Since the switchgears of transmission network are for the most part out of date and now such worn equipment cannot function properly, the issue of selection among switchgear types to be used in the Latvian power system is very urgent. The method proposed in this article allows selection of switchgear type based on technical, economic and power supply reliability criteria, taking into consideration operating variables of the transmission and generation system and its development visions. The primary criterion for switchgear selection is reliability. For estimation of the switchgear reliability factor four types of critical faults are to be taken into account: 1) disconnection of one line, 2) disconnection of one section, 3) 50% disconnection of sections, and 4) 100% disconnection of sections. For all types of faults the probability of operational availability and interruption time should be calculated. To provide calculation of the reliability criterion for several types of switchgears an original algorithm has been worked out, which is based on disintegration of the calculation elements of a switchgear type’s configurations. The probability of operational incapability for all the elements can be calculated by the proposed unified method, which is designed/ conceived as a module of pilot equipment LDM-PG’08 (in the framework of “Information technology for sustainable development management of electric transmission networks and generation systems”). Apart from calculation of operational reliability and interruption time for all types of faults, the amount of non-supplied electricity and the costs of non-delivery can also be calculated.