In this paper, a novel approach for the optimal placement of Phasor Measurement Units (PMUs) and Power Flow Measurement (PFM) devices is presented to ensure continuous observability of the power system network. The method incorporates the impact of contingency, specifically addressing the potential loss of a single PMU, in the optimal allocation strategy, thus ensuring full observability even under adverse conditions. To further optimize the deployment of monitoring devices, the concept of a zero-injection bus is introduced into the proposed technique, resulting in a reduced number of required PMUs and PFMs. Leveraging the Cuckoo Optimization Algorithm (COA), a heuristic technique is employed for the optimal location of these monitoring devices. The effectiveness and efficiency of the proposed method are validated through extensive testing on a 7-bus test system, IEEE-14, 57, and 118 bus test systems, as well as a real-world system. Comparative analysis with existing literature substantiates the superior performance of this approach.