Common cause analysis

Burdick, G. R., and N. H. Wilson, and J. R. Wilson, COMCAN - A Computer Program for Common Cause Analysis, INEL ANCR-1314, May. 

Common cause analysis can also be very useful in incident analyses. If it is found that one factor keeps recurring, then a common cause has been identified. Its elimination is probably likely to have a powerful effect on reducing risk. 

The advisory material to CS25.1309, FAR25.1309 and SAE ARP4761 group these specific studies under the term common cause analysis, which is further subdivided into three approaches ... 

Common Cause Analysis Generic term encompassing zonal safety analysis, particular risk analysis and common mode analysis [RTCA/1X)-178B/ED-12B]. 

Laundry lists of analyses frequently mix types of analyses (preliminary hazard analysis, system hazard analysis, and operating hazard analysis) with the methods or techniques for performing analyses (fault tree analysis, energy trace and barrier analysis, failure modes and effects analysis, common cause analysis, change analysis, and so on). Whether fault hazard analysis is a type or a method depends upon the reference in use. For all practical purposes, fault hazard analysis and system (or subsystem) hazard analysis seem to be the same thing, which is apparently called gross hazard analysis occasionally. 

The recommended techniques for preliminary hazard analysis are energy trace and barrier analysis (ETBA) and failure modes and effects analysis (FMEA). Recommended techniques for system and subsystem hazard analyses are FMEA, fault tree analysis (FTA), common cause analysis, sneak circuit analysis (for electrical, electronic, and some hydraulic or pneumatic circuits) and, of course, software hazard analysis for software. 

Based on the results of the PHA, recommendations made by 30% review boards, and guidance provided in the system safety program plan, detailed hazard analyses are made of specified (critical) subsystems. The techniques for these SSHAs are as outlined in the system safety program plan or as selected by the SSWG. Failure modes and effects analysis (FMEA) and/or fault tree analysis (FTA) are generally the techniques of choice. Software hazard analysis, common cause analysis, and/or sneak circuit analysis may also be appropriate. 

Common cause failure analysis (CCFA) or common cause analysis is used primarily to evaluate multiple failures that may be caused by a single event or causal factor common to or shared by multiple components. It is especially important in evaluating the true reliability pr uced by redundant systems or components. 

Common Cause Failure Analysis A system safety analytical technique (also known as common cause analysis) used primarily in the evaluation of multiple failures that have the occurrence of a single event as a common causal factor. 

ZSA is part of a common cause analysis to discover where design independence is required but violated it is primarily a qualitative analysis. ZSA should be carried out during the development process of a new aircraft or of any major modification to an existing aircraft. The objective of the analysis is to ensure that the equipment installation meets the applicable safety requirements, particularly those relating to CCF. Zonal safety concerns typically arise from design errors, installation errors, or failures that cross physical system zones. 

The common cause analysis for system failures is performed in a generally systematic manner. To quantify the effects of common cause failures, a beta factor approach was used. Conservative screening values of 0.1 for two failures (0.05 for three or more failures) were used for the beta factors. More-realistic estimates for the beta factors will be developed for long-term risk management applications to ensure acceptable value-impact analyses. Despite the coarseness of the present estimates, it is not believed that more accurate values would result in significant new engineering insights. Therefore, this issue does not require resolution prior to restart. 

---