IEC 63435:2025 1.1 Object of this document
This document specifies the characteristics of operator support systems (OSS) used by the
control room staff, maintenance engineers and emergency response staff, establishes general
principles for OSS life cycle and requirements for OSS design following the human factors
engineering (HFE) programme. This document also gives the human factors guidelines and the
verification and validation (VV) requirements for OSS design.
This document is applicable to new nuclear facilities whose conceptual design is initiated after
the publication of this document but it can also be used for designing OSS in existing nuclear
facilities.
1.2 Context leading to development of OSS
Enhancing safety, optimizing operator workload and increasing nuclear facility availability have
always been greatly valued aims which, during nuclear facility operation, rely largely on the
operating staff and on OSS.
In addition, the use of computer technology to provide operator support functions and a
database of operation and maintenance for the operating staff and for teams and individuals
outside of a nuclear facility, on-line and/or off-line, is increasing and becoming current practice.
This can be done as diagnosis and guide formats both in normal operation and abnormal
conditions. When properly implemented and kept up to date, such OSS can provide enhanced
support for greater safety and effectiveness of operation and maintenance of nuclear facilities.
1.3 Use of this document with related standards
This document is intended to deal with aspects that are specific to OSS, as supplementary to
the operator support function specified in IEC 60964. For functional design criteria of safety
parameter display functions, see also IEC 60960; and for requirements of computer-based
procedures (CBP), see also IEC 62646.
In order to design OSS efficiently and properly, some important considerations are addressed
in the following related standards:
a) Control rooms design
– IEC 60964 provides requirements and recommendations for the design of control rooms.
b) Human factors engineering (HFE)
– IEC 63351 provides management of HFE programme, task analysis (TA) and human
machine interface (HMI) design guidance.
c) Functional analysis and assignment (FAA)
– IEC 61839 gives rules for developing criteria for the functional assignment to either
operators or systems.
d) Human factors design guidelines
– IEC 61772 provides guidance on physical implementation of visual display units (VDUs),
display formats, and implementation into the main control room (MCR);
– ISO 11064-1, ISO 11064-4 and ISO 11064-5 provide guidance on human-centered
design activities throughout the life cycle of a computer-based interactive system.
e) Verification and validation (VV)
– IEC 61771 provides guidance on VV of the design of control rooms.

This document assumes simultaneous considerations of the requirements for:
– cybersecurity, which is in compliance with IEC 62645,
– categorization and classification, which are in compliance with IEC 61226,
– safety life cycle aspects, which are in compliance with IEC 61513, IEC 60880, IEC 62138
and IEC 60987 depending on the safety classification of OSS.

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