Large amount of information on maintenance history is accumulated during transformer’s service life including analysis of design, results of measurements and diagnostic tests, technical reports etc. A topical issue is to provide organized system for the viewing, archiving, and storage of above mentioned data to ensure more accurate survey on equipment's technical condition. In order to evaluate possible ways of obtaining additional information from accumulated statistical data, five classification methods of faults developed by various organizations are summarized in this paper, as well as verified on statistical data of power transformers in power transmission network in Latvia. Calculations, performed by Weibull cumulative distribution function, for determining service life of particular part of 2 power transformers are presented as a case study.