SIST EN 61226:2010

SLOVENSKI STANDARD
01-september-2010
Jedrske elektrarne - Merilna in nadzorna oprema za zagotavljanje varnosti -
Klasifikacija funkcij merilne in nadzorne opreme (IEC 61226:2009)
Nuclear power plants - Instrumentation and control important to safety - Classification of
instrumentation and control functions (IEC 61226:2009)
Kernkraftwerke - Leittechnische Systeme mit sicherheitstechnischer Bedeutung -
Kategorisierung leittechnischer Funktionen (IEC 61226:2009)
Centrales nucléaires de puissance - Instrumentation et contrôle-commande importants
pour la sûreté - Classification des fonctions d'instrumentation et de contrôle-commande
(CEI 61226:2009)
Ta slovenski standard je istoveten z: EN 61226:2010
ICS:
27.120.20 Jedrske elektrarne. Varnost Nuclear power plants. Safety
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61226
NORME EUROPÉENNE
March 2010
EUROPÄISCHE NORM
ICS 27.120.20
English version
Nuclear power plants -
Instrumentation and control important to safety -
Classification of instrumentation and control functions
(IEC 61226:2009)
Centrales nucléaires de puissance -  Kernkraftwerke -
Instrumentation et contrôle-commande Leittechnische Systeme
importants pour la sûreté - mit sicherheitstechnischer Bedeutung -
Classification des fonctions Kategorisierung leittechnischer
d'instrumentation Funktionen
et de contrôle-commande (IEC 61226:2009)
(CEI 61226:2009)
This European Standard was approved by CENELEC on 2010-03-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61226:2010 E
Foreword
The text of the International Standard IEC 61226:2009, prepared by SC 45A, Instrumentation and control
of nuclear facilities, of IEC TC 45, Nuclear instrumentation, was submitted to the CENELEC formal vote
for acceptance as a European Standard and was approved by CENELEC as EN 61226 on 2010-03-01.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following dates are proposed:
– latest date by which the EN has to be implemented

at national level by publication of an identical

national standard or by endorsement
(dop) 2011-03-01
– latest date by which the national standards conflicting

with the EN have to be withdrawn
(dow) 2013-03-01
Annex ZA has been added by CENELEC.
As stated in the nuclear safety Directive 2009/71/EURATOM, Chapter 1, Article 2, item 2, Member States
are not prevented from taking more stringent safety measures in the subject-matter covered by the
Directive, in compliance with Community law. In a similar manner, this European Standard does not
prevent Member States from taking more stringent nuclear safety measures in the subject-matter covered
by this European Standard.
__________
– 3 – EN 61226:2010
Endorsement notice
The text of the International Standard IEC 61226:2009 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 61508-1 NOTE  Harmonized as EN 61508-1.
IEC 61508-2 NOTE  Harmonized as EN 61508-2.
IEC 61508-3 NOTE  Harmonized as EN 61508-3.
IEC 61508-4 NOTE  Harmonized as EN 61508-4.
__________
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE  Where an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year
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 - EN 60812 -
Procedure for failure mode and effects analysis
(FMEA)
IEC 60880 2006 Nuclear power plants - Instrumentation and EN 60880 2009
control systems important to safety - Software
aspects for computer-based systems performing
category A functions
IEC 60964 - Nuclear power plants - Control rooms - Design EN 60964 -
IEC 60965 - Nuclear power plants - Control rooms - -
-
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 EN 60987 -
control important to safety - Hardware design
requirements for computer-based systems
IEC 61000-4 Series Electromagnetic compatibility (EMC) - EN 61000-4 Series
Part 4: Testing and measurement techniques
IEC 61000-6-2 - Electromagnetic compatibility (EMC) - EN 61000-6-2 -
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

– 5 – EN 61226:2010
Publication Year Title EN/HD Year
IEC 61771 - Nuclear power plants - Main control-room - - -
Verification and validation of design
IEC 61772 - Nuclear power plants - Control rooms - Application - -
of visual display units (VDUs)
IEC 61839 - Nuclear power plants - Design of control rooms - - -
Functional analysis and assignment
IEC 62138 - Nuclear power plants - Instrumentation and EN 62138 -
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 : safety requirements
IAEA NS-G-1.3 2002 Instrumentation and control systems important to - -
safety in nuclear power plants

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 2-8318-1052-1
– 2 – 61226 © IEC:2009
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

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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
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.

– 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.
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)
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.
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.
—————————
IAEA NS-R-1 requirement 5.1.
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.
– 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.
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.

– 8 – 61226 © IEC:2009
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

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]
– 10 – 61226 © IEC:2009
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 more different
ways or means of achieving a specified objective. Diversity is specifically provided as a defence against common
cause failure. It may be achieved by providing systems that are physically different from each other or by functional
diversity, where similar systems achieve the specified objective in different ways. It is totally consistent with the
IAEA definition given here.
3.6
equipment
one or more parts of a system. An item of equipment is a single definable (and usually
removable) element or part of a system
[IEC 61513, 3.17, modified]
3.7
function
specific purpose or objective to be accomplished, that can be specified or described without
reference to the physical means of achieving it
3.8
functionality
attribute of a function which defines the operations which transform input information into
output information
[IEC 61513, 3.25]
61226 © IEC:2009 – 11 –
3.9
human factor engineering programme
programme that describes at least the human factors organisation, role and mission of human
factors specialists and team, human factors activities and their integration in the design and
validation process, list of deliverables to be provided at each step of the program
3.10
item important to safety
item that is part of a safety group and/or whose malfunction or failure could lead to radiation
exposure of the site personnel or members of the public.
Items important to safety include:
a) those structures, systems and components whose malfunction or failure could lead to
undue radiation exposure of the site personnel or members of the public;
b) those structures, systems and components that prevent anticipated operational
occurrences from leading to accident conditions;
c) those features which are provided to mitigate the consequences of malfunction or failure
of structures, systems or components.
[IAEA Safety Glossary: 2007]
NOTE Items important to safety considered in this standard are mainly I&C systems important to safety.
3.11
non-hazardous stable state
state of the plant, where stabilisation of any transient has been achieved, the reactor is
subcritical, adequate heat removal is ensured and radioactive releases are limited
NOTE A transient is considered to be stabilised when, for all safety significant parameters, the margins (e.g.
between the heat removal capacity and heat generation) are either stable or increasing, or sufficient margin
remains to cover all expected physical processes.
3.12
performance
effectiveness with which an intended function is carried out (e.g. time response, accuracy,
sensitivity to parameter changes)
3.13
plant states
Operational states Accident conditions
Design basis events Beyond design basis
accidents
Normal operation Anticipated operational a) Design basis
occurrences accidents
b) Severe accidents
Accident management
a) Accident conditions which are not explicitly considered design basis accidents but which are encompassed by
them.
b) Beyond design basis accidents without significant core degradation.

NOTE This definition is consistent with the one of the IAEA safety glossary. It just indicates the position of the
concept of “design basis event” compared to the other concepts.

– 12 – 61226 © IEC:2009
3.14
postulated initiating event
PIE
event identified during design as capable of leading to anticipated operational occurrences or
accident conditions
[IAEA Safety Glossary:2007]
3.15
redundancy
provision of alternative (identical or diverse) structures, systems or components, so that any
one can perform the required function regardless of the state of operation or failure of any
other
[IAEA Safety Glossary:2007]
3.16
safety group
assembly of equipment designated to perform all actions required for a particular postulated
initiating event to ensure that the limits specified in the design basis for anticipated
operational occurrences and design basis accidents are not exceeded
[IAEA Safety Glossary:2007]
3.17
safety related system
a system important to safety that is not part of a safety system
[IAEA Safety Glossary:2007]
3.18
safety system
a system important to safety, provided to ensure the safe shutdown of the reactor and the
residual heat removal from the core, or to limit the consequences of anticipated operational
occurrences and design basis accident
[IAEA Safety Glossary:2007]
3.19
single failure
a failure which results in the loss of capability of a system or component to perform its
intended safety function(s), and any consequential failure(s) which result from it
[IAEA Safety Glossary:2007]
3.20
system
set of components which interact according to a design, where an element of a system can be
another system, called a subsystem
[IEC 61513, 3.61]
3.21
type test
conformity test made on one or more items representative of the production
[IEV 394-40-02]
61226 © IEC:2009 – 13 –
3.22
unacceptable consequence
consequence of an operational state or of a PIE, that exceeds specified limits for the
corresponding plant states, in terms of releases at the site or to the wider environment
NOTE Additional limits, such as unacceptable fuel damage, or damage to other main components may also be
specified on a national basis. This might be either a massive, uncontrolled release caused by events with a
frequency that is beyond the NPP's design basis, or events with a frequency that is in the design basis but leading
to a magnitude exceeding specified limits. Additional limits, such as unacceptable fuel damage may also be
specified. This might be damage to the fuel cladding that leads to an unacceptable increase in the activity of the
primary coolant, or structural damage to the fuel that impairs the ability to cool it. Damage to the other barriers may
also be considered as unacceptable consequence.
4 Abbreviations
ALARA As low as reasonably achievable
DBA Design basis accident
DBE Design basis event
FAT Factory acceptance test
FMEA Failure modes and effects analysis
HMI Human machine interface
IAEA International Atomic Energy Agency
I&C Instrumentation and control
NPP Nuclear power plant
PIE Postulated initiating event
PRA Probabilistic risk assessment
QA Quality assurance
SAT Site acceptance test
5 Classification scheme
5.1 General
Functions to be performed by I&C systems shall be assigned to categories according to their
importance to safety. The importance to safety of a function shall be identified by means of
the consequences in the event of its failure when it is required to be performed and the
consequences in the event of a spurious actuation. The category determines the design and
quality requirements for I&C systems and equipment. These requirements shall be defined
independently from the technology of the equipment to be applied. Subclause 5.2 provides the
background to the classification scheme.
Subclause 5.3 describes the three categories that are used to classify functions. The
categories are based upon those defined originally in the first edition of IEC 61226 published
in 1993.
Subclause 5.4 presents the assignment criteria for each category.
Clause 6 provides guidance on the classification process.
Clause 7 provides the technical requirements for each of the three categories. Most of the
requirements apply to the systems and equipment that perform the functions, but some
requirements apply only to the functions.
Annex A contains typical examples of the classification of NPP I&C functions. It is only for
information because it may depend on the reactor type.

– 14 – 61226 © IEC:2009
5.2 Background
The principle of defence in depth is firmly established in the safety design basis of nuclear
power plants. The fundamental idea is that there should be several layers or echelons of
defence in the prevention of unsafe conditions, and that the prevention of unsafe conditions,
before mitigation is required, is always to be preferred. Because of the large number of
functions that are required to operate and keep safe a NPP, a number that increases with the
principle of defence in depth, it is important that the significance to safety of each function is
known.
IAEA safety standard series NS-R-1 establishes the idea of classification of NPP systems
according to their importance to safety, and gives examples of the classification of the major
systems of several types of NPP. All structures, systems and components, including software
for instrumentation and control (I&C), that are items important to safety, shall be first
identified and then classified on the basis of their function and significance with regard to
safety. They shall be designed, constructed and maintained such that their quality and
reliability is commensurate with this classification.
The IAEA safety guide NS-G-1.3 gives guidance on the classification of systems according to
the importance to safety of the functions they perform. It introduces time factors such as
– the duration that the I&C system is needed once it has been initiated;
– the time for which alternative actions can be taken;
– the timeliness by which hidden faults can be detected and remedied.
This standard extends the classification strategy presented in IAEA Safety Guide NS-G-1.3,
and establishes the criteria and methods to be used to assign the I&C functions of a NPP to
one of the three categories A, B and C, depending on their importance to safety, or to an
unclassified category for functions with no direct safety role. I&C functions falling within the
boundary of the safety systems will generally be assigned to category A or B. I&C functions
defined as safety related will generally be assigned to categories B or C.
The safety importance of, and the corresponding requirements placed on, parts of the safety
systems and safety related I&C systems will differ, so that it is appropriate to assign them to
different safety classes. Some I&C systems can have a significant effect on safety and
therefore require appropriate attention. Other I&C systems have intermediate, low, or no
significance to safety. They have correspondingly less stringent requirements for ensuring
system performance and safety justification, and therefore have different technical
requirements.
National application of the principles and criteria of this standard may assign differing
nomenclature to categories A, B and C. The national application shall be according to the
principles, criteria and associated requirements given in this standard. This shall involve
establishing and documenting an appropriate correspondence to the categories defined.
5.3 Description of categories
5.3.1 General
I&C systems in NPPs perform functions with different levels of importance to safety. The
importance to safety of each I&C function depends upon its role in achieving and maintaining
safety, the potential consequence of failure of the function to operate when required, and the
probability of these consequences. Therefore, an initial safety analysis of the specific NPP
design is required to be completed prior to the classification of the I&C functions. The severity
of the potential consequences in the case of a postulated failure of an I&C function, defines
th
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