Standards Development

Scope

This International Standard establishes requirements associated with the defence of Electrical power systems and I&C systems and their support systems against Common Cause Failure (CCF) in nuclear facilities.  It sets out requirements for:Overall I&C and electrical power systems analysis; Analysis and defence against CCF from hazards; Analysis and defence against CCF from fault propagation; Analysis and defence against CCF from systematic faults; and Documentation of the defences against CCF to allow their adequacy to be judged.The standard applies to CCF of individual systems that make use of redundant identical channel/division architecture, (IEC 61523 5.4.2.6 para 1) and also to multiple systems belonging to different lines of defence (IEC 61523 5.4.2.6 para 4 clause f).  This standard applies to all Systems Important to Safety and their necessary supporting systems, such as the electrical power system.This standard does not provide detailed technical design requirements for defence of systems against CCF. Specific design principles will be identified in other level 3 IEC standards where appropriate.This standard does not set requirements for determining the adequacy of the provisions to defend against CCF.  This is due to the differences in national approaches to CCF; some countries treat CCF as a Design Basis Accident requirement (DBA), others as a Design Extension Condition (DEC) and some as a deterministic requirement, and also because of the dependence on the type of facility and the plant specific details. Instead this document identifies the process and information necessary to support a determination of adequacy.  This standard follows the general principles given in IAEA safety requirement SSR-2/1 and IAEA safety guides SSG-34 and SSG-39 for I&C and Electrical power  systems.  It defines a structured method of applying the guidance contained in those codes and standards to the I&C electrical power systems that perform functions important to safety in Nuclear Facilities. This standard is to be read in association with these IAEA guides and IEC 63046 and 61513 in implementing the requirements of basic safety publication IEC 61508.Clause 5 of the standard addresses the overall approach to CCF and its analysis.Clause 6 of this standard addresses the approach to the facility and system analysis that are to be undertaken for provision of defence against CCF from Hazards.Clause 7 of this standard addresses the approach to the facility and system analysis that are to be undertaken for provision of defence against CCF from fault propagation.  This includes fault propagation between divisions of the same system and systems of different lines of defence.Clause 8 of this standard addresses the approach to the defence against CCF from systematic faults through avoidance of systematic faults and the use of diversity to enable the safety targets to be reached.