IEC 61226:2020 establishes, for nuclear power plants , a method of assignment of the functions specified for the plant into categories according to their importance to safety. Subsequent classification of the I&C and electrical power systems performing or supporting these functions, based on the assigned category, then determines relevant design criteria. The design criteria, when applied, ensure the achievement of each function in accordance to its importance to safety. In this document, the criteria are those of functionality, reliability, performance, environmental qualification (e.g. seismic) and quality assurance (QA). This edition includes the following significant technical changes with respect to the previous edition: - to align on IAEA requirements, recommendations and terminology, particularly to take into account the replacement of NS-R-1 by SSR 2/1 and publication of SSG 30; - to extend the scope to electrical power systems; - to move the detailed requirements applying to functions and I&C systems to a normative annex, which will be removed after updating IEC 61513.