Nonferromagnetic Open Shields at Industrial Frequency Rate

Abstract

We have investigated some innovative geometric configurations for shielding extremely low-frequency (ELF) magnetic fields through nonferromagnetic materials. We define proper 2-D and 3-D finite-element (FE) models for the numerical computation of the shielding effectiveness of open conducting shields, in order to assess the mitigation of the magnetic field generated by a single-wire transmission line, two-wire t.l., or three-phase current excitation at industrial frequency rates. For the case of current conductors in air (i.e., in absence of the metallic shield) we validate our numerical model using the Biot-Savart law, while for the case of an infinite plane shield we exploit an analytical validation by means of a direct solution of the Maxwell's equations, with properly defined boundary conditions. We compare our experimental results with those found in literature and prove the accuracy of the computed shielding efficiency. After this preliminary validation of the numerical tool, we study the behavior of several nonplane open shields, characterized by different transversal profiles: by means of the FE model, we directly compare these innovative configurations to the plane one, thus showing that they can guarantee an increase of the shielding effectiveness at constant weight of the shield just by a simultaneous mitigation of both the two components of the magnetic field in the transversal plane. This feature is verified by means of experimental results, in which we have considered a finite shield with a current source constituted by a single long wire.