IEC 61508-6
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 6: Guidelines on the application of IEC 61508-2 and IEC 61508-3
Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 6: Lignes directrices pour l'application de la CEI 61508-2 et de la
CEI 61508-3
IEC 61508-6:2010
®
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IEC 61508-6
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 6: Guidelines on the application of IEC 61508-2 and IEC 61508-3
Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 6: Lignes directrices pour l'application de la CEI 61508-2 et de la
CEI 61508-3
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
XE
ICS 25.040.40
PRICE CODE
CODE PRIX
ISBN 978-2-88910-529-8
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
®
– 2 – 61508-6 © IEC:2010
CONTENTS
FOREWORD...........................................................................................................................6
INTRODUCTION.....................................................................................................................8
1 Scope.............................................................................................................................10
2 Normative references .....................................................................................................12
3 Definitions and abbreviations..........................................................................................12
Annex A (informative) Application of IEC 61508-2 and of IEC 61508-3.................................13
Annex B (informative) Example of technique for evaluating probabilities of hardware
failure ...................................................................................................................................
21
Annex C (informative) Calculation of diagnostic coverage and safe failure fraction –
worked example....................................................................................................................
76
Annex D (informative) A methodology for quantifying the effect of hardware-related
common cause failures in E/E/PE systems............................................................................
80
Annex E (informative) Example applications of software safety integrity tables of
IEC 61508-3 .........................................................................................................................
95
Bibliography........................................................................................................................110
Figure 1 – Overall framework of the IEC 61508 series ..........................................................
11
Figure A.1 – Application of IEC 61508-2 ...............................................................................17
Figure A.2 – Application of IEC 61508-2 (Figure A.1 continued)............................................18
Figure A.3 – Application of IEC 61508-3 ...............................................................................20
Figure B.1 – Reliability Block Diagram of a whole safety loop ...............................................22
Figure B.2 – Example configuration for two sensor channels.................................................26
Figure B.3 – Subsystem structure .........................................................................................29
Figure B.4 – 1oo1 physical block diagram .............................................................................30
Figure B.5 – 1oo1 reliability block diagram............................................................................31
Figure B.6 – 1oo2 physical block diagram .............................................................................32
Figure B.7 – 1oo2 reliability block diagram............................................................................32
Figure B.8 – 2oo2 physical block diagram .............................................................................33
Figure B.9 – 2oo2 reliability block diagram............................................................................33
Figure B.10 – 1oo2D physical block diagram.........................................................................33
Figure B.11 – 1oo2D reliability block diagram .......................................................................34
Figure B.12 – 2oo3 physical block diagram ...........................................................................34
Figure B.13 – 2oo3 reliability block diagram..........................................................................35
Figure B.14 – Architecture of an example for low demand mode of operation........................40
Figure B.15 – Architecture of an example for high demand or continuous mode of
operation ..............................................................................................................................
49
Figure B.16 – Reliability block diagram of a simple whole loop with sensors organised
into 2oo3 logic ......................................................................................................................
51
Figure B.17 – Simple fault tree equivalent to the reliability block diagram presented on
Figure B.1.............................................................................................................................
52
Figure B.18 – Equivalence fault tree / reliability block diagram..............................................52
Figure B.19 – Instantaneous unavailability U(t) of single periodically tested
components ..........................................................................................................................
54
Figure B.20 – Principle of PFD
avg
calculations when using fault trees...................................55
61508-6 © IEC:2010 – 3 –
Figure B.21 – Effect of staggering the tests ..........................................................................56
Figure B.22 – Example of complex testing pattern ................................................................56
Figure B.23 – Markov graph modelling the behaviour of a two component system ................58
Figure B.24 – Principle of the multiphase Markovian modelling .............................................59
Figure B.25 – Saw-tooth curve obtained by multiphase Markovian approach.........................60
Figure B.26 – Approximated Markovian model ......................................................................60
Figure B.27 – Impact of failures due to the demand itself......................................................61
Figure B.28 – Modelling of the impact of test duration...........................................................61
Figure B.29 – Multiphase Markovian model with both DD and DU failures .............................62
Figure B.30 – Changing logic (2oo3 to 1oo2) instead of repairing first failure........................63
Figure B.31 – "Reliability" Markov graphs with an absorbing state ........................................63
Figure B.32 – "Availability" Markov graphs without absorbing states .....................................65
Figure B.33 – Petri net for modelling a single periodically tested component.........................66
Figure B.34 – Petri net to model common cause failure and repair resources........................69
Figure B.35 – Using reliability block diagrams to build Petri net and auxiliary Petri net
for PFD and PFH calculations ...............................................................................................70
Figure B.36 – Simple Petri net for a single component with revealed failures and
repairs ..................................................................................................................................71
Figure B.37 – Example of functional and dysfunctional modelling with a formal
language...............................................................................................................................72
Figure B.38 – Uncertainty propagation principle....................................................................73
Figure D.1 – Relationship of common cause failures to the failures of individual
channels ...............................................................................................................................82
Figure D.2 – Implementing shock model with fault trees........................................................93
Table B.1 – Terms and their ranges used in this annex (applies to 1oo1, 1oo2, 2oo2,
1oo2D, 1oo3 and 2oo3) ........................................................................................................
27
Table B.2 – Average probability of failure on demand for a proof test interval of six
months and a mean time to restoration of 8 h .......................................................................36
Table B.3 – Average probability of failure on demand for a proof test interval of one
year and mean time to restoration of 8 h...............................................................................37
Table B.4 – Average probability of failure on demand for a proof test interval of two
years and a mean time to restoration of 8 h ..........................................................................38
Table B.5 – Average probability of failure on demand for a proof test interval of
ten years and a mean time to restoration of 8 h ....................................................................39
Table B.6 – Average probability of failure on demand for the sensor subsystem in the
example for low demand mode of operation (one year proof test interval and
8 h MTTR) ............................................................................................................................
40
Table B.7 – Average probability of failure on demand for the logic subsystem in the
example for low demand mode of operation (one year proof test interval and
8 h MTTR) ............................................................................................................................
41
Table B.8 – Average probability of failure on demand for the final element subsystem
in the example for low demand mode of operation (one year proof test interval and
8 h MTTR) ............................................................................................................................
41
Table B.9 – Example for a non-perfect proof test ..................................................................42
Table B.10 – Average frequency of a dangerous failure (in high demand or continuous
mode of operation) for a proof test interval of one month and a mean time to
restoration of 8 h ..................................................................................................................
45
