Capability of a product to automatically place itself in a safe operating mode, or to revert to a safe condition in the event of a failure
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safety to a particular hazard, a system being “fail-safe” does not mean that failure is impossible or improbable, but rather that the system’s design prevents or mitigates unsafe consequences of the system’s failure.
That is, if and when a “fail-safe” system fails, it remains at least as safe as it was before the failure.