas to cool the generator in addition to its function in preventing disastrous iires within the casings since combustion can not be maintained in the inert gas. It is also an object of this invention to circulate the inert gases formed by the combustion of fuel through the casings of electric generators. The specific heat of the products of combustion is somewhat greater than the specic heat of air and consequently the cooling eect of the products of combustion on the electrical apparatus is somewhat greater than that of the air.
The spacing between the exposed high-tension electric conductors of the system, and between the conductors and ground depends upon the potentials of the conductors and the dielectric strength of air. For high potential conductors the spacing or distances between the conductors and adjacent objects must be relatively great and, consequently, the house enclosing the conductors and associated apparatus must have substantial dimensions and be correspondingly expensive in construction.
It is an object of this invention to increase the factor or" safety of the insulation of high tension conductors in control houses and to reduce the dimensions and consequently expense of construction of such houses by maintaining the high tension conductors in a gaseous fluid having a substantially higher dielectric strength than air and, specically, in a gaseous uid composed mainly of combustion gas, which has a high dielectric strength and is inert so that danger from res is minimized. Combustion gas is composed mainly of nitrogen and carbon dioxide and has a dielectric strength which is about sixteen percent greater than air, both being at atmospheric pressure. By the use of combustion gas the factor of safety of the gaseous insulation of the conductors is increased by this material extent, thereby reducing very materially the possibility of ash over due to potential surges on the conductors, or reducing by this amount the spacing between the conductors and grounded objects, as partition walls, thereby permitting a material reduction in the dimensions and consequently expense of construction of the houses enclosing the conductors.
By the provision of the circulation of the products of combustion from the furnaces of steam generating boilers through and about the essential apparatus of an electric generating system, I am enabled to provide a unique design for the arrangement of the apparatus of the system and for the house containing the apparatus and this constitutes a yet further object of the invention.
A still further object of the invention is generally to improve the construction and arrangement of electric generating systems and apparatus and the houses containing them.
Fig. 1 is a transverse sectional elevation, diagrammatically illustrated, through a generating and distributing station provided for the performance of this invention.
Fig. 2 is a perspective diagrammatic detail partly in section of a somewhat modied arrangement for conducting the inert gases formed of the waste products of combustion of fuel in the furnace of the steam boiler through the compartments containing translating apparatus and current conductors.
Fig. 3 is a sectional elevation through one of the switches illustrated in Figs. 1 and 2.
In Fig. 2 the invention is illustrated as associated with oil immersed electric switches and the station bus bars of a three phase generating and distributing system. The electric switches 10 one being here shown are of the oil immersed type wherein the movable and stationary contacts are immersed in oil contained in an enclosing casing. The switches are contained within enclosing cells 12, the front openings of which may be closed by doors 14. The doors may be of any suitable type and are adapted to make an approximately gas tight seal with the cell structure when closed so that large amounts of gas will not escape from or leak into the cell. It is not vital, however', to have these doors form an absolutely gas tight seal with the cell structure since the invention provides for such an abundant supply of inert gas that a small amount of leakage will not nullify the invention. The electric switches 10 may be of any suitable type, for instance as shown in Fig. 3, and may be adapted to vent into their enclosing cells through vent passages 11 so that the atmosphere of the cell is also the atmosphere within the switch casing. The terminals of the switch are connected to conductors 16 which extend through openings in the rear wall 18 of the cell and connect with line conductors 20 and 22 through disconnecting switches 24 and 26, which conductors 20 and 22 and switches are disposed on the rear wall 18 of the enclosing cell and within a or less gas tight. The buses 30 of the distribution system are disposed in separate superimposed compartments 32 which are located above the switch cells, and the upper line conductors 20 are arranged for electrical connection with said buses in any usual manner.
In accordance with this invention, provision is made for filling or fiooding the switch enclosing cells, and also the conductor and bus compartments and for circulating therethrough and maintaining therein an inert gaseous l'iuid as a gas which is not combustible with air or with the oil and oil vapors of the switch. In `accordance with this invention the inert gas comprises the waste products of combustion formed in the furnace 34 of the steam generating boiler 36 which may supply steam-to drive the electric generators, the circuits of which may be controlled by said switches 10. The waste products of combustion from the furnace may pass from the boiler header `38 intothe'stack 40V under usual conditions. In accordance with this invention, waste products of combustion are taken from the headerv 38 through a pipe 42.' Said pipemay be provided with a valve 44 therein to control the amount of waste products ofcombustion taken to maintain an inert atmosphere around the essential apparatus of the system. `Preferably, the waste products of combustion are passed through a dust separating and gas cooling apparatus'46 from which the gasespass into-the pipe 48 in a practically dust-free and cool state. 'A blower as the .tan 50 may be located at any suitable place in the gas supplysystem as in the pipe 48 to move the waste products of combustion positively through thefsystem'. The pipe 48 communicates with a header 52 which may be extended injparallel relation with and disposed adjacent the row of. switch cells. `Individual ducts 54 are preferably extended from said header 52 and'terminate in the bottoms of the individual switch cells whereby to deliver the cooled products of combustion into the cells. Said ducts-54 may each be provided with a valvev 56 by Iwhich' the supply of inert gases to each cell may be suitably regulated. The cells l2 may be in conimunication through the top openingsf58 with the bottom of the lowermost bus compartmentBZ and the bus compartments may bein gascommuni- Vcation with each other through similar openings 60 formed in their bottom walls. Inert gas may be caused to pass from the bottom of the switch cells through openings `62 in the rear` wall there-l of into the bottom` of the conductor compartment 28. The upper/.portion of said wall 18 may be provided with openingsA 64 therein which communicate ywith the bus compartments 32` atmosphere.l The particular manner in; which the inert gases are caused to circulate through the Various switch and conductor and bus compartment is herein of no particularconsequence so long as the compartmentsV are `v4*maintained lled with inert gas in suflcientquantities to render atmosphere therein permanently incapable of supporting combustion or at least for during such periods as the possibility'iof res would otherwise be present. It is not essential that the various compartments be completely gas tight as the supply of inert gas is abundant. Preferably the inert atmosphere in the various compartments is at least at atmospheric pressure and preferably slightly .thereabove whereby to prevent-leakage of air into the compartments in quantities to dilute the inert atmosphere 'sufliciently to render it combustible. By reason of the inert atmosphere surrounding the electric switches and current carrying components of the system, the danger of a disastrous fire is practically eliminated. Since the oil immersed electric switches are surrounded by an inert atmosphere andbreathe in the inert atmosphere explosions of the oil gases or Yvapors above the oil within the switch is prevented. Whilehot oil and oil vapors may be expelled ,fromthe switch under severe conditions of v'circuit Vinterruption the hot oil and vapors cannot take re'as they issue from the switch since vthey `are discharged into an inert atmosphere in which combustion can not be supported. Disastrous results ofv flash overs between the adjacent vcurrent carrying conductors 'is minimizeddue to the superior insulationprovided by the inert gases and also since there can be no flame to form hot-gases that might Aof the system and thereby enables it to carry a greater load than is possible without provision for circulation of the atmospheresurrounding the apparatus. The circulation ofr the inert gases also serves to carryaway the gases and vapors formed in theoil immersed apparatus during circuit interruption and to cool them below the ignition point before they are nally liberated into the atmosphere. The supply of inertl gases isabundant and continuous for vso longi as the steam boilers are in operation and tem. In the case of substations and other locations wherein steam boilers are not linvolvedin -the generation of electric energy, I propose to generate an abundant supply of waste products of combustion byburning fuel in a fuel burning furnace especially installed for the purpose and I may utilize the heat resulting from the burning of the fuel in any suitable or desirable manner.
By. the utilization of the waste gaseous products" of combustion of steam-operated electricgenerating systems for the purpose of flooding the electrical'apparatus of the system with inert gases, I am enabled to design a steam-operated electrical-generating and distributing system with-specific regard to the circulation of the waste products of combustion about the electrical apparatus of the system and Fig. l illustrates a steam operated electrical generating and distribution station arranged in accordance with this invention. As illustrated in Fig. l steam boilers may be arranged in parallel rows in the loweris no electric energy to be controlled Vby the sysllG most room '720i the `station and the waste products of combustion from the boilers are adapted to pass into headers 74Vlocated in the upper part of the boiler rooms.
immediately above the boiler rooms Aand said heaters are connected to said headers through conduits 80. Thehot furnace gases are adapted` to heat the boiler feed water and the hot gases are thereby cooled to about the temperature of the feed water. The partially cooled gases are passed. from the eccnomizers into ahorizontally disposedconduit and thence into a gas cooler land washer 82 located in a washer room 84 at one side of and at the same elevation as the economizer room 78. Said washer 82 is adapted to remove objectionable amounts of solid particlesyas dust, in the gases and to cool the gases in anysuitable manner as by water sprays or screens so that the gases emerging from the washer are practically free from dust and are cold. The cold and dust free gases pass from the washer into a duct 86 which is in communicationV with the casing of the turbine-driven velectric generator 88 disposed in the generator room 90 located immediately above the washer `'room 84.` The cold and inert vgases are adapted Economizers or feed water n heaters '76 are located in an economiser room '78 CII for the generator. Inasmuch as the specific heat of the composite mixture of gases passing through the generator is somewhat greater than the specific heat of the circulating air ordinarily employed in cooling the generator, the generator can deliver a greater output than would be the case if air alone were circulated through the generator under the same conditions. lThe heated inert gases are conducted from the outlet of the generator casing into a conduit 92 which is extended to the upper part of the generator room 90 and is placed in communication with a conduit 94. Preferably each generator in the room is provided with separate inlet and outlet conduits. Said conduit 94 may be located above the roof of the generating station and may connect with a header 96 which header ex tends along the roof and is in communication with the stack 98. The inert gases are ultimately discharged from the upper end of said stack into the atmosphere.
The switching equipment is arranged for vertical isolated phase protection and is disposed on three vertical oors 100, 102, and 104i which are disposed one above the other above the economizer room '78 and at the side of the generator room 90. A double bus system is herein shown so that rooms 100e, 100D; 102e and 1021:.; 104e, and 104D are provided by the Vertical walls 106 and 108, which walls are spaced apart in the middle of the fioors to form compartments or galleries 100e, 162e, and 104e in which compartments the switch mechanisms are disposed and which are of sufcient size to permit free working room for attendants. Buses 110 and 116e, corresponding to one phase of the three phase system, are disposed in said compartments 160e and 100D respectively, in the upper portions thereof adjacent said walls 106 and 108 respectively and are located in enclosures 112 and 112er. Switches 114 and 114e are disposed in said compartment 100a and 100D respectively beneath said buses and are electrically connected therewith. Line conductors 115 and 11511, are connected with the respective phase conductors and switches and are extended transversely along said walls 106 and 108. Similar buses, bus compartments and switches are disposed in the upper compartments 10211, and 10217; 10411,- and 1042) for the remaining phases of the three phase system. Said compartments 100e and 100D; 162e and 1821;; 164e and 10413; may extend longitudinally of the station and each compartment may contain a plurality of switches which are connected with the common bus in said compartments. Each switch may be shielded from neighboring switches by walls 105, in the usual manner.
Said isolated phase compartments 100e and 100b, 102a and 102i), 104e and 104B, and the bus compartments 112 and 1120i, are adapted to be ooded with the cold furnace gases. To this end, a plurality of conduits 116 and 116e are disposed in said mechanism compartments 100e, 102e, and 104e adjacent said walls 106 and 108 and are provided with ducts 120- which extend through said walls and into said phase compartments. Other ducts 122 communicate with said conduits and with the bus compartments 112 and 112a. Said bus compartments may be provided with apertures, not shown, by which the gases in said bus compartments are ultimately discharged into said phase compartments. Said conduits 116 and 116@ are extended downwardly into the economizer room 78 and are in communication with a conduit 126 disposed therein.
Valves or dampers 128 are disposed in said conduits 116 and 11Go whereby to regulate the flow of furnace gases therethrough. Said conduit 126 is extended into the washer room 84 and communicates with the conduit 86 therein between said washer 82 and the damper 87 which regulates the ow of furnace gases into the housing 0f the generator 98. A plurality of conduits 130 and 132 are extended vertically in said phase compartments 10Go and 100b, 102e and 10219, 104a and 104D and are provided with inlet openings in the upper regions of said compartments whereby to provide for a circulation of the furnace gases entirely through said phase compartments. Said conduits 130 and 132 may be connected with headers 134 and 136 which extend longitudinally through the uppermost rooms 104e and 104b. Said headers are connected with ducts 138 and 94 which conduct the gases into the common header 96 and thence into the stack 98. As thus arranged, the phase rooms and the electrical equipment therein is completely flooded with inert gases and consequently combustion cannot be maintained in said rooms.
The switches in said rooms are in venting communication therewith and consequently are filled with inert gas so that the oil of the switches cannot burn in the switch casings and neither can the hot oil or Vapor burn if it is expelled from the casings into the rooms. Consequently, the station appears to be entirely immune from danger of disastrous fires.
The mechanism compartments 100e and 102e and 104e are free from the inert gas and contain atmospheric air only. ventilators 140 may be located insaid mechanism rooms to maintain the rooms substantially clear of any furnace which may/'leak therein so that attendants may occupy the rooms without interference. 1f it is necessary for an attendant to enter one of the phase compartments, the iiow of furnace gases thereinto may be stopped by closing one of the valves 128 and permitting the compartment to clear itself from the gases, either by the natural draft of the stack or by suitable Ventilating means not shown. The system may be operated on the natural draft provided by the stack 93 or it may be operated on forced draft provided by the usual blowers which furnish air under pressure to conduits 15G connected with the boiler furnaces. Preferably the system is operated under forced draft so that the gases in the various parts of the station and apparatus is under a pressure somewhat greater than that of the atmosphere so that atmospheric air cannot f leak in and dilute the inert gases to any delcterious extent. However, the supply of inert gases is so abundant that the amount of air that can leak in through the usual crevices of the station structure is the amount of inert gases present that there is no danger of having an atmosphere present that will support combustion.
Since the apparatus of the separate phases of a multi-phase circuit are isolated from cach other by bodies of inert gases, and since the inert gases have a dielectric strength that is materially greater than that of air, danger of trouble on one phase communicating to an adjacent phase is minimized and, as a consequence, the apparatus of the separate phases can be installed in close relation without danger, thereby permitting a material reduction in the size of the switch house; or the factor of safety f the insulation is materially increased.
so small in proportion to I have not herein shown the invention applied specifically to transformers, reactors and other electrical apparatus as the application of the inventionthereto will be the saineas ,tothe electricV switches andlgenerators herein illustrated sothat any` additional showingV is 'consideredl unneces- Whilethefshowing of Figi illustrates .an isolated phasesystem and that cFig. 2- illustrates a grouped-:phase system,l the application oi the broad principle-of my inventionA is not restricts-l to any particular arrangement of the system and of the apparatus thereof.
1. The combination of an electric switch having an oil-containing casing in which the separable switch members are contained,`said casing having avent passage therethrough, and means to normally maintain an inertv gaseousA atmosphere about said casing and the outlet of said vent passage. i
2. The combination of an Aelectric switch having an oil-containing casing in vwhich the separable switch members are contained, said casing having a vent passage therethrough, and means to normally maintain an inert gaseous atmos-l .phere of combustion gas about said Casing and the outlet of said vent passage.
3.V The combination of an electric switch. having an oil-containing casing in which the separable switch members are contained, said casing having a vent passagev therethrough, and means to normally maintain an inert' circulatory gaseous atmosphere about said casing and the outlet of said vent passage.
Within said casing including means providing an inert atmosphere about said casing in communication with the interior thereof through s id passage.
6. A control house having a room, an electrical apparatus contained therein, said apparatus having exposed electrical conductors and means to maintain a constant circulatory inert atmosphere having a dielectricl strength greater than thatvof air in said'room about said conductors to prevent flash-over therebetween.
'7. In a control house, a plurality of compartments containing electrical apparatus, and means to maintain said compartments flooded with an inert atmosphere having a dielectric strength greater than that of air.
8. In a control house for electrical apparatus, an enclosing compartment, an electrical apparatus contained therein, and' means to maintain in said compartment a combustion-gas under a pressure which is elevated abovevthat of the ate mosphere. v
9. In a controlthouse for electrical apparatus, an enclosing compartment, an electrical apparatus contained therein, and means to normallymaintain in said compartment an inert gaseous atmospherehaving a dielectric strength greater deleterious amounts fof atmospheric air.
' v 10-.'In a2 control house ,foreletrizcali apparatus,
a compartment having a' wall, abus vlocated in the'. upper `portion of said compartment adjacent saidwall,l an electric switch connected with and disposed beneath-said bus adjacent' said`-wall,
and means-to normallymaintain an inert gaseous` atmosphere having a dielectric strength greater than that of airwithin said compartment 'to surround .said Y bus land switch.
11. The combination-o a fuelburning furnace,
stack for the gaseous` lproducts ofcombustion of said furnace, a `switch house havingavv room containing an electrical apparatus, fandQ means to conduct gaseous products of combustion from said furnace into said room and out` of said room into said stack.
12. rIhe combination of a fuel burning furnace, a stack for the gaseous products of combustion of said furnace, a switch house having a plurality of compartments containing electrical apparatus, separate means to conduct gaseous products of combustion from said furnace into said compartments and out of said compartments into said stack, and means to regulate the iiow of gas through each compartment.
13.A In a switch house, an isolated switch room,
and agallery extended along side said room,
said gallery having an' atmosphere of air in it, Y
and saidrswitch room having an atmosphere of inertl gas in it.
14. The method of preventing fires in the vicinity of oil-immersed electric apparatus which consists;in circulating acool inert gaseous fiuid about the apparatus to receive hot combustible oil or oil vapors discharged from the apparatus,
and `discharging the gaseous fluid and the' com-l bustible oil or oil vapors into the atmosphere only after the oil or oil vapors has cooled below its ignition point with they atmosphere.: i
l5. An electric generating plant comprising an electric generator, exposed conductors, an oil switch for controlling the circuit through' said generator and conductors, means for generating gaseous combustion products neccessary for the production of power to operate said generator,
- and means for circulating said combustion products to cool and insulate the generator, to insulate said conductors and to provide a non-combustion supporting atmosphere externally about said switch. i
16. Anelectrical distribution system comprising'conductors'of substantial length, an oil switch connected thereto, means for generating'gaseous Y combustion products, and means for circulating said gases externally about the oil switch to pro-Y vide an inert non-combustion supporting atmosphere for preventing combustion of hot oil expelled from the switch and for circulating said gases about said conductors whereby to improve their insulation. Y t
17. The combination of an electric switch having an oil-containing casing infwhich the sepof hot oil and gases ejected from an oil switchV arable switch members are contained and means casing which comprises circulating an inert gase" Vous atmosphere externally'about said casing lin the .vicinity thereof whereby hot oil and gases ejected from said casing come vin contact'with said inert gaseous atmosphere; Ythan that of air and under a pressure which exi cludes the entrance into the compartment of 19. In combination,V a compartment, an oil switch comprising an oilfllled casing within said compartment, and vmeans to normally maintain vented from burning and causing a flash-over between said terminals;
2l. The combination of an electric switch having an oil-containing casing in which the separable switch members are contained, said switch having exposed terminals on the exterior of said casing, and means to normally maintain an inert circulator57 gaseous atmosphere having a dielectric strength greater than that of air about said casing and said exposed terminals to prevent ash-over therebetween.
WILLIAM S. EDSALL.