A special case disclosing the mechanism through which an inadequate result occurs in representing the semiconductor switches by means of piecewise-linear approximation is considered taking as an example the discrete model of a three-phase uncontrolled rectifier. It has been found in analyzing the voltages across the switches of the non-conducting three-phase bridge that with certain ratios of the load counter EMF and the amplitude of the line-to-line network EMF at any moment of time, the voltage across one of the non-conducting bridge switches is positive whereas the voltages across all other switches are negative. As a consequence, fluctuations of the switch state alterations occur that are not typical for the operation of a real circuit. Factors causing the inadequacy of piecewise-linear models to the physical ideas about the circuit operation are revealed, and the consequences arising due to this in the discrete models of rectifies are pointed out. Recommendations on the ways for eliminating the factors causing inadequate modeling are given.