The contingency analysis (CA) has been an important tool in power system planning and security analysis for decades. CA studies the impact of equipment outage on the system. This paper presents the dynamic contingency redefinition in power system security analysis. Contingency definition is usually accomplished in an off-line development environment. The pre-defined contingency cases are studied in the off-line or on-line contingency analysis program. Usually the contingency definition defines hypothetical equipment outage or fault. It does not explicitly specify the isolating breakers of the fault. The dynamic contingency re-definition takes into account the actual response of protection subsystem to the equipment outage. It identifies the breakers required for isolating the contingency equipment. This provides additional accuracy of post contingency results to ensure the analysis accuracy and the remedial action schemes emulated correctly. Each re-defined contingency is simulated in CA by opening these breakers to evaluate their impact on the system. The ability to respect the status of manually operated switches is important to ensure the simulation mimics protection schemes. Any non-contingency equipment forced out as a result of these breaker openings will be added to the contingency equipment list. All outage equipment and breakers will be listed on the contingency definition display for each contingency case. The contingency re-definition function has been integrated into power system security analysis.