The apparatus for fault line selecting based on transient traveling waves in the neutral un-effectual grounded system（NUS） is one of the types of facilities used for practical applications. Performance testing using real time digital simulator（RTDS） provides helpful information for its improvement. This paper introduces methods of using transient traveling wave signal（TTWS） to faulty line selection. The feasibility for testing the apparatus based on TTWS using RTDS is analyzed， and a NUS simulation platform is built for TTWS based on RTDS. Simulation results demonstrate the platform is suitable for testing such apparatus to some extent.