Experimental behavior of a magnetic field shield for an underground power line

Abstract

This paper describes the experimental behavior of a pure iron magnetic shield for a single-phase underground power line. We have built a 0,5 mm-thick 2 m-long cylindrical shell. The measures realized show that this shield is very effective in order to reduce the magnetic field generated by a single-phase power line placed inside the shield, just in its center. 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. This means that the selection of an adequate material to act as a magnetic field is decisive. In order to maximize the absorption losses, such a material must have a high magnetic permeability and a high electric conductivity when dealing with very weak magnetic fields. For these reasons we have selected the purified iron (with a pureness at least 99’95 %). The ferromagnetic material of the shield (in the studied case is the pure iron) absorbs the magnetic field generated by the buried power line and just a very small portion of the magnetic field survives outside the shield. It is due that as the shield is made pure iron, which has a high electrical conductivity, the induced eddy generate a magnetic field opposed to the magnetic field generated by the power line. Due to the opposed magnetic field, the total magnetic field H outside the shield is reduced.