Nuclear power plants - Instrumentation and control systems important to safety - Design and qualification of isolation devices (IEC 62808:2015)

This International Standard establishes requirements for the design, analysis and qualification
of isolation devices used to ensure electrical independence of redundant safety system
circuits, or between safety and lower class circuits, as specified in IEC 60709. This standard
includes guidance on the determination of the maximum credible fault that is applied to the
isolation devices. The maximum credible fault can be used as a basis for the test levels used
in testing based on other standards (e.g. IEC TS 61000-6-5 or IEC 62003).
This standard does not address safety or CCF issues due to functional inter-dependencies
and possible interferences or CCFs that may result from signal exchange or sharing between
systems or sub-systems. It also does not address design or qualification issues related to
digital or programmable logic in isolation devices. For isolation devices containing digital or
programmable logic, additional design and qualification requirements must be considered;
these requirements are outside the scope of this standard.

Kernkraftwerke - Leittechnik mit sicherheitstechnischer Bedeutung - Auslegung und Qualifizierung von Isolationseinrichtungen (IEC 62808:2015)

Centrales nucléaires de puissance - Systèmes d’instrumentation et de contrôle-commande importants pour la sûreté - Conception et qualification des appareils d’isolement (IEC 62808:2015)

L'IEC 62808:2015 établit des exigences pour la conception, les analyses et la qualification des appareils d'isolement utilisés pour garantir l'indépendance électriques entre les circuits des systèmes de sûreté redondants ou entre les circuits des systèmes de sûreté et des systèmes dont la classe de sûreté est inférieure, comme spécifié dans l'IEC 60709. La présente norme comprend des recommandations portant sur la détermination du défaut maximal prévisible qui est pris en compte pour les appareils d'isolement. Le défaut maximal prévisible peut être utilisé comme une base pour le niveau des essais définis à partir d'autres normes (par exemple l'IEC TS 61000-6-5 ou l'IEC 62003).

Jedrske elektrarne - Instrumenti in nadzorni sistemi za zagotavljanje varnosti - Projektiranje in razvrščanje izolacijskih naprav (IEC 62808:2015)

Ta mednarodni standard določa zahteve glede projektiranja, analize in razvrščanja izolacijskih naprav, ki se uporabljajo za električno ločitev odvečnih krogotokov varnostnega sistema ali med varnostnimi krogotoki in krogotoki nižjega razreda, kot je opredeljeno v standardu IEC 60709. Ta standard vključuje smernice za določevanje največje verodostojne napake, ki se uporablja za izolacijske naprave. Največja verodostojna napaka se lahko uporablja kot osnova za preskusne ravni, ki se uporabljajo pri preskušanju na podlagi drugih standardov (npr. IEC TS 61000-6-5 ali IEC 62003).
Ta standard ne obravnava varnostnih težav ali težav z odpovedmi iz normalnih razlogov (CCF) zaradi medsebojnih funkcionalnih odvisnosti in možnih motenj ali odpovedi iz normalnih razlogov, ki so lahko posledica izmenjave ali skupne rabe signalov med sistemi ali podsistemi. Poleg tega ne obravnava težav zaradi projektiranja ali razvrščanja, povezanih z digitalno ali programirljivo logiko v izolacijskih napravah. Za izolacijske naprave, ki vsebujejo digitalno ali programirljivo logiko, je treba upoštevati dodatne zahteve glede projektiranja in razvrščanja; te zahteve so zunaj področja uporabe tega standarda.

General Information

Status
Published
Publication Date
27-Sep-2016
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
21-Sep-2016
Due Date
26-Nov-2016
Completion Date
28-Sep-2016

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Kernkraftwerke - Leittechnik mit sicherheitstechnischer Bedeutung - Auslegung und Qualifizierung von Isolationseinrichtungen (IEC 62808:2015)Centrales nucléaires de puissance - Systèmes d’instrumentation et de contrôle-commande importants pour la sûreté - Conception et qualification des appareils d’isolement (IEC 62808:2015)Nuclear power plants - Instrumentation and control systems important to safety - Design and qualification of isolation devices (IEC 62808:2015)27.120.20Jedrske elektrarne. VarnostNuclear power plants. SafetyICS:Ta slovenski standard je istoveten z:EN 62808:2016SIST EN 62808:2016en01-november-2016SIST EN 62808:2016SLOVENSKI

STANDARD
SIST EN 62808:2016
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 62808
September 2016 ICS 27.120.20
English Version

Nuclear power plants - Instrumentation and control systems important to safety - Design and qualification of isolation devices (IEC 62808:2015)

Centrales nucléaires de puissance - Systèmes d'instrumentation et de contrôle-commande importants pour la sûreté - Conception et qualification des appareils d'isolement (IEC 62808:2015)

Kernkraftwerke - Leittechnik mit sicherheitstechnischer Bedeutung - Auslegung und Qualifizierung von Isolationseinrichtungen (IEC 62808:2015) This European Standard was approved by CENELEC on 2016-07-18. 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 CEN-CENELEC Management Centre 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 CEN-CENELEC Management Centre 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. European Committee for Electrotechnical Standardization

Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung CEN-CENELEC Management Centre: Avenue Marnix 17,

B-1000 Brussels © 2016 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN 62808:2016 E SIST EN 62808:2016

EN 62808:2016 2 European foreword This document (EN 62808:2016) consists of the text of IEC 62808:2015 prepared by SC 45A “Instrumentation, control and electrical systems of nuclear facilities” of IEC/TC 45 “Nuclear instrumentation".

The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2017-07-18 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2019-07-18

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.

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 standard. Endorsement notice The text of the International Standard IEC 62808:2015 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 60880 NOTE Harmonized as EN 60880. IEC 61226:2009 NOTE Harmonized as EN 61226:2010 (not modified). 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. IEC 62138 NOTE Harmonized as EN 62138. SIST EN 62808:2016

EN 62808:2016 3 Annex ZA (normative)

Normative references to international publications with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.

NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available here: www.cenelec.eu Publication Year Title EN/HD Year

IEC 60709 -
Nuclear power plants - Instrumentation
and control systems important to safety - Separation EN 60709 -
IEC/TS 61000-6-5 -
Electromagnetic compatibilty (EMC) -

Part 6-5: Generic standards - Immunity for power station and substation environments - -

IEC 61513 -
Nuclear power plants - Instrumentation
and control important to safety - General requirement for systems EN 61513 -
IEC 62003 -

Nuclear power plants - Instrumentation and control important to safety - Requirements for electromagnetic compatibility testing - -

SIST EN 62808:2016
SIST EN 62808:2016

IEC 62808 Edition 1.0 2015-05 INTERNATIONAL STANDARD NORME INTERNATIONALE Nuclear power plants – Instrumentation and control systems important to safety – Design and qualification of isolation devices

Centrales nucléaires de puissance – Systèmes d’instrumentation et de
contrôle-commande importants pour la sûreté – Conception et qualification

des appareils d’isolement INTERNATIONAL ELECTROTECHNICAL COMMISSION COMMISSION ELECTROTECHNIQUE INTERNATIONALE

ICS 27.120.20
ISBN 978-2-8322-2665-0
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale ®

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé. SIST EN 62808:2016

– 2 – IEC 62808:2015 © IEC 2015 CONTENTS FOREWORD ........................................................................................................................... 3 INTRODUCTION ..................................................................................................................... 5 1 Scope .............................................................................................................................. 7 2 Normative references ...................................................................................................... 7 3 Terms and definitions ...................................................................................................... 7 4 Symbols and abbreviations .............................................................................................. 8 5 General principles for isolation devices ............................................................................ 8 5.1 General ................................................................................................................... 8 5.2 Isolation characteristics .......................................................................................... 9 5.3 Actuation priority ................................................................................................... 10 6 Isolation device design requirements ............................................................................. 10 6.1 Requirements on isolation device application ........................................................ 10 6.1.1 Isolation device power ................................................................................... 10 6.1.2 Maximum credible fault .................................................................................. 10 6.1.3 Energy limiting devices .................................................................................. 11 6.2 Requirements on isolation device design .............................................................. 11 6.2.1 Basic design requirements ............................................................................. 11 6.2.2 Postulated faults ............................................................................................ 12 6.2.3 Physical component arrangement .................................................................. 12 6.3 Power isolation devices ........................................................................................ 13 6.3.1 General ......................................................................................................... 13 6.3.2 Circuit breaker tripped by fault currents ......................................................... 13 6.3.3 Circuit breaker tripped by fault signals ........................................................... 13 6.3.4 Input current limiters ...................................................................................... 13 6.3.5 Fuses ............................................................................................................ 13 7 Qualification test requirements ...................................................................................... 13 7.1 General ................................................................................................................. 13 7.2 Requirements on the test method .......................................................................... 14 7.2.1 Test specification ........................................................................................... 14 7.2.2 Testing energy limiting devices ...................................................................... 14 7.2.3 Qualification test environment ........................................................................ 14 7.3 Application specific testing .................................................................................... 15 7.3.1 General ......................................................................................................... 15 7.3.2 Isolation of safety circuits from lower class circuits ........................................ 15 7.3.3 Isolation between redundant safety circuits .................................................... 15 7.4 Documentation of test requirements and results .................................................... 15 Bibliography .......................................................................................................................... 16

Figure 1 – Application of maximum credible fault .................................................................. 11 Figure 2 – Application of postulated fault .............................................................................. 12

SIST EN 62808:2016

IEC 62808:2015 © IEC 2015 – 3 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________

NUCLEAR POWER PLANTS –
INSTRUMENTATION AND CONTROL SYSTEMS IMPORTANT TO SAFETY –
DESIGN AND QUALIFICATION OF ISOLATION DEVICES

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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 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 62808 has been prepared by subcommittee 45A: Instrumentation, control and electrical systems of nuclear facilities, of IEC technical committee 45: Nuclear instrumentation. The text of this standard is based on the following documents: FDIS Report on voting 45A/1004/FDIS 45A/1019/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. SIST EN 62808:2016

– 4 – IEC 62808:2015 © IEC 2015 The committee has decided that the contents of this publication will remain unchanged until the stability 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 62808:2016

IEC 62808:2015 © IEC 2015 – 5 – INTRODUCTION a) Technical background, main issues and organisation of the standard I&C (instrumentation and control) systems important to safety in nuclear power plants need to tolerate the effects of plant / equipment faults as well as internal and external hazards. IEC 60709 provides requirements to establish independence between redundant portions of safety systems, and between safety systems and systems of a lower class. Among the techniques available to increase the level of tolerability of I&C systems to such effects is the provision of isolation devices where connections are made between redundant divisions of safety equipment, or between safety equipment and systems of a lower class. This standard provides technical requirements and recommendations for the design and qualification of isolation devices that are required by IEC 60709. This standard deals with the criteria and methods used to confirm that the design of isolation devices ensures that credible failures in the connected lower class system or redundant channels will not prevent the safety systems from meeting their required functions. Isolation devices may be required on power or signal interfaces within the system. Guidance for other aspects of isolation device qualification (e.g. electromagnetic compatibility, environmental and seismic qualification) may be found in IEC 60780. The object of this standard is: – in Clause 5: to establish the basic criteria for acceptability of the design and application of isolation devices; – in Clause 6: to establish design requirements on the selection and application of suitable isolation devices; – in Clause 7: to establish requirements on qualification testing done to validate the adequacy of the isolation device design. It is intended that the standard be used by operators of NPPs (utilities), designers of nuclear I&C system and equipment, systems evaluators and regulators. b) Situation of the current standard in the structure of the IEC SC 45A standard series IEC 62808 is the third level IEC SC 45A document tackling the issue of isolation devices. IEC 60709 is 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 a normative standard regarding requirements of separation. IEC 62808 is intended to provide requirements and recommendations relating to the design and qualification of isolation devices which are identified in IEC 60709 as a means of achieving independence between systems when signals are extracted from a system for use in lower class systems, or between independent subsystems of the same classes. For more details on the structure of the IEC SC 45A standard series, see item d) of this introduction. SIST EN 62808:2016

– 6 – IEC 62808:2015 © IEC 2015 c) Recommendations and limitations regarding the application of this 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. IEC 60709 requires the use of isolation devices where connections between independent systems must be made. IEC 62808 provides criteria for the analysis and qualification of the the isolation device. A fundamental criterion for isolation devices is that they be included in, and designed to, the standards of the higher class system for which they provide protection against hazards. Additional requirements relating to design and qualification of an isolation device as an element of a safety system are not given in this standard. d) Description of the structure of the IEC 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. 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. Regarding nuclear safety, it provides the interpretation of the general requirements of IEC 61508-1, IEC 61508-2 and IEC 61508-4, for the nuclear application sector, regarding nuclear safety. 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 GS-R-3 and IAEA GS-G-3.1 and IAEA GS-G-3.5 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 SSR-2/1, 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 terminology and definitions used by SC 45A standards are consistent with those used by the IAEA. NOTE It is assumed that for the design of I&C systems in NPPs that implement conventional safety functions (e.g. to address worker safety, asset protection, chemical hazards, process energy hazards), international or national standards would be applied, that are based on the requirements of a standard such as IEC 61508. SIST EN 62808:2016

IEC 62808:2015 © IEC 2015 – 7 – NUCLEAR POWER PLANTS –
INSTRUMENTATION AND CONTROL SYSTEMS IMPORTANT TO SAFETY –
DESIGN AND QUALIFICATION OF ISOLATION DEVICES

1 Scope This International Standard establishes requirements for the design, analysis and qualification of isolation devices used to ensure electrical independence of redundant safety system circuits, or between safety and lower class circuits, as specified in IEC 60709. This standard includes guidance on the determination of the maximum credible fault that is applied to the isolation devices. The maximum credible fault can be used as a basis for the test levels used in testing based on other standards (e.g. IEC TS 61000-6-5 or IEC 62003). This standard does not address safety or CCF issues due to functional inter-dependencies and possible interferences or CCFs that may result from signal exchange or sharing between systems or sub-systems. It also does not address design or qualification issues related to digital or programmable logic in isolation devices. For isolation devices containing digital or programmable logic, additional design and qualification requirements must be considered; these requirements are outside the scope of this standard. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60709, Nuclear power plants – Instrumentation and control systems important to safety – Separation IEC TS 61000-6-5, Electromagnetic compatibility (EMC) – Part 6-5: Generic standards – Immunity for power station and substation environments IEC 61513, Nuclear power plants – Instrumentation and control important to safety – General requirements for systems IEC 62003, Nuclear power plants – Instrumentation and control important to safety – Requirements for electromagnetic compatibility testing 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1

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 Note 1 to entry: The following definition is given in the IAEA Safety Glossary, edition 2007: “A physical obstruction that prevents or inhibits the movement of people, radionuclides or some other phenomenon (e.g. fire), or provides shielding against radiation”. The IAEA definition is more general and consistent with the definition given in this standard. SIST EN 62808:2016

– 8 – IEC 62808:2015 © IEC 2015 3.2

common mode electrical faults voltage or current faults between both signal terminals and a common reference plane (ground)

Note 1 to entry: These faults should not be confused with common cause failures. Note 2 to entry: This causes the potential of both signal terminals to be changed simultaneously and by the same amount relative to the common reference plane (ground).

3.3

differential mode electrical faults voltage or current faults between signals 3.4

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

Note 1 to entry: As described in IEC 60709, malfunctions can be caused by faults and normal actions. 3.5

maximum credible fault

MCF voltage or current transient that may exist in circuits, as determined by test or analysis, taking into consideration the circuit location, routing, and interconnections combined with failures that the circuit and adjacent circuits may credibly experience Note 1 to entry:

The evaluation shall consider the impact of seismic and flooding conditions. 4 Symbols and abbreviations AC alternating current CCF common cause failure DC direct current EMI electromagnetic interference I&C instrumentation and control MCF maximum credible fault NPP nuclear power plant 5 General principles for isolation devices 5.1 General The requirements for the application of isolation devices are in IEC 60709. Clause 5 is included as a summary and provides additional requirements for the isolation devices. The word "shall" identifies the additional requirements. Isolation devices used in interfaces between I&C systems important to safety or between channels within a system important to safety may have an impact on the integrity of the overall design and in particular, on defence in depth. When used, they may be relied upon to provide electrical isolation between redundant safety functions or safety functions in different layers of defence in the overall architecture. In general, the introduction of such interfaces between systems should be considered carefully based on the principles and approaches outlined in IEC 61513. A systematic analysis of failures at system and overall I&C architecture level is required. Functional inter-dependencies are introduced between systems due to signal interfaces and their associated failure modes shall be considered carefully. SIST EN 62808:2016

IEC 62808:2015 © IEC 2015 – 9 – Where signals are transmitted between a Class 1 system or equipment (performing Category A functions) and systems of a lower class, the transmission of these signals are through isolation devices that are included within the higher class system. When failures or conditions are present at the output terminals of the isolation devices (which are connected to the lower class system) the safety action of the Class 1 system or sub-system to which the isolation device is connected cannot be affected. As an example, a circuit performing a Category A function may be monitored by a lower class circuit utilizing a relay coil in the Class 1 system and the relay contact in a lower class system.

Isolation devices are to be used where signals are transmitted between independent Class 1 systems and between redundant equipment channels of Class 1 systems. Where signals are transmitted from Class 2 or 3 systems for use in lower class systems, or between independent subsystems in these classes, isolation devices may not be required; however, good engineering practices are followed to prevent the propagation of faults. In cases where Class 2 systems need to take on the aspects of Class 1 systems due to the functions performed, isolation is applied. An example of this is a Class 2 system performing a Category B function in support of a Class 1 system performing a Category A function to protect against the same fault.

Temporary connections for maintenance to systems performing Category A functions without isolation devices are only permitted provided that they are connected to only a single redundancy at any given time, that they are disconnected after use, and that the system is capable of withstanding a fault introduced through failure or use of the connection. NOTE This standard discusses isolation devices as stand-alone devices which are separate from the equipment performing safety functions. The isolation device may be part of a module or equipment that performs a sa

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