TS60815-2@IEC:2008(E) - 7 - 3.1 Terms and definitions 3.2 unified specific creepage distance USCD creepage distance of an insulator divided by the r.m.s. value of the highest operating voltage acrosstheinsulator NOTE 1 This definition differs from that of specific creepage distance where the line-to-line value of the highest result in a value that is V3 times that given by the definition of specific creepage distance in IEC/TR 60815 (1986). NOTE 2 For ‘Um' see IEV 604-03-01 [1]1. NOTE 3 It is generally expressed in mm/kV and usually expressed as a minimum. 3.3 referenceunified specificcreepagedistance RUSCD initial value of unified specific creepage distancefora pollution site before correction for size, profile, mounting position, etc. according to this technical specification and generally expressed inmm/kv Ll 3.4 Abbreviations CF creepage factor ESDD equivalent salt deposit density NSDD non soluble deposit density SDD salt deposit density SES site equivalent salinity SPS site pollution severity USCD unified specific creepage distance RUscD referenceunified specificcreepagedistance 4 Principles The overall process of insulation selection and dimensioning can be summarized as follows: Firstly, using IEC/TS 60815-1: determine the appropriate approach: 1, 2 or 3 as a function of available knowledge, time and resources; collect the necessary input data,notably system voltage,insulation application type (line, post, bushing, etc.); collect the necessary environmental data, notably site pollution severity and class. At this stage, a preliminary choice of possible candidate insulators suitable for the applicationsand environmentmaybemade. Then, using this technical specification: refine the choice of possible candidate ceramic or glass insulators suitable for the environment; References in square brackets refer to the bibliography. TS60815-2@IEC:2008(E) - 8 - determine the reference UscD for the insulator types and materials, either using the indications given in this technical specification, or from service or test station experience in the case of approach 1 (Clause 7); choose suitable profiles for the type of environment (Clause 8); verify that the profile satisfies certain parameters,with correction or action according to the degree of deviation (Clause 9); modify, where necessary (approaches 2 and 3), of the RUscD by factors depending on the size, profile, orientation, etc. of the candidate insulator (Clauses 10 and 11); verify that the resulting candidate insulators satisfy the other system and line requirements such as those given in Table 2 of IEC/Ts 60815-1 (e.g. imposed geometry, dimensions,economics); verify the dimensioning,if required in the case of approach 2,by laboratory tests (see Clause 12). HNEAMSNTHISLANNALED BY BOOK SPPLY BURAU. NOTE Without sufficient time and resources (i.e. using approach 3), the determination of the necessary USCD will have less accuracy 5 Materials This technical specification is applicable to ceramic and glass insulators.The guidance given hereassumesthattheinsulatorsareofstandardmanufacturewithoutanysurface modification ortreatment. Technologies exist intended to improve the performance of such insulators under pollution, for NCHI/B example,semi-conducting glaze and hydrophobic coatings.At present it is notpossible to give specific information on the degree and durability of the improvement given by such technologies. As far as the relative performance of ceramic and glass insulators under pollution is concerned, there is no notable consistent difference between these materials; hence the choice of either glass or ceramic material with respect to the other depends purely on factors (e.g.ageing,operating experience, maintenance procedures)which are out of the scope of this technical specification. 6 Siteseveritydetermination For the purposes of standardization, five classes of pollution characterizing the site severity are qualitatively defined in IEC/TS 60815-1,from very lightpollution t