Fail-safe factor

Although direct inquiry about suicide is the single most critical part of an evaluation, one should also ask about other risk factors associated with completed suicide. Although there is no fail-safe method for identifying all patients at serious risk, knowledge of these factors can help estimate its likelihood. 

Wheel-speed sensors do not monitor speed directly they sense the movement of the circumference of the tire. This is done with 48 or 32 pulses per turn, and some correction factor for tire sizes. This information comes from driven and non-driven wheels, and means four independent sources of information describing road surface, friction coefficient, cornering etc. In all cases the information is passed through the electronic control unit (ECU). ABS and all advanced systems are safety systems that do not allow the electrical signal to be split before it goes to the ECU. This is necessary to make a fail-safe system. 

The most important design factor about pressure-relieving devices is the underlying principle of intrinsic safety. They must fail safe or not at all. 

FS Factor of Safety or Fail Safe or Fail Steady... 

Safety integrity level 1 equates to a simple non-redundant single path designed to fail safe with a typical availability of 0.99. Level 2 involves a partially redundant logic structure, with redundant independent paths for elements with lower availability. Overall availability is in the range of 0.999. Level 3 is composed of a totally redundant logic structure. Redundant independent circuits are used for the total interlock system. Diversity is considered an important factor and is used where appropriate. Fault tolerance is enhanced since a single fault of an ESD system component is unlikely to result in a loss of process protection. 

Standard component failure rates are scaled by multiplicative factors to account for the environmental conditions and lead to a rate for faults which could affect a particular drive output on channel 1 of 0.4 faults per "year of continuous control system operation". Assuming the system is used to handle 50 fuel assemblies per reactor year, each complete cycle taking up to 18 hours, this fault rate is equivalent to 4 x 10 per reactor year. For "fail-safe" modes the equivalent rate is calculated to be 10 per reactor year. These rates may be expected to decrease as the period of use builds up and failed components are replaced. 

Design of critical systems should allow for in-service testing from the sensor to the final element and in-service maintenance/calibration. The frequency of testing is important. For example, the reliability of a simple level alarm increases by a factor of 15 with monthly testing as opposed to testing on a 3-year turnaround. Testing is even more critical on energize-to-trip systems where a component failure will not cause a shutdown or alarm, i.e., the system is not completely fail safe. 

The use of only one of these principles or techniques is seldom adequate. A combination of two or more is usually needed to provide a fail-safe design. Effective application of the fail-safe concept can result in the highest level of safety. However, its effectiveness is determined by the quality of the architectural design, the limits imposed by external influences and crew fail-safety. These three factors are now discussed. 

Human error remains a causal factor in the majority of serious aircraft accidents. Human error causes accidents of fail-safe, fully functional designs. 

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