Common-mode failure concerns

A common cause (or a common mode) failure concerns the possibility that system failure involving multiple item failure may occur due to a common cause, i.e., the loss (during some critical period) of multiple or redundant paths/components/parts/ functions due to an underlying common mechanisms/faults/phenomenon. A common mode failure is a failure which has the potential to fail more than one function and to possibly cause an initiating event, or other event(s), simultaneously. 

Although physical separation can reduce the effects of common-mode failures, the owner/operator should also evaluate the likelihood that these common-mode failures occur at the same time. Only simultaneous common-mode failures are of concern. Typically, electronic component stress results in failures at different times, often separated in time by months or years. In these situations, the advantages of physical separation may be outweighed by the economic advantages of installing the redundant hardware in the same physical location or cabinet. 

Two major classes of system failure can occur when component dependence occurs common cause failures and common mode failures. A common cause failure (CCF) is the failure of two or more components in a system due to the same event but where the failures are not consequences of each other. For example, if two components in a system that are both more likely to fail under humid conditions fail on a very humid day, this is a CCF. CCFs are especially likely in systems that use similar components redundantly to increase reliability because such components are affected similarly by environmental conditions. The second type of system failure that occurs due to component dependence is the common mode failure (CMF). A CMF is the failure of two or more components in a system where the failure of one component causes the other components to fail. This form of system failure is of special concern in Perrow s work (1984). Perrow argues that when complex, unanticipated interactions among components occur, redundant safety features can actually reduce the reliability of a system. 

In addition, there remains a non-disprovable concern that there may be a weakness to common-mode failure (CMF) if both sets of logic solvers in two nominally diverse systems are software-based. This concern is related to the complexity of software systems, and the associated difficulties of verification and validation (V V). In some undefined way, similarities in the software code design and production processes may yield the possibihty of CMF - even if different software languages and operating systems are employed in the two systems. Because of this concern, it has become common in some countries to specify that the diverse protection system should be hard-wired. 

Hence both the SIL and Beta factor methods have underlying philosophical concerns. The SIL approach uses unveriflable judgments about the relationship between system reliability and quality control techniques and measures, whereas the Beta factor approach draws unveriflable correlations between single-channel failure rates and common-mode failure rates. 

When Common Cause Failure is not a concern due to the use of diversity, the software failure modes can be considered at processing unit level only. 

The techniques for quantifying the predicted frequency of failures are just the same as those previously applied to plant availability, where the cost of equipment failure was the prime concern. The tendency in the last few years has been towards a more rigorous application of these techniques (together with third-party verification) in the field of hazard assessment. They include Fault Tree Analysis, Failure Mode Effect Analysis, Common Cause Failure Assessment, and so on. These will be explained in Chapters 5 and 6. 

Fire safety covers those safety aspects involved with fire, fire prevention, fire protection, and fire suppression. This safety aspect crosses many different boundaries and interfaces of a system. For example, a particular system may have common fire safety concerns during facility storage, transportation, handling, operation, maintenance, and disposal. Fire is a potential common-cause failure mode that can cause the simultaneous failure of redundant equipment. 

These cyclic mechanical stresses lead to fatigue or ageing of the concerned materials. Crack formation or wear-out is a typical failure mode that can be observed. These failure modes are common if mechanical components are considered. This does not surprise as the principle failure mechanism is purely on the material mechanical side. 

In this paper simulations of the crack tip region under mode II loading are presented. Of central interest are common features with previously obtained results under mode I loading [5] concerning plastic deformation and the ductile failure of Co-ligaments as well as differences in the deformation of the crack tip region. 

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