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The purpose of power-system back-up protection is to disconnt faulty primary equipment not isolated by the main protection and associated circuit breakers. Conventional back-up techniques are difficult to apply in a large power system and heavy reliance on main protection is required. In this paper, both substation-based and centrally co-ordinated back-up protection methods, using microprocessors and minicomputers, are described. In a power system with a reliable communication network between substations, a fault-directing method is proposed to perform centralised digital back-up protection. If the communication network is not available, a substation-based back-up method is suggested. Both methods can reduce back-up fault-clearance time with correct discrimination, high reliability and cost effectiveness.