Common-mode failure avoidance

Common-mode failure Avoid common-mode failure possibilities with services, control systems, safety systems etc. 

Avoid common-mode failures between the BPCS and the SIS. Do not share components within a BPCS and SIS. 

A control system designed to automatically change the operating conditions such that the COP is no longer outside the Never-Exceed-Limit (N-E-L). The system may shutdown the operation or it may change conditions such that operation continues in a different mode. The Safety Interlock System must be independent from the normal control system to avoid "common mode" failure where one failure defeats both the normal control system and the Safety Interlock System. 

Bothhydrauhc subsystems are made different (sensors, pressures to be applied for control etc.) to avoid common mode failures, thus applying the principle of diversity. 

To avoid the so-called common mode failures it is better that the subsystems in system combinations are not identical. 

Mounting consideration This is very important to avoid common-mode failure because of environmental conditions, attaining true parameters, etc. 

For each risk assessment/SIL determination study, dutyholders should be able to justify each claim, and data used in the risk assessment, and ensure that appropriate management systems and procedures are implemented to support those claims. For COMAEI top-tier sites this will form part of the demonstration required within the safety report. Of particular importance is the reliability and diversity of the independent layers of protection. To avoid common mode failures extreme care should be taken when claiming high reliability and diversify, particularly for multiple human interventions. 

Two types of aircraft are taken in account Lear jet 23 (twin-jet of 5.7 tons) and Cessna 210 (single-engined aircraft of 1500 kg). Generally NPP sites are outside vicinity of airports where fall frequency is higher, furthermore civil or military reglementation forbid flights at a low altitude. Solutions consist in reinforcement of civil structures and separation of safety trains to avoid common mode failures. 

All areas of the cooling water system where a specific form of damage is likely to be found are described. The corrosion or failure causes and mechanisms are also described. Especially important factors influencing the corrosion process are listed. Detailed descriptions of each failure mode are given, along with many common, and some not-so-common, case histories. Descriptions of closely related and similarly appearing damage mechanisms allow discrimination between failure modes and avoidance of common mistakes and misconceptions. 

It is predictive and target setting (i.e. it determines the system s safety objectives without any architectural limitations). It should be used to identify design precautions necessary to ensure independence, to determine the required software level and to avoid common mode and cascade failures (see Chapters 6-8). 

The appearance of crazes in a material is generally a precursor to brittle failure. While such a mode is normally to be avoided, the presence of crazes can provide some beneficial effects. Because the crazes contain highly drawn fibrils of material, considerable plastic deformation and hence energy goes in to their formation and this can be a major source of fracture toughness. Indeed, the deliberate inclusion of small, typically rubber, particles into inherently brittle polymers is commonly undertaken to produce tough materials (112-115), because the presence of the particles dramatically increases the craze density. 

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