Rated delay and phase error of digital sampling are important factors affecting the performance of relay protection.The composition of rated delay in each link of digital sampling is analyzed.The implementation of synchronous sampling is described in smart substations and the differences and similarities between rated delay and phase error of digital sampling are specified in detail.The indirect and direct methods of rated delay testing are put forward and their advantages and disadvantages are compared.Finally，precautions and suggestions regarding the application of digital sampling in smart substations are given.