The study presented in this paper investigated online data loss in a substation automation system (SAS) under network faults or failures. Online data loss refers to the amount of data that become inaccessible when a failure occurs in the SAS. It is a mathematical index related to the network's topology and architecture, and the calculation of which requires knowledge of the physical characteristics of SAS elements. The elements that record data were identified, and then the online data lost due to failures in those elements were determined. Also, as a case study, a typical SAS was examined under various states of failure, and the online data loss occurring under each scenarios was calculated.