“Active mitigation system is equipment, devices, or technologies that need human, mechanical, or other energy input to function. An example is an automatic shutoff valve that limits the duration of a hazardous material release.”
Reference: Walter, R. J. (2010). Local Emergency Planning Committee Guidebook: Understanding the EPA Risk Management Program Rule: Wiley. [Retrieved on October-12-2015]
Note: An active system is formed by a loop consisting of a sensor, a processor and an actuator. Full protection is expected from high integrity systems. These are active and highly reliable, they do not contain a human-in-the-loop, and are called Safety Instrumented System. According to standard IEC 61511 there are three Safety Integrity Levels (SILs): 10-1-10-2; 10-2-10-3; 10-3-10-4