Abstract: The possibility and the severity of the consequences for the failure of OCS(Overhead Contact Line System) is considered and analyzed accurately, and the failure risk is assessed in time to provide a theoretical basis for the development of risk control measures. Firstly, the failure fault tree of OCS is established, and then the BDD structure is generated by the ITE rules. The top event risk, the Birnbaum importance and the critical importance of the basic events are calculated by recursively accessing the nodes of the BDD structure from top to bottom. According to the BDD algorithm, the C# program is finished, thus the occurrence probability of failure accident for OCS and the key factors causing the accident can be obtained. Compared with the cut set method, the BDD method can not only get the exact values of the occurrence probability of top event and the importance of basic events, but also have the advantages of fast calculation speed and simple procedure.

Key words: BDD algorithm, fault tree, risk assessment, Birnbaum importance, critical importance

摘要： 准确衡量与分析接触网系统失效的可能性与后果严重程度，并及时评估其失效风险，为制定风险控制措施提供理论依据。首先建立接触网失效故障树，然后由ITE规则生成BDD结构。通过自上而下递归访问BDD结构各结点来计算顶事件风险、基本事件的Birnbaum重要度和关键重要度。根据BDD算法编制C#程序，可得到接触网失效事故的发生概率和引发事故的关键因素。与割集方法相比，BDD方法不但能得到顶事件发生概率和基本事件重要度的精确值，而且计算速度快，过程简便。

关键词: BDD算法, 故障树, 风险评估, Birnbaum重要度, 关键重要度