Magnetic Shields for Underground Power Lines.

Abstract

This paper describes a simple method for shielding the magnetic fields generated by underground power lines. It consists on placing a cylindrical shell made of a ferromagnetic material. The principle for reducing the magnetic field outside the shield is that the ferromagnetic material absorbs the magnetic field generated by the buried power line and only a very small portion of the magnetic field survives outside the shield. As the shield is made of a high-conductivity ferromagnetic material, eddy currents are induced inside it and they generate a magnetic field opposed to the magnetic field generated by the power line. It results that the total magnetic field H inside the shield is reduced.

It is well known that the relative magnetic permeability of ferromagnetic materials decreases sharply when magnetic fields intensities H are very weak (below 0,1 A/m) and when they are very strong (due to saturation phenomena). In these situations the shielding efficiency of these materials is dramatically reduced.

The selection of an adequate material to act as a magnetic field is decisive. Such a material must have a high magnetic permeability when dealing with very weak magnetic fields and a high electric conductivity in order to maximize the absorption losses. For these reasons the selected material has been the purified iron (at least 99’95 % pureness).