Dead man's switch - AbsoluteAstronomy.com

A dead man's switch is a switch that is automatically operated in case the human

Human

Humans are the only living species in the Homo genus...

operator becomes incapacitated, such as through death

Death

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

or loss of consciousness.

The switch usually stops a machine, and is a form of fail-safe

Fail-safe

A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....

. They are commonly used in locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

s, aircraft refuelling, freight elevator

Elevator

An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...

s, lawn mower

Lawn mower

A lawn mower is a machine that uses a revolving blade or blades to cut a lawn at an even length.Lawn mowers employing a blade that rotates about a vertical axis are known as rotary mowers, while those employing a blade assembly that rotates about a horizontal axis are known as cylinder or reel...

s, tractor

Tractor

A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction...

s, personal watercraft, outboard motor

Outboard motor

An outboard motor is a propulsion system for boats, consisting of a self-contained unit that includes engine, gearbox and propeller or jet drive, designed to be affixed to the outside of the transom and are the most common motorized method of propelling small watercraft...

s, chainsaw

Chainsaw

A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...

s, snowblowers, tread machine

Treadmill

A treadmill is an exercise machine for running or walking while staying in one place. The word treadmill traditionally refers to a type of mill which was operated by a person or animal treading steps of a wheel to grind grain...

s, snowmobile

Snowmobile

A snowmobile, also known in some places as a snowmachine, or sled,is a land vehicle for winter travel on snow. Designed to be operated on snow and ice, they require no road or trail. Design variations enable some machines to operate in deep snow or forests; most are used on open terrain, including...

s, and many medical imaging

Medical imaging

Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

devices.

A dead man's switch may also be used to activate a harmful device, such as a bomb

Bomb

A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

or IED

Improvised explosive device

An improvised explosive device , also known as a roadside bomb, is a homemade bomb constructed and deployed in ways other than in conventional military action...

. The user holds down a switch of some sort in their hand which arms the device. When the switch is released, the device will activate, so that if the user is killed while holding the switch, the switch will be released and the bomb will detonate (ie Fail-deadly

Fail-deadly

Fail-deadly is a concept in nuclear military strategy which encourages deterrence by guaranteeing an immediate, automatic and overwhelming response to an attack. The term fail-deadly was coined as a contrast to fail-safe.-Military usage:...

). The Special Weapons Emergency Separation System

Special Weapons Emergency Separation System

Special Weapons Emergency Separation System , also known informally as the dead man's switch, was a nuclear bomb release system that Strategic Air Command built into bombers such as the B-52 Stratofortress...

is an application of this concept in the field of nuclear weapons.

Background

Interest in dead-man's controls increased with the introduction of electric streetcars

Tram

A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

and especially electrified rapid transit

Rapid transit

A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

trains, though dead-man equipment was quite rare on US streetcars until comparatively recently. In conventional steam railroad

Steam railroad

Steam railroad is a term used in the United States to distinguish conventional heavy railroads from street railways, interurban streetcar lines, and other light railways usually dedicated primarily to passenger transport....

trains, there was always a second person with the engineer, the fireman, who could almost always bring the train to a stop if necessary. For many decades this practice continued on electric and diesel

Diesel locomotive

A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...

locomotive

Locomotive

s, even though a single person could theoretically operate them.

With modern urban

Urban area

An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...

and suburban railway systems, the driver is typically alone in an enclosed cab. Automatic devices were already beginning to be deployed on newer installations of the New York City Subway

New York City Subway

The New York City Subway is a rapid transit system owned by the City of New York and leased to the New York City Transit Authority, a subsidiary agency of the Metropolitan Transportation Authority and also known as MTA New York City Transit...

system in the early 20th century. The Malbone Street Wreck

Malbone Street Wreck

The Malbone Street Wreck, also known as the Brighton Beach Line Accident of the Brooklyn Rapid Transit Company , was a rapid transit railroad accident that occurred November 1, 1918, beneath the intersection of Flatbush Avenue, Ocean Avenue, and Malbone Street, in the community of Flatbush, Brooklyn...

on the Brooklyn Rapid Transit

Brooklyn Rapid Transit

The Brooklyn Rapid Transit Company was a public transit holding company formed in 1896 to acquire and consolidate lines in Brooklyn and Queens, New York City, United States. It was a prominent corporation and industry leader using the single-letter symbol B on the New York Stock Exchange...

system in 1918, though not caused by driver incapacitation, did spur the need for universal deployment of such devices to halt trains in the event of the operator's disability. According to Manhattan borough historian Michael Miscione, there have been at least three instances where the dead-man's switch was used successfully - in 1927, 1940, and 2010.

Handle

Pneumatically or electrically linked dead-man's controls, still used today, involve relatively simple modifications of the controller handle, the device that regulates traction power. The main requirement is that the train's emergency brakes are applied if pressure is not maintained on the controller.

Typically, the controller handle is a horizontal bar, rotated to apply the required power for the train. Attached to the bottom of the handle is a rod which, when pushed down, contacts a solenoid

Solenoid

A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

or switch inside the control housing. The handle springs up if pressure is removed, releasing the rod's contact with the internal switch, instantly cutting power and applying the brakes.

Though there are ways that this type of dead-man's control could conceivably fail, they have proven highly reliable.

On some earlier equipment, pressure was not maintained on the entire controller, but on a large button protruding from the controller handle. This button also had to be pressed continuously, typically with the palm of the hand so that the button was flush with the top of the handle. Another method used, particularly with some lever-type controllers, which are pushed or pulled rather than rotated, requires that the handle on the lever be turned through 90 degrees and held in that position while the train is in operation.

Some dead-man's controls require the motorman to hold it in the mid-position rather than apply full pressure (see pilot valve

Pilot valve

A pilot valve is a small valve that controls a limited-flow control feed to a separate piloted valve. Typically, this valve controls a high pressure or high flow feed...

).

In modern New York City Subway

New York City Subway

trains, for example, the dead man's switch is incorporated into the train's speed control. On the R142A

R142A (New York City Subway car)

The R142A, along with the R142 is the newest generation of cars for the A Division of the New York City Subway.-Background:The R142A is built by Kawasaki and has many of the same features as the R142, built by Bombardier. The main order of 400 R142A cars are numbered 7211-7610; the 120 option cars...

car, the train operator must continually hold the lever in place.

Every lawn mower sold in the US since 1982 has an "operator-presence" device, which by law must stop the blades within 3 seconds after the user lets go of the controls.

Touch sensor

On the diesel-electric railway locomotives in Canada, and on Nottingham Express Transit

Nottingham Express Transit

Nottingham Express Transit is a light-rail tramway in the Nottingham area in England. The first line opened to the public on 9 March 2004, having cost £200 million to construct. The scheme took sixteen years from conception to implementation...

vehicles, the tram

Tram

's speed controller is fitted with a capacitive touch sensor to detect the driver’s hand. If the hand is removed for more than a short period of time, the track brake

Track brake

Track brakes are a form of brakes unique to railborne vehicles. The braking force derives from the friction resulting from the application of wood or metal braking shoes directly to the tracks...

s are activated. Gloves, if worn, have to be finger-less for the touch sensor to operate. A back up dead-man's switch button is provided on the side of the controller for use in the case of a failed touch sensor or if it is too cold to remove gloves.

Pedal

A pedal can be used instead of a handle. In the Waterfall train disaster

Waterfall train disaster

The Waterfall rail accident was a train accident that occurred on 31 January 2003 near Waterfall, New South Wales, Australia. The train derailed, killing seven people aboard, including the train driver.-Incident:...

, it appeared that the driver slumped on his seat, keeping the pedal depressed when he died suddenly of a heart attack. This also happened to a Canadian National Railway

Canadian National Railway

The Canadian National Railway Company is a Canadian Class I railway headquartered in Montreal, Quebec. CN's slogan is "North America's Railroad"....

Railliner

Budd Rail Diesel Car

The Budd Rail Diesel Car, RDC or Buddliner is a self-propelled diesel multiple unit railcar. In the period 1949–62, 398 RDCs were built by the Budd Company of Philadelphia, Pennsylvania, United States...

passenger train in the 1970s, but the problem was noticed by other crew members and the train safely halted. In the movie Silver Streak, a man hijacks the train and keeps it running by placing a heavy toolbox on the pedal.

The pedal can also have a vigilance function built in, where drivers must release and re-press the pedal in response to an audible signal. This prevents it from being defeated by the above circumstances, and is a standard feature on British DSD systems.

Some types of locomotive are fitted with a three-position pedal, which must normally be kept in the mid-position. This also eliminates the possibility of accidentally defeating it, although it may still be possible to deliberately do so. Adding a vigilance function to this type of pedal results in a very safe system. However, isolation devices are still provided in case of equipment failure, so a deliberate override is still possible. These isolation devices usually have tamper-evident seals fitted for that reason.

Seat switches

On modern tractors, the switch is beneath the seat, and will cut the engine if the operator gets off the tractor while the transmission is engaged or the power take-off

Power take-off

A power take-off or power takeoff is a splined driveshaft, usually on a tractor or truck, that can be used to provide power to an attachment or separate machine. It is designed to be easily connected and disconnected...

is spinning.

Key switches

On recreational vehicles, such as boats with outboard motor

Outboard motor

s, snowmobile

Snowmobile

s, jet ski

Jet ski

Jet Ski is the brand name of a personal watercraft manufactured by Kawasaki Heavy Industries. The name is sometimes mistakenly used by those unfamiliar with the personal watercraft industry to refer to any type of personal watercraft; however, the name is a valid trademark registered with the...

s and waverunner

WaveRunner

WaveRunner is a trademarked name and type of personal water craft produced by the Yamaha Motor Company. Unique to the WaveRunner among PWCs is the spout of water that shoots into the air from the rear of the vehicle....

s, the user has the ignition key attached to his wrist or waist by a leash. The key will be removed from the ignition switch if the rider falls off the vehicle, thus turning off the engine or setting the throttle position to "idle". Some luggage carts at airports and exercise treadmill

Treadmill

s have this feature. In the case of treadmills, they are designed to stop the treadmill if the user falls. The dead man switch on a treadmill usually consists of an external magnet controlling the circuitry that provides power to the treadmill belt.

Altimeter switches

Strategic Air Command

The Strategic Air Command was both a Major Command of the United States Air Force and a "specified command" of the United States Department of Defense. SAC was the operational establishment in charge of America's land-based strategic bomber aircraft and land-based intercontinental ballistic...

developed a dead man's switch for its nuclear bombers, known as Special Weapons Emergency Separation System

Special Weapons Emergency Separation System

(SWESS), that ensured the nuclear payload detonated in the event of the crew becoming incapacitated through enemy action. The purpose of this device, unlike other examples mentioned above, was to fail-deadly

Fail-deadly

rather than fail-safe. Once armed, the system would detonate the onboard nuclear weapons if the aircraft's altitude dropped below a predetermined level.

Vigilance control

The main safety failing with the basic dead man’s system is the possibility of the operating device being held permanently in position, either deliberately or accidentally. Vigilance control was developed to detect this condition by requiring that the dead man’s device be released momentarily and re-applied at timed intervals.

There has also been a proposal to introduce a similar system to automotive cruise control

Cruise control

Cruise control is a system that automatically controls the speed of a motor vehicle. The system takes over the throttle of the car to maintain a steady speed as set by the driver.-History:...

s.

A hybrid between a dead man's switch and a vigilance control device is a dead-man's vigilance device

Dead-man's vigilance device

A dead-man's vigilance device is a railroad safety device that operates in the case of incapacitation of the engineer. It is a hybrid between a dead-man's switch and a vigilance control....

Software uses

Software versions of dead man's switches are generally only used by people with technical expertise, and can serve several purposes; such as sending a notification to friends or deleting and encrypting

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

data. The "non-event" triggering these can be almost anything, such as failing to log in for 7 consecutive days, not responding to an automated e-mail

E-mail

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

, ping

Ping

Ping is a computer network administration utility used to test the reachability of a host on an Internet Protocol network and to measure the round-trip time for messages sent from the originating host to a destination computer...

, a GPS-enabled telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

not moving for a period of time, or merely failing to type a code within a few minutes of a computer's boot

Booting

In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...

. An example of a software based dead man's switch is Dead Man's Switch which starts when Windows boots up and can encrypt or delete user specified data if an unauthorised user should ever gain access to the protected computer.

Spacecraft uses

Many spacecraft use a form of dead man's switch to guard against command system failures. A timer is established that is normally reset by the receipt of any valid command (including one whose sole function is to reset the timer.) If the timer expires, the spacecraft enters a "command loss" algorithm that cycles through a predefined sequence of hardware and/or software modes (such as the selection of a backup command receiver) until a valid command is received. The spacecraft may also enter a safe mode

Safe mode (spacecraft)

Safe mode is an operating mode of a modern spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.-Triggering events:...

to protect itself while waiting for further commands.

While having some similarities to a dead man's switch, this type of device (a command loss timer) is not actually a dead man's switch, because it aims to recover from a hardware failure rather than the absence of human operators. It is generally called a watchdog timer

Watchdog timer

A watchdog timer is a computer hardware or software timer that triggers a system reset or other corrective action if the main program, due to some fault condition, such as a hang, neglects to regularly service the watchdog A watchdog timer (or computer operating properly (COP) timer) is a computer...

, and is also used extensively in nuclear power control systems. System components on a spacecraft that put it into a safe mode or cause it to execute default behaviors when no command is received within a predefined time window can be considered a dead man's switch, but hardware or software that attempts to receive a command from human operators through an alternate channel is an auto-recovering or adaptive communications system, not a dead man's switch.

Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

recovered from a command receiver failure with a command loss timer.

Alternative names

Alerter system
Driver's Safety Device (DSD) is the official UK railway term.
Dead man's button/trigger/pedal/handle/control/brake
Vigilance control
Live-man switch (commonly used in the robotics industry)
Operator Presence Control (OPC)

Event recording

The status and operation of both the vigilance and dead-man's may be recorded on the train's event recorder

Event recorder

A Train event recorder is similar to the flight data recorder found on aircraft. It records data about the operation of train controls and performance in response to those controls and other train control systems.Data storage is provided by magnetic tape, battery-backed RAM and, most recently,...

(commonly known as a black box).

External links

Deadmans on French trams and guided (trolley) buses (PDF)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.