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To prevent wide-area disturbances, and enhance power system reliability, various forms of system protection scheme (SPS) have been designed and implemented by utilities. One of the main concerns in SPS deployment is to ensure that the system will fit with the reliability requirement specification in terms of dependability, and security. After major changes in system operating practices, or physical changes of the protected system, a refinement of the SPS decision process may be required. This paper uses a currently operational SPS as an example to demonstrate a refinement study. By incorporating an interval theory, a risk reduction worth importance concept, and a probabilistic risk-based index, the proposed procedure conducts parameter uncertainty analysis, identifies critical factors in the reliability model, performs risk assessments, and determines a better option for the refinement of the studied SPS decision process logic module.