This paper presents a decentralized-approach-based wide-area back-up protection scheme to identify the faulted line in a power transmission network. The proposed scheme divides the whole power system under study into different protection zones (PZs) corresponding to generator bus(es). Furthermore, a 'gain in momentum' (GIM) index at each generator bus is used to minimize the search set. This information helps in reducing the search space to the PZ corresponding to the generator bus with highest GIM following a fault. This area is termed as a vulnerable protection zone (VPZ). Once the VPZ is identified, positive-sequence voltage magnitudes of each bus within the VPZ are monitored to locate the bus closest to the fault (BCF). Finally, reactive power-flow information of each line connected to the BCF is used to identify the faulted line. The performance of the proposed scheme is extensively validated on the Western System Coordinating Council-9 bus system, the IEEE-39 bus New England system, and 246-bus North India Grid using the Power System Simulation for Engineering. The test results indicate that the proposed scheme is highly effective for the WABP of transmission lines. © 2007-2012 IEEE.