EP1166287B1 - Process of manufacture of lightning arrestors and lightning arrestor with electrical varistors - Google Patents

Abstract

The present invention relates to a method of manufacturing surge arrestors, the method including stacking varistors; and forming a coating of composite material on the stack. Between these steps, also included is placing a bead of flexible, adhesive, and dielectric material on the previously formed stack in register with the various interfaces between each pair of adjacent varistors.

Description

The present invention relates to the field of surge arresters.

It applies in particular to medium-voltage surge arresters, typically to electrical networks with a nominal voltage greater than 1 kV effective between phases.

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

These overvoltages may be due for example to atmospheric phenomena, such as lightning, or induction phenomena 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 nowadays by stacking several zinc oxide-based disks, the electrical resistivity of which is highly nonlinear depending on the applied voltage.

More precisely, these varistors pass virtually no current as long as the voltage at their terminals is below a conduction threshold and, on the other hand, pass a very strong current, which can reach several tens of kA, when the voltage applied to their terminals exceeds the above-mentioned conduction threshold.

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

Thus, the surge arrester can withstand the operating voltage permanently, without significant leakage of current, and allows on the other hand to discharge the very high intensity discharge currents that can appear temporarily on the line in case of surge accidental.

Many types of surge arresters have already been proposed.

The field of surge arresters has given rise to a very large 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 usually 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 has itself been the subject of a very abundant literature.

For example, GB-A-2 073 965 proposed to make this envelope using a heat-shrinkable material.

US-A-4,298,900, DE-A-3,001,943, DE-A-3,002,014 have proposed to further place a porcelain outer casing on the heat-shrinkable casing.

US-A-4,092,694 and US-A-4,100,588 have proposed placing each varistor in a silicone-based ring and arranging the stack of varistors thus surrounded in a porcelain casing.

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

Documents EP-A-0 008 181, EP-A-0 274 674, EP-A-0 231245 and US-A-4,456,942 have proposed to make the envelope surrounding the varistors using a material elastomer, formed especially by overmolding.

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

US-A-4 161 012 has also proposed to have an elastomeric casing on the varistors. This document proposes to make this envelope by depositing the elastomer on the outer surface of the varistors, or by molding the casing on the varistors, or by preforming the elastomer casing, and then inserting the varistors into it. .

The document US Pat. No. 3,018,406 proposed, as early as 1958, to produce the envelope in the form of two preformed complementary shells and an external injection-molded plastic casing on the varistors.

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

EP-A-0 196 370 proposes to make the casing on a body of varistors by casting a synthetic resin formed for example of epoxy resin, polymer concrete, silicone resin, or an elastomer, or covering the body of varistors with a plastic shrinkable tube, or by providing this stack with a layer of synthetic resin.

Furthermore, 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 have proposed to make the envelope surrounding the stack of varistors composite materials composed of fibers, usually glass fibers impregnated with resin.

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

This document proposes more precisely either to preform the weather-resistant polymer housing, then to engage the stack of varistors provided with the winding of fibers in this housing, or to initially form the winding of fibers on the stack of varistors, then to realize the housing of weather-resistant polymeric material, by molding on the winding, spraying polymer on the winding or insertion of the stack of varistors provided with the winding in a polymer bath.

US-A-4 404 614 has proposed to arrange successively 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 outer shell based on EPDM rubber, or butyl rubber.

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

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

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

Alternatively, the casing may be molded in situ based on synthetic resin or polymeric material.

The document indicates that the shell can be preformed. This document also proposes to use a web of pre-impregnated fibers.

The document EP-A-0 304 690 proposes firstly to produce a filament winding of glass fibers impregnated with resin, then to form on the outside of the winding a coating of elastomer material of the EPDM type, by injection.

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

The document EP-A-0 397 163 proposed to place successively on the stack of varistors a filament winding impregnated with resin, then to form on this winding a finned coating, elastomer, for example EPDM, by injection.

The technique of using a composite material is very old.

DE-A-0 898 603 indeed proposed in 1964 to use glass fibers impregnated with resin to wrap varistors.

More recently, it has been proposed in the document FR-A-2 698 736, a method of manufacturing a surge arrester comprising the steps of stacking varistors, forming a first envelope of composite material, on the stack of varistors, which first envelope at least semi-rigid present a constant outer section, over its length, compensating in particular for the surface unevenness of the stack of varistors due to misalignment and dispersion of the dimensions of varistors and placing an outer fin envelope on the first composite material by extruding a substantially constant outer casing on the first casing, and then returning annular fins to the extruded outer casing.

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

dans lequel l'étape de formation d'une enveloppe en matériau composite consiste à :

• placer un tissu de fibres sur l'extérieur de l'empilement de 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.

Furthermore, WO-A-97/39 462 discloses a method of manufacturing surge arresters comprising the steps of:

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

wherein the step of forming a composite material envelope comprises:

• placing a fiber fabric on the outside of the stack of varistors, in contact therewith,
• place a flexible outer envelope on the outside of the stack of varistors, and
• injecting a material capable of impregnating the fiber fabric in the annular space formed between the stack of varistors and the flexible outer envelope.

The surge arresters hitherto proposed have been of great service.

However, they do not always give full satisfaction.

In particular the Applicant has observed that almost all surge arresters manufactured today, is made based on enamelled varistors.

Enameling varistors has so far been considered essential to improve the dielectric strength and incidentally constitute a chemical barrier to their immediate environment.

And the Applicant has found that this enamelling poses two problems.

In the first place, enamelling, which generally comprises a high proportion of lead, typically greater than 50%, leads to a major risk of environmental pollution, if precautions, inevitably costly, are not taken at the manufacturing level. , recovery and / or recycling of used arresters.

Secondly, enamelling which can only be achieved before stacking, therefore individually varistor varistor, is a significant part of the overall cost of current surge arresters.

On the basis of this observation, the Applicant proposes, by the present invention, to improve the existing surge arresters.

A main object of the present invention is to produce reliable surge arresters based on varistors, without requiring enamelling.

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

• empiler des varistances, et
• former un enrobage en matériau composite sur l'empilement de varistances,

caractérisé par le fait que :

• entre les étapes d'empilement et de formation de l'enrobage en matériau composite, le procédé comprend l'étape consistant à disposer un cordon de matériau souple, adhésif et diélectrique sur l'empilement précédemment formé, en regard des différentes interfaces entre chaque paire de varistances adjacentes.

For this purpose, the present invention provides a method of manufacturing surge arresters of the type comprising the steps of:

• stack varistors, and
• forming a coating of composite material on the stack of varistors,

characterized by the fact that:

• between the steps of stacking and forming the coating of composite material, the method comprises the step of arranging a bead of flexible, adhesive and dielectric material on the stack previously formed, opposite the different interfaces between each pair adjacent varistors.

The present invention also relates to surge arresters obtained by the implementation of the aforementioned method.

• les figures 1 à 4 annexées représentent schématiquement les diverses étapes successives de réalisation d'un parafoudre conforme à un mode de réalisation préférentiel de la présente invention.

Other features, 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 non-limiting example, and in which:

• Figures 1 to 4 attached schematically show the various successive steps of making a surge arrester according to a preferred embodiment of the present invention.

• empiler des varistances 10 centrées sur un axe commun 12 (éventuellement avec interposition d'entretoises entre certaines au moins des paires de varistances 10 adjacentes), des pièces électriquement conductrices formant électrodes, et dénommées ci-après cimblots 20, étant disposées respectivement sur chaque extrémité de l'empilement, puis maintien en compression axiale de l'ensemble ;
• déposer des cordons annulaires 30 d'un agent de collage/étanchéité, sur l'empilement ainsi formé en regard de chaque interface définie au niveau de la zone de jonction entre deux varistances 10 adjacentes (voir figure 1) :
• réaliser un enroulement hélicoïdal d'un ruban tissé préimprégné (préférentiellement à base de fibres de verre/résine époxy ou équivalent) autour de l'ensemble, typiquement avec chevauchement de 50 % et un seul passage afin d'obtenir l'équivalent de deux épaisseurs de rubans tissés préimprégnés en tous points de l'empilement (comme illustré sur la figure 2 sur laquelle le ruban tissé préimprégné est référence 40) ;
• déposer des cordons 50 d'un agent de collage/étanchéité à intervalles réguliers sur l'ensemble précédemment enrubanné (tel qu'illustré sur la figure 3) ;
• enfiler par expansion une enveloppe extérieure en élastomère 60 (typiquement à base d'une gomme en silicone ou équivalent) sur le corps précédemment enduit (tel qu'illustré sur la figure 4);
• puis passage en étuve de l'ensemble pour assurer la polymérisation du matériau composite interne 40.

The method of manufacturing surge arresters illustrated in the accompanying FIGS. 1 to 4 comprises the steps of:

• stacking varistors 10 centered on a common axis 12 (possibly with the interposition of spacers between at least pairs of adjacent varistors 10), electrically conductive pieces forming electrodes, and hereinafter called cimblots 20, being respectively disposed on each end stacking and maintaining in axial compression of the assembly;
• depositing annular beads 30 of a bonding / sealing agent, on the stack thus formed opposite each interface defined at the junction zone between two adjacent varistors 10 (see FIG. 1):
• make a helical winding of a woven ribbon prepreg (preferably based on glass fibers / epoxy resin or equivalent) around the assembly, typically with 50% overlap and a single pass in order to obtain the equivalent of two thicknesses of woven ribbons pre-impregnated at all points stack (as illustrated in Figure 2 on which the preimpregnated woven tape is referenced 40);
• depositing beads 50 of a bonding / sealing agent at regular intervals on the previously wrapped assembly (as illustrated in Figure 3);
• threading an outer elastomeric shell 60 (typically based on a silicone rubber or equivalent) on the previously coated body (as shown in FIG. 4) by expansion;
• then passing in an oven of the assembly to ensure the polymerization of the internal composite material 40.

Of course, the invention is not limited to the number or dimensions of varistors illustrated in the accompanying figures.

The bonding / sealing agent 30 deposited on the interfaces of the stack of varistors 10 is a flexible, adhesive and dielectric material. It is advantageously an elastomer or a gel, for example based on silicone or equivalent.

According to a preferred embodiment of the present invention, the varistors 10 are unglazed varistors.

The use of unglazed varistors 10 is made possible, in the context of the present invention, by the addition of this specific material 30 at the level of the peripheral junctions of varistors 10, said material 30 making it possible at the same time to guarantee good dielectric withstand (glue function) and total absence of axial partial discharge (sealing function).

However, alternatively, the varistors 10 can include a thin enamel protective film intended to protect the varistors during the manufacturing process of the surge arrester, in particular in order to avoid any pollution of the varistors during the perfume manufacturing process.

The conventional enamel layers typically have a thickness of the order of 100 .mu.m. Such a protective film according to the present invention may typically have a thickness in the range of ½ to 1/3 of this conventional thickness.

In the context of the present invention, the protective film may be removable, that is to say removed, once the risks of pollution become non-existent during the manufacturing process.

Furthermore, in the context of the present invention, the protective film is made of a material free of lead.

• de supprimer toute bulle ou poche d'air entre l'empilement de varistances 10 et l'enroulement 40 ultérieur; les cordons 30 permettent en quelque sorte de réparer tout défaut éventuel au niveau des arêtes des varistances ; les cordons 30 de mastic silicone remplacent ainsi avantageusement l'émaillage classique au niveau des interfaces entre les paires de varistances adjacentes ;
• éviter toute pénétration de la résine époxyde du matériau composite 40 enroulé sur l'empilement de varistances, au niveau de l'interface entre les paires adjacentes de varistances (la Demanderesse a constaté en particulier que si l'on n'évite pas la pénétration de la résine époxyde entre les varistances 10, lors du passage d'un courant dans le parafoudre, la force électrodynamique résultante qui tend à séparer les varistances adjacentes 10 conduit à un arrachement de la couche superficielle des varistances, au niveau de ses interfaces, en raison de la forte adhérence définie entre cette couche superficielle des varistances et la résine époxyde ayant pénétré entre celles-ci) ;
• assurer une bonne adhésion entre l'empilement de varistances 10 et l'enrobage en composite 40 superposé grâce à la liaison élastique assurée par les cordons 30, d'une part, avec la surface externe des varistances 10 et, d'autre part, avec l'enrobage superposé 40.

More specifically, it will be noted that the cords of material 30 preferably formed based on silicone sealant allow:

• removing any bubble or air pocket between the stack of varistors 10 and the subsequent winding 40; cords 30 allow somehow to repair any defect at the edges of the varistors; the silicone sealant cords 30 thus advantageously replace the conventional enamelling at the interfaces between the pairs of adjacent varistors;
• to avoid any penetration of the epoxy resin of the composite material wound on the stack of varistors, at the interface between the adjacent pairs of varistors (the Applicant has found in particular that if one does not avoid the penetration of the epoxy resin between the varistors 10, during the passage of a current in the arrester, the resulting electrodynamic force which tends to separate the adjacent varistors 10 leads to tearing of the surface layer of the varistors, at its interfaces, due to of the strong adhesion defined between this superficial layer of varistors and the epoxy resin having penetrated between them);
• ensure good adhesion between the stack of varistors 10 and the composite coating 40 superimposed thanks to the elastic connection provided by the cords 30, on the one hand, with the outer surface of the varistors 10 and, on the other hand, with the superimposed coating 40.

Typically, in the context of the present invention, each silicone sealant bead 30 has an axial width of about 5 mm and a comparable thickness, preferably less than 5 mm.

The cords 30 are preferably deposited in the form of a single turn into a one-component silicone mastic, necessarily compatible with the material of the varistors 10 formed very advantageously based on zinc oxide. For this purpose, preferably, the material does not contain acetic acid.

As shown schematically in FIG. 1, preferably to ensure the deposition of the annular beads 30, several suitably offset nozzles or a dispensing nozzle 32 fixed in rotation but capable of being displaced axially cyclically with a pitch equal to distance separating two cords 30, and the stack of varistors 10 and end cimblots 20 held in axial compression of a rotational movement about the axis 12 is animated.

In FIG. 1, the parts ensuring, on the one hand, maintaining the stack in axial compression and the driving in rotation thereof, are referenced 70 and 72.

In the context of a preferred but nonlimiting implementation of the present invention, the material constituting the cords 30 is formed of a silicone sealant marketed by the company Dow Corning under the reference 7091. This silicone sealant has excellent adhesion without primary and neutral polymerization base. This silicone sealant allows a temperature range of -55 ° C to + 150 ° C, and it has an elongation of 500% and a dielectric strength of 16 kV per mm.

The ribbon 40 of preimpregnated fabric wound preferably at 45 ° with an overlap of 50% preferably has a resin content of between one third and one half by weight.

In the context of a preferred but non-limiting embodiment of the invention, the preimpregnated fabric tape 40 consists of a fabric whose mass content of resin is of the order of 35% has a nominal mass of 470. g / m ² and a reinforcing structure based on glass fibers having a nominal mass of 305 g / m ² in a satin-like structure 8.

It should be noted that such a ribbon 40 is self-extinguishing.

The bonding / sealing agent forming the cords 50 deposited on the composite tape 40 may be formed of the same material, preferably silicone mastic or equivalent, that used to make the cords 30 and described above.

Preferably, the material 50 is chosen to have properties of lubricant, adhesive, dielectric, and filler, in order to facilitate the engagement of the outer shell 60, to adhere perfectly thereto, to provide dielectric functions, and to perform perfectly the interface between the composite coating 40 and the outer casing 60.

The material 50 deposited on the composite coating 40 may optionally be placed in a helix. However, as illustrated in FIG. 3, the material 50 is preferably deposited in the form of annular beads.

Again the material 50 is deposited preferably with the aid of several nozzles or a nozzle 32 fixed in rotation but cyclically displaced axially with a pitch equal to the distance separating two cords 50, and rotating the preformed assembly surge arrester around the axis 12.

Note also that, preferably, such cords 50 are deposited respectively at the two ends of the stack to ensure a complete filling of the surface irregularities between the composite tape 40 and the outer shell 60, over the entire length of the surge arrester, as well as to guarantee a perfect seal of the arrester.

As schematized in FIG. 4, preferably the outer shell 60 of elastomer is an outer annular fin envelope for extending the creepage line on the outside of the arrester. The number of fins and their profile and spacing may vary depending on the requirements for resistance to pollution, and of course the nominal voltage surge arrester.

It will be noted that such an outer envelope 60 provided with annular fins is characterized by zones of greater rigidity at its fins.

The use of the outer casing 60 as a molding mold by radial compression of the body of the surge arrester during the polymerization step of the prepreg ribbon resin 40 provides a perfect adhesion between the different layers of internal materials and allows both to guarantee a good seal and the absence of partial radial discharge for the device.

In the context of the present invention, preferably, this polymerization step is carried out under axial compression at a temperature of the order of 130 ° C. and for a duration of the order of 1 hour.

• un empilement de varistances 10 préférentiellement à l'oxyde de zinc, non émaillées (ou seulement revêtues d'une fine pellicule d'émail exempte de plomb et donc non polluante, utilisée uniquement pour faciliter le processus de fabrication des varistances), éventuellement associées à une ou plusieurs entretoises métalliques intermédiaires,
• une électrode métallique 20 de contact à chaque extrémité,
• un enrobage composite 40 obtenu par enroulement hélicoïdal simple d'un ruban de tissu de fibres de verre préimprégnées de résine synthétique,
• une enveloppe extérieure en élastomère 60, et
• un agent de collage 30 aux interfaces de jonction entre les varistances 10 et l'enrobage composite 40, et un agent de collage 50 aux interfaces entre l'enrobage composite 40 et l'enveloppe élastomère 60.

Finally, the arrester obtained for the implementation of the aforementioned method comprises:

• a stack of varistors 10 preferentially with zinc oxide, unglazed (or only coated with a fine enamel film which is lead-free and therefore non-polluting, used only to facilitate the process of manufacturing the varistors), possibly associated with one or more intermediate metal spacers,
• a metal contact electrode 20 at each end,
• a composite coating 40 obtained by simple helical winding of a ribbon of fiberglass fabric pre-impregnated with synthetic resin,
• an outer shell of elastomer 60, and
• a bonding agent 30 at the junction interfaces between the varistors 10 and the composite coating 40, and a bonding agent 50 at the interfaces between the composite coating 40 and the elastomeric casing 60.

Of course, the present invention is not limited to the particular embodiment which has just been described but extends in all variants according to the definition that is made in the claims.

Thus, in a variant, the ribbon 40 of glass fiber fabric pre-impregnated with synthetic resin may be deposited on the stack of varistors 10 provided with beads 30 of silicone putty, not in a helix, but in the form of rings placed opposite of the interface between each pair of adjacent varistors. In this case, however, to ensure a satisfactory dielectric withstand by arrester, the resulting assembly is preferably further provided with an outer casing having a good dielectric strength, for example based on a silicone overmoulding, or of an envelope deposited by expansion and then retraction, or a heat-shrinkable envelope.

According to yet another embodiment according to the present invention, the ribbon 40 of preimpregnated fabric can be deposited both in the form of rings opposite the connection interfaces between each pair of adjacent varistors, then a superimposed helix as shown in FIG. 2.

Claims (26)

1. A method of manufacturing surge arrestors, the method comprising the steps consisting in:

   · making a stack of varistors (10); and

   · forming a coating (40) of composite material on the stack of varistors (10);

   the method being characterized by the fact that:

   · between the steps of making the stack and forming the coating of composite material (40), the method includes the step of depositing a bead (30) of flexible, adhesive, and dielectric material on the previously-formed stack in register with the various interfaces between each adjacent pair of varistors.
2. A method according to claim 1, characterized by the fact that the beads (30) of flexible, adhesive, and dielectric material are made on the basis of an elastomer or a gel, preferably of silicone material.
3. A method according to claim 1 or claim 2, characterized by the fact that each bead (30) has a typical width of 5 mm and a thickness of less than 5 mm.
4. A method according to any one of claims 1 to 3, characterized by the fact that the material constituting the beads (30) has no acetic acid.
5. A method according to any one of claims 1 to 4, characterized by the fact that it further comprises the steps consisting in depositing an outer envelope (60) on the coating (40) of composite material and using said outer envelope (60) as a mold for shaping the body of the arrestor by a radial compression effect during a polymerization step.
6. A method according to claim 5, characterized by the fact that the outer envelope (60) possesses annular fins.
7. A method according to any one of claims 1 to 6, characterized by the fact that it further comprises the step consisting in depositing beads of adhesive/sealing agent (50) on the coating of composite material (40) prior to installing the outer envelope (60).
8. A method according to claim 7, characterized by the fact that the beads (50) of adhesive/sealing agent deposited on the coating of composite material (40) are made of silicone mastic.
9. A method according to claim 7 or claim 8, characterized by the fact that the beads (50) of adhesive/sealing agent deposited on the coating of composite material (40) are shaped as rings.
10. A method according to any one of claims 1 to 9, characterized by the fact that the coating of composite material (40) is wound helically.
11. A method according to any one of claims 1 to 10, characterized by the fact that the coating of composite material (40) is made by helically winding a preimpregnated woven tape with overlap of 50%.
12. A method according to any one of claims 1 to 11, characterized by the fact that the coating of composite material (40) has rings of preimpregnated woven tape deposited in register with the interfaces between adjacent pairs of varistors (10).
13. A method according to claim 12, characterized by the fact that the arrestor also has an envelope deposited on the coating of composite material (40) to reinforce the dielectric behavior of the arrestor.
14. A method according to any one of claims 1 to 13, characterized by the fact that the coating of composite material (40) preferably based on glass fibers and epoxy resin, has a resin content lying in the range one-third to one-half by weight.
15. A method according to any one of claims 1 to 14, characterized by the fact that the coating of composite material (40) is made under axial compression of the stack of varistors (10).
16. A method according to any one of claims 1 to 15, characterized by the fact that the varistors (10) are not enameled.
17. A method according to any one of claims 1 to 15, characterized by the fact that the varistors (10) are coated in a fine protective film of a lead-free enamel.
18. A surge arrestor comprising a stack of varistors (10) and a coating of composite material (40), the arrestor being characterized by the fact that it further comprises beads (30) of flexible, adhesive, and dielectric material on the stack of varistors in register with the various interfaces between each adjacent pair of varistors (10).
19. An arrestor according to claim 18, characterized by the fact that the beads (30) of flexible, adhesive, and dielectric material are based on silicone material.
20. An arrestor according to claim 18 or claim 19, characterized by the fact that it further comprises an outer envelope (60) having annular fins.
21. An arrestor according to any one of claims 18 to 20, characterized by the fact that it further comprises beads (50) of an adhesive/sealing agent between the coating of composite material (40) and an outer envelope (60).
22. An arrestor according to claim 21, characterized by the fact that the beads (50) of adhesive/sealing agent deposited on the coating of composite material (40) are made of silicone mastic.
23. An arrestor according to any one of claims 18 to 22, characterized by the fact that the coating of composite material (40) is made by helically winding a preimpregnated woven tape with overlap of 50%.
24. An arrestor according to any one of claims 18 to 23, characterized by the fact that the coating of composite material (40) has a resin content lying in the range one-third to one-half by weight.
25. An arrestor according to any one of claims 18 to 24, characterized by the fact that the varistors (10) are not enameled.
26. An arrestor according to any one of claims 18 to 24, characterized by the fact that the varistors (10) are coated in a fine protective film of lead-free enamel.

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