Fail-Safe

Design features which provide for the maintenance of safe operating conditions in the event of a malfunction of control devices or an interruption of an energy source (e.g., failure direction of a motor operated value on loss of motive power).

A designed property of an item which prevents its failures being critical failures.

Describing a circuit or device which fails in such a way as to maintain circuit continuity or prevent damage. A feature of a system or device which initiates an alarm or trouble signal when the system or device either malfunctions or looses power.

Any protection against effects of failure of the equipment, such as, fuel shut off in the event of loss of flame in a furnace.

Describes an equipment fault which, irrespective of inputs, causes the ESD system to move nearer the shut-down point or initiate shut-down action. The fail-safe fault may lead to an undesired frequency of shut-downs which could be costly and lead to diminished confidence in the ESD system.

The desired failed-state (e.g., closed, open for a final control element, or any control system component).

A feature incorporated for automatically counteracting the effect of an anticipated possible source of failure. A system is fail-safe if failure of a component, signal, or utility, initiates action that return the system to a safe condition.