Low frequency magnetic shielding: Present and future measurement

Effective magnetic shielding from DC to power frequencies is of growing importance in many applications. Electrical steel is often used as a shielding material since it offers a cost effective solution over this frequency range. There is no standard or commonly accepted approach to measuring the shielding capabilities of different grades of sheet materials or accurately assessing their effectiveness in practical applications. This paper briefly reviews recent experimental approaches to measuring the shielding factors of electrical steels. Research results obtained from one type of simple test system are presented to show the dependence of the shielding factors of electrical steel panels on thickness and relative permeability. Multiple-layer shielding is often advocated and its effectiveness, in the case of grain oriented steel, is demonstrated. Depending on the application, the selection of shielding material based on typical B-H curves presented by manufacturers can be misleading so a clearer understanding of specific material properties which control the shielding effectiveness is needed. The standardized method for evaluation of shielding effectiveness used for small formed shields is not suitable for the evaluation of sheet materials. Some suggestions are presented on the approach to a standardized method of measurement of the shielding effectiveness of sheet materials