For the time being there are three different structures between process level and bay level for digital substations below 35 kV such as: IEC 61850-9-1 standard， IEC 61850-9-2 standard and small analog signal transmission standard. Considering that the characteristics of the three kinds of structures and special requirements for the earth fault line selection device， the design scheme of line selection device is proposed for digital substations with three different structures. The advantages， disadvantages， commonality and technical economy index of three design schemes are contemplated. Considering that the feature of electronic instrument transformer and demand for technical economy index of the system below 35 kV， a distributed line selection design scheme based on small analog signal sampling is proposed for digital substation below 35 kV， and the design is of highest commonality and best technical economy index.