Apart from the obvious meaning (adjective describing something that cannot fail), the term if often used with a different connotation when describing the behaviour of critical systems.

A fail-safe device is designed to revert to a safe state when malfunctioning - for instance by using low or high asserting circuits. Which state is safe depends on the circumstances - this entry from The Evil Overlord list might shed some light on the subject:

"96. My door mechanisms will be designed so that blasting the control panel on the outside seals the door and blasting the control panel on the inside opens the door, not vice versa."
The phrase fail-safe was impressed into the American public consciousness by SAC in the 1950s. It referred to the fail-safe (as opposed to fail-deadly) planning structure of the USAF bombers' role in the SIOP. On receipt of war warning, U.S. SAC bombers would sortie from their bases and fly north towards their targets in the Soviet Union. However, they would only progress as far as a map line in northern Canada (the Greenwood line?) at which point they would begin to orbit predefined positions until they received a coded radio signal indicating that they were to continue on their missions. They could also be recalled by signal from these positions.

It is not generally known (due to being classified) if the bombers had contingency orders that would allow them to continue on with their mission if they were unable to communicate with the NCA for longer than a certain period of time (indicating the destruction of the command structure in the CONUS). This would seem probable; however, elaborate systems of communications were set up to ensure that the NCA could communicate with the bombers under all sorts of lousy conditions including nuclear strikes. Note that the act of sending the bombers away from their bases to the hold positions was popularly known as flushing the bombers, presumably in reference to the act of flushing game birds from a field.

The USAF bomber Fail-Safe protocol inspired a worst-case scenario book (and movie) by the same name, and caused general discomfort in cold war Americans. The exact same scenario was also the subject of the brilliant Dr. Strangelove.

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