FR2867318A1 - Low voltage installation protection device e.g. lightning arrester, has line-to ground voltage and neutral phase varistors arranged between phase and ground, and phase and neutral respectively and mounted successively within insulating case - Google Patents

Abstract

The device has a line-to ground voltage (PT) varistor arranged between a phase (L1) to be protected and ground (T), and a neutral phase (PN) varistor arranged between the phase and neutral (N). An electrically insulating case (2) is adapted to receive the two varistors that form an electrical binomial. The varistors are mounted successively within the case to form a mounting binomial. An independent claim is also included for a process of manufacturing a protection device.

Description

OVERVOLTAGE PROTECTION DEVICE

  The present invention relates to the general technical field of devices for protecting electrical installations against overvoltages, in particular transient overvoltages due to lightning.

  The invention relates more particularly to an overvoltage protection device adapted to provide a protection according to the common and differential modes, and comprising: at least one electrically insulating housing, at least one electrical pair, formed by a so-called phase-earth varistor, disposed between a first phase to be protected and the earth, and a so-called phase-neutral varistor disposed between said first phase to be protected and the neutral.

  The invention also relates to a method of manufacturing an overvoltage protection device adapted to provide a common and differential mode protection, comprising the steps of: - a) fabricating at least one electrically insulating housing, b) manufacture at least one electrical pair, formed by a so-called phase-earth varistor, intended to be arranged between a first phase to be protected and earth, and a so-called phase-neutral varistor, intended to be arranged between said first phase to be protected and the neutral.

  Electrical surge protection devices are widely used, and may be commonly referred to as surge arresters. They have the essential purpose of grounding the lightning currents, and possibly clipping additional voltages induced by these currents to levels compatible with the behavior of equipment and devices to which they are connected.

  There are, in a conventional way, two overvoltage protection modes, namely the common mode MC, in which the protection components are connected between the active conductors (phase or neutral) and earth, and the differential mode MD, in which the protection components are connected between the active conductors (ie between the phases and the neutral).

  To ensure the protection of the installation according to these two common and differential modes, protection devices 15 having varistors are generally used.

  In the known devices, a varistor is generally mounted per branch, that is to say that to protect a single-phase network, the protection device must comprise an electrical pair formed by at least two varistors in order to ensure the protection according to the two modes mentioned above. Thus, in single-phase, the electrical pair will generally include a phase-ground varistor connected between the phase to be protected and earth, and a phase-neutral varistor connected between said phase to be protected and the neutral. In the case of a three-phase network, it will take at least three electrical pairs to perform the same function.

  In accordance with the requirements of the standardization (NF-EN61643-11 and NFC 61740/95), the end of life of the varistors must be imperatively controlled, which requires the use of integrated disconnectors, for example of thermal type, each varistor being then equipped with its own disconnection system. The varistor and its disconnection system are then mounted in a housing-support, the assembly constituting what will be referred to hereafter as a motor.

  The known common mode / differential mode protection devices thus generally comprise at least two housings (or motors) to provide protection according to the two modes mentioned above in the case of a single-phase network, and at least six boxes (or motors). to ensure the same function in the case of a three-phase network, each housing (or motor) having a single varistor.

  The known devices, if they properly protect the network or the electrical installation, nevertheless suffer from several disadvantages.

  First, such devices are particularly bulky, each housing (or motor) generating an individual footprint multiplied by the number of housings used. Therefore, it is understood that in the case of a polyphase network, these devices can occupy a significant volume, while for an increasing number of applications, it is precisely to reduce the dimensions of the protective devices and to make them more discreet possible.

  In addition, withdrawable versions of known devices are generally made with two cartridges per phase, that is to say with at least two cartridges interchangeable in the case of a single-phase network and with at least six cartridges interchangeable in the case of a three-phase network.

  However, such a configuration is particularly complex for the user who logically expects to find a single cartridge per phase and not two cartridges per phase as in known devices.

  To overcome these drawbacks, it is known to combine the varistors with a spark gap.

  In the case of a single-phase network, the surge arrester then consists of a phase-neutral varistor connected between the phase to be protected and the neutral point, and a spark gap, connected between the neutral point and the ground. In withdrawable versions of the device, each protection component is integrated in an individual cartridge so that the surge arrester comprises two interchangeable cartridges, only one of which is connected to the phase to be protected, which corresponds to the logical configuration expected by the device. user. Equivalently, in the case of a three-phase network, the surge arrester will comprise three varistors each connected between a phase to be protected and the neutral, and a spark gap, connected between the neutral and the ground, each protection component being integrated into a cartridge individual. The arrester thus has, in three-phase, four detachable cartridges, among which three cartridges are connected to the three phases to protect, one cartridge per phase, which corresponds to what the user intuitively expects to find.

  Such devices thus make it possible to ensure the protection of the network on the one hand in common mode, by placing the two phaseuper and neutral-earth branches in series, and on the other hand in differential mode, thanks to the phase-neutral branch. .

  However, this assembly is not usable for all types of network, and in particular those for which the connection diagram of the neutral to the ground is of the type IT (isolated neutral of the ground or impedant).

  In addition, this assembly is not suitable for cascading surge arresters. In particular, when a first surge arrester, located upstream, comprises a varistor in its neutral-earth branch, and a second surge arrester, placed downstream comprises a spark gap in its neutron branch, the spark gap placed downstream will flow, in overvoltage, the greater part of the lightning current, and the varistor placed upstream will be under-solicited, which goes against the objectives sought when we coordinate two surge arresters.

  We therefore see the whole point of trying to achieve an overvoltage protection device capable of operating in both common and differential modes which, while being compact, is compatible with all types of connection diagrams of the neutral to the earth, in particular the TT, TN-S, TN-C or IT regimes, thus facilitating the choice of the end-user, especially since the information relating to the neutral system of the network is not necessarily known to the user. the user.

  The objects assigned to the invention therefore aim to remedy the various disadvantages listed above and to propose a new surge protection device adapted to provide a protection according to the common and differential modes which does not have the drawbacks listed above, and which, while being compatible with all connection diagrams of the neutral to the earth, has a limited space requirement.

  Another object of the invention is to propose a new protection device whose maintenance is particularly easy.

  Another object of the invention is to propose a new protection device which, in its withdrawable version, is particularly easy to use, in a logical and intuitive manner.

  Another object of the invention is to propose a new protection device whose adaptation to each type of network is particularly easy.

  The objects assigned to the invention also aim at proposing a novel method of manufacturing a surge protection device adapted to provide a protection according to the common and differential modes that is particularly simple to implement.

  The objects assigned to the invention are achieved by means of an overvoltage protection device adapted to provide protection in common and differential modes, and comprising: at least one electrically insulating housing, at least one electrical pair formed by a so-called phase-to-earth varistor disposed between a first phase to be protected and earth, and a so-called phase-neutral varistor disposed between said first phase to be protected and the neutral, characterized in that the housing is specifically adapted to receive the electrical pair, the phase-to-ground and phase-to-neutral varistors forming said electrical pair being mounted next to each other within said housing so as to form a mounting binomial.

  The objects assigned to the invention are also achieved by means of a method of manufacturing an overvoltage protection device adapted to provide a protection according to the common and differential modes, comprising the steps of: - a) to manufacture at least one electrically insulating casing, b) to manufacture at least one electrical pair, formed by a so-called phase-earth varistor, intended to be arranged between a first phase to be protected and the earth, and a so-called phase-neutral varistor, intended for being arranged between said first phase to be protected and the neutral, characterized in that it comprises the following steps: c) specifically arranging the housing so that it can receive the electrical pair, d) associating the two varistors in a pair of mounting , and mount them next to each other within the case.

  Other features and advantages of the invention will appear and will appear in more detail on reading the following description, with reference to the accompanying drawings, given for purely illustrative and non-limiting purposes, in which: - Figure 1 illustrates, according to a partial sectional view, a housing in which is mounted the mounting pair.

  - Figure 2 illustrates, in a general perspective view, the housing according to the invention in a withdrawable version.

  FIG. 3 illustrates, in a perspective view, a base on which the housing illustrated in FIG. 2 is intended to be mounted.

  FIG. 4 illustrates an electrical circuit diagram of the overvoltage protection device according to the invention in the case of a single-phase network.

  FIG. 5 illustrates an electrical circuit diagram of the overvoltage protection device according to the invention in the case of a three-phase network.

  The overvoltage protection device according to the invention is intended to be connected bypass to the equipment or electrical installation to be protected.

  The term "electrical installation" refers to all types of devices or networks susceptible to voltage disturbances of various origins, and in particular transient overvoltages due to lightning.

  The protection device according to the invention is preferably a low-voltage surge arrester, that is to say that it is preferably intended to protect low voltage installations (for example from 50 volts to 750 volts).

  The overvoltage protection device 1 will now be described with reference to FIGS. 1 to 5.

  The overvoltage protection device 1 is adapted to provide protection according to the common and differential modes, that is to say that it must be able to be used either in common mode or in differential mode, without this being necessary. necessary to modify in any way the device, including the nature and mounting of the protective components constituting it.

  According to the invention, and as shown in FIGS. 1 and 2, the protection device 1 comprises at least one electrically insulating casing 2, that is to say advantageously made from a non-conductive material of electricity.

  According to the invention, the protection device 1 also comprises at least one electrical pair, formed by a so-called phase-to-earth varistor PT, arranged between a first phase L1 to be protected and earth T and a so-called phase-neutral varistor PN, arranged between said first phase L1 to be protected and the neutral N. Thus, the phase-earth varistor PT is arranged to ensure the protection of the electrical installation in common mode (ie between an active conductor and the earth), while the phase varistor -Neutre PN is arranged to provide protection in differential mode (ie between two active conductors).

  According to the invention, and as shown in FIG. 1, the housing 2 is specifically adapted to receive the electrical pair formed by the phase-to-earth and phase-to-neutral varistors PN, that is to say that it is sized to accommodate, at most, two varistors.

  According to the invention, the phase-to-earth and PN phase-neutral varistors PN forming the electrical pair are mounted next to each other within the housing 2 so as to form a pair of mounting 3. The protection device 1 thus designed advantageously has a small footprint, the phase-to-earth varistors PT and phase-neutral PN is no longer, as in 1.0 devices of the prior art, mounted in individual housings but grouped within the same housing 2.

  In accordance with the requirements of the standardization, each PT phase or neutral phase-PN varistor is advantageously provided with its own means of disconnection, preferably of the thermal type, such that the thermal runaway is stopped sufficiently early in the event of aging of the varistors.

  Particularly advantageously, and as shown in FIG. 1, the disconnection means may be formed by a disconnection blade 4 which extends between two ends 4A, 4B, one of said ends 4A being preferably fixed using a hot-melt weld on one of the poles of the associated varistor. The end 4A of the disconnection blade 4 is thus preferably welded in stress, so that the heating of the varistor at the end of life causes the fusion of the weld which, once broken, allows the release of the blade disconnection 4 and thus the disconnection of the associated varistor of the electrical installation. The casing 2, provided with the phase-to-earth and phase-to-neutral PN varistors provided with their disconnection means 4, advantageously then forms a double motor, that is to say a motor provided with two varistors.

  Of course, the housing 2 may be formed by a simple support, not necessarily closed, but specifically adapted to receive the electrical pair formed by the pair of phase-to-earth varistors PT and phase-neutral PN.

  According to a particularly advantageous characteristic of the invention, the protective device 1 thus designed can be easily adapted to be in a withdrawable version.

  For this purpose, the protection device 1 advantageously comprises a base 6 which is preferably fixed, that is to say permanently connected to the electrical installation to be protected (FIG. 6).

  In a particularly advantageous manner, each housing 2 is then provided with racking / unplugging means 7 (FIG. 2) adapted to allow the removable electrical connection of the housing 2 relative to the base 6. As shown in FIG. 2, the racking-in / racking-out means 7 are advantageously formed by four pads 8A, 8B, 9A, 9B allowing the case 2 to be plugged onto the base 6. The poles of the phase-to-earth and phase-to-neutral varistors PN are therefore advantageously connected respectively to the pads 8A, 8B on the one hand and 9A, 9B on the other hand.

  In addition, the base 6 is preferably provided with a housing 10 arranged to receive the housing 2, and on the bottom 10A of which are formed four openings 8'A, 8'B, 9'A, 9'B specifically adapted for receive respectively the pads 8A, 8B, 9A, 9B.

  Particularly advantageously, the housing 2 provided with its racking / unplugging means 7 constitutes an interchangeable cartridge 11. Thus, when one of the phase-to-earth or phase-to-neutral varistors PN is degraded and disconnected, the user can unclip the cartridge 11 relative to the base 6, in order to replace it. This operation is made particularly easy and logical because of the unambiguous link existing between the cartridge 11 and the phase to be protected.

  Several embodiments of the invention will now be described with reference to FIGS. 4 and 5.

  According to a first embodiment of the invention shown in FIG. 4, the protection device 1 is designed to protect a single-phase network. For this purpose, the protection device 1 comprises at least one housing 2 (in dashed lines in FIG. 4) provided with a mounting pair formed by a phase-to-earth varistor PT and a phase-to-neutral varistor PN, said mounting pair. being electrically connected to the phase L1 to be protected. As shown in FIG. 4, the phase-earth varistor PT is electrically connected between the phase L1 to be protected and the earth T, thus providing a common mode protection, and the phase-neutral varistor PN is connected between the phase L1 and neutral point N so as to provide protection in differential mode.

  According to a second embodiment of the invention shown in FIG. 5, the protection device 1 is designed to protect a three-phase network and for this purpose comprises at least three housings 2A, 2B, 2C (in dashed lines on FIG. 5) each provided with a mounting pair connected electrically to one of the phases L1, L2, L3 to be protected. Thus, the housing 2A contains a phase-to-earth varistor PT connected between the first phase L1 to be protected and the ground, and a phase-to-neutral varistor PN connected between said phase L1 and the neutral point N. Likewise, the housing 2B comprises a phase-to-earth varistor PT connected between the second phase L2 to be protected and the ground, and a phase-to-neutral varistor PN connected between the second phase L2 and the neutral point N. Finally, the third case 2C comprises a phase-to-ground varistor PT, connected between the third phase L3 to be protected and earth, and a phase-to-neutral varistor PN connected between said third phase L3 and the neutral point N. In general, the protection device 1 according to the invention can be adapted to ensure the protection of a polyphase network comprising a number N of phases. In this case, the protection device 1 advantageously comprises a number of housings 2 at least equal to the number N of phases, each housing 2 being provided with a mounting pair 3.

  For networks whose ground neutral connection diagram is not of the IT type (neutral isolated from the earth or impedant), the protection device 1 may comprise a so-called neutral-earth NT varistor, arranged between the neutral and the earth, said varistor being mounted within an additional housing 2S, as shown in Figures 4 and 5. The additional housing 2S may either be specifically designed to contain a single varistor, or be similar in all respects to housing 2, with the difference that it will receive only one neutral-earth varistor NT as shown in Figures 4 and 5. In the latter case, a 2V portion of the additional housing 2S will then be unused.

  Of course, for networks with an earth-neutral connection scheme of type IT (impedant neutral or isolated from earth), simply remove or remove (in the case of a withdrawable version) the additional enclosure 2S, the protective device 1 then having more than one housing 2 in the case of a single-phase network and three boxes 2A, 2B, 2C in the case of a three-phase network.

  The protection device 1 according to the invention thus advantageously has a small footprint, with a maximum of two 2867318 14 boxes 2, 2S single-phase and at most four housings 2A, 2B, 2C, 2S three-phase. In addition, the protection device 1 having only one housing 2, 2A, 2B, 2C (or motor) per active conductor, the manipulation of withdrawable versions of this device is particularly easy, the user intuitively combining an L1 phase , L2 or L3 to be protected to a single corresponding cartridge 11.

  This aspect of the invention is particularly interesting in the case of polyphase networks comprising a large number of active conductors. Thus, the greater the number of active conductors is high, and the protection device 1 according to the invention is advantageous compared to the devices of the prior art in terms of size and ease of use.

  In cases where the protection device 1 comprises several interchangeable cartridges 11, each cartridge 11 being associated with either a phase L1, L2 or L3 to be protected or at the neutral point N, the base 6 (shown in dashed lines in FIGS. 5) may comprise several housings, each housing being capable of receiving a cartridge 11, or a single housing arranged to receive all the cartridges 11 mentioned above.

  As shown in FIGS. 4 and 5, each phase-to-earth, phase-neutral, or neutral-to-earth varistor NT is connected to the network by means of electrical connection means 15, the electrical connection to the earth being performed using other electrical connection means 16. The electrical connection means 15, 16 are, preferably and conventional, wire-like.

  Advantageously, each phase-earth varistor PT comprises a so-called phase terminal p electrically connected to the phase L1, L2 or L3 to be protected, and a so-called earth terminal t, electrically connected to the earth T. Likewise, each phase-to-earth varistor PN neutral comprises a so-called phase terminal p electrically connected to the phase L1, L2 or L3 to protect and a so-called neutral terminal n electrically connected to the neutral N. As shown in Figures 4 and 5, the phase-varistors PT earth and phase-neutral PN are arranged next to each other within the housing 2, 2A, 2B, 2C. However, the neutral n and earth terminals t are not at the same potential so that if the latter are too close to each other, an electric arc is likely to be formed between said terminals n, t, thus short-circuiting the protection device 1. In order to avoid this phenomenon, the protection device 1 advantageously comprises electrical insulation means 20, adapted to electrically isolate the earth terminal t from the neutral terminal n and preferably arranged within the housing 2, 2A, 2B, 2C.

  By way of illustrative and nonlimiting example, the electrical insulation means 20 may be formed by an electrically insulating separating partition 21 interposed between the earth terminal t and the neutral terminal n so as to guarantee the electrical insulation said earth terminals t and neutral n relative to one another.

  The partition wall 21 may for example extend over a surface just sufficient to provide isolation between earth terminals t and neutral n.

  Preferably, and as shown in FIG. 1, the separation partition 21 will preferably be disposed inside the housing 2 so as to separate the latter into two housings 22, 23 that are substantially symmetrical with respect to the housing 22. separating partition 21, each housing 22, 23 being capable of receiving a varistor provided with its disconnecting means. Even more preferably, and as shown in FIGS. 1 and 2, the partition wall 21 can extend outside the housing 2, so as to separate the pair of pads 8A, 8B associated with the phase-earth varistor PT, of the pair of pads 9A, 9B associated with the phase-neutral varistor PN.

  In this case, the base 6 advantageously comprises a central housing 60, specifically adapted to receive the outgoing end of the partition wall 21 during the racking of the housing 2 on the base 6.

  Such a configuration makes it possible in particular to avoid arcing between the pads 8A, 9A of different potentials, respectively connected to the earth terminal t and the neutral terminal n during plugging.

  According to a preferred embodiment of the invention, the phaseterre phaseterre and phase-neutral PN varistors of the same pair of mounting 3 have different operating voltages from one another. Thus, by way of illustrative and nonlimiting example, the phase-to-earth varistor PT may be of the 420-volt type, the phase-to-neutral varistor PN being rather of the 275-volt type. In the case of withdrawable versions of the protection device 1, each cartridge 11 will thus advantageously comprise a pair of varistors of different values, and this without creating any confusion in the mind of the user, since each active conductor remains associated with a single cartridge 11.

  The invention also relates to a method of manufacturing a protection device 1 against overvoltages adapted to provide a protection according to the common and differential modes, said method comprising the steps of: - a) manufacture at least one housing 2 electrically insulating b) manufacture at least one electrical pair, formed by a so-called phase-to-earth varistor PT intended to be arranged between a first phase L1 to be protected and the earth T and a so-called phase-neutral varistor PN intended to be arranged between said first phase L1 phase to be protected and the neutral N. According to the invention, the method also comprises the following steps: - c) specifically arrange the housing 2 so that it can receive the electrical pair, - d) associate the two varistors PT, PN in a pair of mounting 3 and mount them next to each other within the housing 2.

  Advantageously, the method also comprises a step in which the housing 2 is provided with racking / unplugging means 7 for connecting said housing 2 removably to a fixed base 6.

  Particularly advantageously, the phase-earth varistor PTincluding a phase terminal p electrically connected to the phase L1, L2 or L3 to be protected and a so-called earth terminal t, electrically connected to earth T, and the phase-neutral varistor PN comprising a so-called phase terminal p, electrically connected to said phase L1, L2 or L3 to be protected and a so-called neutral terminal n, electrically connected to the neutral, the method also includes a step in which insulation means are available electrical 20 between the earth terminal t and the neutral terminal n.

  The method also advantageously comprises the following steps: - e) manufacture a so-called neutral-earth NT varistor intended to be arranged between the neutral N and the ground T, - f) mounting said neutral-earth varistor NT within a housing additional 2S, preferably withdrawable.

  The invention thus makes it possible to significantly reduce the dimensions and therefore the bulk of the protection device 1 of the common mode / differential mode type.

  Another advantage of the invention is to allow easy and intuitive removal of the worn out protection components, with a view to their replacement.

  Another advantage of the protective device 1 according to the invention is that it is usable whatever the connection diagram of the neutral to earth.

  Another advantage of the invention stems from the fact that the protection device 1 according to the invention requires a number of parts, including engines, reduced compared to the devices of the prior art, while providing the same functions. The solution offered by the invention is therefore more economical than existing solutions.

  Another advantage of the invention is that it makes it possible to rationalize the method of manufacturing the protection device 1 according to the invention, in particular by systematically resorting to double motors.

Claims (12)

  1 - protection device (1) against overvoltages adapted to provide a protection according to the common and differential modes, and comprising: at least one housing (2) electrically insulating, - at least one electrical pair, formed by a varistor called phase- earth (PT), disposed between a first phase (L1) to be protected and earth (T), and a so-called phase-neutral varistor (PN) disposed between said first phase (L1) to be protected and the neutral (N), characterized in that the housing (2) is specifically adapted to receive the electrical pair, the phase-earth (PT) and phase-neutral (PN) varistors forming said electrical pair being mounted next to one another said housing (2) so as to form a mounting pair (3).   2 - Device according to claim 1 characterized in that it comprises at least one base (6) fixed, and in that each housing (2) is provided with racking / removal means (7), adapted to allow the removable electrical connection of said housing (2) relative to said base (6).   3 - Device according to claim 2 characterized in that the housing (2) 20 provided with its racking / removal means (7) is an interchangeable cartridge (11).   4 - Device according to one of claims 1 to 3 characterized in that it comprises electrical isolation means (20), adapted to electrically isolate the earth terminal (t) of the phase-earth varistor (PT) of the neutral terminal (n) of the phase-neutral resistor (PN).   Device according to Claim 4, characterized in that the electrical insulating means (20) are formed by an electrically insulating separating partition (21) interposed between the earth terminal (t) and the neutral terminal (n) in such a way to guarantee the electrical isolation of said earth terminals (t) and neutral (n) relative to each other.   6 - Device according to one of claims 1 to 5 characterized in that it is designed to ensure the protection of a single-phase network and in that it comprises at least one housing (2) provided with a pair of mounting (3) electrically connected to the phase (L1) to be protected.   7 - Device according to one of claims 1 to 5 characterized in that it is designed to provide protection of a three-phase network and in that it comprises at least three housings (2A, 2B, 2C), each provided a pair of mounting (3) electrically connected to a phase (L1, L2, 1_3) to be protected.   8 - Device according to one of claims 1 to 5 characterized in that it is designed to provide protection of a polyphase network comprising an N number of phases, and in that it comprises a number of housings (2) at least equal to the number N of phases, each housing (2) being provided with a mounting pair (3).   9 - Device according to one of claims 1 to 8 characterized in that it also comprises a so-called neutral-earth (NT) varistor, disposed between the neutral (N) and the earth (T), said neutral-earth varistor ( NT) being mounted within an additional housing (2S).   -Dispositif according to one of claims 1 to 9 characterized in that the varistors phase-earth (PT) and phase-neutral (PN) of the same pair of mounting (3) have different operating voltages one of the other.   11-A method of manufacturing an overvoltage protection device adapted to provide a protection according to the common and differential modes, said method comprising the steps of: - a) fabricating at least one electrically insulating casing (2), b) fabricating at least one electrical pair, formed by a so-called phase-earth (PT) varistor, intended to be arranged between a first phase (L1) to be protected and earth (T), and a phase-neutral varistor (PN), intended to be arranged between said first phase (L1) to be protected and the neutral (N), characterized in that it comprises the following steps: c) specifically arranging the housing (2) so that it can receive the electrical pair , d) combining the two varistors (PT, PN) into a mounting pair (3), and mounting them next to each other within the housing (2).   12 -Procédé according to claim 11 characterized in that it provides the housing (2) racking / removal means (7) for its removable electrical connection to a fixed base (6).   13 -Procédé according to claim 12 characterized in that one has the electrical isolation means (20) between the earth terminal (t) of the phase-earth varistor (PT) and the neutral terminal (n) of the phase-neutral varistor (PN).   14 -Procédé according to one of claims 11 to 13 characterized in that it comprises the following steps: - e) manufacture a so-called neutral-earth varistor (NT), intended to be arranged between the neutral (N) and the earth (T), f) mounting said neutral-earth varistor (NT) within an additional box (2S), preferably withdrawable.