Criteria
Providing fault-tolerant design for every component is normally not an option. In such cases the following criteria may be used to determine which components should be fault-tolerant:
- How critical is the component? In a car, the radio is not critical, so this component has less need for fault-tolerance.
- How likely is the component to fail? Some components, like the drive shaft in a car, are not likely to fail, so no fault-tolerance is needed.
- How expensive is it to make the component fault-tolerant? Requiring a redundant car engine, for example, would likely be too expensive both economically and in terms of weight and space, to be considered.
An example of a component that passes all the tests is a car's occupant restraint system. While we do not normally think of the primary occupant restraint system, it is gravity. If the vehicle rolls over or undergoes severe g-forces, then this primary method of occupant restraint may fail. Restraining the occupants during such an accident is absolutely critical to safety, so we pass the first test. Accidents causing occupant ejection were quite common before seat belts, so we pass the second test. The cost of a redundant restraint method like seat belts is quite low, both economically and in terms or weight and space, so we pass the third test. Therefore, adding seat belts to all vehicles is an excellent idea. Other "supplemental restraint systems", such as airbags, are more expensive and so pass that test by a smaller margin.
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