The installation is flush mounted on a surface and has a base that engages with a wall socket inserted in the surface. There is a carrier ring with keyholes supported on the surface and a cover frame for the front side of the apparatus. The apparatus has a hollow cylindrical sealing insert surrounding the base and on this insert is formed a collar (16) in one-piece fashion. This collar is located between the mounting surface and the carrier ring. It has a peripheral coaming which forms a seal with the cover in a labyrinth fashion. The coaming is formed by a sealing lip bent back from the collar.
