Fail-safe

A fail-safe or fail-secure describes a device which, if (or when) it fails, fails in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel.

Definitions

“A device that will provide its intended function upon loss of power.” (2007 NFPA-80)
An operation which ensures that a failure of equipment, process, or system does not propagate beyond the immediate environs of the failing entity.

Examples

Mechanical or physical

Luggage carts in airports in which the hand-brake must be held down at all times. If it is released, the cart will stop. See dead man's switch.
Air brakes on railway trains and trucks. The brakes are held in the 'off' position by air pressure created in the brake system. Should a brake line split, or a carriage become de-coupled, the air pressure will be lost and the brakes applied. It is impossible to drive a train or truck with a serious leak in the air brake system.
Motorized gates - In case of power outage the gate can be pushed open by hand with no crank or key required. However, as this would allow virtually anyone to go through the gate, a fail-secure design is used: In a power outage, the gate can only be opened by a hand crank that is usually kept in a safe area.

An F/A-18A Hornet lights its afterburners to maintain full power following a night arrested landing aboard the aircraft carrier USS Harry S. Truman

Aircraft landing on an aircraft carrier increase the throttle to full power at touchdown. If the arresting wires fail to capture the plane, it safely takes off again.^[1]
During the Apollo program of Moon landings, the spacecraft was put on a free return trajectory – if the engines failed at lunar orbit insertion, the craft would safely coast back to Earth.

Electrical or electronic

Avionics using redundant systems to perform the same computation with voting logic to determine the "safe" result.
Traffic light controllers use a Conflict Monitor Unit to detect faults or conflicting signals and switch an intersection to all flashing red, rather than displaying potentially dangerous signals.^[2]
The automatic protection of programs and/or processing systems when a hardware or software failure is detected in a computer system. See fail-safe (computer).
A control operation or function that prevents improper system functioning or catastrophic degradation in the event of circuit malfunction or operator error; for example, the failsafe track circuit used to control railway block signals.
A precautionary secondary mechanism that achieves the same task as the primary mechanism; for example, the activation of grenades when the primary detonator is destroyed, or the release of lethal gas when a device that activates explosives is destroyed.^{[citation needed]}
The iron pallet ballast on the Bathyscaphe is dropped to allow the submarine to ascend. The ballast is held in place by electromagnets. If electrical power fails the ballast is released, and the submarine then ascends to safety.

Strategic

A system that has been structured such that it cannot fail (or that the probability of such failure is extremely low) to accomplish its assigned mission, regardless of environmental factors; for example, the hardening of a nuclear missile bunker, or the dispersion of nuclear bombers to multiple secret locations.

Other Terminology

Fail-safe (foolproof) devices are also known as Poka-Yoke devices. Poka-yoke, a Japanese term, was coined by Shigeo Shingo, a quality guru.

References

^ Harris, Tom. "How Aircraft Carriers Work". HowStuffWorks, Inc. Retrieved 2007-10-20.
^ Manual on Uniform Traffic Control Devices, Federal Highway Administration, 2003

[1] Harris, Tom. "How Aircraft Carriers Work". HowStuffWorks, Inc. Retrieved 2007-10-20.

[2] Manual on Uniform Traffic Control Devices, Federal Highway Administration, 2003

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