<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Field penetration into a metallic enclosure with a large aperture protected by a wire mesh screen is analyzed in this paper, using an in-house finite-difference time-domain code with a thin-wire approximation that accurately accounts for the cross between wires of the mesh. All results are experimentally validated using a reverberation chamber. Different enclosure dimensions and mesh sizes are considered, highlighting their implication on shielding performances. </para>