- Protection system failure is one of the main causes of cascading outages. This paper proposes an integrated scheme to study power system vulnerability considering protection system failures. In this scheme, both adequacy and security based reliability analysis are conducted. A new protection system reliability model including two major failure modes is established to demonstrate their effects on power system reliability. The mechanism and scheme of protection systems have been analyzed for their contribution to cascading outages as well as system stability after a fault occurs. All contingencies and the responses in the power system are depicted in their inherent stochastic manner. The power system vulnerability is assessed by both adequacy indices, such as Bus Isolation Probability (BIP), Loss of Load Probability (LOLP) and Expected Power Loss (EPL), and the security index Probability of Stability (POS). In addition, a new vulnerability index, Integrated System Vulnerability (ISV), is introduced to give a more comprehensive description of the system vulnerability. A nonsequential Monte Carlo simulation approach is used to implement the stochastic properties of contingencies, protective response and protection system failures. The IEEE Reliability Test System is used to illustrate the methodology and present the results. 2004 IEEE.