IEC 61226:2009

Nuclear power plants - Instrumentation and control important to safety - Classification of instrumentation and control functions

IEC 61226:2009 establishes a method of classification of the information and command functions for nuclear power plants, and the instrumentation and control systems and equipment that provide those functions, into categories that designate the importance to safety of the function. The resulting classification then determines relevant design criteria. Is applicable to all the information and command functions and the instrumentation and control systems and equipment that provide those functions. The main changes with respect to the previous edition are listed below:
- to introduce a definition for "non-hazardous stable state";
- to clarify limits of categories;
- to clarify requirements related to equipment used for beyond design events.

Centrales nucléaires de puissance - Instrumentation et contrôle-commande importants pour la sûreté - Classement des fonctions d'instrumentation et de contrôle-commande

La CEI 61226:2009 établit une méthode de classement des fonctions d'information et de commande des centrales nucléaires de puissance et des systèmes d'instrumentation et de contrôle-commande (I&C) et matériels qui assurent ces fonctions en catégories indiquant l'importance pour la sûreté de la fonction. Le classement qui en résulte permet alors de déterminer les critères de conception appropriés. Est applicable à toutes les fonctions d'information et de commande, à tous les systèmes d'I&C et les matériels supports de ces fonctions. Les modifications techniques majeures par rapport à l'édition précédente sont les suivantes:
- introduction d'une définition pour "état contrôlé";
- clarification des limites des catégories;
- clarification des exigences portant sur les matériels utilisés hors dimensionnement.

General Information

• Status: Published
• Publication Date: 07-Jul-2009
• ICS: 27.120.20 - Nuclear power plants. Safety
• Technical Committee: SC 45A - Instrumentation, control and electrical power systems of nuclear facilities
• Drafting Committee: WG 7 - TC 45/SC 45A/WG 7
• Current Stage: DELPUB - Deleted Publication
• Completion Date: 29-Apr-2020

Relations

• Revised: IEC 61226:2020 - Nuclear power plants - Instrumentation, control and electrical power systems important to safety - Categorization of functions and classification of systems
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61226


®

Edition 3.0 2009-07



INTERNATIONAL



STANDARD



NORME
INTERNATIONALE


Nuclear power plants – Instrumentation and control important to safety –
Classification of instrumentation and control functions

Centrales nucléaires de puissance – Instrumentation et contrôle-commande
importants pour la sûreté – Classement des fonctions d’instrumentation et de
contrôle-commande


IEC 61226:2009

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IEC 61226


®

Edition 3.0 2009-07



INTERNATIONAL



STANDARD



NORME
INTERNATIONALE


Nuclear power plants – Instrumentation and control important to safety –
Classification of instrumentation and control functions

Centrales nucléaires de puissance – Instrumentation et contrôle-commande
importants pour la sûreté – Classement des fonctions d’instrumentation et de
contrôle-commande


INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
V
CODE PRIX
ICS 27.120.20 ISBN 978-2-88910-448-2
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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CONTENTS

FOREWORD.3


INTRODUCTION.5

1 Scope.8

2 Normative references .8


3 Terms and definitions .9

4 Abbreviations .13

5 Classification scheme.13

5.1 General .13
5.2 Background .14
5.3 Description of categories.14
5.3.1 General .14
5.3.2 Category A .15
5.3.3 Category B .15
5.3.4 Category C .15
5.4 Assignment criteria.16
5.4.1 General .16
5.4.2 Category A .16
5.4.3 Category B .16
5.4.4 Category C .17
6 Classification procedure .17
6.1 General .17
6.2 Identification of design basis .18
6.3 Identification and classification of functions.18
7 Assignment of technical requirements to categories .21
7.1 General requirements.21
7.2 Requirements related to functions .21
7.2.1 Basic requirements.21
7.2.2 Specific requirements .22
7.3 Requirements related to I&C systems.22
7.3.1 Basic requirements.22
7.3.2 Specific requirements .23
7.4 Requirements related to equipment .25

7.4.1 Basic requirements.25
7.4.2 Specific requirements .25
7.5 Requirements related to quality aspects .26
7.5.1 Basic requirements.26
7.5.2 Specific requirements .26
Annex A (informative) Examples of categories .30
Bibliography.32

Figure 1 – Method of classification.20

Table 1 – Tabular correlation between categories and other IEC standards .29

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61226 © IEC:2009 – 3 –


INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________



NUCLEAR POWER PLANTS –

INSTRUMENTATION AND CONTROL IMPORTANT TO SAFETY –

CLASSIFICATION OF INSTRUMENTATION AND CONTROL FUNCTIONS





FOREWORD


1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61226 has been prepared by subcommittee 45A: Instrumentation
and control of nuclear facilities, of IEC technical committee 45: Nuclear instrumentation.

This third edition cancels and replaces the second edition published in 2005 and constitutes a
technical revision. The main changes with respect to the previous edition are listed below:
• to introduce a definition for “non-hazardous stable state”;
• to clarify limits of categories;
• to clarify requirements related to equipment used for beyond design events.

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– 4 – 61226 © IEC:2009


The text of this standard is based on the following documents:


FDIS Report on voting

45A/745/FDIS 45A/767/RVD



Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.



This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

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61226 © IEC:2009 – 5 –


INTRODUCTION


a) Technical background, main issues and organisation of the standard


This International Standard responds to an International Atomic Energy Agency (IAEA)
1
requirement to classify nuclear power plants instrumentation and control systems according

to their importance to safety. With distributed computer based I&C systems now being used

for NPP instrumentation and control systems, the functions important to safety are distributed

over several systems or subsystems. Therefore, it is the intent of this standard to


– classify the I&C functions important to safety into categories, depending on their

contribution to the prevention and mitigation of postulated initiating events (PIE), and to
develop requirements that are consistent with the importance to safety of each of the
categories;
– assign specification and design requirements to I&C systems and equipment concerned
which perform the classified functions.
2
According to IAEA recommendation, the methods of classification are primarily based on the
deterministic safety analysis, and should be complemented where appropriate by probabilistic
methods. Several possible approaches for use of probabilistic safety assessment (PSA) for
classification are described in IEC/TR 61838, “Nuclear power plants – Instrumentation and
control important to safety – Use of probabilistic safety assessment for the classification of
functions”.
This revision of the standard enables quantitative assessment to be partly taken into account.
b) Situation of the current standard in the structure of the SC 45A standard series
IEC 61226 is directly referenced by IEC 61513 and is the second level SC 45A document
tackling the issue of categorization of functions and classification of systems.
For more details on the structure of the SC 45A standard series see item d) of this
introduction.
c) Recommendation and limitation regarding the application of this standard
Correct classification of functions directs the appropriate degree of attention by the plant's
designers, operators and regulatory authorities to the specification, design, qualification,
quality assurance (QA), manufacturing, installation, maintenance, and testing of the systems
that ensure the safety functions.

—————————
1
 IAEA NS-R-1 requirement 5.1.
2
 The NS-R-1, section 5.2 requires that the method for classifying the safety significance of a structure, system
or component shall be primarily based on deterministic methods complemented where appropriate by
probabilistic methods and sound engineering judgment taking into account factors such as
a) the safety function(s) to be performed;
b) the consequences of failure to perform the function;
c) the probability that it (the I&C system) will be required to perform a safety function;
d) the time following a PIE at which, or the period throughout which it (the I&C system) will be called upon to
operate.

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– 6 – 61226 © IEC:2009


This standard establishes the criteria and methods to be used to assign the I&C functions of a
NPP to three categories A, B and C, which depend on the importance of the function for

safety, and an unclassified category for functions with no direct safety role. It outlines generic

requirements for each category, and specifies basic technical requirements for matters such

as QA, reliability, testing and maintenance.


The category to which a function is assigned determines generic and specific technical

requirements. Generic requirements for each function are based on providing the appropriate

level of assurance that it will be executed on demand with the required performance and

reliability level. This applies to the aspects of functionality, reliability, performance,

environmental durability and QA. The level of assurance to be shown for each of these

aspects must be consistent with the importance of the function to safety.
i) Assurance of functionality is established by the creation of a complete and comprehensive
requirements specification, and the application of appropriate standards and codes.
ii) Assurance of reliability is provided by the selection of appropriate components, structures
and levels of redundancy and diversity in association with physical separation and/or
barriers, electrical isolation and periodic testing during service.
iii) Assurance of performance is gained by the creation of specifications of the required
performance, the application of QA procedures, verification and validation processes
during design and manufacture, pre-service testing of the individual and integrated
systems and equipment, and testing during service.
iv) Assurance of environmental durability is established by equipment qualification
programmes to ensure that ageing effects and environmental conditions that exist when
the equipment is required to operate do not degrade its performance below that required.
v) Assurance that the aspects of functionality, performance, environmental durability and
reliability have been properly considered at each stage from conception, through design,
manufacture, test, installation, commissioning and entry into service is provided by
carrying out each stage of the work under the control of an appropriate QA program.
Throughout this standard, the auxiliary "shall" indicates requirements that are mandatory for
compliance with the standard, the auxiliary "should" indicates requirements that are not
mandatory for compliance with the standard but are strongly recommended and the auxiliary
"may" indicates requirements that are optional.
d) Description of the structure of the SC 45A standard series and relationships with
other IEC documents and other bodies documents (IAEA, ISO)
The top-level document of the IEC SC 45A standard series is IEC 61513. It provides general
requirements for I&C systems and equipment that are used to perform functions important to
safety in NPPs. IEC 61513 structures the IEC SC 45A standard series.

IEC 61513 refers directly to other IEC SC 45A standards for general topics related to
categorization of functions and classification of systems, qualification, separation of systems,
defence against common cause failure, software aspects of computer-based systems,
hardware aspects of computer-based systems, and control room design. The standards
referenced directly at this second level should be considered together with IEC 61513 as a
consistent document set.
At a third level, IEC SC 45A standards not directly referenced by IEC 61513 are standards
related to specific equipment, technical methods, or specific activities. Usually these
documents, which make reference to second-level documents for general topics, can be used
on their own.
A fourth level extending the IEC SC 45A standard series, corresponds to the technical reports
which are not normative.

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61226 © IEC:2009 – 7 –


IEC 61513 has adopted a presentation format similar to the basic safety publications of

IEC 61508 series with an overall safety life-cycle framework and a system life-cycle

framework and provides an interpretation of the general requirements of IEC 61508-1,

IEC 61508-2 and IEC 61508-4, for the nuclear application sector. Compliance with IEC 61513

will facilitate consistency with the requirements of IEC 61508 as they have been interpreted

for the nuclear industry. In this framework, IEC 60880 and IEC 62138 correspond to

IEC 61508-3 for the nuclear application sector.


IEC 61513 refers to ISO, as well as to IAEA 50-C-QA (now replaced by IAEA GS-R-3) for

topics related to quality assurance (QA).


The IEC SC 45A standards series consistently implements and details the principles and
basic safety aspects provided in the IAEA code on the safety of NPPs and in the IAEA safety
series, in particular the requirements NS-R-1, establishing safety requirements related to the
design of nuclear power plants, and the safety guide NS-G-1.3 dealing with instrumentation
and control systems important to safety in nuclear power plants. The terms and definitions
used by SC 45A standards are consistent with those used by the IAEA.

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NUCLEAR POWER PLANTS –

INSTRUMENTATION AND CONTROL IMPORTANT TO SAFETY –

CLASSIFICATION OF INSTRUMENTATION AND CONTROL FUNCTIONS








1 Scope


This International Standard establishes a method of classification of the information and

command functions for nuclear power plants, and the I&C systems and equipment that
provide those functions, into categories that designate the importance to safety of the
function. The resulting classification then determines relevant design criteria.
The design criteria are the measures of quality by which the adequacy of each function in
relation to its importance to plant safety is ensured. In this standard, the criteria are those of
functionality, reliability, performance, environmental durability (including seismic) and quality
assurance (QA).
This standard is applicable to all the information and command functions and the instrument-
ation and control (I&C) systems and equipment that provide those functions. The functions,
systems and equipment under consideration provide automated protection, closed or open
loop control and information to the operating staff. They 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 functions that fulfil these roles safeguard the health and safety of
the NPP operators and the public.
This standard follows the general principles given in IAEA safety code NS-R-1 and safety
guide NS-G-1.3, and it defines a structured method of applying the guidance contained in
those codes and standards to the I&C systems that perform functions important to safety in a
NPP. This standard should be read in association with the IAEA guides and IEC 61513 in
implementing the requirements of IEC 61508 series.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.

IEC 60671:2007, Nuclear power plants – Instrumentation and control systems important to
safety – Surveillance testing
IEC 60709, Nuclear power plants – Instrumentation and control systems important to safety –
Separation
IEC 60780, Nuclear power plants – Electrical equipment of the safety system – Qualification
IEC 60812, Analysis techniques for system reliability – Procedure for failure mode and effects
analysis (FMEA)
IEC 60880:2006, Nuclear power plants – Instrumentation and control systems important to
safety – Software aspects for computer-based systems performing category A functions
IEC 60964, Nuclear power plants – Control rooms – Design

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61226 © IEC:2009 – 9 –


IEC 60965, Supplementary control points for reactor shutdown without access to the main

control room


IEC 60980, Recommended practices for seismic qualification of electrical equipment of the

safety system for nuclear generating stations


IEC 60987, Nuclear power plants – Instrumentation and control important to safety –

Hardware design requirements for computer-based systems


IEC 61000-4 (all parts), Electromagnetic compatibility (EMC) – Part 4: Testing and

measurement techniques

IEC 61000-6-2, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards –
Immunity for industrial environments
IEC 61513:2001, Nuclear power plants – Instrumentation and control for systems important to
safety – General requirements for systems
IEC 61771, Nuclear power plants – Main control room – Verification and validation of design
IEC 61772, Nuclear power plants – Main control room – Application of visual display units
(VDU)
IEC 61839, Nuclear power plants – Design of control rooms – Functional analysis and
assignment
IEC 62138, Nuclear power plants – Instrumentation and control important for safety –
Software aspects for computer-based systems performing category B or C functions
IAEA NS-R-1:2000, Safety of nuclear power plants: Design
IAEA GS-R-3:2006, The management system for facilities and activities (available in English
only)
IAEA NS-G-1.3:2002, Instrumentation and Control Systems Important to Safety in Nuclear
Power Plants

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.
3.1
anticipated operational occurrence
operational process deviating from normal operation which is expected to occur at least once
during the operating lifetime of a facility but which, in view of appropriate design provisions,
does not cause any significant damage to items important to safety nor lead to accident
conditions
[IAEA Safety Glossary:2007]

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3.2
common cause failure

CCF

failure of two or more structures, systems or components due to a single specific event or

cause


[IAEA Safety Glossary:2007]


3.3

design basis accident

DBA

accident conditions against which a facility is designed according to established design
criteria, and for which the damage to the fuel and the release of radioactive material are kept
within authorized limits
[IAEA Safety Glossary:2007]
3.4
design basis event
DBE
group of design basis accidents and anticipated operational occurrences
NOTE See also 3.13.
3.5
diversity
presence of two or more redundant systems or components to perform an identified function,
where the different systems or components have different attributes so as to reduce the
possibility of common cause failure, including common mode failure
[IAEA Safety Glossary:2007]
NOTE The following definition was given in 3.5 of IEC 60880 for the term “diversity”: Existence of two or
...