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This paper presents a very fast and effective "lambda-logic" based algorithm for reserve constrained economic dispatch (RED) problem when some of the on-line generating units have prohibited operating zones. The operating region of units having prohibited zones is broken into isolated feasible sub-regions, which results in multiple decision spaces for the economic dispatch problem. The optimal solution will lie in one of the feasible decision spaces and can be found using the conventional lambda-delta iterative method based on the equal- incremental-cost criterion in each of the feasible decision spaces. However, for a system with a large number of decision spaces, such exhaustive search method would not be acceptable in real time operation due to high computational time requirement. In this paper, a two executions of "lambda-logic based" algorithm is used. It uses an efficient approach to determine the most advantageous space using the average power of prohibited operating region. The proposed method is very efficient for both small and large size of generating units with prohibited operating zones in the problem and it remarkably reduces the computation time. The proposed algorithm has been tested successfully on 4 units, two different 15 units test system with some of the on-line units having prohibited operating zones including spinning reserves and test results are reported in this paper.