Abstract: In order to ensure the safety operation of relay protection and automatic safety device in smart substation, safety isolation measures are mainly adopted, such as putting into maintenance pressing plate, exiting device soft pressing plate, outlet hard pressing plate and disconnecting optical fiber between devices.Taking line outage maintenance for example, starting from the information connection between equipment such as voltage and current sampling, tripping（including remote tripping）,starting failure etc.,it focuses on the analysis of the dangerous points affecting the operation equipment and the possible results. The applicable secondary safety measures are summarized according to various dangerous points, and the reliable isolation between the maintenance devices and the operation devices are completed, which provides valuable references for the operation and maintenance personnel.