This paper proposes an optimal strategy for EDR implementation in the power distribution system against large contingencies. First of all, the evaluation methods and application steps are developed and presented to determine the maximum potentials of EDR, which are serving as the constraints for the system security dispatch. Secondly, the mathematical problem is formulated as an optimization model where the objective is to minimize the total cost of the EDR program, and the constraints include the network power flow and bus voltage, as well as other operational limits such as the upper bound of the EDR for different buses. Finally, an optimal power flow (OPF) method is employed to solve the optimization model. Numerical case studies results show that the proposed method can achieve the minimum EDR amount while satisfying the security control requirements.