As the leading features in smart substations, information-sharing and interoperability can provide rich information about operation and condition of secondary devices, which greatly facilitates secondary device condition monitoring and its condition-maintenance. However, it is still insufficient in current research such as information modeling of switches. This paper firstly discusses the concept and object of secondary device condition monitoring. Then the key techniques and methods of information modeling and monitoring data analysis and processing are summarized. Besides, a switch and a network topology modeling are proposed. Several methods with their advantages, shortcomings and suitable occasion of secondary device fault diagnosis using condition monitoring data are also summarized and compared. At last, considering current research and application situation of secondary device condition monitoring, future research directions and potential applications are discussed.