Failure/fault

Thus a failure occurs when an error passes through the observation interface and affects the service delivered by the system a system being composed of components that are themselves systems. The manifestation of failures, faults and errors follows a fundamental chain ... 

A description of a system s structure identifies its component systems, their observable boundaries and functions, and their means of interaction. Careful identification of system boundaries is fundamental to understanding and distinguishing between failures, faults and errors, so as to analyze possible or actual failure situations and find means of reducing their likelihood, especially in complex systems. System structuring thus plays a central role in dependability. 

Given that an infrastructure is just a special kind of system, or rather a system viewed from a particular vantage point, the diagram can be used in connection with certain types of failure — fault —> error chain between an infrastructure and the systems the infrastructure is supporting, namely chains related to failures in the systems being supported, or by infrastructure service failures. 

Devise adequate check-out procedures to cater for maintenance errors which could result in a failure/fault/hazard. 

Note that there is some variation in assumed definitions of these terms (failure, fault and error) between different standards and guidance. 

Fault Tree Analysis was one of the earliest systems safety techniques developed for examining equipment failures. Fault Tree Analysis is a top down procedure that identifies undesirable events and their contributing factors. Once a tree has been developed, probabilities of failures can be determined for individual components in the tree. With the individual probabilities, overall probabilities of failures and can be calculated for event paths using Boolean algebra. 

Hardware failure and software failure are two kinds of failures encountered in programmable systems, as already discussed. In cases of hardware failure, fault tolerant designs such as redundancy could be applied. Software failure, as discussed, has to overcome certain procedures, but certain failures (design failure) could include behaviors that can be unsafe. A new technique known as system theoretic process analysis is applied in nuclear installations. This is required to identify the control requirements and then check conditions caused hy inadequate control actions such as ... 

Also, a subsystem hazard analysis (SSHA) examines each major subsystem (such as shown on the functional organizational tree in Figure 5.3) and identifies specific hazards and safety concerns including failures, faults, processes, or procedures and human errors. An SSHA also should address hazard controls and how those controls are verified. 

Keywords Model checking, nuclear power plants, architecture, hardware failure, fault-tolerance. 

Dependencies of fault, error, failure FAULT -K (Security) ERROR (Security/ Safety) l hv FAILURE (Safety)... 

Control systems are safe and chosen taking account of expected failures, faults and constraints in the planned use of the work equipment Control systems do not create increased risk to health or safety and that any fault in or damage to any part of the control system or loss of supply of energy used by the work equipment cannot result in additional or increased risk to health or safety or impede the operation of stop and emergency stop controls... 

Usually in the theory of reliability and safety of a lot of attention is dedicated to failures, especially to dangerous ones. However, apart from them, there are other events that seem to be much less important at first sight. These are transient failures (faults), see Shubinski (2012), Qi et al. (2008) and the references cited in Shubinski (2012). Transient failures are defined as follows. A functional transient failure is an event consisting of a unique distortion of information that is either processed or stored in an information processing and transmitting system under the influence of interior or exterior destabilizing influences (e.g. noise) Shubinski (2012). 

As indicated in [LAP 92], the functional safety of a complex system can be compromised by three types of incidents failures, faults, and errors. The system elements are subjected to failures, which can potentially result in accidents. 

As noted in Section 2.2.3, most core damage accidents involve multiple failures. Fault trees provide a systematic approach for identifying many of these failures. Most multiple independent failures and explicitly dependent failures, such as support system dependencies and shared equipment dependencies (see Section 2.2.3.2), are readily identified. However, some... 

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