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 .