A connection device for connecting a multicore branch line to a flat cable has a connection device housing that allows the flat cable to pass through, and penetration contacts for contacting the flat cable without stripping of the insulation. The penetration contacts are situated in the connection device housing. The connection device also has an electrical disconnection point between the branch line and the flat cable, and at least one seal in the area of at least one opening of the connection device housing for protection from penetration of dust and/or water. The connection device housing is made, at least partially, of plastic, and on its outer side or inner side is coated with a diffusion barrier layer, wherein the diffusion barrier layer includes one or more layers, one of which is a metallic layer.