Purpose. Finding dependence between reliability indexes and power quality. Working out methodology for calculation of reliability index reduction depending on poor power quality based on found dependency. Methodology. The method of harmonic analysis, the principle of superposition for instantaneous power and the methods of the theory of electric machines. The research objects are distribution processes, processes of transformation, transmitting and consumption of electrical energy in power systems of 6 and 0.4 kV with non-sinusoidal voltage in it. The subjects of the research are reliability indexes of separate power supply system units and its dependency of indexes of electromagnetic compatibility. Findings. Scientific explanation of equipment reliability index reduction depending on poor power quality in power system. Detection of the effect of non-sinusoidal voltage on the reliability of transformers and cable lines. Originality. The article analyzes the problems of electricity quality at the moment and describes the importance of solving these problems. The solutions are mainly based on the change in the level of harmonics affecting the power grids, thus improving the quality of electricity. In the work, the analysis of non-sinusoidal voltage influence on power system units reliability indexes is conducted based on typical city power supply scheme. Practical value. Finding of regularities of electromagnetic processes flaws in power system units of 6 and 0.4 kV depending on non-sinusoidal voltage level. Based on researched regularities, the process of reliability index change should be described depending on power quality. The research is conducted for such power system elements as transformers and cable lines. Based on the research, the characteristics of power system units reliability index change are built depending on the load level.