IEC 62425:2025

IEC 62425 ®
Edition 2.0 2025-05
COMMENTED VERSION
INTERNATIONAL
STANDARD
Railway applications – Communication, signalling and processing systems –
Safety related electronic systems for signalling
ICS 45.060.01 ISBN 978-2-8327-0420-2
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CONTENTS
FOREWORD . 6
INTRODUCTION . 9
1 Scope . 10
2 Normative references . 11
3 Terms, definitions and abbreviated terms . 12
3.1 Terms and definitions . 12
3.2 Abbreviated terms . 22
4 Overall framework of this document . 23
5 Requirements for developing safety-related electronic systems . 25
5.1 General . 25
5.2 The quality management process . 25
5.3 The safety management process. 27
5.3.1 General . 27
5.3.2 Guideline for structuring documentation . 27
5.3.3 Safety life cycle . 28
5.3.4 Safety organization . 28
5.3.5 Safety plan . 30
5.3.6 Hazard log . 31
5.3.7 Safety requirements specification . 31
5.3.8 System design for safety . 31
5.3.9 Safety operation and maintenance plan . 32
5.3.10 Safety verification . 32
5.3.11 Safety validation . 33
5.3.12 Safety qualification tests . 34
5.3.13 Management of safety-related application conditions . 35
5.3.14 Safety justification . 36
5.3.15 Independent safety assessment . 37
6 Requirements for elements following different life cycles . 37
6.1 General . 37
6.2 Use of pre-existing items . 38
6.2.1 General . 38
6.2.2 Requirements for use of complete pre-existing systems . 39
6.2.3 Requirements for use of pre-existing equipment . 39
6.3 Safety-related tools for electronic systems . 40
6.4 Physical security and cybersecurity. 41
7 The safety case: structure and content . 42
7.1 The safety case structure . 42
7.2 The technical safety report . 44
7.3 Generic and specific safety cases . 53
7.4 Provisions for the specific application safety case . 53
7.5 Dependencies between safety cases . 54
8 System safety acceptance and subsequent phases . 55
8.1 System safety acceptance process . 55
8.2 Operation, maintenance and performance monitoring . 59
8.3 Modification and retrofit . 59
8.4 Decommissioning and disposal . 59

Annex A (normative) Safety integrity levels . 60
A.1 General . 60
A.2 Safety requirements . 60
A.3 Safety integrity . 61
A.4 Determination of safety integrity requirements . 61
A.4.1 General . 61
A.4.2 Risk assessment . 63
A.4.3 Hazard control . 65
A.4.4 Identification and treatment of new hazards arising from design . 71
A.5 Allocation of SILs . 72
A.5.1 General aspects . 72
A.5.2 Relationship between SIL and associated TFFR . 73
Annex B (normative) Management of faults for safety-related functions . 75
B.1 General . 75
B.2 General concepts . 75
B.2.1 Detection and negation times . 75
B.2.2 Composition of two independent items . 76
B.3 Effects of faults . 77
B.3.1 Effects of single faults . 77
B.3.2 Independence of items. 79
B.3.3 Detection of single faults . 84
B.3.4 Action following detection (retention of safe state) . 87
B.3.5 Effects of multiple faults . 88
B.3.6 Defence against systematic faults . 91
Annex C (normative) Identification of hardware component failure modes . 92
C.1 General . 92
C.2 General procedure . 92
C.3 Procedure for integrated circuits . 92
C.4 Procedure for components with inherent physical properties . 93
C.5 General provisions concerning component failure modes . 93
Annex D (informative) Example of THR/TFFR/FR apportionment and SIL allocation. 111
Annex E (normative) Techniques and measures for the avoidance of systematic faults
and the control of random and systematic faults . 113
E.1 General . 113
E.2 Tables of techniques and measures . 115
Annex F (informative) Guidance on User Programmable Integrated Circuits. 123
F.1 General . 123
F.1.1 Purpose . 123
F.1.2 Terminology and context . 123
F.2 UPIC life cycle . 124
F.2.1 General . 124
F.2.2 Organization, roles, responsibilities and personnel competencies . 126
F.2.3 UPIC Requirements . 126
F.2.4 UPIC Architecture and Design . 127
F.2.5 Logic Component Design . 128
F.2.6 Logic Component Coding . 128
F.2.7 Logic Component Verification . 128
F.2.8 UPIC Physical Implementation . 128

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F.2.9 UPIC Integration . 128
F.2.10 UPIC Validation . 128
F.2.11 Requirements for use of pre-existing logic components . 128
F.3 Detailed technical requirements for UPIC . 128
F.3.1 Guidance on safety architecture . 128
F.3.2 Protection against random faults – architectural principles . 129
F.3.3 Protection against systematic faults – techniques and measures . 129
Annex G (informative) Changes in this document compared to IEC 62425:2007. 138
Bibliography . 141
List of comments . 143

Figure 1 – Scope of the main IEC and CENELEC railway application standards . 11
Figure 2 – Structure of IEC 62425 . 24
Figure 3 – Example of system life cycle . 26
Figure 4 – Example of design and validation portion of system life cycle . 28
Figure 5 – Independence of roles for different SILs and BI, phases 5 to 10 . 30
Figure 6 – Structure of safety case . 43
Figure 7 – Structure of technical safety report . 45
Figure 8 – Examples of different usage of safety cases . 55
Figure 9 – Examples of different safety acceptance processes . 58
Figure A.1 – Safety requirements and safety integrity . 60
Figure A.2 – The hourglass model . 63
Figure A.3 – Definition of hazards with respect to the system boundary . 65
Figure A.4 – Example of a hazard analysis process . 67
Figure A.5 – Common cause failures (CCF) . 69
Figure A.6 – Treatment of CCF by FTA . 70
Figure A.7 – Relationship between SILs and techniques . 73
Figure B.1 – Detection and negation times . 75
Figure B.2 – Control of single and multiple faults . 79
Figure B.3 – Influences affecting the independence of items . 81
Figure B.4 – Detection and negation of single faults – composite fail-safety . 87
Figure B.5 – Detection and negation of single faults – reactive fail-safety . 88
Figure C.1 – Example of a 4-terminal resistor, using a hybrid thick layer technique . 95
Figure D.1 – Example of THR/TFFR/FR breakdown and related SIL allocation . 111
Figure F.1 – UPIC architecture. 124
Figure F.2 – UPIC development context . 124
Figure F.3 – Example of UPIC development life cycle . 125
Figure F.4 – Example of UPIC development life cycle with pre-existing components . 125
Figure F.5 – UPIC development techniques and measures . 130

Table 1 – Example of SRAC template . 36
Table 2 – Sections and contents of the technical safety report . 45
Table A.1 – The SIL table . 73

Table B.1 – Measures to detect faults in integrated circuits by means of periodic online
testing . 89
Table C.1 – Resistors . 95
Table C.2 – Capacitors . 96
Table C.3 – Electromagnetic components . 97
Table C.4 – Diodes . 100
Table C.5 – Transistors . 101
Table C.6 – Controlled rectifiers . 103
Table C.7 – Surge suppressors . 104
Table C.8 – Opto-electronic components . 105
Table C.9 – Filters . 107
Table C.10 – Interconnection assemblies . 108
Table C.11 – Fuses . 109
Table C.12 – Switches and push/pull buttons . 109
Table C.13 – Lamps . 109
Table C.14 – Batteries . 110
Table C.15 – Transducers and sensors (excluding those with internal electronic
circuitry) . 110
Table E.1 – Safety planning and quality assurance activities . 115
Table E.2 – Safety requirements specification . 116
Table E.3 – Safety organization . 116
Table E.4 – Architecture of system, subsystem or equipment . 117
Table E.5 – Design features . 118
Table E.6 – Failure and hazard analysis methods . 119
Table E.7 – Design and development of system, subsystem or equipment . 120
Table E.8 – Safety verification and validation of the system, subsystem or equipment . 121
Table E.9 – Application, operation and maintenance . 122
Table F.1 – Example of documentation generated during each phase . 126
Table F.2 – Simplified techniques and measures for protection against systematic
failures . 130
Table F.3 – Design and verification (including all activities before synthesis) . 131
Table F.4 – Synthesis . 131
Table F.5 – Placement, routing and layout generation . 132
Table F.6 – Description of techniques for design . 133
Table F.7 – Description of techniques for synthesis . 136
Table F.8 – Description of techniques for placement, routing and layout generation . 137
Table G.1 – Clauses and subclauses – correspondence with IEC 62425:2007 . 138
Table G.2 – Figures and tables – correspondence with IEC 62425:2007 . 140

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RAILWAY APPLICATIONS –
COMMUNICATION, SIGNALLING AND PROCESSING SYSTEMS –
SAFETY RELATED ELECTRONIC SYSTEMS FOR SIGNALLING

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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This commented version (CMV) of the official standard IEC 62425:2025 edition 2.0
provides the user with comments from IEC TC 9 experts to explain the reasons of the
most relevant changes made to the previous IEC 62425:2007 edition 1.0.
Experts' comments are identified by a blue-background number. Mouse over a number to
display a pop-up note with the comment.
This publication contains the CMV and the official standard. The full list of comments is
available at the end of the CMV.

IEC 62425 has been prepared by IEC technical committee 9: Electrical equipment and systems
for railways. It is an International Standard.
EN 50129:2018 has served as a basis for the development of this document.
This second edition cancels and replaces the first edition published in 2007. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) a better alignment with the life cycle phases defined in IEC 62278-1 has been made;
b) Clause 5 describes the requirements that apply to the development of safety-related
electronic systems (until phase 9 of the life cycle);
c) Clause 8 focuses on the requirements for safety acceptance and approval of safety-related
electronic systems and subsequent phases;
d) requirements and guidance have been added in Clause 6 on the following topics:
1) reuse of pre-existing systems,
2) safety-related tools,
3) impact of cybersecurity threats on functional safety,
4) specific application safety cases;
e) requirements for the structure and content of the safety case are now defined in a dedicated
Clause 7;
f) Annex A has been updated for the specification and allocation of safety integrity
requirements;
g) the content of former Annex D has been merged with Annex B, and the content has been
changed from informative to normative;
h) the status of Annex E has been changed from informative to normative;
i) an Annex F has been added as an informative annex on User Programmable Integrated
Circuits.
A more detailed comparison of changes between IEC 62425:2007 and this document can be
found in Annex G.
The text of this International Standard is based on the following documents:
Draft Report on voting
9/3113/FDIS 9/3141/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.

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The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
INTRODUCTION
This International Standard defines requirements for the development and acceptance of safety-
related electronic systems in the railway signalling field.
Safety-related electronic systems for signalling include hardware and software aspects. To
develop complete safety-related systems, both aspects need to be taken into account
throughout the whole life cycle of the system. The requirements for the overall safety-related
electronic system and for its hardware aspects are defined in this document. Other requirements
are defined in associated IEC and CENELEC standards. For safety-related systems which
include software, additional conditions are defined in IEC 62279:2015.
NOTE 1 IEC 62279:2015 is derived from the European Standard EN 50128:2011.
Additional requirements for safety-related communication are defined in IEC 62280:2014.
This document is the common base for safety acceptance and approval of electronic systems
for railway signalling applications. The aim of railway authorities and railway industry is to
develop railway systems based on common standards. The safety authorities having jurisdiction
can apply this document to the relevant matters they choose. On this basis, cross-acceptance
of safety approvals for sub-systems and equipment can be applied by the different national
safety authorities.
Cross-acceptance is applicable to generic approval, not to specific applications.
This document is concerned with the evidence to be presented for the acceptance of safety-
related systems. However, it specifies not only those life cycle activities which need to be
completed before the acceptance stage, but also the additional planned activities to be carried
out afterwards. In this way, safety justification will cover the whole life cycle.
This document is concerned with what evidence is to be presented. Except where considered
appropriate, it does not specify who carries out the necessary work. The necessary work can
be carried out by different people, in different circumstances or organisational structures,
provided that independence of roles is respected.
This document consists of Clauses 1 to 8, which form the main part, and Annexes A, B, C, D,
E, F and G. The requirements defined in Clauses 5 to 8 and in Annexes A, B, C and E are
normative, whilst Annexes D, F and G are informative.
1 1
This document is in line with, and contain references to IEC 62278-1:— and IEC 62278-2:— .
NOTE 2 IEC 62278-1:— and IEC 62278-2:— are derived from the European Standards EN 50126-1:2017 and EN
50126-2:2017 respectively.
This document is based on the system life cycle described in IEC 62278 series and is in line
with the IEC 61508 series. IEC 62278, IEC 62279 and IEC 62425 comprise the railway sector
equivalent of the IEC 61508 series so far as railway communication, signalling and processing
systems are concerned. Given that compliance with these documents has been demonstrated,
there are no requirements in this document for further evaluation of compliance with the
IEC 61508 series.
___________
Under preparation. Stage at the time of publication: IEC/FDIS 62278-1:2025 and IEC/FDIS 62278-2:2025.

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RAILWAY APPLICATIONS –
COMMUNICATION, SIGNALLING AND PROCESSING SYSTEMS –
SAFETY RELATED ELECTRONIC SYSTEMS FOR SIGNALLING

1 Scope
This document is applicable to safety-related electronic systems (including subsystems and
equipment) for railway signalling applications.
This document applies to generic systems (i.e. generic products or systems defining a class of
applications), as well as to systems for specific applications.
The scope of this document, and its relationship with other IEC and CENELEC standards, are
shown in Figure 1.
This document is applicable only to the functional safety of systems. It does not deal with other
aspects of safety such as the occupational health and safety of personnel. While functional
safety of systems clearly can have an impact on the safety of personnel, there are other aspects
of system design which can also affect occupational health and safety and which are not
covered by this document. Cybersecurity aspects of functional safety are addressed only to a
limited extent. 1
This document applies to all the phases of the life cycle of a safety-related electronic system,
focusing in particular on phases 5 (architecture and apportionment of system requirements) to
10 (system acceptance) as defined in IEC 62278-1:—.
Requirements for systems which are not related to safety are outside the scope of this document.
This document is not applicable to existing systems, subsystems or equipment which had
already been accepted prior to the development of this document. However, so far as
reasonably practicable, it is applicable to modifications and extensions to existing systems,
subsystems and equipment.
This document is primarily applicable to systems, subsystems or equipment which have been
specifically designed and manufactured for railway signalling applications. It is also applicable,
so far as reasonably practicable, to general-purpose or industrial equipment (e.g. power
supplies, display screens or other commercial off the shelf items), which is procured for use as
part of a safety-related electronic system.
This document is aimed at railway duty holders, railway suppliers, and assessors as well as at
safety authorities, although it does not define an approval process to be applied by the safety
authorities.
Figure 1 – Scope of the main IEC and CENELEC railway application standards
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 60664-1:2020, Insulation coordination for equipment within low-voltage supply systems –
Part 1: Principles, requirements and tests
IEC 62278-1:— , Railway Applications – The Specification and Demonstration of Reliability,
Availability, Maintainability and Safety (RAMS) – Part 1: Generic RAMS Process
IEC 62278-2:— , Railway Applications – The Specification and Demonstration of Reliability,
Availability, Maintainability and Safety (RAMS) – Part 2: Systems Approach to Safety
IEC 62279:2015, Railway applications – Communication, signalling and processing systems –
Software for railway control and protection systems
IEC 62280:2014, Railway applications – Communication, signalling and processing systems –
Safety related communication in transmission systems
IEC 62497-1 :2010, Railway applications – Insulation coordination – Part 1: Basic requirements
– Clearances and creepage distances for all electrical and electronic equipment
IEC 62497-1:2010/AMD1:2013
IEC 62498-1:2010, Railway applications – Environmental conditions for equipment – Part 1:
Equipment on board rolling stock
IEC 62498-3:2010, Railway applications – Environmental conditions for equipment – Part 3:
Equipment for signalling and telecommunications
___________
Under preparation. Stage at the time of publication: IEC/FDIS 62278-1:2025 and IEC/FDIS 62278-2:2025.
There exists a consolidated edition 1.1:2013 that includes IEC 62497-1:2010 and its Amendment 1:2013.

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3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1.1
accident
unintended event or series of events that results in harm
[SOURCE: IEC 60050-821:2017, 821-12-02, modified – “that results in death, injury, loss of a
system or service, or environmental damage” has been replaced with “that results in harm”]
3.1.2
basic insulation
insulation of hazardous-live-parts which provides basic protection
Note 1 to entry: This concept does not apply to insulation used exclusively for functional purposes.
[SOURCE: IEC 60050-826:2021, 826-12-14, modified – The words “of hazardous live-parts”
have been added.]
3.1.3
basic integrity
integrity attribute for a safety-related function with a tolerable functional failure rate equal to, or
−5 −1
higher than (less demanding), 10 h ; or for a non-safety-related function
Note 1 to entry: In this document basic integrity requirements relate only to safety-related functions. If a non-safety-
related function has been given basic integrity requirements on the basis of the process described in
IEC 62278-1:—, no additional requirements are defined in this document.
[SOURCE: IEC 62278-1:—, 3.7, modified – "Note 1 to entry has been added.]
3.1.4
causal analysis
analysis of the reasons how and why a particular hazard can come into existence
[SOURCE: IEC 60050-821:2017, 821-12-07]
3.1.5
common cause failures, pl
failures of multiple items, which would otherwise be considered independent of one another,
resulting from a single cause
[SOURCE: IEC 60050-192:2015, 192-03-18, modified – Notes 1 and 2 to entry have been
deleted.]
3.1.6
configuration
structuring and interconnection of the hardware and software of a system for its intended
application
[SOURCE: IEC 60050-821:2017, 821-12-12]
3.1.7
consequence analysis
analysis of events which are likely to happen after a hazard has occurred
[SOURCE: IEC 60050-821:2017, 821-12-14]
3.1.8
cross-acceptance
status achieved by a product that has been accepted by one authority to the relevant standards
and is acceptable to other authorities without the necessity for further assessment
[SOURCE: IEC 60050-821:2017, 821-12-15]
3.1.9
DC fault model
fault category that includes the following failure modes: stuck-at faults, stuck-open, open or
high impedance outputs and short circuit between signal lines, and for integrated circuits short
circuit between any two connections (pins)
3.1.10
design
activity applied in order to analyse and transform specified requirements into acceptable
solutions
[SOURCE: IEC 60050-821:2017, 821-12-16, modified – The end of the definition "design
solutions which have the required safety integrity level" has been replaced with "solutions".]
3.1.11
diversity
existence of two or more different ways or means of achieving a specified objective
Note 1 to entry: Diversity is specifically provided as a defence against common cause failures. It can 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.
[SOURCE: IEC 60050-395:2014, 395-07-115, modified – The supplementary information has
been moved to a new note 1 to entry, which replaces the original note 1 to entry.]

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3.1.12
double insulation
insulation comprising both basic insulation and supplementary insulation
Note 1 to entry: In double insulation, each layer shall be able to be tested or analysed separately. In particular:
– the clearance distance shall be the basic distance. In addition, also the supplementary solid layer shall be
dimensioned taking into account the same rated impulse voltage (U ).
Ni
– the creepage distance shall be the sum of basic and supplementary distances. The basic distance shall be
evaluated against the rated insulation voltage (U ). In addition, also the supplementary distance shall be
Nm
evaluated against the same U . The U shall not be apportioned.
Nm Nm
Note 2 to entry: With respect to Note 1, for definition of rated insulation voltage and rated impulse voltage, see
IEC 62497-1:2010 and IEC 62497-1:2010/AMD1:2013. Similar definitions can also be found in IEC 60664-1:2020.
[SOURCE: IEC 60050-195:2021, 195-06-08, modified – Notes to entry 1 and 2 have been
added.]
3.1.13
electronic component
hardware component
electronic device that cannot be taken apart without destruction or impairment of its intended
use
EXAMPLE: Resistors, capacitors, diodes, integrated circuits, hybrids, application specific integrated circuits, wound
components and relays.
[SOURCE: IEC 60050-904:2014, 904-01-09, modified – The preferred terms "electronic part"
and "piece part" have been deleted and a new preferred term "hardware component" has been
added.]
3.1.14
equipment
single apparatus or set of devices or apparatuses, or the set of main devices of an installation,
or all devices necessary to perform a specific task
Note 1 to entry: Examples of equipment are a power transformer, the equipment of a substation, measuring
equipment.
[SOURCE: IEC 60050-151:2001, 151-11-25]
3.1.15
error
discrepancy between a computed, observed or measured value or condition and the true,
specified or theoretically correct value or condition
Note 1 to entry: An error can be caused by a faulty item, e.g. a computing error made by faulty computer equipment.
Note 2 to entry: A human error can be seen as a human action or inaction that can produce an unintended result.
[SOURCE: IEC 60050-192:2015, 192-03-02, modified – Notes 1 and 2 to entry have been
modified.]
3.1.16
fail-safe
able to enter or remain in a safe state in the event of a failure
[SOURCE: IEC 60050-821:2017, 821-01-10]

3.1.17
failure
loss of ability to perform as required
Note 1 to entry: Qualifiers, such as catastrophic, critical, major, minor, marginal and insignificant, may be used to
categorize failures
...