One of the most important factors that impair the electricity reliability is the outdating of electric networks and substation equipment. The equipment in the Latvian power system has been in use since 1960-1970, although its permissible operating time is approximately 30 years. The use of outdated equipment that does not meet modern requirements worsens the technically-economical indices of the power system, increases the duration and frequency of equipment repairs and the frequency of incidents. All this influences negatively the reliability of electricity supply. Therefore, high voltage network equipment of Latvenergo should be replaced almost completely. An element of power stations and substations, from the reliability point of view, is the circuit-breaker. In the last years, Latvenergo has been realizing reconstruction of 110 and 330 kV substations, with replacement of the old air-blast circuit-breakers by new SF6 gas circuit-breakers. In Latvia, the existing 330 kV substations are mostly equipped with breakers whose number exceeds that of feeder bays. This means that in Latvia substations with repeated electric circuit feeders (two breakers or one-and-a-half breaker on line) are preferable. In the paper, the method for choice of substation is recommended taking into account the reliability of energy supply. In accordance with this method, reliability criteria can be calculated for all substation types: the four-switch mesh substation, the substation with one-and-a-half breaker arrangement and the double busbar substation with a transfer busbar. The probability is calculated for the following situations: 1) all breakers are in operating state, 2) one breaker is in non-operating state, 3) two breakers are in non-operating state, etc. In the calculations, formulas of binomial dividing are used. The recommended method is envisaged to be realized on the basis of the software for optimization and analysis of high and medium voltage network development worked out at the Laboratory of Power System Mathematical Modelling (Latvian Academy of Sciences, the Institute of Physical Energetics). The software can be employed for each 330 and 110 kV substation to calculate the following: 1) the probability of incident condition, 2) the probability of operating state, 3) the admissibility of the chosen substation, 4) the probability of unsupplied power, 5) the probability of unsupplied energy, 6) the costs of unsupplied electric energy for various types of substations and circuit-breakers