A mechanical interlock assembly for a network transformer primary disconnect assembly is provided. The mechanical interlock assembly includes a blocking member assembly with a body. The a blocking member assembly body includes an obstruction portion and a lacunar portion. The blocking member assembly is movably disposed adjacent to a movable operation contact carriage and a ground contact carriage. The blocking member assembly body is movable between a first position, wherein the blocking member assembly body obstruction portion is disposed in the path of the ground contact carriage, a neutral position, wherein the blocking member assembly body obstruction portion is disposed in the path of the ground contact carriage and the operation contact carriage, and a second position, wherein the blocking member assembly body obstruction portion is disposed in the path of the operation contact carriage.