In this article is examined a question of substations` power supply reliability. The formulas of calculation of the main parameters that are used to evaluate level of reliability are given at this work. The principle of probability of disabled state calculation depending on some objects` configuration is given at this work too. At the second part of the article the principles of determination of necessary reliability level are given. These principles are based on economical criteria and reliability level is determined by minimum costs. The ways of improving of reliability level by using automatics, bus sectioning and using more reliable network constructions are given at the second part too. At the third part of article the principles of determination of probability of disabled state for one middle voltage busbar and for two middle voltage busbars are given. At this part of article dependence of probability of disabled state of whole 110 kV substation of some factors as place of substations between main substations, substations` number connected in series (with probability of transit of power) and power supply lines` length is given too. First two factors are important for substations that are located in cities because of high level of density of substation. The third factor is important for rural areas.