In order to reduce disaster rate due to machine in working place, new methodology in safety technology is expected. With this background, international standards on machine safety are highlighted. This paper concerns the function of safety devices for machines and their validation according to the standards. The purpose of this study is to understand core concepts included in them through a case-study. In this paper, 3 typical circuits for safeguards are focused and their safety functions are analyzed by FMEA, ETA, and FTA and importance index. Through this study, following points are shown:(1) procedure of categorization of safety devices, (2) importance of failure detection especially in Category 4, and (3) ranking of components by importance index.