The reduction of the magnetic field emitted by industrial sources, such as MV/LV substations, has been faced by means of computation and experimentation. The proper selection of parameters and dimensions of the magnetic screens is critical for their efficiency. In this paper, the influence of the thickness is analyzed varying it for different plane shape ferromagnetic and conductive materials (Fe, Al and Cu). A typical three phase conductor distribution has been used as magnetic source. The results of the measurements validate those of the simulations. The graphics presented show the limitation phenomenon appearing in the efficiency of the different materials which demonstrates that shielding effectiveness is not lineally improved by an increment in screen thickness. For each of the materials there is one optimal value for the thickness which is shown and analyzed in this paper.