The Kluwer Elektrotechniek/Elektronika woordenboek suggests 'middle conductor', 'inner main' or 'third wire' as the translation of 'middelleider' for dc systems (which is what you have) and 'neutral conductor' for ac systems. However, the first three terms strike me as being not all that common (and googling them doesn't suggest that they are used particularly often in this particular context), so I suggest 'neutral conductor' or 'neutral lead' (presumably this is the conductor that serves as the reference and/or ground return for the other related conductors).