CLC/TR 50126-2:2007

SLOVENSKI STANDARD
01-september-2007
Železniške naprave – Specifikacija in prikaz zanesljivosti, razpoložljivosti,
vzdrževalnosti in varnosti (RAMS) – 2. del: Vodilo za uporabo EN 50126-1 za
varnost
Railway applications - The specification and demonstration of Reliability, Availability,
Maintainability and Safety (RAMS) -- Part 2: Guide to the application of EN 50126-1 for
safety
Bahnanwendungen - Spezifikation und Nachweis der Zuverlässigkeit, Verfügbarkeit,
Instandhaltbarkeit, Sicherheit (RAMS) -- Teil 2: Leitfaden zur Anwendung der EN 50126-
1 für Sicherheit
Applications ferroviaires - Spécification et démonstration de la fiabilité, de la disponibilité,
de la maintenabilité et de la sécurité (FDMS) -- Partie 2:Guide pour l’application de l’EN
50126-1 à la sécurité
Ta slovenski standard je istoveten z: CLC/TR 50126-2:2007
ICS:
29.280 (OHNWULþQDYOHþQDRSUHPD Electric traction equipment
45.020 Železniška tehnika na Railway engineering in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CLC/TR 50126-2
RAPPORT TECHNIQUE
February 2007
TECHNISCHER BERICHT
ICS 45.020
English version
Railway applications -
The specification and demonstration of Reliability, Availability,
Maintainability and Safety (RAMS) -
Part 2: Guide to the application of EN 50126-1 for safety

Applications ferroviaires -  Bahnanwendungen -
Spécification et démonstration Spezifikation und Nachweis
de la fiabilité, de la disponibilité, der Zuverlässigkeit, Verfügbarkeit,
de la maintenabilité Instandhaltbarkeit, Sicherheit (RAMS) -
et de la sécurité (FDMS) - Teil 2: Leitfaden zur Anwendung
Partie 2:Guide pour l’application der EN 50126-1 für Sicherheit
de l’EN 50126-1 à la sécurité
This Technical Report was approved by CENELEC on 2007-01-22.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, 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: rue de Stassart 35, B - 1050 Brussels

© 2007 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50126-2:2007 E

Foreword
The European Standard EN 50126-1:1999, which was prepared jointly by the Technical Committees
CENELEC TC 9X, Electric and electronic applications for railways, and CEN TC 256, Railway applications,
under mode 4 co-operation, deals with the specification and demonstration of Reliability, Availability,
Maintainability and Safety (RAMS) for railway applications.
A guide to the application of EN 50126-1 for safety of railway systems (this CLC/TR 50126-2) and a guide for
the application to EN 50126-1 for rolling stock RAM (CLC/TR 50126-3:2006) have been produced to form
informative parts of EN 50126-1:1999. Whilst this CLC/TR 50126-2 is applicable to all railway systems,
including rolling stock, CLC/TR 50126-3:2006 is applicable to rolling stock RAM only.
This Technical Report, which was prepared by WG 8 of the Technical Committee CENELEC TC 9X, forms
an informative part of EN 50126-1:1999 and contains guidelines for the application of EN 50126-1 for the
safety of railway systems.
The text of the draft was submitted to the vote and was approved by CENELEC as CLC/TR 50126-2 on
2007-01-22.
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Contents
Introduction.8
1 Scope.9
2 References.11
3 Definitions and abbreviations.12
3.1 Guidance on the interpretation of terms and definitions used in EN 50126-1 .12
3.2 Additional safety terms.15
3.3 Abbreviations.17
4 Guidance on bodies/entities involved and concepts of system hierarchy and safety.17
4.1 Introduction.17
4.2 Bodies/entities involved in a system.18
4.3 Concepts of system hierarchy.18
4.3.1 Rail transport system environment and system hierarchy .19
4.4 Safety concepts.19
4.4.1 Hazard perspective .19
4.4.2 Risk.21
4.4.3 Risk normalising .22
5 Generic risk model for a typical railway system and check list of common functional hazards .23
5.1 Introduction.23
5.2 Generic risk model .23
5.3 Risk assessment process.24
5.3.1 Introduction.24
5.3.2 Generic process .24
5.4 Application of the risk assessment process .28
5.4.1 Depth of analysis.29
5.4.2 Preliminary hazard analysis .29
5.4.3 Qualitative and Quantitative assessment.30
5.4.4 Use of historical data.31
5.4.5 Sensitivity analysis .32
5.4.6 Risk assessment during life cycle phases.32
5.5 Check-list of common functional hazards and hazard identification .33
5.5.1 Introduction.33
5.5.2 Hazard grouping structures.34
5.5.3 Check-list of “Hazards”.35
6 Guidance on application of functional safety, functional safety requirements and SI targets,
risk apportionment and application of SILs.36
6.1 Introduction.36
6.2 Functional and technical safety.36
6.2.1 System characteristics .36
6.2.2 Railway system structure and safety requirements .37
6.2.3 Safety related functional and technical characteristics and overall system safety .37

6.3 General considerations for risk apportionment .38
6.3.1 Introduction.38
6.3.2 Approaches to apportionment of safety targets .38
6.3.3 Use of THRs.40
6.4 Guidance on the concept of SI and the application of SILs .40
6.4.1 Safety integrity.40
6.4.2 Using SI concept in the specification of safety requirements.42
6.4.3 Link between THR and SIL .46
6.4.4 Controlling random failures and systematic faults to achieve SI.46
6.4.5 Use and misuse of SILs .49
6.5 Guidance on fail-safe systems .51
6.5.1 Fail-safe concept .51
6.5.2 Designing fail-safe systems.52
7 Guidance on methods for combining probabilistic and deterministic means for safety
demonstration .54
7.1 Safety demonstration .54
7.1.1 Introduction.54
7.1.2 Detailed guidance on safety demonstration approaches.54
7.1.3 Safety qualification tests.65
7.2 Deterministic methods.65
7.3 Probabilistic methods .65
7.4 Combining deterministic and probabilistic methods.65
7.5 Methods for mechanical and mixed (mechatronic) systems.66
8 Guidance on the risk acceptance principles.67
8.1 Guidance on the application of the risk acceptance principles .67
8.1.1 Application of risk acceptance principles .67
8.1.2 The ALARP principle.68
8.1.3 The GAMAB (GAME) principle.69
8.1.4 Minimum Endogenous Mortality (MEM) safety principle (EN 50126-1, Clause D.3) .70
9 Guidance on the essentials for documented evidence or proof of safety (Safety case) .71
9.1 Introduction.71
9.2 Safety case purpose.72
9.3 Safety case scope .72
9.4 Safety case levels .72
9.5 Safety case phases .74
9.6 Safety case structure.75
9.7 Safety assessment .78
9.7.1 The scope of the safety assessor .78
9.7.2 The independence of a safety assessor .78
9.7.3 Competence of the safety assessor.79
9.8 Interfacing with existing systems.79
9.8.1 Systems developed according to the EN 50126-1 process .79
9.8.2 System proven in use.79
9.8.3 Unproven systems.80

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9.9 Criteria for cross acceptance of systems .80
9.9.1 The basic premise.80
9.9.2 The framework .81
Annex A (informative) Steps of risk assessment process.82
A.1 System definition .82
A.2 Hazard identification.83
A.2.1 Empirical hazard identification .83
A.2.2 Creative hazard identification.83
A.2.3 Foreseeable accident identification.83
A.2.4 Hazards .84
A.3 Hazard log .86
A.4 Consequence analysis .87
A.5 Hazard control .87
A.6 Risk ranking.88
A.6.1 Qualitative ranking.89
A.6.2 Semi-quantitative ranking approach.89
Annex B (informative) Railway system level HAZARDs - Check lists .92
B.1 General.92
B.2 Example of hazard grouping according to affected persons.94
B.2.1 “C-hazards” – Neighbours group.94
B.2.2 “C-hazards” - Passengers group.95
B.2.3 “C-hazards” - Workers group.96
B.3 Example of functional based hazard grouping.96
Annex C (informative) Approaches for classification of risk categories .99
C.1 Functional breakdown approach (a).99
C.2 Installation (constituent) based breakdown approach (b) .99
C.3 Hazard based breakdown approach (c) .100
C.4 Hazard causes based breakdown approach (d) .101
C.5 Breakdown by types of accidents (e) .102
Annex D (informative) An illustrative railway system risk model developed for railways in UK.103
D.1 Building a risk model .103
D.2 Illustrative example of a risk model for UK railways.104
D.2.1 Modelling technology.104
D.2.2 Usage and constraints.105
D.2.3 Model forecasts .105
Annex E (informative) Techniques & methods .108
E.1 General.108
E.2 Rapid ranking analysis .109
E.3 Structured What-if analysis .109
E.4 HAZOP .110
E.5 State transition diagrams.110
E.6 Message Sequence Diagrams .111
E.7 Failure Mode Effects and Criticality Analysis - FMECA .112
E.8 Event tree analysis .112

E.9 Fault tree analysis .113
E.10 Risk graph method .114
E.11 Other analysis techniques .115
E.11.1 Formal methods analysis .115
E.11.2 Markov analysis.115
E.11.3 Petri networks.115
E.11.4 Cause consequence diagrams.115
E.12 Guidance on deterministic and probabilistic methods.115
E.12.1 Deterministic methods and approach.115
E.12.2 Probabilistic methods and approach .116
E.13 Selection of tools & methods.117
Annex F (informative) Diagramatic illustration of availability concept .119
Annex G (informative) Examples of setting risk acceptance criteria .120
G.1 Example of ALARP application .120
G.2 Copenhagen Metro.123
Annex H (informative) Examples of safety case outlines .124
H.1 Rolling stock .124
H.2 Signalling .126
H.3 Infrastructure .128
Bibliography.131

Figures
Figure 1 – Nested systems and hierarchy.18
Figure 2 – Definition of hazards with respect to a system boundary and likely accident.20
Figure 3 – Sequence of occurrence of accident, hazard and cause.21
Figure 4 – Risk assessment flow chart.25
Figure 5 – Hazard control flow chart .26
Figure 6 – Safety allocation process .39
Figure 7 – Factors influencing SI.41
Figure 8 – Process for defining a code of practice for the control of random failures.48
Figure 9 – Process for defining a code of practise for the control of systematic faults.49
Figure 10 – Differential risk aversion.71
Figure 11 – Safety case levels .73
Figure A.1 – Risk ranking for events with potential for significantly different outcomes .91
Figure D.1 – Illustrative annual safety forecasts generated by an integrated risk model .106
Figure D.2 – Illustrative individual risk forecasts generated by an integrated risk model .107
Figure E.1 – State transition diagram – Example.111
Figure E.2 – Example of message collaboration diagram.111
Figure E.3 – Example of consequence analysis using event tree.113
Figure E.4 – Fault tree analysis – Example.114
Figure F.1 – Availability concept and related terms .119
Figure G.1 – Risk areas and risk reducing measures .121
Figure G.2 – ALARP results of options 1 to 4 .123

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Tables
Table 1 – Cross-reference between certain life cycle phase activities and clauses of the report.10
Table 2 – Clauses of the report covering scope issues .10
Table 3 – Comparison of terms (duty holders).13
Table 4 – Structured approach to allocation of SI (refer to 6.4.2.2) .43
Table 5 – THR/SIL relationship .46
Table 6 – Possible states of a fail safe system .53
Table 7 – Approaches for system safety demonstration .56
Table 8 – Criteria for each of the risk acceptance principles .67
Table 9 – List of EN 50129 clauses and their applicability for documented evidence to systems other
than signalling .75
Table A.1 – Example of frequency ranking scheme.89
Table A.2 – Example of consequence ranking scheme .90
Table A.3 – Risk ranking matrix.90
Table B.1 – Railway neighbour “c-hazards” .94
Table B.2 – List railway passenger “c-hazards” .95
Table B.3 – List of railway worker “c-hazards” .96
Table B.4 – System level hazard list based on functional approach.97
Table D.1 – Sample parametric data for a risk forecasting model .105
Table E.1 – Failure and hazard analysis methods .108
Table E.2 – Example of a hazard-ranking matrix .109
Table E.3 – Hazop guide words .110
Table G.1 – Upper and lower ALARP limits .123

Introduction
EN 50126-1 was developed in CENELEC under a mode 4 co-operation with CEN and is now regularly called
up in specifications. In essence, it lists factors that influence RAMS and adopts a broad risk-management
approach to safety. The standard also gives examples of some risk acceptance principles and defines a
comprehensive set of tasks for the different phases of a generic life cycle for a total rail system.
Use of EN 50126-1 has enhanced the general understanding of the issues involved in dealing with safety
and in achieving RAMS characteristics within the railway field. However, a number of issues have arisen that
suggest that there are differences in the way that safety principles and/or requirements of this standard are
being interpreted and/or applied to a railway system and its sub-systems.
Therefore, the guidelines included are to remove such differences and to enable a coherent and pragmatic
approach, within Europe, for setting safety targets, assessing risks and generally dealing with safety issues.
The report is not intended to set any specific safety targets (which will remain the responsibility of the
relevant regulatory authorities) but only to provide guidance on different methods that can be used for setting
targets, assessing risks, deriving safety requirements, demonstrating satisfactory safety levels, etc., with
examples, where appropriate. The responsibility for accepting the methods to be used and for setting targets
remains with the Railway Authority (RA) in conjunction with the Safety Regulatory Authority (SRA).
Furthermore the introduction of the proposed safety directive (European Directive on the development of
safety on the Community’s railways through development of common safety targets and common safety
methods) should lead to a common safety regulatory regime within Europe. Such a regime will require that
there is a common European approach to the methods for setting safety targets and for assessing risks.
The Technical Report is intended to cover the full spectrum of railway systems and for use by all the different
user groups of the standard EN 50126-1. User groups may be part of any of the different players
(bodies/entities) involved during the life cycle phases of a system, from its conception to disposal.
However, this Technical Report deals with only those items covered by the standard EN 50126-1 that are
identified by the scope of work and with clarification of areas where EN 50126-1 could be misinterpreted.
Clauses in the report are structured to cover clarifications of definitions and concepts and then to reflect the
items in the scope and in order of the risk assessment process. But the contents are limited to include
guidance and explanations for only those items that were remitted by resolution 26/5 of TC 9X and any
related issues.
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1 Scope
1.1 This Technical Report provides guidance on specific issues, listed under 1.3 below, for applying the
safety process requirements in EN 50126-1 to a railway system and for dealing with the safety activities
during the different system life cycle phases. The guidance is applicable to all systems covered within the
scope of EN 50126-1. It assumes that the users of the report are familiar with safety matters but need
guidance on the application of EN 50126-1 for safety issues that are not or could not be addressed in the
standard in detail.
1.2 EN 50126-1 is the top-level basic RAMS standard. This application guide, CLC/TR 50126-2 forms an
informative part of EN 50126-1 dealing explicitly with safety aspects as limited by the scope defined in 1.3
below.
1.3 Limitation of scope
The scope is limited to providing guidance only for the following issues related to EN 50126-1.
i) Production of a top-level generic risk model for the railway system down to its major constituents (e.g.,
signalling, rolling stock, infrastructure, etc.) with definition of the constituents of the model and their
interactions.
ii) Development of a checklist of common functional hazards within a conventional railway system
(including high speed lines, Light Rail Train’s, metro’s, etc.).
iii) Guidance on the application of the risk acceptance principles in EN 50126-1.
iv) Guidance on the application of functional safety in railway systems and qualitative assessment of
tolerable risk with examples.
v) Guidance for specifying relevant functional safety requirements and apportionment of safety targets to
the requirements for sub-systems (e.g. for rolling stock: door systems, brake systems, etc.).
vi) Guidance on the application of safety integrity level concept, through all the life cycle phases of the
system.
vii) Guidance on methods for combining probabilistic and deterministic means for safety demonstration.
viii) Guidance on the essentials (incl. maintenance, operation, etc.) for documented evidence or proof of
safety (safety case) with proposals for a common structure for such documentation.
1.4 A diagrammatic representation of the scope and limitations of the scope cross linking with the safety
activities within the life cycle phases of EN 50126-1 and the roles/responsibilities of the principal players is
given in Table 1 below. However, for full comprehension it is suggested that these clauses are considered
only after the whole document has been read:

Table 1 – Cross-reference between certain life cycle phase activities and clauses of the report
Lifecycle phase of EN 50126-1 Bodies/Entities involved Relevant clause
1. CONCEPT Not in the scope
2. SYSTEM DEFINITION AND APPLICATION Generally, Railway Authority (RA) for 4.3, 5.3.2.1
CONDITIONS railway system level, Railway
Support Industry (RSI) for lower
system levels.
3. RISK ANALYSIS RA or RSI, depending on the life 4.4, 5.3, 5.4
cycle phase.
4. SYSTEM REQUIREMENTS Generally, RA for railway system 5.3.2.1, 6.2
level. RSI for lower system levels.
5. APPORTIONMENT OF SYSTEM REQUIREMENTS Body/entity responsible for the 5.4.6, 6.2, 6.3, 8
design of the system under
consideration.
6. DESIGN AND IMPLEMENTATION RSI 4.3, 5.4, 6
7. MANUFACTURING Not in the scope
8. INSTALLATION Not in the scope
9. SYSTEM VALIDATION (INCLUDING SAFETY SRA and RSI 7.1, 9
ACCEPTANCE AND COMMISSIONING)
10. SYSTEM ACCEPTANCE RA and SRA 7.1, 9
11. OPERATION AND MAINTENANCE RA 5.4.6, 9.5
12. PERFORMANCE MONITORING Not in the scope
13. MODIFICATION AND RETROFIT RA, SRA and RSI as relevant Part of 9.8
14. DECOMMISSIONING AND DISPOSAL Not in the scope

1.5 This Technical Report is structured generally to reflect the order of the safety process. However, the
issues within the scope of the report, as listed under 1.3 above, are covered in the clauses as tabulated
below.
Table 2 – Clauses of the report covering scope issues
Clause 1 Scope.
Clause 2 References.
Clause 3 Interpretations and explanations of the definitions in EN 50126-1 and definition of
additional terms and abbreviations used in the report.
Clause 4 Provides guidance on system hierarchy, on bodies/entities involved and their
responsibilities and on safety concepts implicit in the safety process as covered by the
scope.
Clause 5 Items i) and ii) of the scope.
Clause 6 Items iv), v) and vi) of the scope.
Clause 7 Item vii) of the scope.
Clause 8 Item iii) of the scope.
Clause 9 Item viii) of the scope.

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2 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.
EN 50126-1:1999 Railway applications – The specification and demonstration of Reliability,
Availability, Maintainability and Safety (RAMS) – Part 1: Basic requirements
and generic process
CLC/TR 50126-3:2006 Railway applications – The specification and demonstration of Reliability,
Availability, Maintainability and Safety (RAMS) – Part 3: Guide to the
application of EN 50126-1 for rolling stock RAM
EN 50128:2001 Railway applications – Communication, signalling and processing systems –
Software for railway control and protection systems
EN 50129:2003 Railway applications – Communication, signalling and processing systems –
Safety related electronic systems for signalling
1)
CLC/TR 50506 series Railway applications – Communication, signalling and processing systems –
Application Guide for EN 50129
EN 60300-3-1:2004 Dependability management – Part 3-1: Application guide – Analysis
techniques for dependability – Guide on methodology (IEC 60300-3-1:2003)
EN 61508:2001 (series) Functional safety of electrical/electronic/programmable electronic safety-
related systems (IEC 61508 series)
EN 61078:1993 Analysis techniques for dependability – Reliability block diagram method
(IEC 61078:1991)
EN 61160 Design review (IEC 61160)
EN 61703 Mathematical expressions for reliability, availability, maintainability and
maintenance support terms (IEC 61703)
IEC 60050-191 International Electrotechnical Vocabulary – Chapter 191: Dependability and
quality of service
IEC 60300-3-9:1995 Dependability management – Part 3: Application guide – Section 9: Risk
analysis of technological systems
IEC 60812:1985 Analysis techniques for system reliability – Procedure for failure mode and
effects analysis (FMEA)
IEC 61025:1990 Fault tree analysis (FTA)
IEC 61165:1995 Application of Markov techniques
IEC 61882:2001 Hazard and operability studies (HAZOP studies) – Application guide
ISO/IEC Guide 51:1999 Safety aspects – Guidelines for their inclusion in standards

At draft stage.
3 Definitions and abbreviations
The definitions in EN 50126-1 are a necessary prerequisite for the correct understanding and application of
the standard. User experience has shown however, that in some cases definitions in the standard can be
interpreted in more than one way. In other cases, the definitions differ from those used in other safety related
standards, e.g. EN 50128, EN 50129 or EN 61508.
Furthermore, user feedback suggests that some translated definitions of EN 50126-1 (in a language other
than English), are not sufficiently accurate with the consequence that misinterpretations have occurred.
Consequently some clarification of the terms and definitions used in EN 50126-1 is included in this report to
ensure a coherent interpretation of these terms.
Some additional safety terms used in the report have also been defined. Use of these terms in the report is
to further ensure a coherent interpretation of certain safety management concepts of EN 50126-1 and to
enhance their understanding.
3.1 Guidance on the interpretation of terms and definitions used in EN 50126-1
The following paragraphs provide clarifications to the definitions in EN 50126-1. The respective clause
numbers of EN 50126-1 are shown in brackets.
3.1.1
apportionment (3.1)
EN 50126-1 defines apportionment as:
a process whereby the RAMS elements for a system are sub-divided between the various items which
comprise the system to provide individual targets.
In this definition the term “RAMS elements” can usually be interpreted as “targets” or “requirements” for
Reliability, Availability, Maintainability and Safety. The overall RAMS targets (e.g. risk acceptance criteria)
has to be apportioned to the individual system elements in order to enable these elements to be constructed
in a way that allows the overall target to be achieved
3.1.2
availability (3.4)
In EN 50126-1 this term is defined as:
The ability of a product to be in a state to perform a required function under given conditions at a given
instant of time or over a given time interval assuming that the required external resources are provided.
Availability is related to failed states/failure-modes (see Figure 3 of EN 50126-1) of functions that the system
is supposed to provide. Considering only the subset of safety-related failure modes the direct influence of
safety on availability becomes obvious.
NOTE  Terms contributing to the definition of availability are sometimes used incorrectly. Figure F.1 (Annex F) illustrates the concept of
availability and clarifies the correct use of contributory terms.
Prior to the determination of the availability the system boundaries have to be defined to be able to decide
whether external resources (e.g. the supplied power) are part of the system
3.1.3
failure rate (3.14)
The definition used in EN 50126-1 is abstract, formulated in mathematical language as:
the limit, if this exists, of the ratio of the conditional probability that the instant of time, T, of a failure of a
product falls within a given time interval (t, t+Δt) and the length of this interval, Δt, when Δt tends towards
zero, given that the item is in an up state at the start of the time interval.
&
R(t) − R(t + Δt) R(t)
λ()t = lim = −
Δt→0
Δt ⋅ R(t) R(t)
R(t) means the reliability function
For better understanding of this definition, the following might be useful:
The product of the failure rate (at a certain time t in the components live) and the following very small interval
(∆t →0) of time λ(t) ∆t describes the conditional probability that an item which has survi
...