A design principle where if a system fails, it does so in a way that defaults to a safe condition. For example, a fail-safe valve might close (shutting off flow) if power is lost, to prevent an uncontrolled process. In OT, fail-safe mechanisms ensure that if something goes wrong, whether a component fails or communication is lost, the process will shut down or revert to a state that avoids harm to people, environment, or equipment. (Contrast with fail-secure, where on failure a system locks in a secure state, like a door locking during a power outage – but in industrial control, fail-safe is usually about safety.)
