ed ▁with ▁the ▁more ▁reliable ▁W 2 4 2 , ▁however , ▁on ▁some ▁units , ▁only ▁one ▁compress or ▁was ▁replaced , ▁leaving ▁one ▁of ▁each ▁type ▁in ▁use ▁on ▁a ▁single ▁locomot ive ; ▁the ▁C 3 8 ▁produced ▁air ▁until ▁ 8 ▁bars ▁in ▁main ▁res er voir ▁t anks ▁were ▁reached , ▁while ▁the ▁W 2 4 2 ▁from ▁ 8 ▁to ▁ 9 ▁bars . ▁In ▁fact , ▁on ▁these ▁units , ▁only ▁the ▁W 2 4 2 ▁was ▁used ▁on ▁normal ▁conditions ; ▁the ▁other ▁one ▁only ▁if ▁pressure ▁dropped ▁below ▁ 7 ▁bars . ▁ ▁Main - t anks ▁and ▁the ▁ 2 4   V ▁batter ies ▁supplies ▁air ▁and ▁current ▁to ▁several ▁systems : ▁▁ ▁B rak ing ▁system , ▁which ▁takes ▁air ▁from ▁the ▁main ▁res er voir ▁t anks ▁and ▁which ▁is ▁generally ▁kept ▁under ▁a ▁pressure ▁of ▁about ▁ 5 ▁bar ▁( when ▁bra kes ▁aren ' t ▁un re leased ). ▁ ▁Main ▁break er ▁( , ▁IR ), ▁which ▁is ▁the ▁" high ▁voltage ▁switch " ▁that ▁connect s ▁the ▁ 3   k V ▁of ▁the ▁line ▁to ▁the ▁locomot ive . ▁It ▁can ▁be ▁closed ▁only ▁when ▁pressure ▁is ▁greater ▁than ▁ 5 ▁bars . ▁Contact ors . ▁They ▁are ▁widely ▁employed ▁on ▁Italian ▁electro - me chan ic ▁locomot ives , ▁including ▁E . 6 3 6 . ▁They ▁are ▁used ▁in ▁various ▁systems , ▁mainly ▁to ▁insert / ex clude ▁the ▁r he ost at