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levels, the voltages for the impulse voltage wavetest (see curve d on figure 21).c Overvoltage solutionsThese are applied both when equipment isimplemented and chosen:v on installation, it is advisable to removeovervoltage sources away from the mostsensitive items of equipment and to monitorequipment equipotentiality;v when choosing, it may prove useful to provide:- electrostatic shielding or capacitors(see appendix 3),- screen transformers,- filters,- surge limiters (see fig. 23 ).Low voltage surge limiters are also availabledesigned for installations with an IT earthingsystem (with unearthed neutral). Defined inFrance by standard NF C 61-740, they areintended to prevent all voltage rises due to aMV fault: “Cahier Technique” no. 179 describestheir implementation.Level1 0,52 13 24 4xTest voltage ± 1% (kV)SpecialLevel “x” is open: it must be defined forspecial equipment specifications.Fig. 22 : test voltages according to standardIEC 61000-4-5.Limited U(kV)I max (kA)(8/20 wave)Varistors 14 to 1,200 10 to 5,000Rectifier diode 5 to 400 10 to 1,000Fig. 23 : characteristics of the various surge limiter types.Cahier Technique Schneider Electric no. 141 / p.15