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Difino
| • | Safety engineering is an applied science strongly related to systems engineering. Safety engineering assure that a life-critical system behaves as needed even when pieces fail.
Safety engineers distinguish different extents of defective operation: A "fault" is said to occur when some piece of equipment does not operate as designed. A "failure" only occurs if a human being (other than a repair person) has to cope with the situation. A "critical" failure endangers one or a few people. A "catastrophic" failure endangers, harms or kills a significant number of people.
Safety engineers also identify different modes of safe operation: A "probabilistically safe" system has no single point of failure, and enough redundant sensors, computers and effectors so that it is very unlikely to cause harm (usually "very unlikely" means, on average, less than one human life lost in a billion hours of operation). An "inherently safe" system is a clever mechanical arrangement that cannot be made to cause harm – obviously the best arrangement, but this is not always possible. For example, "inherently safe" airplanes are not possible. A "fail-safe" system is one that cannot cause harm when it fails. A "fault-tolerant" system can continue to operate with faults, though its operation may be degraded in some fashion.
These terms combine to describe the safety needed by systems: For example, most biomedical equipment is only "critical," and often another identical piece of equipment is nearby, so it can be merely "probabilistically fail-safe". Train signals can cause "catastrophic" accidents (imagine chemical releases from tank-cars) and are usually "inherently safe". Aircraft "failures" are "catastrophic" (at least for their passengers and crew,) so aircraft are usually "probabilistically fault-tolerant". Without any safety features, nuclear reactors might have "catastrophic failures", so real nuclear reactors are required to be at least "probabilistically fail-safe", and some such as pebble bed reactors are "inherently fault-tolerant".
Source: [wikipedia: safety engineering]
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