Fail safe means that when the power source is lost, the controller causes the process go to a safe state.
Safe state mostly requires equipment to be shutdown, production to be halt and sometime process material to be released.
Fail safe makes the process system in safer state if either of the following condition occurs: abnormal process condition, loss of power, loss of instrument air supply, or field cable faults.
To have system which operates in fail safe, the following condition should be fulfilled:
Switch shall have sensor contacts to be closed and the loop is energized during normal process operation.
In the event of power failure, valve will move to a predetermined position, which could be open or closed and other final element would stop to operate such as motor would stop, heater would be turned off.
Status indication contact shall be closed to indicate running and open to indicate stop.