The experimental results of the harmonic content of uncompensated electric currents in the building's electrical system. Found that these currents have frequency harmonics and interharmonics of electrical power frequency. They are generated by switching power supplies hardware and cause the increase of the level of magnetic fields on the premises. The methods to reduce these fields. The main of them is non-linear decrease in the proportion of consumers in the total electrical load of the building and shielding portions mains with uncompensated electric currents.