TS 60815-3 @IEC:2008(E) - 15 - 10.2 Correction for insulator diameter Kad CorrectforaverageinsulatordiameterDaasfollows: Kad = 1 when Da is smaller than 300 mm; Kad from the figure below when Da is equal to or larger than 300 mm. Where Da = (2Dt + Ds1 +Ds2)/4 (Ds1=Ds2 for regular sheds). For complex shed repetitions, add each extra diameter to the numerator and add 2 to the denominator. This correction may be reduced in certain conditions, as described in 5.2 for reduced creepage distance.This is reflected in Figure 7 below by the different curves for HTM, non- HTM and intermediate cases where hydrophobicity of HTM materials may be temporarily lost. HOFENTETAMUSENI THIs LOCANONBANLYASORLED YBOOK SLY BURAU. HTMwith 1,4 Non HTM possible loss of 1,3 hydrophobicity 1,2 1,1 3 1,0 HTM 0,9 0.,8 0,7 0,6 0 200 400 600 800 1000 1200 IEC1989/08 Average diameter (mm) IEC1990/08 NOTE CIGRE 33.13 TF 01 documents [1, 2], provide a background to this diagram. Figure7a-llustrationofparameters Figure 7b - Kad versus average diameter Dt. Ds1, and Ds2 Figure7-Kadversusaveragediameterand illustrationofparameters 11 Determination of the final minimum creepage distance Once the RUscD has been corrected according to Clause 10, the final minimum creepage distance is determined for the candidate insulator by rounding up to the nearest creepage distance available for that type of insulator within the constraints (system, dimensional, etc.). If the considerations of both 5.2 and Clause 7 concerning HTM materials are applicable,then by agreement the UscD maybe adjusted downwards by upto one pollution class. In cases of SPs in classes d and e, obtaining the necessary RUscD by using longer creepage distance profiles may lead to degradation problems (see 5.2.2 and AnnexA), notably if the profile changes are detrimental (e.g. reduced shed spacing which compromises self-cleaning and increases local field stress). 12 Confirmation by testing There are no available testing methods for polymer insulators in current IEC publications.Nor is precise information available from CIGRE at the present time [4] although further work is underway. In the absence of recommended tests, tests can be agreed between utility and manufacturer,taking into account thefollowing considerations: - 16 - TS60815-3@IEC:2008(E) for solid layer tests (generally representative of type A pollution),the testing ofHTM insulatorsmay require investigation of the performance inboth hydrophilic and hydrophobic states; the measurement of SDD can be problematic for HTM insulators; treatments enabling the application of solid layer may affect hydrophobicity; for salt fog tests (generally representative of type B pollution), standard pre-conditioning techniques may temporarily destroy hydrophobicity; withstand determination tests where flashovers occur may also destroy hydrophobicity, e.g. the "up and down" method. HEAEDYOOKSPLYBURAU AME NCH SLUP TS60815-3@IEC:2008(E) 17 Annex A (informative) Background information on pollutioninduced degradation of polymers As mentioned in 5.1, the kind of housing material itself and the design of the insulator can be decisive factors for the successful use of composite insulator in polluted conditions, notably concerningtheirlong-termpollution withstandcharacteristics,stability of hydrophobicityand comparison to conventional insulators is,for a given environment, due to several influences smaller average diameter, notably in the case of suspension insulators; finer, more open profiles due to reduced material thickness; AEDBYBPPLYBURAU. different material physical characteristics giving reduced thermal lag (less wetting due to dew/mist)and altering pollution accumulation and arc propagation; hydrophobic surfaces which reduce surface conductance and leakage current activity. Because of the lack of experience with hydrophobic housing materials and the uncertainty of the long-termhydrophobicity behaviour,the linear creepage principle given by IEC/TR has been successful