Editing Fail-safe (section)

==Examples==

===Mechanical or physical===
Examples include:
*[[File:Pl control valve.jpg|thumb|upright|Globe control valve with pneumatic diaphragm actuator. Such a valve can be designed to fail to safety using spring pressure if the actuating air is lost.]]Safety valves – Various devices that operate with [[fluid]]s use [[fuse (hydraulic)|fuses]] or [[safety valve]]s as fail-safe mechanisms.
*Roller-shutter fire doors that are activated by building alarm systems or local smoke detectors must close automatically when signaled regardless of power. In case of power outage the coiling fire door does not need to close, but must be capable of automatic closing when given a signal from the building alarm systems or smoke detectors. A temperature-sensitive [[fusible link]] may be employed to hold the fire doors open against gravity or a closing spring. In case of fire, the link melts and releases the doors, and they close.
*Some airport [[baggage]] [[cart]]s require that the person hold down a given cart's handbrake switch at all times; if the handbrake switch is released, the brake will activate, and assuming that all other portions of the braking system are working properly, the cart will stop. The handbrake-holding requirement thus both operates according to the principles of "fail-safety" and contributes to (but does not necessarily ensure) the fail-security of the system. This is an example of a ''[[dead man's switch]]''.
*[[Lawnmower]]s and [[snow blower]]s have a hand-closed lever that must be held down at all times. If it is released, it stops the blade's or rotor's rotation. This also functions as a ''dead man's switch''.
*[[Air brake (rail)|Air brakes]] on railway [[train]]s and [[air brake (road vehicle)|air brakes]] on [[truck]]s. 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 uncoupled, the air pressure will be lost and the brakes applied, by springs in the case of trucks, or by a local air reservoir in trains. It is impossible to drive a truck with a serious leak in the air brake system. (Trucks may also employ [[Wig wag (truck braking systems)|wig wags]] to indicate low air pressure.)
*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 or under lock and key. When such a gate provides vehicle access to homes, a fail-safe design is used, where the door opens to allow fire department access.
*{{anchor|semaphore}}[[File:130330 Thomas ELC and Wansbeck Railtour Northumberlandia 030.jpg|thumb|upright|Railway semaphore signals. "Stop" or "caution" is a horizontal arm, "Clear to Proceed" is 45 degrees upwards, so failure of the actuating cable releases the signal arm to safety under gravity.]] A [[railway semaphore signal]] is specially designed so that, should the cable controlling the signal break, the arm returns to the "danger" position, preventing any trains passing the inoperative signal.
*[[Isolation valve]]s, and control valves, that are used for example in systems containing hazardous substances, can be designed to close upon loss of power, for example by spring force. This is known as fail-closed upon loss of power.
*An [[elevator]] has brakes that are held off brake pads by the tension of the elevator cable. If the cable breaks, tension is lost and the brakes latch on the rails in the shaft, so that the elevator cabin does not fall.
* Vehicle air conditioning – Defrost controls require vacuum for diverter damper operation for all functions except defrost. If vacuum fails, defrost is still available.

===Electrical or electronic===
Examples include:
*Many devices are protected from [[short circuit]] by [[fuse (electrical)|fuses]], [[circuit breaker]]s, or [[current limiting]] circuits. The electrical interruption under overload conditions will prevent damage or destruction of wiring or circuit devices due to overheating.
*[[Avionics]]<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=127}}</ref> using [[redundancy (engineering)|redundant systems]] to perform the [[triple modular redundancy|same computation using three different systems]]. Different results indicate a fault in the system.<ref>{{cite book
 |last         = Bornschlegl
 |first        = Susanne
 |title        = Ready for SIL 4: Modular Computers for Safety-Critical Mobile Applications
 |publisher    = MEN Mikro Elektronik
 |year         = 2012
 |url          = https://www.menmicro.com/downloads/search/dl/sk/%22White%20Paper%3A%20Ready%20for%20SIL4%3A%20Modular%20Computers%20for%20Safety-Critical%20Mobile%20Applications%22/dx/1/
 |format       = pdf
 |access-date  = 2015-09-21
 |archive-date = 2019-06-09
 |archive-url  = https://web.archive.org/web/20190609205912/https://www.menmicro.com/downloads/search/dl/sk/%22White%20Paper%3A%20Ready%20for%20SIL4%3A%20Modular%20Computers%20for%20Safety-Critical%20Mobile%20Applications%22/dx/1/
 |url-status   = dead
}}</ref>
*[[Drive-by-wire]] and [[fly-by-wire]] controls such as an Accelerator Position Sensor typically have two potentiometers which read in opposite directions, such that moving the control will result in one reading becoming higher, and the other generally equally lower.  Mismatches between the two readings indicates a fault in the system, and the [[engine control unit|ECU]] can often deduce which of the two readings is faulty.<ref>{{cite web|url=http://www.obd-codes.com/p2138|title=P2138 DTC Throttle/Pedal Pos Sensor/Switch D / E Voltage Correlation|website=www.obd-codes.com}}</ref>
*[[Traffic light]] controllers use a ''Conflict Monitor Unit'' to detect faults or conflicting signals and switch an intersection to an all flashing error signal, rather than displaying potentially dangerous conflicting signals, e.g. showing [[green]] in all directions.<ref>Manual on Uniform Traffic Control Devices, Federal Highway Administration, 2003</ref>
*The automatic protection of programs and/or processing systems when a [[computer hardware]] or [[software]] failure is detected in a [[computer system]]. A classic example is a [[watchdog timer]]. See [[Fail-safe (computer)]].
*A [[control operation]] or function that prevents improper system functioning or [[catastrophic failure|catastrophic]] degradation in the event of [[electronic circuit|circuit]] malfunction or operator error; for example, the failsafe [[track circuit]] used to control [[railway signaling|railway block signal]]s. The fact that a flashing amber is more permissive than a solid amber on many railway lines is a sign of a failsafe, as the relay, if not working, will revert to a more restrictive setting.
*The iron pellet ballast on the [[Bathyscaphe]] is dropped to allow the submarine to ascend. The ballast is held in place by [[electromagnet]]s. If electrical power fails, the ballast is released, and the submarine then ascends to safety.
*Many [[nuclear reactor]] designs have neutron-absorbing control rods suspended by electromagnets. If the power fails, they drop under gravity into the core and shut down the chain reaction in seconds by absorbing the neutrons needed for fission to continue.
*In [[industrial automation]], alarm circuits are usually "[[normally closed]]". This ensures that in case of a wire break the alarm will be triggered. If the circuit were normally open, a wire failure would go undetected, while blocking actual alarm signals.
*Analog sensors and modulating actuators can usually be installed and wired such that the circuit failure results in an out-of-bound reading – see [[current loop]].  For example, a potentiometer indicating pedal position might only travel from 20% to 80% of its full range, such that a cable break or short results in a 0% or 100% reading.
*In control systems, critically important signals can be carried by a complementary pair of wires (<signal> and <not_signal>). Only states where the two signals are opposite (one is high, the other low) are valid. If both are high or both are low the control system knows that something is wrong with the sensor or connecting wiring. Simple failure modes (dead sensor, cut or unplugged wires) are thereby detected. An example would be a control system reading both the [[normally open]] (NO) and [[normally closed]] (NC) poles of a [[switch#Contact terminology|SPDT]] selector switch against common, and checking them for coherency before reacting to the input.
*In [[HVAC control system]]s, [[actuators]] that control dampers and valves may be fail-safe, for example, to prevent coils from freezing or rooms from overheating. Older [[pneumatic actuators]] were inherently fail-safe because if the air pressure against the internal diaphragm failed, the built-in spring would push the actuator to its home position – of course the home position needed to be the "safe" position. Newer electrical and electronic actuators need additional components (springs or capacitors) to automatically drive the actuator to home position upon loss of electrical power.<ref>{{cite web|title=When Failure Is Not an Option: The Evolution of Fail-Safe Actuators |date=29 October 2015 |url=https://www.kmccontrols.com/blog/when-failure-is-not-an-option-the-evolution-of-fail-safe-actuators/ |publisher=KMC Controls |access-date=12 April 2021 }}</ref>
*[[Programmable logic controller]]s (PLCs). To make a PLC fail-safe the system does not require energization to stop the drives associated. For example, usually, an emergency stop is a normally closed contact. In the event of a power failure this would remove the power directly from the coil and also the PLC input. Hence, a fail-safe system.
*If a [[voltage regulator]] fails, it can destroy connected equipment. A [[crowbar (circuit)]] prevents damage by short-circuiting the power supply as soon as it detects overvoltage.

===Procedural safety===
As well as physical devices and systems fail-safe procedures can be created so that if a procedure is not carried out or carried out incorrectly no dangerous action results. 
For example:
*Spacecraft trajectory - During early [[Apollo program]] missions to the Moon, the spacecraft was put on a [[free return trajectory]]&nbsp;— if the engines had failed at [[lunar orbit]] insertion, the craft would have safely coasted back to Earth.
*[[File:FA-18-Afterburners.jpg|thumb|right|An aircraft lights its [[afterburner]]s to maintain full power during an [[arrested landing]] aboard an [[aircraft carrier]]. If the arrested landing fails, the aircraft can safely take off again.]]The pilot of an aircraft landing on an [[aircraft carrier]] increases the throttle to full power at touchdown. If the [[arresting wire]]s fail to capture the aircraft, it is able to take off again; this is an example of ''fail-safe practice''.<ref>{{cite web|first=Tom|last=Harris|title=How Aircraft Carriers Work|url=http://science.howstuffworks.com/aircraft-carrier4.htm|work=HowStuffWorks, Inc|date=29 August 2002 |access-date=2007-10-20}}</ref>
*In [[railway signalling]] signals which are not in active use for a train are required to be kept in the 'danger' position. The default position of every controlled absolute signal is therefore "danger", and therefore a positive action&nbsp;— setting signals to "clear"&nbsp;— is required before a train may pass. This practice also ensures that, in case of a fault in the signalling system, an incapacitated signalman, or the unexpected entry of a train, that a train will never be shown an erroneous "clear" signal.
*Railroad engineers are instructed that a railway signal showing a confusing, contradictory or unfamiliar aspect (for example a [[railway signalling#Colour light signals|colour light signal]] that has suffered an electrical failure and is showing no light at all) must be treated as showing "danger". In this way, the driver contributes to the fail-safety of the system.