2 . 5   mm ▁are ▁surrounded ▁by ▁the ▁die lect ric ▁oil . ▁ ▁A ▁test ▁voltage ▁is ▁applied ▁to ▁the ▁electro des ▁and ▁is ▁continu ously ▁increased ▁up ▁to ▁the ▁break down ▁voltage ▁with ▁a ▁constant , ▁standard - comp li ant ▁sle w ▁rate ▁of ▁e . g . ▁ 2 ▁k V / s . ▁ ▁At ▁a ▁certain ▁voltage ▁level ▁break down ▁occurs ▁in ▁an ▁electric ▁arc , ▁leading ▁to ▁a ▁collapse ▁of ▁the ▁test ▁voltage . ▁ ▁An ▁instant ▁after ▁ign ition ▁of ▁the ▁arc , ▁the ▁test ▁voltage ▁is ▁switched ▁off ▁automatically ▁by ▁the ▁testing ▁device . ▁Ult ra ▁fast ▁switch ▁off ▁is ▁highly ▁des irable , ▁as ▁the ▁carbon isation ▁due ▁to ▁the ▁electric ▁arc ▁must ▁be ▁limited ▁to ▁keep ▁the ▁additional ▁poll ution ▁as ▁low ▁as ▁possible . ▁ ▁The ▁transform er ▁oil ▁testing ▁device ▁measures ▁and ▁reports ▁the ▁root ▁mean ▁square ▁value ▁of ▁the ▁break down ▁voltage . ▁ ▁After ▁the ▁transform er ▁oil ▁test ▁is ▁completed , ▁the ▁ins ulation ▁oil ▁is ▁stir red ▁automatically ▁and ▁the ▁test ▁sequence ▁is ▁performed ▁repeatedly : ▁typically ▁ 5 ▁repet itions , ▁depending ▁on ▁the ▁standard . ▁ ▁As ▁a ▁result ▁the ▁break down ▁voltage ▁is ▁calculated ▁as ▁mean ▁value ▁of ▁the ▁individual ▁measurements . ▁ ▁Types ▁of ▁test ▁Color ; ▁e . g ., ▁A ST M ▁D 1 5 0 0 . ▁Die lect ric ▁break down ▁voltage ; ▁e . g ., ▁D ▁ 8 7 7 , ▁A ST M ▁D