Arc furnaces are used in metallurgical plants in the production of steel and various alloys. The furnaces are usually supplied with 50 Hz industrial frequency and their power ranges from hundreds of kVA to more than 100 MVA. Employees during typical operations related to furnace operation are exposed in particular to 50 Hz magnetic fields, which may also be present in the vicinity of electrode current paths. Due to the source instability, it is necessary to analyze the frequency spectrum and find the maximum intensity value - FFT and MAX HOLD functions.