Nuclear power plants - Instrumentation and control systems important to safety - Separation (IEC 60709:2004)

This standard is applicable to nuclear power plant instrumentation and control (I&C) systems, and their cables, that are important to safety, as defined in IAEA Safety Guide NS-G-1.3. It is also applicable to temporary installations which are part of those I&C systems important to safety (for example, auxiliary equipment for commissioning tests and experiments). Clause 6 is intended particularly for the cabling of the I&C systems important to safety. This standard applies to the I&C of new nuclear power plants as well as to I&C upgrading or back-fitting of existing plants. For existing plants, only a subset of the requirements is applicable; this subset is to be identified at the beginning of any project. Where independence is required by general safety standards such as IAEA safety guides or IEC 61513, one aspect of achieving this independence is physical separation between the systems and their equipment that perform functions important to safety. This standard defines the assessments needed and the technical requirements to be met for I&C systems important to safety and their cables, in order to achieve adequate physical separation between redundant sections of a system and between a system and another system. This separation is needed to prevent or minimise the impact on safety that could result from faults and failures which could be propagated or affect several sections of a system or several systems.

Kernkraftwerke - Leittechnische Systeme mit sicherheitstechnischer Bedeutung - Physikalische und elektrische Trennung (IEC 60709:2004)

Centrales nucléaires de puissance - Systèmes d'instrumentation et de contrôle commande importants pour la sûreté - Séparation (CEI 60709:2004)

Définit les exigences techniques qui doivent être satisfaites par les systèmes d'instrumentation et de contrôle commande importants pour la sûreté et leurs câbles, de façon à obtenir une séparation physique appropriée entre les parties redondantes de système ou entre un système et un autre système.

Jedrske elektrarne - Merilna in nadzorna oprema za zagotavljanje varnosti - Ločevanje (IEC 60709:2004)

Ta standard se uporablja za merilno in nadzorno opremo za jedrske elektrarne in pripadajoče kable, za zagotavljanje varnosti, kot je opredeljeno v varnostnih navodilih IAEA NS-G-1.3. Uporablja se tudi za začasne naprave in napeljave, ki so del merilne in nadzorne opreme za zagotavljanje varnosti (na primer, pomožna oprema za preskuse in poskuse ob usposobitvi za zagon).  Klavzula 6 je namenjena zlasti okabljenju merilne in nadzorne opreme za zagotavljanje varnosti. Ta standard se uporablja za merilno in nadzorno opremo novih jedrskih elektrarn, kot tudi nadgradnjo ali posodabljanje merilne in nadzorne opreme obstoječih elektrarn. Za obstoječe elektrarne se uporablja samo podmnožica zahtev; ta podmnožica se odpredeli ob začetku vsakega projekta. Kadar splošni varnostni standardi, kot so varnostna navodila IAEA ali IEC 61513 zahtevajo neodvisnost, je en vidik doseganja takšne neodvisnosti fizična ločitev med sistemi in njihovo opremo za zagotavljanje varnosti. Ta standard določa potrebne presoje in tehnične zahteve, ki jih mora izpolnjevati merilna in nadzorna oprema za zagotavljanje varnosti in pripadajoči kabli, da bi dosegli zadostno fizično ločitev med neuporabljenimi deli sistema in med sistemom in drugim sistemom.  Ta ločitev je potrebna, da bi preprečili ali minimizirali vpliv na varnost, do katerega bi lahko prišlo zaradi napak in okvar, ki bi se lahko razširile ali vplivale na več delov sistema ali več sistemov.

General Information

Status
Withdrawn
Publication Date
22-Jul-2010
Withdrawal Date
15-Aug-2022
Technical Committee
Current Stage
9900 - Withdrawal (Adopted Project)
Start Date
29-Jul-2022
Due Date
21-Aug-2022
Completion Date
16-Aug-2022

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Kernkraftwerke - Leittechnische Systeme mit sicherheitstechnischer Bedeutung - Physikalische und elektrische Trennung (IEC 60709:2004)Centrales nucléaires de puissance - Systèmes d'instrumentation et de contrôle commande importants pour la sûreté - Séparation (CEI 60709:2004)Nuclear power plants - Instrumentation and control systems important to safety - Separation (IEC 60709:2004)27.120.20Jedrske elektrarne. VarnostNuclear power plants. SafetyICS:Ta slovenski standard je istoveten z:EN 60709:2010SIST EN 60709:2010en01-september-2010SIST EN 60709:2010SLOVENSKI
STANDARD



SIST EN 60709:2010



EUROPEAN STANDARD EN 60709 NORME EUROPÉENNE
EUROPÄISCHE NORM May 2010
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Management Centre: 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 60709:2010 E
ICS 27.120.20
English version
Nuclear power plants -
Instrumentation and control systems important to safety -
Separation (IEC 60709:2004)
Centrales nucléaires de puissance -
Systèmes d'instrumentation et de contrôle commande importants pour la sûreté -
Séparation (CEI 60709:2004)
Kernkraftwerke -
Leittechnische Systeme
mit sicherheitstechnischer Bedeutung -
Physikalische und elektrische Trennung (IEC 60709:2004)
This European Standard was approved by CENELEC on 2010-05-01. 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. SIST EN 60709:2010



EN 60709:2010 – 2 – Foreword The text of the International Standard IEC 60709:2004, 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 60709 on 2010-05-01. 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 were fixed: – latest date by which the EN has to be implemented at national level by publication of an identical national standard or by endorsement
(dop)
2011-05-01 – latest date by which the national standards conflicting with the EN have to be withdrawn
(dow)
2013-05-01 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.” __________ Endorsement notice The text of the International Standard IEC 60709:2004 was approved by CENELEC as a European Standard without any modification. SIST EN 60709:2010



– 3 – EN 60709:2010 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 60332 Series Tests on electric and optical fibre cables under fire conditions
EN 60332 Series IEC 60964 - Design for control rooms of nuclear power plants - - IEC 61000 Series Electromagnetic compatibility (EMC)
EN 61000 Series IEC/TS 61000-6-5 - Electromagnetic compatibilty (EMC) -
Part 6-5: Generic standards - Immunity for power station and substation environments - - IEC 61226 - Nuclear power plants - Instrumentation and control systems important to safety - Classification
- - IEC 61513 - Nuclear power plants - Instrumentation and control for systems important to safety - General requirements for systems - - IEC/TR 62096 - Nuclear power plants - Instrumentation and control - Guidance for the decision on modernization - - IAEA safety guide NS-G-1.3 - Instrumentation and control systems important to safety in nuclear power plants - -
SIST EN 60709:2010



SIST EN 60709:2010



NORME INTERNATIONALECEIIEC INTERNATIONAL STANDARD 60709Deuxième éditionSecond edition2004-11 Centrales nucléaires de puissance – Systèmes d'instrumentation et de contrôle commande importants
pour la sûreté – Séparation
Nuclear power plants – Instrumentation and control systems important to safety – Separation
Pour prix, voir catalogue en vigueur For price, see current catalogue IEC 2004
Droits de reproduction réservés

Copyright - all rights reserved Aucune partie de cette publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch
Web: www.iec.ch CODE PRIX PRICE CODE R Commission Electrotechnique InternationaleInternational Electrotechnical CommissionSIST EN 60709:2010



60709  IEC:2004 – 3 – CONTENTS FOREWORD.5 INTRODUCTION.9 1 Scope.13 2 Normative references.13 3 Terms and definitions.15 4 General principles for separation within I&C systems important to safety.17 4.1 General.17 4.2 Design errors.19 4.3 I&C system failure events.19 4.3.1 Single random failure.19 4.3.2 Multiple failures from a single common cause.19 4.4 Plant failure events.21 4.4.1 Environmental conditions.21 4.4.2 Electromagnetic interference.21 4.4.3 Failure of plant systems, equipment or structures.21 4.4.4 Operator error.21 4.5 External failure events.21 4.5.1 Natural events.21 4.5.2 External man-made causes.23 4.6 Special operating conditions.23 4.7 Separation issues at existing plants.23 5 Design basis.25 5.1 Fire protection.25 5.2 Environmental conditions during and after accidents.25 5.3 Isolation devices.25 5.3.1 General.25 5.3.2 Isolation characteristics.27 5.3.3 Actuation priority.27 5.4 Independence from control systems.29 6 Requirements for cabling separation.31 6.1 General requirement.31 6.2 Separation.31 6.2.1 Separation of redundant cables inside the I&C system important to safety.31 6.2.2 Lesser separation distances.31 6.2.3 Associated circuits.33 6.2.4 Separation of system cables of different safety categories.35 6.2.5 Separation of signal cables from power cables.35 6.2.6 Separation of cables from tubes or pipes.35 6.2.7 General routing considerations.35 6.2.8 Control room cabinets, desks, panels and related cables.35 6.3 Thermal and physical protection.39 6.4 Fire protection.39 6.5 Identification.39
SIST EN 60709:2010



60709  IEC:2004 – 5 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________
NUCLEAR POWER PLANTS –
INSTRUMENTATION AND CONTROL SYSTEMS
IMPORTANT TO SAFETY – SEPARATION
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 60709 has been prepared by subcommittee 45A: Instrumentation and control of nuclear facilities, of IEC technical committee 45: Nuclear instrumentation. This second edition cancels and replaces the first edition published in 1981 and constitutes a technical revision. This revision of IEC 60709 is intended to accomplish the following: – adjust the document format to follow current IEC/ISO directives on style of standards; – expand to cover all systems important to safety, and separation between and within category A, B, C and un-categorised systems; – align with the new revisions of IAEA documents (replacing D3 and D8) and broaden the scope to include other aspects of independence; – provide references to relevant normative standards; SIST EN 60709:2010



60709  IEC:2004 – 7 – – cover new technologies that either present unique separation issues or provide new means of achieving independence; – include provisions from IEC 60639 (which will be withdrawn as a standard) that are not adequately covered in IEC 61513 or that required further expansion, e.g. isolation, control/protection interaction, etc.; – expand concepts of electromagnetic disturbance as a failure initiator with CCF potential and the use of separation as a means to minimise risk of interference; – enhance requirements and guidance for areas of cable congestion, e.g. control room, cable spreading galleries, etc.; – introduce the concept of “associated circuits” (from US practice) to deal with non-safety cables that are not separated from safety cables; – provide guidance for the application of fire qualification standards (such as the IEC 60332 series) to cables important to safety. This includes the topics: • barriers as an alternative to separation, • cables themselves as potential barriers, and • similar concepts such as fibre optics as a barrier to EMI, armour as a barrier to physical damage, etc.; – address the implications of low energy circuits, such as the possible use of analysis to reduce the minimum separation distance; – review existing requirements, update terminology and definitions; – provide guidance for the application of the standard to existing plants. The text of this standard is based on the following documents: FDIS Report on voting 45A/537/FDIS 45A/545/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. SIST EN 60709:2010



60709  IEC:2004 – 9 – INTRODUCTION Background, main issues and organization of the standard I&C systems important to safety in nuclear power plants need to tolerate the effects of plant / equipment faults as well as internal and external hazards. Various techniques are available to increase the level of tolerability of I&C systems to such effects, including the provision of independent systems, subsystems and equipment. For claims to be made of independence between such systems and equipment, adequate separation must be provided and maintained. This standard provides technical requirements and recommendations for the implementation of separation in the design of I&C systems. The object of this standard is as follows: – in Clause 4, to identify a certain number of possible causes of failures and to lay down, taking these causes into consideration, a set of requirements to be followed when designing an I&C system important to safety in order to ensure that its purpose is fulfilled in the best possible way. These requirements apply to the I&C system as a whole. Clause 4 also presents guidance on separation when modernising I&C systems at existing nuclear power plants; – in Clause 5, to establish design basis criteria for I&C systems important to safety that take the causes of failure identified in Clause 4 into consideration; – in Clause 6, to give requirements to be fulfilled for cabling separation within an I&C system important to safety.
Situation of the current standard in the structure of the SC 45A standard series IEC 60709 is a document of the second level, directly referenced by IEC 61513 in regard to physical and electrical separation being required between subsystems of different safety trains of I&C systems important to safety, and between I&C systems important to safety and those that are not important to safety. IEC 61226 establishes the principles of categorization of I&C functions, systems and equipment according to their level of importance to safety. It then requires that adequate separation be provided between functions of different categories. IEC 61226 refers to IEC 60709 as the normative standard regarding requirements of separation. For more details on the structure of the SC 45A standard series, see the last paragraph of this introduction. Recommendations and limitations regarding the application of the Standard IEC 60709 applies to I&C systems and equipment important to safety. It establishes requirements for physical and electrical separation as one means to provide independence between the functions performed in those systems and equipment. Other aspects of independence that may be required to address concerns of common cause failure are not included in this standard. Additional requirements relating to availability and detailed requirements for the elimination of electrical interference are not given in this standard. SIST EN 60709:2010



60709  IEC:2004 – 11 – 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 SC 45A standard series is IEC 61513. It provides general requirements for instrumentation and control systems and equipment (I&C systems) that are used to perform functions important to safety in nuclear power plants (NPPs). IEC 61513 structures the SC 45A standard series.
IEC 61513 refers directly to other 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, SC 45A standards generally 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 SC 45A standard series corresponds to the technical reports, which are not normative. IEC 61513 has adopted a presentation format similar to the basic safety publication IEC 61508 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, -2 and -4, for the nuclear application sector. Compliance with this standard 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 for topics related to quality assurance. The SC 45A standards series consistently implements and details the principles and basic safety aspects provided in the IAEA Code on the safety of nuclear power plants and in the IAEA safety series, in particular the Requirements NS-R-1, “Safety of Nuclear Power Plants: Design” and the Safety Guide NS-G-1.3, “Instrumentation and control systems important to safety in Nuclear Power Plants”. The terminology and definitions used by SC 45A standards are consistent with those used by the IAEA. SIST EN 60709:2010



60709  IEC:2004 – 13 – NUCLEAR POWER PLANTS –
INSTRUMENTATION AND CONTROL SYSTEMS
IMPORTANT TO SAFETY – SEPARATION
1 Scope
This standard is applicable to nuclear power plant instrumentation and control (I&C) systems, and their cables, that are important to safety, as defined in IAEA Safety Guide NS-G-1.3. It is also applicable to temporary installations which are part of those I&C systems important to safety (for example, auxiliary equipment for commissioning tests and experiments). Clause 6 is intended particularly for the cabling of the I&C systems important to safety. This standard applies to the I&C of new nuclear power plants as well as to I&C upgrading or back-fitting of existing plants. For existing plants, only a subset of the requirements is applicable; this subset is to be identified at the beginning of any project.
Where independence is required by general safety standards such as IAEA safety guides or IEC 61513, one aspect of achieving this independence is physical separation between the systems and their equipment that perform functions important to safety. This standard defines the assessments needed and the technical requirements to be met for I&C systems important to safety and their cables, in order to achieve adequate physical separation between redundant sections of a system and between a system and another system. This separation is needed to prevent or minimise the impact on safety that could result from faults and failures which could be propagated or affect several sections of a system or several systems. 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 60332 (all parts), Tests on electric cables under fire conditions IEC 60964, Design for control rooms of nuclear power plants IEC 61000 (all parts), Electromagnetic compatibility (EMC)
IEC 61000-6-5, Electromagnetic compatibility (EMC) – Part 6-5: Generic standards – Immunity for power station and substation environments IEC 61226, Nuclear power plants – Instrumentation and control systems important for safety – Classification IEC 61513, Nuclear power plants – Instrumentation and control for systems important to safety – General requirements for systems IEC 62096, Nuclear power plants – Instrumentation and control – Guidance for the decision on modernisation IAEA Safety Guide NS-G-1.3, Instrumentation and Control Systems Important to Safety in Nuclear Power Plants
SIST EN 60709:2010



60709  IEC:2004 – 15 – 3 Terms and definitions For the purposes of this document, the following definitions apply. 3.1 associated circuit circuit of a lower safety category that is not physically separated or is not electrically isolated from the circuit(s) of the higher category by acceptable separation distances, safety class structures, barriers, or electrical isolation devices but meets suitable criteria for safety 3.2 barrier device or structure interposed between redundant equipment or circuits important to safety, or between equipment or circuits important to safety and a potential source of damage to limit damage to the I&C system important to safety to an acceptable level 3.3 cable route physical pathway through the plant along which multiple cables can be laid, such as through a room or duct in the plant building, or a metal duct, tray, or tube, or a duct below or gantry over roads 3.4 common cause failure (CCF) failure of two or more structures, systems or components due to a single specific event or cause [IAEA NS-G-1.3] 3.5 isolation device device in a circuit that prevents malfunctions in one section of a circuit from causing unacceptable influences in other sections of the circuit or other circuits 3.6 postulated initiating event (PIE) event identified during design as capable of leading to anticipated operational occurrences or accident conditions [IAEA NS-G-1.3]
3.7 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 NS-G-1.3] 3.8 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 NS-G-1.3] SIST EN 60709:2010



60709  IEC:2004 – 17 – 4 General principles for separation within I&C systems important to safety 4.1 General IEC 61226 defines how safety functions are classified according to their significance to safety, and requires physical separation to provide protection against propagation of failures due to physical effects, and against jeopardising redundant systems simultaneously.
An I&C system may perform functions in more than one category. In such a case, the category designation of the system shall be of the highest category function performed by it. For example, a system performing both category A and B functions is identified as a category A system. As a design basis for the I&C systems that are important to safety, the following general principles shall be applied to maintain the independence of redundant systems and between different systems, and to ensure that the redundancy and diversity (provided to achieve high reliability of systems important to safety) are effective. – Systems performing category A functions shall be protected from consequential physical effects caused by faults and normal actions within a) redundant parts of those systems, and
b) systems of a lower category.
The faults considered shall include those internal to the I&C system and its power supply as well as those that occur as a result of events external to the I&C systems.
In some cases, it may be necessary to provide physical separation between different systems performing category A functions where those functions are required to be independent. – Systems performing category B functions shall be protected from consequential physical effects caused by faults and normal actions within a) redundant parts of those systems, and
b) systems of a lower category.
The faults considered shall include those internal to the I&C system and its power supply, but may exclude those that occur as a result of events external to the I&C systems. In cases where category B functions are claimed to provide protection in the event of specific hazards, then tho
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