US3569651A

US3569651A - Circuit breaker having pressurized liquified gas continuously maintained above instantaneous vapor pressure

Info

Publication number: US3569651A
Application number: US867959A
Authority: US
Inventors: Jean Louis Gratzmuller
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-01-21
Filing date: 1969-10-20
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: GB1104228A; SE331852B; BE675378A; NL149941B; DE1640124A1; DK123677B; JPS4840940B1; NL6600732A

Abstract

A circuit breaker having a circuit interrupting chamber containing a liquefied dielectric gas with a fixed contact member disposed so as to lie in the liquefied gas and a movable contact member disposed in the chamber so as to have its contact making surface within the liquefied gas for all of its positions relative to the fixed contact, and having means, such as a hydraulic accumulator or a compressed gas compartment communicating with the circuit interrupting chamber; i.e., other than the gaseous phase of the liquefied dielectric gas for continuously maintaining the liquid pressure above the critical pressure of the dielectric gas.

Description

United States Patent [72] lnventor Jean Louis Gratzmuller [56] References Cited NeuilIy-sur-Seinc, Hauts-de-Seine, France UNITED STATES PATENTS [21] PP'- 867,959 3,150,245 9/1964 Leeds et al 200/14s .7 [22] Filed Oct. 20, 1969 FOREIGN PATENTS [45] Patented Mar. 9, 1971 [32] Priority Jan.2l, 1965 1,281,324 12/1961 France 200/1508 [33] France 1,323,669 3/1963 France 200/1508 [31] 2 1,349,314 12/1963 France 200/1508 Continuation application sen 1,143,890 2/1963 Germany.... 200/150 521,852, Jan. 20, 1966, abandoned, 1,164,536 3/1964 Germanvm ZOO/148.5

609,589 10/1948 Great Britain ZOO/148.7

Primary Examiner-Robert S. Macon Attorney-Jacobi, Davidson, Lilling & Siegel ABSTRACT: A circuit breaker having a circuit interrupting chamber containing a liquefied dielectric gas with a fixed con [54] PSRESSURIZED tact member disposed so as to lie in the liquefied gas and a INSTANTANEOUSUVAU S E E movable contact member disposed in the chamber so as to 11 9 D PDR R s URE have its contact making surface within the liquefied gas for all of its positions relative to the fixed contact, and having means, [52] US. Cl. 200/148, such as a hydraulic accumulator or a compressed gas compart- 200/150 ment communicating with the circuit interrupting chamber; [51] Int. Cl. H0lh 33/68 i.e., other than the gaseous phase of the liquefied dielectric gas [50] Field of Search ZOO/148.2, for continuously maintaining the liquid pressure above the 148.7, 148, 150 (foreign), 150.1, 150 (G), 148.1

critical pressure of the dielectric gas.

PATENTEU MAR 919m sum 1 a; 3

PATENTEDNAR 91971 SHEET 2 U? 3 FIGS FIGS

PATENTED MAR 9 I971 SHEET 3 OF 3 a15:5 :E .i

FIGS

CIRCUIT BREAKER HAVING PRESSURIZED LIQUEFIED GAS CONTINUOUSLY MAINTAINED ABOVE INSTANTANEOUS VAPOR PRESSURE CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation of application Ser. No. 521,852, filed Jan. 20, 1966, now abandoned.

FIELD OF INVENTION This invention is for improvements in or relating to electric circuit breakers.

On the basis of the insulating fluid in which the circuit is broken, the known kinds of circuit breakers can be divided into two main kinds with rupturing properties compatible with the ever increasing short circuit powers of highand mediumvoltage networks. These two kinds are:

1. oil circuit breakers containing either a large volume of oil, as in US. practice, or a reduced volume of oil as in European practice, and

2. gas circuit breakers such as air blast circuit breakers and more recently sulfur hexafluoride (SF6) circuit breakers.

This invention relates to circuit breakers using a novel dielectric agent for arc extinction.

The invention relates to a circuit breaker wherein the insulating fluid in which the arc is quenched is a liquefiable dielectric gas kept liquid, at least in the circuit breaker rupture chamber, at normal outside ambient temperatures by pressurization, the fluid being chosen amongst those having, when in the gas state and liquid state, physical, chemical and/or electrical properties better than air and oil, respectively, for are quenching.

The only liquid insulants so far used in circuit breakers i.e., oils and similar products -have the advantage inherent in their liquid state, including a higher heat transfer coefficient than gases and therefore a greater rate of heat dissipation of the arc column, and therefore a short time constant which helps to break AC arcs as the voltage is passing through zero because of the rapidity with which the dielectric medium is restored. However, a disadvantage of oil is that the arc decomposes it, conductive carbonaceous products being formed.

In the case of gas circuit breakers, endeavors have been going on for a number of years to replace air by a gas having better properties (time constant, dielectric strength, rupturing capacity) than air, leading to the construction of SF circuit breakers. As well as having advantages over air, SF has the advantage over oil that arcing does not produce any conductive decomposition product. Also, most of the decomposition products recombine immediately to form SF However, a disadvantage of SF circuit breakers is that they must have some form of heating to keep the sulfur hexafluoride in the gas state during cold weather; SP circuit breakers are therefore dependent upon external power and not substantially independent in the same way as oil circuit breakers.

The circuit breakers according to the invention combine the advantages of oil circuit breakers and gas circuit breakers but are free from the disadvantage of both kinds.

According to the invention, the new circuit breakers are of very similar construction to small-oil-volume circuit breakers, but the oil is replaced by a pressurized liquefied gas such as sulfur hexafluoride SF selenium hexafluoride, SeF C F and CF;, SP gases, electronegative gases, and perhalogenated hydrocarbons, such as CCl F known under the name of F reons, and so on. These gases will be referred to hereinafter by the general term of liquefiable dielectric gases.

SUMMARY OF THE INVENTION In a circuit breaker according to the invention, the liquefiable dielectric gas is kept liquid at a substantially constant pressure which for some gases can be near or above the critical pressure of the gas. As a rule, the pressurizing means are such that the pressure variations produced by temperature variations are of limited amplitude.

In circuit breakers of this kind, one advantageous way of pressurizing the fluid sufficiently to maintain the same permanently liquid is to use a known hydraulic accumulator of the spring or compressed gas kind and with or without a separating piston. The circuit breaker need not then be dependent upon an external source of power in normal operation. More particularly, no heating or cooling means are required for the circuit breaker to operate outside in all the extreme temperature conditions found, for instance, in continental France of from -40 to +60 C.

A circuit breaker according to the invention can comprise a hermetic rupture chamber in which at least one fixed contact and one moving contact are disposed and which is tilled with a liquefiable dielectric gas in the liquid state. A circuit breaker according to the invention can also comprise a hydraulic accumulator whose liquid compartment is filled with the same gas in the liquid state and is hydraulically connected to the rupture chamber and experiences the pressure applied by the resilient means at the accumulator, such pressure being enough to keep the gas liquid in normal outside temperature conditions.

According to the invention, connecting means between the rupture chamber, the accumulator and/or the outside are provided to enable air to be bled from the system and taking into operation and to deal with volume variation caused by temperature variations.

The invention will be more clearly understood from the following detailed description and from the accompanying drawings which show various embodiments of the invention as nonlimitative examples and in which:

BRIEF DESCRIPTION OF THE DRAWING VIEWS FIG. 1 is a diagrammatic sectioned view of a circuit breaker according to the invention;

FIG. 2 shows a preferred embodiment of the invention;

FIG. 3 is a separate view of a spring-biased hydraulic accumulator for pressuring the system;

FIG. 4 is a diagrammatic sectioned view of a circuit breaker according to the invention;

FIGS. 57 each show a way of filling the circuit breaker with liquefiable gas;

FIG. 8 shows a level-indicating system for a circuit breaker according to the invention, and

FIG. 9 shows another embodiment of a circuit breaker according to the invention wherein the liquid dielectric is pressurized by a pocket of gas disposed at the top of the rupture chamber.

DETAILED DESCRIPTION FIG. 1 shows by way of example a ground-mounted circuit breaker having a rupture chamber 2 at the top of an insulating column 4. In construction the circuit breaker resembles the small-oil-volume type circuit breakers. A movable contact 6 can move in the chamber 2 and is borne on by rubbing contacts 8 connected to a part 10 of a line to be broken and which can engage in a fixed contact or tulip contact 12 connected to another part 14 of such line. The moving contact 6 is operated for closing and tripping by a conventional mechanism 16 including for instance, a hydraulic jack to which the contact 6 is connected by an insulated rod 18.

In the embodiment shown in FIG. 1, the internal volume of the chamber 2 is closed hermetically; more particularly, a gland 20 is provided where the moving contact 6 passes through the chamber end 22. An insulated tube 24 extends into the internal volume or space 19 of the chamber 2, goes through the column 4 and terminates at a liquid compartment 26 of a hydraulic accumulator 28 which is at earth potential. The liquid compartment 26, which can be bounded by a free piston 30, is kept pressurized by the conventional resilient means of the accumulator, such as a pressurized gas cushion 32 trapped in the second compartment of the accumulator. The resilient means can take the form of a spring 33, as shown in FIG. 3.

The internal volume or spaces of the chamber 2, tube 24 and compartment 26 are filled completely by a liquefiable dielectric gas, such as SF or Freon, which is kept liquid by the pressure applied by the gas cushion 32. Of course, the gas used for the cushion, e. g. nitrogen, has a liquefaction pressure appreciably greater than the liquefaction pressure of the dielectric gas used. A conventional valve 34 is provided for reinflatingthe accumulator, and a pressure gauge 36 gives a check on the pressure of the dielectric fluid in the circuit breaker and helps to estimate the quantity of liquid contained in the compartment 26; the pressure gauge 36 can, if required, control safety or pressure-restoring operations automatically. An escape valve 38 at the top of the rupture chamber 2 is used to bleed air from the system when the same is taken into 1 operation.

In a ground-mounted circuit breaker of the kind just described, the usual practice is for the column 4 to be filled with insulating oil, as is normal practice with known circuit breakers. However, the gland 20 may be difficult to keep tight after a time because of erosions of the moving contact 6 by the arc at rupture. In another embodiment of the invention, therefore, which is shown in FIG. 2, the hermetic separation, provided by the gland 20 and the chamber end 22, between the rupture chamber 2 and the interior of the column 4 is omitted and all these spaces are filled with pressurized liquefied dielectric gas. Only one seal between a stationary element and a moving element then remains and it is provided by a gland 40 at the bottom of the column 4, the gland 40 cooperating with jack rod 42 which can be completely smooth and does not experience erosion.

Another feature of the embodiment shown in FIG. 2 is the provision of bleeding of a kind, making it unnecessary to operate the safety valve 38, the same not being readily accessible and being in the live part of the circuit breaker when the same is in operation. Accordingly, an escape tube 44 is disposed at the top of the rupture chamber 2 and comprises a nonreturn valve 46 through which fluid can flow only in the direction indicated by the arrow 48. The tube or duct 44 is insulated and forms a return to the liquid compartment 26 of the accumulator 28. Similarly, the duct 24 connecting the liquid compartment 26 to the interior of the column 4 and chamber 2 has a nonreturn valve 50 through which fluid can flow only from the accumulator towards the circuit breaker.

A draincock 52 which is disposed at the bottom of the duct 44 and which is therefore at earth potential and readily accessible, can connect the interior of the circuit breaker to atmosphere for air bleeding purposes at startup. The accumulator 28' can be isolated by a stop cock 54 during this operation.

Instead of the stop clock 54, a simple nonreturn valve can be used, in which event there is no need to open the stop cock 54 at startup. Of course, these two

valve devices

52, 54 can be combined as a single three-way valve. The duct 24' has fitted to it a pressure gauge 36 and a branch 56 which can be connected to a source of liquefiable dielectric fluid to fill or topup the system. A safety valve 57 can be provided somewhere in the system. Since most liquefiable dielectric gases are much heavier than air, opening the drain cock 52 at the start of filling ensures removal of all the air in the system.

Another variant shown in FIG. 2 is the accumulator 28 since the liquid compartment 26 and the gas compartment 32 are not separated from one another by moving pistons; instead, the resilient cushion 32 of compressed gas, for instance, nitrogen, acts directly on the liquified dielectric gas contained in the compartment 26. The circuit just described which connects the interior of the circuit breaker to the accumulator and selectively to atmosphere and a dielectric fluid source, also provide a one-way flow of the fluid in closed circuit and automatic bleeding of foreign gases.

The accumulator is charged to a pressure P (or else, in the case shown in FlG. 3, the accumulator spring applies a pressure P to the piston) such that the liquefiable dielectric gas is kept liquid at normal ambient temperatures. For instance, if the liquefiable dielectric gas is sulfur hexafluoride, SF the accumulator can be pressurized to at least 35 kg/cm.2 in normal temperature conditions. For Freons the pressure can be merely about from 15 to 20 kg/cmF. lf the temperature drops, the volume of liquefiable dielectric gas in the circuit breaker decreases and the nonreturn valve 46 stays closed, but the valve 50 opens and a corresponding volume of liquefiable dielectric gas is transferred from the liquid chamber 26 of the accumulator to the circuit breaker; Of course, the volume of the gas compartment 32 increases correspondingly and the pressure decreases, but since the temperature has dropped the dielectric fluid remains in the liquid state. In the event of a temperature rise, there is a transfer from the circuit breaker to the accumulator through the duct 44, in the direction indicated by the arrow 48. The transfer is accompanied by a general pressure increase throughout the system, the dielectric fluid therefore tending to remain in the liquid state despite the temperature rise. Clearly, therefore, there is a slow closedcircuit flow of the dielectric fluid in accordance with tempera ture variations. Because of this flow, the liquid can be filtered and the gases return to the accumulator. This leads to a continuous and automatic purging or deaeration of the system.

With some liquefiable dielectric gases having a critical temperature about 45 C. in the case of SP near the temperature to which some circuit breaker parts may rise when exposed to the sun, the pressure can readily go above the critical pressure; indeed, it may be advantageous for the pressure to be kept permanently above the critical level to reliably prevent volume variations causing changes of state. Advantageously, in all cases, accumulator volume is so adapted to the volume of the circuit breaker containing the liquefied dielectric that pressure variations caused by temperature variations always maintain the dielectric in the liquid state, at least in that part of the circuit breaker where the arcis produced.

Although some Freon gases have dielectric properties worse than the dielectric properties of SF the use of all or some Freons" in circuit breakers according to the invention may be advantageous, since their freezing temperatures are very low and their critical temperatures are considerably above all the outside temperatures met with in practice. For the rest, they have a relatively low liquefaction pressure and, depending upon the particular kind actually used, the system can operate at pressures of from 15 to 2'0 kg/cm The circuit breaker shown in FIG. 4 is similar to the circuit breaker shown in FIG. 1 in that the interior 19 of the rupture chamber 2 is filled with a liquefied dielectric gas, such as SF and is separated, by the chamber end 22 and a gland 20, from the interior of the insulated column 4 which can be filled with insulating oil. The circuit breaker also resembles the circuit breaker shown in FIG. 2 to the extent that it comprises a closed circuit for the flow of liquid dielectric between the accumulator 28 and the rupture chamber 2. The closed circuit comprises a duct 44 which extends at the top into the rupture chamber 2 and which communicateswith a duct 44 connected to the accumulator 28, a filter'60 being provided between the ducts 44 and 44', stop

cocks

62, 54 being provided one each on each side of the filter 60. The closed circuit also comprises

ducts

24, 24 connected to the bottom of the chamber 2 and to the accumulator 28, respectively. As already stated, temperature variations produce a flow of the dielectric, the flow being possible only in the direction indicated by the arrows beside the ducts 44 and 24 (since the

nonreturn valves

46, 50 are provided), so that continuous filtering of the dielectric is provided. To replace the filter, the two

stop cocks

54, 62 are closed whereafter the filter element can be changed. If the temperature differences which the system experiences are small or if it is required to speed up filtering, a circulating pump 64 can be provided for the closed circuit.

It is advantageous to filter the SP or similar substance through the filter 60 in operation but it is also advantageous to make the first fill with a fluid thoroughly freed from foreign substances which may be present, for instance, in the cylinders in which the liquefied gas is supplied. Advantageously, there fore, a circuit breaker according to the invention is filled for the first time by fractionated distillation, for instance, by one of the procedures shown in FIGS. 5 and 6. Preferably, in order not to waste SF nitrogen is injected from a cylinder connected to the branch 56, to remove all the air from the system before filling begins. Nitrogen is relatively cheap and, even if it does stay in the circuit, will not cause disturbances, particularly if the accumulator 28 is of the kind not having a separating piston. Once the air has been removed from the system, a cylinder 66 of liquefied SF can be connected to the. branch 56 (FIG. 5) with the interposition of a compressor 68. Delivery from the cylinder is in the gas phase, any impurities in the liquid SF not entering the system. When the SP gas has expelled all the nitrogen, the drain cock 52 is closed (preferably, the drain cock 52 is placed very near the filling branch 56 to obtain a thorough purging, as shown in FIG. 4) and the compressor 68 presses the system to the required value.

In the filling procedure shown in FIG. 6, the cylinder 66 of SP is just connected to the branch 56 via a line 70 comprising a safety valve 72, and the cylinder 66 is heated; for instance, in awaterbath 74. As in the procedure shown inFIG. 5, filling is by distillation, the system therefore being filled with purified dielectric.

In the procedure shown in FIG. 7, the system can be filled.

directly with liquid SF contained in a reservoir 76 similar to a hydraulic accumulator. A free piston 78 divides the reservoir 76 into a first compartment 80-which is filled with liquid SP and which can be connected by a duct 82 to thefilling branch 56-and a second compartment 84 which can be connected to a source of pressurized liquid, such as oil. The source can comprise, for instance, a pump which draws oil from a reservoir 88 and delivers the oil at pressure to the compartment 84. Should the pressure become excessiveand when the piston 78 has reached the end of its travel, the pressurized oil discharges through a safety valve90. A cock 92 of the reservoir 76 is then closed and a cock 94 of a duct 96 is opened to allow the oil in the compartment 84 to return to the tank 88. This procedure is useful for topping up the system, using just a small capacity reservoir 76.

A circuit breaker according to the invention can have provision for visual indication of the amount of liquefied dielectric fluid present in the system, for liquid SF looks-like water and is fully visible in a conventional sight glass type of water gauge. One such system is shown in FIG. 8, only an accumulator 28, of the kind shown in FIG 2, beingshown with the two fluid flow ducts 44, 24' connected to it. Of course, the same system can be used for an accumulator to which only one pressurizing duct, such as the duct 24 in FIG. 1, is connected. The accumulator contains a volume 26 of liquid SF and thereabove a cushion 32 of compressed gas, for instance, nitrogen, which pressurizes the liquid SF directly and not via a piston. A sight glass 98 is connected to the accumulator 28, preferably via two isolating

cocks

100, 102. A visual check is therefore provided on the quantity of liquid dielectric and the quantity of cushioning gas. The accumulator can be recharged with nitrogen via the valve 34.

In the foregoing, the use of sulfur hexafluoride, SF as a liquefied dielectric gas has been more particularly considered, but everything said in the foregoing also applies of course to cases where the dielectric is some other liquefiable gas having satisfactory dielectric properties, such as Freons," or a mixture of several gases. For instance, it may be advantageous in some cases to use mixtures of this kind in order to reduce the freezing point of C. dielectric (about 50 C in the case of SP for circuit breakers installed outdoors in very. cold countries.

In the variant shown in FIG. 9, the rupture chamber 2 is filled with liquid SF depending upon whether or not a hermetic partition is provided between the chamber 2 and the insulated column 4, the same can be filled either with liquid SF or with insulating or some other dielectric, such as gaseous SF In this embodiment, the liquid SF is kept pressurized by a cushion 106 of pressurized gas, such as nitrogen; instead of being trapped in an accumulator as in the previous cases, the cushion 106 is immediately above the level 106 of the dielectric in a compartment 108 disposed above the: rupture chamber 2. The compartment 108 has a sight glass 98and a safety valve 110.

This form of pressurization may cause difficulties in outdoor circuit .breakers eptperiencing very wide daily temperature variations, for unless the compartment 108 were relatively large, there would be large pressure variations. However, this simple solution is very useful for circuit breakers disposed indoors or, even better, in underground stations where the temperatures are substantially constant and pressures vary little.

Also provided are a pressure gauge 36 and a thermometer 112 which, in cooperation with the sight glass 98, enable the actual quantity of nitrogen contained in the system to be known in all circumstances.

In a circuit breaker in accordance with this variant, it might appear simpler for the gas cushion 104 to be formed by gaseous SF above the liquid SF However, when the moving contact 6 moves away from the fixed contact 12at breaking, there would then be a risk of cavitation at the place previously occu-. pied by the moving contact; and so it would be different to. quench the are. On the other hand, if the gas used for the. cushion 104 has a higher liquefaction pressure than SP and is more particularly nitrogen, the pressure above the level 106, such pressure being above the vapor tension of the SP forces the liquid dielectric to fill the gap left by the moving contact 6 as the. same separates from the fixed contact 12; the arc is therefore really quenchedin the liquid dielectric with an effect similar to blowing, since unionized dielectric is supplied" during the break action.

Only a single schematic kind of circuit breaker is shown in thedrawings, but the invention is of course of use with widely differing kinds of circuit breakers having single or multiple rupture chambers.

The invention is not of course limited to the embodiments described and shown and can be varied in many ways withinthe knowledge of the engineer in the art to suit the particular uses intended and without departure from the scope of the invention.

Iclaim:

1. A circuit breaker comprising a circuit interrupting chamber containing a liquefied dielectric gas, a fixed contact member disposed in said chamber so as to lie within said liquefied gas, a movable contact member disposed in said chamber so as to be in the liquefied gas for all of its positions relative to the fixed contact member, and pressurizing means for continuously maintaining the liquefied gas under a pressure higher than the instantaneous vapor pressure of the liquefied dielectric gas, whereby all of said liquified gas is constantly maintained in its liquid state.

2. A circuit breaker as set forth in claim 1 wherein the said liquefied dielectric gas is sulfur hexafiuoride SF 3. A circuit breaker according to claim 1, in which said pressure is continuously maintained above the critical pres.- sure of said liquefied gas.

4. A circuit breakeras set forth in claim 1, wherein the saidliquefied dielectric gas is a perhalogenated hydrocarbon or flugene.

5. A circuit breaker as set forth in claim 1, in which said circuit interrupting chamber is hydraulically connected to the liquid compartment of a hydraulic accumulator, defining said pressurizing means, the compartment being filled with liquefied dielectric. gas which is kept liquid by the pressure applied by the resilient means of the accumulator.

6. A circuit breaker asset forth in claim 5, wherein the hydraulic accumulator is at earth potential and the hydraulic connection between the circuit interrupting chamber and the: accumulator is an insulated duct.

7. A circuit breaker as set forth in claim 5, wherein the resilient means for pressurizing the accumulator take the form" of a pressurized gas contained in the second compartment of the accumulator, the liquefaction pressure of the. latter gasbeing considerably greater than the liquefaction pressure of the liquefied dielectric gas at equal temperatures.

8. A circuit breaker as set forth in claim 5, wherein the hydraulic communication between the liquid compartment of the accumulator and the circuit interrupting chamber includes a nonreturn valve inhibiting liquid flow from the chamber to the accumulator; and a draining and flow duct connects the top part of the circuit interrupting chamber to the liquid compartment of the accumulator and also has a nonreturn valve inhibiting flow from the accumulator to the circuit interrupting chamber, so that the fluid can flow in closed circuit but only unidirectionally.

9. A circuit breaker as set forth in claim 8, wherein the draining duct comprises a changeover means for selectively connecting the circuit interrupting chamber either-the normal case-to the liquid compartment of the accumulator or to atmosphere, when it is required to purge the chamber of gases other than the dielectric gas.

10. A circuit breaker as set forth in claim 1, wherein the circuit interrupting chamber is disposed at the top of an insulated column with insulating liquid, and wherein the circuit interrupting chamber and the internal space of the column are filled with said liquefied dielectric gas, the circuit interrupting chamber and the inside space of the column being intercommunicating.

11. A circuit breaker as set forth in claim 1, wherein said pressurizing means comprises, above the circuit interrupting chamber containing the liquid dielectric, a compartment filled with a compressed gas, such as nitrogen, whose liquefaction pressure is appreciably greater than the liquefaction pressure of the liquefied dielectric gas at equal temperatures, the compressed gas forming a resilient cushion for pressurizing the liquefied dielectric gas contained in the circuit interrupting chamber.

Claims

2. A circuit breaker as set forth in claim 1 wherein the said liquefied dielectric gas is sulfur hexafluoride SF6.

3. A circuit breaker according to claim 1, in which said pressure is continuously maintained above the critical pressure of said liquefied gas.

4. A circuit breaker as set forth in claim 1, wherein the said liquefied dielectric gas is a perhalogenated hydrocarbon or flugene.

5. A circuit breaker as set forth in claim 1, in which said circuit interrupting chamber is hydraulically connected to the liquid compartment of a hydraulic accumulator, defining said pressurizing means, the compartment being filled with liquefied dielectric gas which is kept liquid by the pressure applied by the resilient means of the accumulator.

6. A circuit breaker as set forth in claim 5, wherein the hydraulic accumulator is at earth potential and the hydraulic connection between the circuit interrupting chamber and the accumulator is an insulated duct.

7. A circuit breaker as set forth in claim 5, wherein the resilient means for pressurizing the accumulator take the form of a pressurized gas contained in the second compartment of the accumulator, the liquefaction pressure of the latter gas being considerably greater than the liquefaction pressure of the liquefied dielectric gas at equal temperatures.

9. A circuit breaker as set forth in claim 8, wherein the draining duct comprises a changeover means for selectively connecting the circuit interrupting chamber either- the normal case- to the liquid compartment of the accumulator or to atmosphere, when it is required to purge the chamber of gases other than the dielectric gas.