This document establishes, for nuclear power plants2, 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 document is applicable to:
- the functions important to safety that are performed by I&C systems and supported by
electrical power systems (categorization of I&C functions),
- the I&C systems that enable those functions to be implemented (classification of I&C
- the electrical power systems that support those functions (classification of electrical power
The systems under consideration provide automated protection, closed or open loop control,
information to the operating staff, and electrical power supply to systems. These systems
keep the NPP conditions inside the safe operating envelope and provide automatic actions, or
enable manual actions, that prevent or mitigate accidents, or that prevent or minimize
radioactive releases to the site or wider environment. The I&C and electrical power systems
that fulfil these roles safeguard the health and safety of the NPP operators and the public.
This document follows the general principles given in IAEA Safety Requirement SSR-2/1 and
Safety Guides SSG-30, SSG-34 and SSG-39, and it defines a structured method of applying
the guidance contained in those codes and standards to the I&C and electrical power systems
that perform functions important to safety in a NPP. This document is read in association with
the IAEA guides together with IEC 61513 and IEC 63046 in implementing the requirements of
the IEC 61508 series. The overall classification scheme of structures, systems and
components for NPPs can be summarized as follows by Figure 1.