Recent blackouts have induced a sense of urgency among protection engineers to revisit the concept of zone-3 protection in power transmission network. Zone-3 mal-operation is reported to be one of the primary causes behind most of the cascaded outages. The main purpose of this paper is to present a supervisory control based wide area back-up protection scheme. A data-mining algorithm, known as decision trees, is used to classify the state of power transmission system. Further, phase angle of positive sequence integrated impedance is utilised to assist the existing zone-3 protection in order to avoid unnecessary tripping during stressed power system condition. The studies performed on western system coordinating council (WSCC)-9-bus system in Real Time Digital Simulator platform validate the proposed algorithm under various faults and stressed conditions. © 2016 IEEE.