Safety-critical system
A safety-critical system or life-critical system is a system whose failure or malfunction may result in one (or more) of the following outcomes: death or serious injury to people loss or severe damage to equipment/property environmental harm A safety-related system (or sometimes safety-involved system) comprises everything (hardware, software, and human aspects) needed to perform one or more safety functions, in which failure would cause a significant increase in the safety risk for the people or environment involved. Safety-related systems are those that do not have full responsibility for controlling hazards such as loss of life, severe injury or severe environmental damage. The malfunction of a safety-involved system would only be that hazardous in conjunction with the failure of other systems or human error. Some safety organizations provide guidance on safety-related systems, for example the Health and Safety Executive in the United Kingdom. Risks of this sort are usually managed with the methods and tools of safety engineering. A safety-critical system is designed to lose less than one life per billion (109) hours of operation. Typical design methods include probabilistic risk assessment, a method that combines failure mode and effects analysis (FMEA) with fault tree analysis. Safety-critical systems are increasingly computer-based. Safety-critical systems are a concept often used together with the Swiss cheese model to represent (usually in a bow-tie diagram) how a threat can escalate to a major accident through the failure of multiple critical barriers. This use has become common especially in the domain of process safety, in particular when applied to oil and gas drilling and production both for illustrative purposes and to support other processes, such as asset integrity management and incident investigation. Reliability regimes Several reliability regimes for safety-critical systems exist: Fail-operational systems continue to operate when their control systems fail. Examples of these include elevators, the gas thermostats in most home furnaces, and passively safe nuclear reactors. Fail-operational mode is sometimes unsafe. Nuclear weapons launch-on-loss-of-communications was rejected as a control system for the U.S. nuclear forces because it is fail-operational: a loss of communications would cause launch, so this mode of operation was considered too risky. This is contrasted with the fail-deadly behavior of the Perimeter system built during the Soviet era. Fail-soft systems are able to continue operating on an interim basis with reduced efficiency in case of failure. Most spare tires are an example of this: They usually come with certain restrictions (e.g. a speed restriction) and lead to lower fuel economy. Another example is the "Safe Mode" found in most Windows operating systems. Fail-safe systems become safe when they cannot operate. Many medical systems fall into this category. For example, an infusion pump can fail, and as long as it alerts the nurse and ceases pumping, it will not threaten the loss of life because its safety interval is long enough to permit a human response. In a similar vein, an industrial or domestic burner controller can fail, but must fail in a safe mode (i.e. turn combustion off when they detect