This document presents a new method for steady-state security assessment that considers the dynamic thermal ratings of grid assets. The method uses both system-level load flow analysis and component-level time-domain simulations to determine the maximum permissible operation durations for given scenarios before thermal limits are violated. By accounting for short-term emergency ratings, it may be possible to reduce the need for remedial actions by allowing small exceedances of steady-state limits for longer periods. The results identify both the minimum duration a specific contingency can be tolerated, as well as the maximum emergency operation time of the entire grid.