The scope of this paper is to comparatively present the extremely low-frequency (ELF) measurements performed at four power plants in Greece, focusing on: (a) the worst-case exposure conditions, (b) the existence of magnetic field harmonic components, (c) the technical similarities among the power plants and (d) comparison of the measured percentages of reference levels at typical working areas in the power plants. A detailed measurement methodology is proposed, including broadband on-site inspection of the working areas, weighted averaged root-mean-square and peak values of magnetic flux density, percentage of reference levels, according to 1998 ICNIRP guidelines and harmonic analysis of the multi-frequency magnetic fields. During the analysis of the occupational exposure in all power plants, the new Directive 2013/35/EU has been taken into account. The study concludes by proposing a mapping procedure of working areas into certain zones, in order to take measures for workers safety.