Incorporating Common Cause Failures in Mission Critical Facilities Reliability Analysis

Common cause events are those specific groups of dependent events that might adversely affect the operation of a redundant system. Common cause failures (CCF) are considered a subset of dependent failures with a major difference that they cannot be explicitly modeled. A CCF is a single point of failure (SPOF) causing a unit and its "perceived" redundant unit to fail simultaneously. During the first few years of commercial nuclear power plants operations, many probability risk assessment (PRA) studies revealed that CCF have significantly contributed to core damages. Consequently, the U.S. Nuclear Regulation Commission (NRC) initiated various CCF analyses in 1980's. Since then, CCF analyses have been synonymous with nuclear facilities, and majority of CCF analysis efforts have rightfully been focused on the safety of nuclear plants. Given that many industrial and commercial mission critical facilities and high reliability organizations now require high degree of availability, therefore, CCF cannot be ignored. The objectives of this paper are to present: (a) the importance of considering CCF in reliability, availability and maintainability (RAM) analysis for industrial and commercial mission critical facilities and high reliability organizations, (b) simple quantitative analysis methods of CCF for industrial and commercial mission critical facilities, and (c) the need for including CCF as part of future failure data collection efforts.