A three-positions disconnector comprising, for each phase an insulating casing which comprises a first and a second cavity separated by an insulated partition wall, a through hole being present in said partition wall between said first and second cavity. The first cavity houses: a first fixed contact electrically connected to a corresponding branch conductor (of a bus-bar system; at least a portion of a movable contact pivotally mounted on a support positioned in the first cavity and electrically connected to a first electrical terminal positioned in said second cavity. The disconnector further comprises, for each phase, a second fixed contact suitable for ground connection. Also, the disconnector comprises an operating mechanism and a kinematical chain linking the movable contact with the operating mechanism; the movable contact is movable between a first position in which it is coupled to the first fixed contact, a second position in which it is isolated from both the first and second fixed contacts, and a third position in which it coupled to the second fixed contact.