EP0275772B1 - Housing for an electric device, particularly for a surge arrester, comprising an insulating moulded envelope - Google Patents

Abstract

An enclosure for electric device, in particular for surge arrester, comprises an outer, cylindrical envelope and an inner wall both made of electrically non conducting materials. One of the ends of the envelope is closed while the other is open. An electrode with a principal portion inside the enclosure extends through the envelope and projects outside the latter. Bolt anchors are used for fixing the enclosure on a mechanical support and for mounting a closure device on the open end of the envelope. The material constituting the inner wall is impervious to humidity and protects the envelope against breaking thereof by thermal shock caused for example by the production of an electric arc within the enclosure, while the material constituting the cylindrical envelope is a synthetic insulating material capable of withstanding a high mechanical tension. The envelope is molded on the inner wall and around the electrode and the bolt anchors, whereby the inner wall and the electrode are integrated to the envelope, and the bolt anchors are fixedly attached to the synthetic insulating material.

Description

The present invention relates to an electrical device box of the type comprising an outer casing made of molded insulating material. The present invention relates more particularly but not exclusively to a cylindrical arrester housing of this type.

It should be noted that in this disclosure and in the appended claims, terms such as "insulator" and "insulation" refer to electrical insulation between live or grounded electrically conductive parts.

The surge arrester is an electrical device that is connected in parallel with another electrical device and which has the function of reducing the overvoltages that may occur at the terminals of the latter. The surge arrester therefore makes it possible to reduce the level of insulation of the electrical appliance and consequently its production cost. More specifically, the arrester is normally an open circuit which becomes a closed circuit parallel to the protected device as soon as a significant overvoltage appears across its terminals.

The surge arresters currently available on the market and which are used in electrical energy transport or distribution networks are largely made up of a porcelain shell having the general appearance of a cylindrical tube sometimes closed at one end and which has inside a column of varistors in the form of pellets. It is well known that the varistors are electrically active elements made up of metal oxide or else silicon carbide, and whose impedance varies non-linearly under the effect of an overvoltage so as to provide adequate protection. When an arrester suffers an internal fault, the varistors are permanently short-circuited and this results in an electric arc inside the enclosure which generates explosive overpressures as well as temperatures exceeding the melting point of all known metals. In order to avoid the explosion of the lightning protection enclosures following an internal short circuit, pressure relief valves have been designed in the past, which transfer the electric arc outside using diaphragms and nozzles d 'orientation of hot gases, so as to eliminate internal overpressures.

Obviously, these pressure limiting mechanisms must be mounted on a porcelain casing, which contributes to making them expensive. Indeed, the porcelain envelopes used exclusively until now in the construction of surge arresters cannot tolerate, at moderate cost, the mechanical tensions required for such pressure limiting mechanisms. This is why, above all, these mechanisms are found in the surge arresters of high voltage transmission stations, such surge arresters currently having a unit cost ten times higher than that of the distribution surge arresters used at voltages below 35 kV.

Thus, as the surge arresters currently used in the distribution network are not equipped with pressure limiting mechanisms, they risk exploding following an internal overpressure. However, their cost remains lower than the comparative cost of increasing the level of insulation of the device to be protected. However, this would no longer be the case if their cost increased tenfold to make them non-explosive. Therefore, conventional non-explosive arresters are not currently profitable in the distribution network.

Another disadvantage of existing distribution surge arresters is that, in the majority of cases, they are mechanically supported by a metal strip which encircles their porcelain shell near its center and which is connected to a mechanical support structure which is often electrically placed To the earth. This type of design results in an exaggerated elongation of the porcelain casing for the sole purpose of leaving behind the two electrical ends of the arrester of the metal support strip in order to obtain adequate electrical insulation between this support strip and the two electrical ends of the arrester. This obviously contributes to increasing the cost of a surge arrester of this type.

Another limitation of conventional distribution surge arresters is their poor moisture resistance. Obviously, to increase the pressure applied on the seals, it is not possible to use the same types of high mechanical tension anchors suitable for porcelain and used in surge arresters for high voltage substations because of their prohibitive cost.

In recent years, many synthetic insulators using aggregates and a binder of epoxy, polymeric or other type have produced dielectric characteristics comparable to that of porcelain. These also have two indisputable advantages over porcelain, the ability to hold a very high mechanical tension close to that of concrete, as well as the ability to be molded on metal or other parts.

The present invention therefore proposes to replace the use of porcelain, which has the various drawbacks discussed above, in particular but not exclusively by the use of a synthetic insulator of the type described above in the manufacture of an envelope. insulating molded for surge arrester, and more generally in the manufacture of an insulating envelope molded for an electrical device.

• une enveloppe externe;
• une chemise interne constituée d'un matériau non conducteur d'électricité qui présente une étanchéité à l'humidité et qui assure une protection contre le bris de l'enveloppe externe par choc thermique causé par de la chaleur produite à l'intérieur du boîtier; et
• des moyens d'ancrage pour fixer le boîtier sur un support mécanique.

More specifically, the present invention relates to an electrical device box, comprising:

• an external envelope;
• an inner jacket made of an electrically non-conductive material which is waterproof against humidity and which provides protection against breakage of the outer casing by thermal shock caused by heat produced inside the housing; and
• anchoring means for fixing the housing to a mechanical support.

The envelope is made of an insulating material capable of withstanding a high mechanical tension, and molded on the internal jacket and around the anchoring means, so that the internal wall is integrated into the envelope and the anchoring means are firmly fixed to the insulating material constituting the external envelope.

• une enveloppe externe cylindrique ayant une première extrémité fermée et une seconde extrémité ouverte;
• une chemise interne constituée d'un matériau non conducteur d'électricité qui présente une étanchéité à l'humidité et qui assure une protection contre le bris de l'enveloppe cylindrique par choc thermique causé par la production d'un arc électrique à l'intérieur du boîtier;
• une électrode située à l'extrémité fermée de l'enveloppe cylindrique et qui comporte une portion principale interne au boîtier et un prolongement qui s'étend à partir de la portion interne de l'électrode jusqu'à l'extérieur du boîtier à travers l'enveloppe externe;
• des premiers moyens d'ancrage montés à l'extrémité fermée de l'enveloppe externe pour fixer le boîtier sur un support mécanique; et
• des seconds moyens d'ancrage montés à l'extrémité ouverte de l'enveloppe cylindrique pour fixer un dispositif de fermeture du boîtier.

The invention also relates to a cylindrical arrester housing, comprising:

• a cylindrical outer shell having a first closed end and a second open end;
• an inner jacket made of an electrically non-conductive material which is waterproof against humidity and which provides protection against breakage of the cylindrical envelope by thermal shock caused by the production of an electric arc inside of the housing;
• an electrode located at the closed end of the cylindrical envelope and which has a main portion internal to the housing and an extension which extends from the internal portion of the electrode to the outside of the housing through the 'external envelope;
• first anchoring means mounted at the closed end of the outer casing to fix the housing on a mechanical support; and
• second anchoring means mounted at the open end of the cylindrical casing to fix a device for closing the case.

Again, the outer shell is made of an insulating material capable of withstanding a high mechanical tension. This envelope is molded on the inner jacket and around the electrode and the first and second anchoring means, so that the inner jacket and the electrode are integrated into the cylindrical envelope, and that the first and second means d anchorages are firmly attached to the insulating material constituting the outer casing.

The insulating material constituting the envelope of the housing according to the invention can be, as already mentioned, a synthetic insulator in particular of concrete-epoxy, concrete-polymer or other type. For epoxy concrete, the aggregate is sand and the binder is epoxy, while in the case of polymer concrete, the aggregate is inter alia sand and the binder is a synthetic resin .

By eliminating all the drawbacks caused by porcelain, the insulating material constituting the envelope, which can in particular be constituted by a synthetic insulator, makes it possible to construct lightning arresters of electrical energy distribution networks totally resistant to explosions and envelope breakage, at a cost comparable to conventional distribution surge arresters which are liable to explosion.

Of course, any other material having properties similar to synthetic insulators of the concrete-epoxy and concrete-polymer type could be used in the manufacture of the envelope, without departing from the scope of the invention.

• la Figure 1 représente une coupe selon un plan vertical d'un boîtier cylindrique de parafoudre selon la présente invention;
• la Figure 2 représente une coupe selon un plan vertical d'un parafoudre comprenant le boîtier cylindrique de la Figure 1;
• la Figure 3 représente une vue de dessous du parafoudre de la Figure 2;
• la Figure 4 illustre une coupe selon un plan horizontal d'un dispositif de fermeture du parafoudre de la Figure 2; et
• la Figure 5 représente une vue en plan du parafoudre de la Figure 2.

The advantages and other characteristics of the present invention will emerge from the following detailed description of an example embodiment thereof, applied to surge arresters and given by way of non-limiting example only with reference to the appended drawings in which:

• Figure 1 shows a section along a vertical plane of a cylindrical arrester housing according to the present invention;
• Figure 2 shows a section along a vertical plane of a surge arrester comprising the cylindrical housing of Figure 1;
• Figure 3 shows a bottom view of the arrester of Figure 2;
• Figure 4 illustrates a section along a horizontal plane of a device for closing the arrester of Figure 2; and
• Figure 5 shows a plan view of the arrester of Figure 2.

As shown in Figure 1 of the drawings, the arrester housing according to the invention comprises an outer insulating jacket 1 which has the general shape of a vertical cylindrical tube. The casing 1 comprises a closed lower end and an open upper end. As already mentioned, the casing 1 is made of an insulating material, in particular a synthetic insulator of the concrete-epoxy, concrete-polymer or other type. This envelope 1 is molded on an internal jacket 2 and around an electrode 3 and anchors for bolts 4 and 5. In this way, the internal wall 2 and the electrode 3 are integrated into the envelope 1, while that the anchors 4 and 5 are firmly fixed to the insulating material constituting the casing 1 since the latter is capable of withstanding a high mechanical tension.

The inner wall of the envelope 1 and therefore the inner jacket 2 have the shape of a truncated cone which defines an angle 6 suitable for easily removing the interior mold when the molding of the envelope 1 is completed. The angle 6 of the truncated cone also facilitates the expansion of the gases produced by an electric arc occurring inside the housing of FIG. 1 towards an upper pressure limiting mechanism, which will be explained in more detail in the description which follows.

The outer profile of the casing 1 defines a plurality of annular fins such as 7. As the surge arresters are intended to be mounted outside, it is well known that the fins 7 provide dielectric maintenance of the casing 1 under rain and pollution conditions. Obviously, these fins 7 also contribute to increasing the mechanical resistance of the envelope to internal pressures. The angles 8 and 9 of the fins 7 allow the external mold to be easily removed following the molding of the envelope 1.

As shown in Figure 1, the electrode 3 has a main portion internal to the housing and centered on the vertical geometric axis 10 thereof. The electrode 3 further comprises an extension which crosses the casing 1 radially and which extends up to a certain distance outside the casing 1 which is suitable for making an external electrical connection and for attracting and receiving the electric arc as will be described below.

Figures 2 to 5 illustrate a surge arrester using the housing described above with reference to Figure 1 of the drawings.

As illustrated in Figures 2 and 3, the casing 1 is mounted on a mechanical support 12 using three bolts 13 inserted in the three anchors 4. Figure 3 shows precisely the position of the three anchors 4 and associated bolts 13. In order to obtain a more solid fixing of the anchors 4 in the insulating material constituting the envelope 1, bulges such as 14 are provided around each of the anchors 4.

The outer profile of the casing 1 lower than the electrode 3 is designed so as to ensure adequate dielectric rigidity between the electrode 3 and the conductive parts associated with the mechanical support 12 over the distances 15, 16 and 17 while minimizing the volume required of the insulating material in the manufacture of the casing 1, and consequently the mass and the cost of the arrester. To this end, the external profile of the casing 1 lower than the electrode 3 comprises the lower fin 7 ', a flange 18 and a recess 19 identified in Figure 2 of the drawings.

The arrester further comprises a column of varistors such as 20 each having the shape of a patch, this column being centered on the geometric axis 10. The column of varistors 20 is retained in place by means of a spring. mechanical support 21 suitably mounted between the internal portion of the electrode 3 and the varistor column 20, and using the arrester closing device.

The internal portion of the electrode 3 comprises an upper part of reduced horizontal section which makes it possible to hold the spring 21. In addition, a connector 23 parallel to the mechanical holding spring 21 establishes electrical contact between the underside of the column of varistors 20 and the electrode 3.

The device for closing the arrester, which consists of a pressure limiting mechanism, firstly comprises an annular cover 24 which is electrically conductive and securely fixed to the casing 1 by means of three bolts 25 associated with the three anchors 5. three holes 26 drilled through the cover 24 to allow its attachment to the casing 1 using the anchors 5 and bolts 25 are provided with an upper recess such as 27 so that the head of the bolts 25 does not exceed the upper surface of the cover 24 so as not to interfere with the fitting of the other elements of the pressure limiting mechanism which are described below.

It should be noted that Figures 4 and 5 clearly show the position of the three holes 26, the three bolts 25, and therefore the three anchors 5. More specifically, the anchors 5 are separated from each other d '' an angle of 120 _° to the vertical axis 10.

An annular rubber seal 28 (see Figure 2) seals against humidity between the cover 24 and the casing 1.

The cover 24 defines an annular corner 29 in which is positioned a centering and holding piece 30 of the varistor column 20. The upper pad 20 ′ (varistor) rests on the piece 30 so that the varistor column 20 can be kept centered thanks to the compressive force exerted by the spring 21.

As illustrated in Figure 4, the centering and holding part 30 has a central opening 30 ', and three peripheral passages 31 allowing the escape of gases when an overpressure occurs inside the arrester housing.

Figure 4 shows several holes 32 each provided with a thread and which are formed in the cover 24. These holes are used for fixing screws 33 (Figure 5) provided for fixing on the top of the arrester a diaphragm 34 and a hot gas exhaust nozzle 35.

The diaphragm 34 is usually made of a sheet of plastic or of thin aluminum, and is mounted between the circular lower contour 35 ′ of the nozzle 35, and the cover 24.

An annular seal 36 of rubber or other elastic material (FIG. 2) seals against humidity between the diaphragm 34 and the cover 24.

The cover 24 finally comprises, as illustrated in FIG. 4, a cylindrical hole 37 provided with a thread and making it possible to mount the upper electrical terminal 38 of the arrester.

The electrode 3 (Figure 2) extends outside the casing 1 to a distance sufficient to make an electrical connection with the external circuit using an explosive bolt 39 mounted in a hole 11 ( see Figures 1 and 3) made at the free outer end of the extension of the electrode 3. The extension distance of the electrode 3 outside the casing 1 must also be adequate to attract and receive the arc electric transferred from the inside to the outside of the arrester box.

In normal condition, the current from the external circuit to which the surge arrester is electrically connected enters via the upper terminal 38 (arrow 40 in Figure 2). It then passes through the cover 24 to which the terminal 38, the part 30 is connected, and is transmitted to the varistor column 20 (see arrows 41, 42 and 43). It then leaves the varistor column 20 to be transmitted to the electrode 3 through the connector 23 (see arrow 44). The current then feeds the circuit connected to the electrode 3 by the bolt 39 (see arrow 45).

In the event of a failure of the varistors 20 of the column disposed inside the casing 1, an internal electric arc 46 occurs and creates an overpressure which perforates the diaphragm 34 thus allowing the hot gases to be evacuated through the passages 31 and the nozzle 35 towards the integrated electrode 3 thus creating an arc 47 between the nozzle 35 which is made of an electrically conductive material and the external extension of the electrode 3. The arc is thus transferred from the interior to the outside of the arrester housing. Such an arc transfer makes it possible to release the interior of the envelope 1 from the pressures and temperatures which could cause it to explode.

A steel blade 48 can also be mounted inside the nozzle 35 in order to facilitate the perforation of the diaphragm 34 during the production of a overpressure inside the arrester housing. More specifically, the blade 48 cuts the diaphragm 34 during its deformation due to internal removal.

Although the inner liner 2 can be made of several materials, such a frosted glass liner ensures both the sealing of the casing 1 against humidity and the protection of this casing against breakage by thermal shock due to the contact of the interior electric arc 46. In fact, during the production of the arc 46, the glass will be touched and will break, thereby preventing the envelope 1 from breaking, which could cause it to explode.

Following the production of the arc and its transfer from the inside to the outside of the housing, the circuit connected to the bolt 39 will be separated from the electrode 3. In fact, it is well known that a bolt explosive such as 39 contains a powder charge, the explosion of which is caused by too large a current (current flowing in the electric arc 46 or 47). A sufficient distance will then be created between the external electrical circuit and the electrode 3 to isolate the arrester from earth when the fault current is interrupted by the circuit breaker provided for this purpose in the electrical supply network. It should be noted that the internal electric arc 46 will usually create a permanent conductive path between the part 30 and the electrode 3 so that following the recharging of the external circuit, the upper terminal 38 is connected to the electrode 3 and the normal network voltage is found on this electrode during normal reclosing of the circuit breaker. The envelope 1 of the surge arrester must then provide adequate electrical insulation between the live electrode 3 and the conductive parts associated with the metallic mechanical support 12 which in several cases are connected to earth. This is the reason why, as already mentioned in the present description, the electrical insulation over the distances 15, 16 and 17 of FIG. 2, must be optimized to ensure adequate dielectric strength between the electrode 3 and the conductive parts associated with the support 12.

• - une économie due à l'usage de matériaux isolants autres que la porcelaine pour fabriquer l'enveloppe 1 ;
• - les matériaux isolants utilisés permettent sans complexité additionnelle l'usage d'ancrages pour boulons peu coûteux qui produisent une meilleure étanchéité du couvercle 24 en permettant l'application d'une pression adéquate sur le joint d'étanchéité 28, résistent à la tension mécanique du mécanisme limiteur de pression monté à la partie supérieure du parafoudre, et permettent de fixer le parafoudre à un support mécanique situé à l'extrémité inférieure de celui-ci et donc avantageusement loin des pièces métalliques sous tension (ceci est rendu possible grâce à la résistance mécanique élevée du matériau constituant l'enveloppe 1 qui maintient fermement en place les ancrages pour boulons);
• - les ancrages pour boulons 5 permettent un montage facile du mécanisme limiteur de pression, simplifiant donc le système d'attache d'un tel mécanisme à l'enveloppe en comparaison avec les systèmes d'attache conventionnels adaptés à la porcelaine pour ainsi permettre de munir à faible coût un parafoudre d'un mécanisme limiteur de pression pour le rendre non explosif;
• - les matériaux isolants utilisés permettent l'intégration de l'électrode 3 agissant comme borne électrique intégrée à l'enveloppe isolante 1 limitant ainsi le nombre et l'encombrement créé par les pièces conventionnelles de raccords ainsi que les frais d'assemblage; et
• - les matériaux isolants utilisés peuvent être moulés sur une paroi intérieure de verre dépoli ou en un autre matériau adéquat qui assure une parfaite étanchéité à l'humidité du boîtier et constitue un écran contre les bris par chocs thermiques auxquels sont sensibles la porcelaine et certains matériaux isolants lorsqu'ils sont mis en contact direct avec un arc électrique de court-circuit interne.

The main advantages of the housing according to the invention can be summarized as follows:

• - savings due to the use of insulating materials other than porcelain to manufacture the casing 1;
• - the insulating materials used allow without additional complexity the use of inexpensive bolt anchors which produce a better seal of the cover 24 by allowing the application of an adequate pressure on the seal 28, resist mechanical tension of the pressure limiting mechanism mounted on the upper part of the arrester, and allow the arrester to be fixed to a mechanical support located at the lower end of the latter and therefore advantageously far from live metal parts (this is made possible by high mechanical strength of the material constituting the casing 1 which firmly holds the anchors for bolts);
• - the anchors for bolts 5 allow easy mounting of the pressure limiting mechanism, thus simplifying the system for attaching such a mechanism to the casing in comparison with conventional fastening systems suitable for porcelain, thereby enabling to provide at low cost a surge arrester with a pressure limiting mechanism to make it non-explosive;
• - the insulating materials used allow the integration of the electrode 3 acting as an electrical terminal integrated into the insulating envelope 1, thus limiting the number and size created by the conventional pieces of fittings as well as the assembly costs; and
• - the insulating materials used can be molded on an inner wall of frosted glass or another suitable material which ensures a perfect seal against humidity of the case and constitutes a screen against breakage by thermal shock to which porcelain and certain materials are sensitive insulators when brought into direct contact with an internal short circuit electric arc.

It has been found that the housing according to the invention makes it possible to produce distribution surge arresters which are totally explosion-proof and casing breakage by thermal shock, and at a lower cost than conventional distribution surge arresters liable to explosion. , as previously mentioned.

Although the present invention has been described using a preferred embodiment thereof, it should be noted that any modification of this embodiment or other use thereof may be made, provided that: respect the scope of the appended claims, without changing the nature of the present invention.

Claims (22)

1. An enclosure for an electrical device, characterized in that it comprises:

- an outer envelope (1);

- an inner wall (2) made of an electrically non conducting material impervious to humidity and protecting said outer envelope (1) against breaking thereof by thermal shock caused by heat produced within the enclosure; and

- anchor means (4) for fixing the enclosure on a mechanical support (12);

- said envelope (1) being made of an insulting material having high mechanical tensile strength, and being molded on said inner wall (2) and around said anchor means (4) so that the inner wall (2) is integrated to said envelope (1) and said anchor means (4) are fixedly attached to said insulating material forming said outer envelope (1).

2. An enclosure according to claim 1, characterized in that said outer envelope (1) is cylindrical.

3. An enclosure according to claim 2, characterized in that said cylindrical envelope (1) comprises an inner surface which is frusto-conical.

4. A cylindrical enclosure according to claim 2, said cylindrical envelope (1) has a geometrical axis (10) and comprises a plurality of external, annular flanges (7) each located in a plane perpendicular to said geometrical axis (10).

5. An enclosure according to claim 1, characterized in that said anchor means comprise a plurality of bolt anchors (4).

6. An enclosure according to claim 1, characterized in that is comprises an opening and additional anchor means (5) for mounting on the enclosure a device for closing said opening, said envelope (1) being molded around said additional anchor means (5) so that said additional anchor means rigidly fixed within the insulating material constituting the outer envelope (1).

7. An enclosure according to claim 6, characterized in that said additional anchor means comprise a plurality of bolt anchors (5).

8. An enclosure according to claim 1, characterized in that it further comprises an electrode (3) having a principal portion inside the enclosure, and an extension attached to said inside, principal portion traversing said outer envelope (1), said outer envelope (1) being molded around the electrode (3) whereby said electrode is integrated to the outer envelope (1).

9. An enclosure according to claim 1, characterized in that the material constituting the outer envelope (1) comprises a synthetic insulating material.

10. An enclosure according to claim 9, characterized in that said synthetic insulating material comprises epoxy-concrete.

11. An enclosure according to claim 9, characterized in that said synthetic insulating material comprises polymeric concrete.

12. A cylindrical enclosure for a surge arrester, characterized in that comprises:

- an outer, cylindrical envelope (1) having a first, closed end and a second, open end;

- an inner wall (2) made of an electrically nonconducting material impervious to humidity and protecting said cylindrical envelope (1) against breaking thereof by thermal hock caused by the production of an electric arc (46) within the enclosure;

- an electrode (3) located at the closed end of the cylindrical envelope (1) and comprising a principal portion inside said enclosure and an extension attached to said inside, principal portion, traversing the outer envelope (1) and projecting outside the enclosure;

- first anchor means (4) mounted on the closed end of the outer envelope (1) for fixing the enclosure to a mechanical support (12) and;

- second anchor means (5) mounted on the open end of the cylindrical envelope (1) for fixing a closure device of the said enclosure;

- said envelope (1) being made of an insulating material capable of withstanding a high mechanical tension, and being molded on the Inner wall (2) and around the electrode (3) and the first (4) and second (5) anchor means, whereby said inner wall (2) and said electrode (5) are integrated to the cylindrical envelope (1), and the first (4) and second (5) anchor means are fixedly attached to the insulating material constituting the outer envelope (1).

13. A cylindrical enclosure according to claim 12, wherein the first and second anchor means comprise bolt anchors (4, 5).

14. A cylindrical enclosure according to claim 12, wherein the closed end of the cylindrical envelope (1) comprises an external cavity (19) and at least one annular, external flange (21) surrounding said envelope (1) for adequately and electrically insulating said electrode (3) from said mechanical support (12), the cylindrical envelope (1) having a geometrical axis (10) and said annular flange (7') being located in a plane perpendicular to said geometrical axis (10).

15. A cylindrical enclosure according to claim 12, characterized in that said cylindrical envelope (1) has a geometrical axis (10) and comprises a plurality of external, annular flanges (7) each located in a plane perpendicular to said geometrical axis (10).

16. A cylindrical enclosure according to claim 12, characterized in that the cylindrical envelope (1) defines a frusto-conical inner surface having a diameter which increases from the closed end towards the open end of the envelope (1), said inner wall (2) being also frusto-conical.

17. A cylindrical enclosure according to claim 12, characterized in that the electrically non conducting material constituting said inner wall comprises frosted glass.

18. A cylindrical enclosure according to claim 12, characterized in that the insulating material constituting the outer envelope (1) comprises a synthetic insulating material.

19. A cylindrical enclosure according to claim 18, characterized in that said synthetic insulating material comprises epoxy-concrete.

20. A cylindrical enclosure according to claim 12, wherein said synthetic insulating material comprises polymeric concrete.

21. A cylindrical enclosure according to claim 12, wherein the extension of said electrode (3) radially traverses the cylindrical envelope (1).

22. A cylindrical enclosure according to claim 12, wherein said closure device of the enclosure comprises pressure limiting means (31, 34, 35, 48).

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