Operating parameters such as voltages, leakage, and ground fault currents are decisive for a correct and safe operation of low-voltage unearthed networks with diode rectifiers. The knowledge of the maximum possible values of these parameters is necessary for correct assessment of numerous hazards. So far, no simple methods for determining the highest levels of these parameters in AC/DC IT three-phase networks have been available. The paper presents a new approach to the evaluation of the variation ranges of diode rectifiers’ voltages and currents with the help of Mathcad.