Abstract: Apparatus for shielding high frequency electrical and electromagnetic waves for tight wall lead-ins for groups of lines. A pressure-resistant frame is filled by modular superimposed and juxtaposed partly or wholly elastic neutral pieces and fitting pieces. The number of the latter corresponds to that of the lines and is in each case in two parts. A circumferential slot extending to the metal shield of the line is made in the vicinity of the frame plane. The fitting pieces contain a web insertable in the slot. By deformation, the fitting pieces and neutral pieces are subsequently pressed against one another and against the frame in such a way that electrical conductive particles incorporated into the elastic material, which is swellable under heat action, are so displaced with respect to one another that fitting pieces and neutral pieces are made electrically conductive and water, gas and fire-proof over the entire cross-section thereof.