# Process safety time Process safety time (PST) is the sum of early response time, operator reaction time, and SIF (Safety Instrumented Function) response time[^1]. [^1]: Is defined in BS EN 61511-1. ## Early response time The time after a failure has occurred where the process variable is changing and no alarm has been triggered. ## Operator reaction time The time between alarm and SIF activation in which the operator could take manual corrective action. ## SIF response time SIF response time is the time between the SIF setpoint being reached, and the hazardous condition occurring. It should consider [[Sensor response time|sensor response time]], and time to fully operate final elements such as shutdown valves. >[!info] >A common rule-of-thumb for shutdown valves is to assume 1 second per inch of nominal bore for preliminary assessment.