Expert systems appearance in power engineering has been called to life not only by development of information technology but also be growing urgency of precise and timely diagnostics of equipment. The latter was connected with transition from the system of scheduled preventing maintenance to equipment maintenance according to its technical state, both with shortage of high-qualified specialists due to natural generations replacement. Work on ES designing in power engineering of Russia began in the end of 80-s. Expert - diagnostic system for electrical equipment technical conditions evaluation worked out by the specialists of the USTU and joint-stock company was one of the first such systems. Today it is the most promoting system among expert - diagnostic systems for oil-filled electrical equipment in Russia; it is used at the enterprises of 21 power grids (58 sets).