Power-frequency magnetic shielding of heavy-current conductors by rectangular shields

Abstract

Metallic enclosures of rectangular shape (e.g. galvanised iron trunking) are often installed in large buildings for the purpose of mechanical protection. Recently shielding performance of such enclosures has been of interest as a result of concerns about power-frequency magnetic field interference. The paper addresses shielding characteristics of rectangular shields, based on theoretical discussion, numerical simulations and laboratory experiments. It is found that both field reflection and field attenuation are two key factors, determined by impedance and skin depth ratios, respectively. Galvanised iron trunking can be approximately modelled as a linear shield with a constant equivalent relative permeability, the value of which is proportional to current magnitude. Shielding impact of shield parameters and source orientation, as well as guidelines for trunking design also discussed.