Magnetic Shielding Performance of Thin Metal Sheets Near Power Cables

In this study, wrapping conductors with thin magnetic materials is proposed as a magnetic shielding method. The 0.1-mm-thick metal sheets of mu-metal, grain-oriented electrical steel, and nonoriented electrical steel were prepared from commercial alloy sheets through cold rolling and followed high temperature annealing. For magnetic fields generated by three-phase electric currents, mu-metal was the best in shielding performance when magnetic field at the shield point is about 90 mu T, yielding an excellent shielding factor lower than 0.1 in the vicinity of the shield. Above 490 mu T, however, silicon steels were better than mu-metal. Double-layer shielding with grain-oriented silicon steel (inner) and mu-metal (outer) resulted in an excellent shielding factor up to 490 mu T. These results are due to change in hierarchy of magnetic permeabilities of the materials with increasing magnetic field strength. For magnetic fields by single-phase electric current, on the other hand, a magnetic shield rather increased magnetic field or had no practical effect on shielding performance. This result is explained by superposition of original field vectors and secondary field vectors created by eddy currents within the materials.