EP0958584B1 - Enhanced varistor-based lighting arresters - Google Patents

Abstract

The present invention relates to a method of manufacturing a lightning arrester, the method being of the type comprising steps consisting in making a stack of varistors (100), and in forming a cover (300) made of a composite material over the stack of varistors (100), said method being characterized by the fact that the step of forming a cover (300) made of a composite material consists in placing a woven fiber fabric (310) on the outside of the stack of varistors (100) and in contact therewith, in placing a flexible outer cover (400) on the outside of the stack of varistors (100), and in injecting a material (350) suitable for impregnating the fiber fabric (310) into the annular space formed between the stack of varistors (100) and the flexible outer cover (400).

Description

The present invention relates to the field of surge arresters.

It applies in particular to surge arresters for high voltage, typically in electrical networks of nominal voltage greater than 1kV effective between phases.

Surge arresters are devices designed to be connected between a power line, in particular medium or high voltage, and the ground, to limit the amplitude and the duration of the overvoltages appearing on line.

These overvoltages can be due for example to atmospheric phenomena, such as lightning, or inductions in the conductors.

These overvoltages can also be due to maneuvers on the live line.

Surge arresters are generally formed by stacking different varistors, most often these days by stacking several discs based on zinc oxide, whose electrical resistivity is strongly non-linear depending on the applied voltage.

More precisely, these varistors do not allow practically to pass no current as long as the voltage across their terminals is less than one ignition threshold and on the other hand, allow a very strong current to pass. up to several tens of kA, when the voltage applied to their terminals exceed the aforementioned priming threshold.

The number of varistors used in the arrester is such the nominal operating voltage on the power line is less than starting threshold across the stack of varistors.

Thus, the arrester can permanently withstand the voltage nominal service, without leakage of current, and allows on the other hand to drain discharge currents of very high intensity which may appear temporarily on the line in case of accidental overvoltage.

Many types of surge arresters have already been proposed.

The field of surge arresters has in fact given rise to a very abundant literature.

• un empilement de varistances,
• deux pièces de contact en matériau électriquement conducteur placées respectivement sur les extrémités de l'empilement de varistances, et
• une enveloppe en matériau électriquement isolant entourant l'empilement de varistances.

Surge arresters known today generally include:

• a stack of varistors,
• two contact pieces of electrically conductive material placed respectively on the ends of the stack of varistors, and
• an envelope of electrically insulating material surrounding the stack of varistors.

The envelope of electrically insulating material mentioned above itself is the subject of a very abundant literature.

GB-A-2,073,965 has proposed, for example, make this envelope using heat shrinkable material.

Documents US-A-4,298,900, DE-A-3,001,943, DE-A-3,002 014 proposed to place in addition an external porcelain case on the heat-shrinkable envelope.

Documents US-A 4,092,694 and US-A-4,100,588 have proposed place each varistor in a silicone ring and arrange the stack of varistors thus surrounded in a case of porcelain.

Document US-A-2,050,334 has proposed to place a stack of varistors in a porcelain case and fill the space formed between the porcelain case and the stack of varistors using a filling material formed for example of a compound halogenated with wax.

Documents EP-A-0 008 181, EP-A-0 274 674, EP-A-0 231 245 and US-A-4 456 942 have proposed to produce the envelope surrounding the varistors using an elastomeric material, formed in particular by overmolding.

More specifically, document EP-A-0 274 674 proposed to overmolding an envelope of elastomer-based composite material, EPDM, silicone, or other resin, loaded or not, on a stack of varistors.

Document US-A-4 161 012 also proposed to have an elastomer envelope on the varistors. This document proposes make this envelope by depositing the elastomer on the outer surface varistors, or by molding the envelope on the varistors, or again by preforming the elastomer shell, then inserting the varistors in it.

Document US-A-3,018,406 proposed, as early as 1958, to carry out the envelope in the form of two complementary preformed shells and an outer shell of plastic injection molded on the varistors.

Document US-A-3,586,934 proposed to produce the envelope using a synthetic resin, for example based on epoxy or polyester, or even a silicone or polyester varnish.

Document EP-A-0 196 370 proposed to produce the envelope on a body of varistors by pouring a synthetic resin formed by example of epoxy resin, polymer concrete, silicone resin, or an elastomer, or by covering the body of varistors with a tube shrinkable plastic, or by providing this stack a layer of synthetic resin.

In addition, the documents, US-A-4,656,555, US-A-4,905,118, US-A-4 404 614, EP-A-0 304 690, EP-A-0 335 479, EP-A-0 335 480, EP-A-0 397 163, EP-A-0 233 022, EP-A-0 443 286 and DE-A-0 898 603 proposed to make the envelope surrounding the stack of varistors in materials composites composed of fibers, usually impregnated glass fibers resin.

More specifically, document US-A-4 656 555 proposed to first form a plastic-based fiber winding, such as polyethylene, or glass, or even ceramic possibly impregnated with resin, for example epoxy, then fomer on the outside of this winding a housing made of weather-resistant polymer material, for example based on elastomeric polymers, synthetic rubber, thermoplastic elastomers, EPDM.

This document specifically proposes either to preform the weather-resistant polymer housing, then engaging the stack of varistors provided with the winding of fibers in this housing, either to form initially the winding of fibers on the stack of varistors, then make the housing in polymer material resistant to bad weather, by molding on the winding, projection of polymer on winding or inserting the stack of varistors provided with winding in a polymer bath.

Document US-A-4,404,614 has proposed to successively dispose on a stack of varistors a first envelope based on glass fibers impregnated with resin, for example epoxy resin, then a second envelope based on glass flakes and epoxy resin, and finally an elastic external envelope based on EPDM rubber, or butyl rubber.

This document indicates that the first envelope, the second envelope and the outer envelope can be placed successively on the stack of varistors, or the envelopes can be formed in reverse order.

This document also mentions the possibility of molding the outer envelope on the second envelope based on glass flakes and epoxy resin.

Document EP-A-0 233 022 proposed to train on a stack of varistors a shell made of glass fibers reinforced with epoxy resin, then a heat-shrinkable envelope, or can be released by equivalent mechanical means on said shell, elastomer-based.

As a variant, the envelope can be molded in situ based on synthetic resin or polymer material.

The document states that the shell can be preformed. This document also suggests using a sheet of fibers prepregs.

Document EP-A-0 304 690 proposes first of all to carry out a filament winding of glass fibers impregnated with resin, then form a coating of material on the outside of the winding EPDM type elastomer, by injection.

Document EP-A-0 355 479 proposes to place successively on the stack of varistors, first of all a barrier formed by a plastic film, for example based on propylene, then a winding of non-conductive filaments, and finally an elastomer housing resistant to severe weather.

Document EP-A-0 397 163 has proposed successively placing on the stack of varistors an impregnated filament winding resin, then form a finned coating on this winding, elastomer, for example EPDM, by injection.

The technique of using a composite material is very Ancient.

Document DE-A-0 898 603 proposed in 1946 to use resin impregnated glass fibers to wrap varistors.

More recently, document FR-A-2 698 736 has proposed, a method of manufacturing a lightning arrester comprising the steps which consist in stacking varistors, forming a first envelope in composite material, on the stack of varistors, which first at least semi-rigid envelope has a constant external section, along its length, compensating in particular for the surface unevenness of the stack of varistors due to misalignments and dispersion of the varistor dimensions and placing an external envelope with fins on the first envelope of composite material by extruding a substantially constant outer envelope on the first envelope, then by adding annular fins on the envelope extruded exterior.

The surge arresters hitherto proposed have made large services.

However, the Applicant proposes to improve the existing surge arresters.

A main object of the present invention is to improve the reliability of existing surge arresters, in particular avoiding any presence gas at the interface (s) between the stack of varistors and the envelope covering them.

A more incidental object of the present invention is to reduce the cost of known surge arresters.

• empiler des varistances, et
• former une enveloppe en matériau composite sur l'empilement de varistances,

caractérisé par le fait que l'étape de formation d'une enveloppe en matériau composite consiste à :

• placer un tissu de fibres sur l'extérieur de l'empilement des varistances, en contact avec celui-ci,
• placer une enveloppe externe souple sur l'extérieur de l'empilement des varistances, et
• injecter un matériau apte à imprégner le tissu de fibres, dans l'espace annulaire formé entre l'empilement de varistances et l'enveloppe externe souple.

To this end, the present invention provides a method of manufacturing lightning arresters of the type comprising the steps which consist in:

• stack varistors, and
• forming an envelope of composite material on the stack of varistors,

characterized in that the step of forming an envelope of composite material consists in:

• place a fabric of fibers on the outside of the stack of varistors, in contact with it,
• place a flexible external envelope on the outside of the stack of varistors, and
• inject a material capable of impregnating the fabric with fibers, into the annular space formed between the stack of varistors and the flexible external envelope.

As will be understood later, the process according to present invention allows to expel all air at the interface between the stack of varistors and the flexible external envelope.

According to another advantageous characteristic of the invention, the outer envelope is an envelope with annular fins having areas of greater rigidity at the fins.

According to another advantageous characteristic of the invention, the material injected is a thermoplastic material, advantageously a polyester.

According to another advantageous characteristic of the invention, the injection is made through bores formed in contact parts attached to the ends of the stack of varistors.

According to another advantageous characteristic of the invention, the injection is produced by two bores provided respectively in contact parts placed on the ends of the stack of varistors, which bores are diametrically opposite with respect to the axis of the stack.

• les figures 1 à 4 annexées représentent schématiquement, et selon des vues en coupes axiales longitudinales du parafoudre, les diverses étapes successives de réalisation d'un parafoudre conforme à une variante de réalisation préférentielle de la présente invention,
• la figure 5 représente une vue schématique en coupe axiale longitudinale d'une pièce de contact selon le plan de coupe référencé V-V sur la figure 6, et
• la figure 6 représente une vue en coupe transversale de cette même pièce de contact selon le plan de coupe référencé VI-VI sur la figure 5.

Other objects and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting example, and in which:

• FIGS. 1 to 4 appended schematically represent, and according to views in longitudinal axial sections of the arrester, the various successive stages in the production of a arrester in accordance with a preferred variant embodiment of the present invention,
• FIG. 5 represents a schematic view in longitudinal axial section of a contact piece according to the cutting plane referenced VV in FIG. 6, and
• FIG. 6 represents a cross-sectional view of this same contact part according to the section plane referenced VI-VI in FIG. 5.

The arrester obtained according to the process of the present invention represented on Figure 4 attached and obtained using the intermediate steps represented in FIGS. 1 to 3, which will be described later, includes a stack of varistors 100, two contact parts 200, an envelope 300 of composite material comprising a fabric of fibers 310 and an injected material 350 impregnating the fabric with fibers 310, and a outer finned enclosure 400.

If necessary, the arrester can be supplemented with caps end of electrically conductive material attached to the ends of the arrester. These caps that allow you to participate at the same time the electrical contact and the waterproofing of the arrester have not been shown in the accompanying figures to simplify the illustration.

Varistors 100 are preferably formed of discs of constant diameter made from zinc oxide.

Varistors based on zinc oxide are well known from one skilled in the art.

Their production process and their composition will therefore not be described later.

As shown in Figure 1 attached, the varistors 100 are first of all stacked along their axis 102, to be perfectly coaxial.

If applicable, although this is not shown on the attached figures, spacers made of electrically material conductor, for example in the form of discs, or even if necessary comprising an elastic member, can be inserted between certain at least pairs of adjacent varistors 100.

As shown in Figure 2, once this stack two contact parts 200 are placed respectively on the ends of the stack of varistors.

The geometry of a particular embodiment and not limitation of the contact parts 200 will be described in more detail below in see Figures 5 and 6.

For now, we will simply note that the contact parts 200 preferably include an annular groove 210 and a bore longitudinal 250 parallel to the axis 202 of the parts 200 and which leads to a on the outer surface 206 of the contact parts 200 and on the other hand at the bottom of the throat 210.

As illustrated in Figure 2, after performing the stack of varistors 100, a fabric of fibers 310 is placed on the outside of the stack of varistors 100 in contact with the latter.

It is preferably a 310 fiber fabric, very preferably glass fibers, wrapped around the stack of varistors 100, and on the basis of the two contact parts 200.

More precisely still, it is preferably a fabric of fibers 310 having weft yarns and orthogonal warp yarns arranged respectively parallel to the axis 102 of the stack of varistors and transversely to this axis.

The 310 fiber fabric typically has 3.5 x 5 mesh mm. The size of these meshes must be adapted to allow evacuation of the arc and / or gases created by the arc in the event of a fault in the arrester.

Preferably, two ties 320 are tightened on the fiber fabric 310, opposite the aforementioned grooves 210.

More precisely still, preferably, these two links 320, placed in the form of loops in the grooves 210 come from a ribbon common which runs along the stack of varistors 100 on the outside of the fiber fabric 310. This ribbon can be wound in a spiral around the fiber fabric 310 between the two links 320, or even extend in a straight line parallel to the axis of the arrester, on the outside of the fiber fabric 310 between the two links 320, in which case preferably this straight ribbon is placed opposite the free edge of the fiber fabric 310. Thus, the ribbon fulfills a function of maintaining the fabric of fibers 310 the along the arrester.

Then, as illustrated in FIG. 3, an external envelope flexible 400 is arranged on the outside of the stack of varistors 100 fitted with 310 fiber fabric.

It is preferably, as seen in the appended figures, an outer casing 400 of elastomer, for example silicone, to annular fins. Such external finned envelopes are good known to those skilled in the art and will therefore not be described in detail thereafter.

In known manner, the purpose of the annular fins 410 is to extend the line of flight on the outside of the arrester. Their numbers, profiles and spacings may vary depending on holding requirements under pollution, and of course the nominal voltage of the arrester.

It will be noted that such an external envelope 400 provided with fins annular 410 is characterized by higher stiffness zones, at level of these fins 410.

Once the outer envelope 400 has been put in place, injecting a material capable of impregnating the fabric with fibers 310 into the annular space formed between the stack of varistors 100 and the flexible external envelope 400.

The injected material 350 is preferably an epoxy resin, for example example a polyester.

Even more precisely, this material is injected by through one of the bores 250 formed in a contact part 200, the bore 250 of the other part 200 serving for the evacuation of the air.

Furthermore, preferably, as has been shown schematically on the figures, care is taken to have the bore 250 provided in the other part of contact 200 in diametrically opposite position relative to bore 250 serving as an injection nozzle.

According to another advantageous characteristic of the invention, material 350 is injected in a horizontal position of the stack of varistors 100 or in a slightly inclined position by relative to the horizontal, for example of the order of 35 to 45 °.

However, as a variant, the surge arrester can be made in vertical position.

The composite material thus formed by the combination of the fabric 310 fibers and injected material 350 ensures a firm bond between the two contact pieces and maintains, by axial stress, good contact electric between the main faces 104, transverse to the axis 102, of each pair of adjacent varistors on the one hand, and between the faces main external 104 of the varistors placed at the ends of the stack, and respectively the contact parts 200, on the other hand.

In addition, the combination of fibers 310 plus injected material 350 according to the present invention has the property, in case of default of the surge arrester and therefore in case of creation of an electric arc, to allow the "volatilization" of the injected material, while having the fiber fabric 310 which mechanically holds the arrester.

It will be noted that the arrangement of the screened fibers parallel to the axis 102 of the arrester allows that, when a force is applied transverse to axis 102 on one end of the arrester, some of these fibers work in elongation, while others diametrically opposed, work in compression.

Glass fibers in particular have excellent resistance properties, both in elongation and in compression.

They therefore ensure good performance mechanics of the lightning arrester.

The link 320 can be formed of numerous variants, by example of a ribbon, such as fibers impregnated with resin.

The establishment of links 320 allows to immobilize firmly the fiber fabric 310 in translation on each contact piece 200 and so ensure good immobilization in relative translation of the parts of contact 200.

The fiber fabric 310 can be formed from different layers of overlapping fibers.

According to a particular and nonlimiting embodiment, the pitch fins 410 is of the order of 24 mm.

The use of an elastomer 400 envelope divided into zones flexible between two fins 410 and in more rigid zones opposite the fins generates, when injecting the material 350, the two following phenomena.

First of all the presence of more rigid annular zones on the outer casing 400 leads to a variation in pressure drop which allows the resin 350 injected by the bore 250 to standardize annularly wetting of the fiber fabric 310. Indeed, the flow 350 resin causes deformation of the flexible areas between two fins 410. Thus, if an offset in the progression of the resin 350 takes place, this resin is braked when it reaches a more annular zone rigid next to a 410 fin, which makes it possible to recover a vein ring injection of substantially constant section.

Such a refresher occurs each time a fin 410, which prevents a drift of the flow which would risk entrainment of an air bubble difficult to evacuate by the after.

In addition, the pressure drop varies continuously as measure the progress of material 350, which causes variations local deformation of the flexible zone between two fins. This deformation causes a stressing effect on the resin 350 radially inward of the stack of varistors, so that the resin wets the fiber fabric 310 during the injection phase. In addition, such solicitation also takes place, after injection, of the peristaltic type, when the outer casing 400 pushes the excess fluid during the elastic recovery (return to its original geometry).

During the evacuation of this surplus fluid, it is observed that small bubbles of residual gas may escape from the annular space defined between the stack of varistors and the outer envelope 400 by through the second bore 250.

It will also be noted that the injection of material 350 into the hollow of the annular groove 210, as well as the evacuation of resin, at the opposite end of the stack, require complete wetting of the fibers arranged in this area, particularly difficult to impregnate since it corresponds to a larger annular section and with a higher fiber concentration.

Without limitation, the injection of 350 polyester material is performed under a pressure of the order of 2 Bars.

• amélioration de l'homogénéité du matériau 350 grâce à la retenue temporaire résultant des zones de plus forte rigidité correspondant aux ailettes 410, y compris lorsque le matériau 350 est obtenu par mélange de deux fluides en amont du site d'injection, et
• diminution de risque de décantation de charges éventuelles contenues dans le matériau 350, pour les mêmes raisons.

It will also be noted that the arrangements in accordance with the present invention, in particular injection through an external envelope 400 having areas of greater rigidity, allow the following effects:

• improvement of the uniformity of the material 350 thanks to the temporary retention resulting from the zones of higher rigidity corresponding to the fins 410, including when the material 350 is obtained by mixing two fluids upstream of the injection site, and
• reduced risk of settling of any charges contained in the material 350, for the same reasons.

If necessary, it is also injected through one of the bores 250, in the annular space between the outer casing 400 and the stack of varistors 100, a compound such as a silicone resin, improving the bond between the injected polyester material and the outer silicone casing 400.

According to another variant, one can operate beforehand mechanical treatment, for example by abrasion, sandblasting ..., of the interior of the outer silicone casing 400, see a chemical treatment, or the deposit on the internal surface of the envelope 400 of a primer allowing to ensure chemical bridging between the silicone material of the envelope 400 and the material 350 injected.

It will also be noted that the present invention makes it possible to make surge arresters in economical conditions particularly favorable, in particular because all of the manufacturing steps can be performed without requiring atmospheres controlled.

We will now describe in more detail the contact part 200 according to a particular embodiment of the present invention shown in Figures 5 and 6.

Preferably, the two contact pieces 200 placed respectively at the ends of the arrester are identical.

Each contact piece 200 is formed from a block of metal unique with a general symmetry of revolution around an axis 202.

In use, this axis 202 is coaxial with the axis 102 of stacking varistors.

In FIG. 5, the principal faces of contact part 200.

These main faces 204 and 206 are planar and orthogonal to axis 202.

The main face 204 rests for use on the face main external 104 of a varistor 100 placed at the end of stacking.

The main face 206 is directed towards the outside of the arrester.

The contact piece 200 comprises a cylinder 220 adjacent to the main face 206 and extended towards the main face 204 by a barrel 230 of smaller section.

Preferably, the section of the barrel 230 is equal to the external section varistors 100.

Thus, when the contact parts 200 are placed on the stack of varistors 100, the barrel 230 extends the external surface of stacking.

The abovementioned annular groove 210 is formed in the barrel 230, substantially mid-length of it.

The bottom 211 of the groove 210 preferably has a section polygonal, for example a hexagonal section, as shown in the figure 6.

The first side 212 of the groove 210 placed on the side of the face main 204 is preferably plane and perpendicular to axis 202.

The second flank 213 of the groove 210 placed on the side of the face main 210 is preferably conical centered on the axis 202 and at concavity directed towards the main face 206.

In addition, helical threads 232 are formed on the surface. external of barrel 230.

Preferably, the threads 232 extend on either side of the throat 210.

However, the threads 232 are advantageously interrupted before the main face 204.

The limit of the threads 232 on the side of the main face 204 is formed by an annular groove 234.

Each contact piece 200 has a blind tapped bore 240 centered on the axis 202 and opening onto the main face 206.

This 240 threaded bore is designed to receive a stud connection.

The polygonal bottom 211 of the groove 210 and the threads 232 form non-symmetrical structures of revolution around the axis 202.

These structures in engagement with the envelope 300 allow to prohibit any relative rotation between the contact parts 200 and the envelope 300.

Furthermore, the annular grooves 210 in which are engaged the ends of the sheet of fabric forming the envelope 300 allow stable translation fixation between said envelope 300 and contact parts 200.

Finally, each part 200 has a bore 250 parallel to the axis 202, which connects the external face 206 and the bottom of the groove 210.

In conclusion, the structure described above and shown in the accompanying figures provides rigidity arrester, flexion, rotation around the axis 102 of stacking and relative translation along this axis.

Where appropriate, it is possible, as a variant, to envisage forming areas of weakness in the outer shell 400.

Alternatively, the arrester according to the present invention can be fitted with a fault signaling device.

This device can be placed for example on one end of the arrester.

Such a fault signaling device is designed to visualize the passage of a line current to earth via surge arrester, i.e. visualize the passage of a leakage current to through the arrester.

The applicant has already described and shown such a device for signaling in its patent application in France FR-A-2 685 533.

For this reason, this fault signaling device will not be not described in detail later.

• un goujon centré sur l'axe 102 de l'empilement de varistances et relié électriquement à l'une des pièces de contact 200,
• un capteur de courant faible perte comportant un bobinage entourant le goujon,
• un circuit électronique comportant :
• 1. un pont redresseur dont les entrées sont reliées au bobinage et,
• 2. un condensateur relié aux sorties du pont redresseur pour intégrer l'énergie du courant de fuite détecté,
• un ensemble de signalisation, par exemple à base de composants pyrotechniques, conçu pour être initié par l'énergie intégrée dans le condensateur.

It should however be noted that preferably such a fault signaling device comprises:

• a stud centered on the axis 102 of the stack of varistors and electrically connected to one of the contact parts 200,
• a low loss current sensor comprising a winding surrounding the stud,
• an electronic circuit comprising:
• 1. a rectifier bridge whose inputs are connected to the winding and,
• 2. a capacitor connected to the outputs of the rectifier bridge to integrate the energy of the detected leakage current,
• a signaling assembly, for example based on pyrotechnic components, designed to be initiated by the energy integrated in the capacitor.

According to another variant, the fault detector can be based on a principle of increasing the volume of the envelope 400 during a short circuit of the arrester. Such a detector also known in itself will not be described in more detail below.

The arrester manufactured according to the process of the present invention offers many advantages compared to known prior lightning arresters.

First of all, the present invention allows a manufacturing economic.

In addition, the present invention makes it possible to easily adapt the length of the arrester at the nominal voltage of the line to be protected.

The present invention in no way requires adaptation of any mold.

The present invention makes it possible to avoid any layer of air or gas inclusion at the interface between the stack of varistors 100 and the envelope 400, and therefore makes it possible to avoid any surface discharge at this level.

Of course, the present invention is not limited to the mode of particular embodiment which has just been described, but extends to any variant conforms to his spirit.

So, for example, a fabric 310 of prepreg fibers can be used. a resin, placed on the outside of the stack of varistors 100 before injecting material 350 into space annular formed between the stack of varistors 100 and the envelope flexible outer 400.

According to a variant, each bore 250 can be closed by example using a plug or putty, in order to perfect the seal of the arrester, once the material injection operation 350 has been completed.

According to another advantageous characteristic of the invention, one may provide for placing a film impermeable to the injected resin 350, around stacking varistors 100 before the injection step. Such a film avoids the penetration of resin between two varistors 100.

Such a film can be installed only at the level of each interface between two adjacent varistors 100 as well as between the end varistors and contact parts 200, or even cover in one piece the whole of the active part formed by the stack of varistors 100.

According to another variant, contact parts can be provided 200 having several bores 250 to ensure both the injection of the material 350 that the evacuation of the annular space formed between the stack of varistors 100 and the flexible external envelope 400.

Claims (18)

1. A method for manufacturing an overvoltage protector of the type comprising the steps which consist in:

   stacking varistors (100), and

   forming an enclosure (300) of composite material on the stack of varistors (100),

      characterised by the fact that the step of forming an enclosure (300) of composite material consists in:

   placing a fibre fabric (310) on the outside of the stack of varistors (100), in contact with it,

   placing a flexible external enclosure (400) on the outside of the stack of varistors (100), and

   injecting a material (350) able to impregnate the fibre cloth (310), in the annular space formed between the stack of varistors (100) and the flexible external enclosure (400).
2. A method according to Claim 1, characterised by the fact that the external enclosure (400) is an enclosure provided with annular fins (410) defining areas of rigidity greater than the said enclosure disposed around the stack of varistors (100).
3. A method according to one of Claims 1 or 2, characterised by the fact that the injection of impregnation material (350) is performed through contact pieces (200) placed on the ends of the stack of varistors (200).
4. A method according to one of Claims 1 to 3, characterised by the fact that the impregnation material (350) is a thermoplastic material, preferably an epoxy resin, such as polyester.
5. A method according to Claim 4, characterised by the fact that an injection of impregnation material (350) is performed by means of at least one bore (250) provided in a contact piece (200) provided on a first end of the stack of varistors (100), the contact piece (200) provided on the second end of the stack of varistors (100) having at least one homologous bore (250) for the escape of the air contained before injection in the said annular space and where necessary a drainage of impregnation material, the said bore provided in the second contact piece (200) preferably being diametrically opposed with respect to the injection bore.
6. A method according to one of Claims 1 to 5, characterised by the fact that it also comprises the step consisting in first carrying out a treatment of the internal surface of the flexible external enclosure (400), for example a mechanical treatment, a chemical treatment or the deposition of a primer, providing a chemical bridging between the material of the flexible external enclosure (400) and the injected material (350).
7. A method according to one of Claims 1 to 6, characterised by the fact that the impregnation material (350) comprises a compound able to reinforce the connection between the injected thermoplastic material (350) and the external enclosure (400).
8. A method according to one of Claims 1 to 7, characterised by the fact that the external enclosure (400) is made from elastomer, preferably silicon.
9. A method according to one of Claims 1 to 8, characterised by the fact that the bores (250) formed in the contact pieces of the end (200) in order to provide the injection open out in an annular groove (210) provided on these contact pieces (200).
10. A method according to one of Claims 1 to 9, characterised by the fact that the fibre cloths (310) have fibres parallel to the axis (102) of the stack of varistors and fibres transverse to this axis.
11. A method according to one of Claims 1 to 10, characterised by the fact that the fibre cloth (310) has meshes of around 3.5 × 5 mm.
12. A method according to one of Claims 1 to 11, characterised by the fact that the fibre cloth (310) placed on the outside of the stack of varistors (100) before performing the injection of the material (350) in the annular space formed between the stack of varistors (100) and the flexible external enclosure (400) is a cloth preimpregnated with resin.
13. A method according to one of Claims 1 to 12, characterised by the fact that it also comprises the step consisting in closing off each injection bore (250), for example by means of a plug or filler, before completing the watertightness of the overvoltage protector, once the operation of injecting the material (350) has been completed.
14. A method according to one of Claims 1 to 13, characterised by the fact that it also comprises the step consisting in placing two connections (320), in the form of loops in grooves (210) provided in contact pieces (200) provided on the ends of the stack of varistors (100), on the outside of the fibre cloth (310) in order to hold the latter.
15. A method according to Claim 14, characterised by the fact that the two connections (320) come from a common strip which runs along the stack of varistors (100) on the outside of the fibre cloth (310).
16. A method according to Claim 15, characterised by the fact that the strip is wound in a spiral around the fibre cloth (310) between the two connections (320).
17. A method according to Claim 16, characterised by the fact that the strip extends in a rectilinear fashion parallel to the axis of the overvoltage protector, on the outside of the fibre cloth (310) between the two connections (320), preferably opposite the free edge of the fibre cloth (310).
18. A method according to one of Claims 1 to 17, characterised by the fact that it also comprises the step consisting in placing a film impervious to the injected resin (350) around the stack of varistors (100) before the injection step.

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