Life-cycle cost management is the foundation and techniques for life-cycle cost management. Petri nets has a graphical expression; it can improve and add more features and information to fault tree analysis, Markov equations and so on, which raises the speed and accuracy of the entire system of risk assessment. The paper proposes a risk assessment based on the life-cycle management approach, establishes four state models to analyze transformer substation main wiring operation, discusses elements occurring planned maintenance events and breaker tripping phenomenon, and proposes using the basic equations of Petri nets to determine system status and calculate the corresponding probabilities, quantitatively analyzes the outage costs of the substation main connection off for running, and seeks the best value under the run. In the example, it discusses and calculates the risk indicators of outage costs for different wiring schemes; the results demonstrate the feasibility of the proposed algorithm.