In a process for operating an air-cooled mains supply unit for industrial application, temperatures influenced by outer heat sources and by heat sources arranged in the mains supply unit are measured by thermo-elements and reported to a control system. Temperatures measured by at least two thermo-elements arranged at different measurement points are reported to the control system, and the measured temperatures are compared with temperature patterns. The comparison with known temperature patterns makes it possible to sense the totality of the thermal conditions continuously and to derive therefrom ratings for the control system.