oSIST prEN 50126-1:2012

SLOVENSKI STANDARD
oSIST prEN 50126-1:2013
01-januar-2013
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Railway applications - The Specification and Demonstration of Reliability, Availability,
Maintainability and Safety (RAMS) - Part 1: Generic RAMS process
Bahnanwendungen - Spezifikation und Nachweis von Zuverlässigkeit, Verfügbarkeit,
Instandhaltbarkeit und Sicherheit (RAMS) - Teil 1: Generischer RAMS- Prozess
Applications ferroviaires - Spécification et démonstration de la fiabilité, de la disponibilité,
de la maintenabilité et de la sécurité (FDMS) - Partie 1: Processus FDMS générique
Ta slovenski standard je istoveten z: prEN 50126-1:2012
ICS:
29.280 (OHNWULþQDYOHþQDRSUHPD Electric traction equipment
45.020 Železniška tehnika na Railway engineering in
splošno general
oSIST prEN 50126-1:2013 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST prEN 50126-1:2013
oSIST prEN 50126-1:2013
DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2012
EUROPÄISCHE NORM
ICS 45.020 Will supersede EN 50126-1:1999 (partially), CLC/TR 50126-2:2007, CLC/TR 50126-3:2008

English version
Railway applications -
The Specification and Demonstration of Reliability, Availability,
Maintainability and Safety (RAMS) -
Part 1: Generic RAMS process
Applications ferroviaires -  Bahnanwendungen -
Spécification et démonstration de la fiabilité, Spezifikation und Nachweis von
de la disponibilité, de la maintenabilité et de Zuverlässigkeit, Verfügbarkeit,
la sécurité (FDMS) - Instandhaltbarkeit und Sicherheit (RAMS) -
Partie 1: Processus FDMS générique Teil 1: Generischer RAMS- Prozess

This draft European Standard is submitted to CENELEC members for CENELEC enquiry.
Deadline for CENELEC: 2013-03-29.

It has been drawn up by CLC/TC 9X.

If this draft becomes a European Standard, 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.
This draft European Standard was established by CENELEC 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.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they
are aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change
without notice and shall not be referred to as a European Standard.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2012 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Project: 21752 Ref. No. prEN 50126-1:2012 E

oSIST prEN 50126-1:2013
1 Contents Page
2 Foreword . 6
3 In tro d uct i on . 7
4 1 Scope . 9
5 2 Normative references. 10
6 3 Terms and definitions . 10
7 4 A bbr e v i a t i ons . 19
8 5 Process overview . 20
9 5.1 Purpose of overview . 20
10 5.2 Objective . 20
11 5.3 Management responsibility . 20
12 5.4 Adaptability to project scope and size . 20
13 5.5 Interrelation of RAMS management process and life-cycle . 21
14 5.6 Short description of life-cycle phases. 23
15 6 Railway RAMS . 24
16 6.1 Introduction . 24
17 6.2 System-level approach . 24
18 6.2.1 Concepts of system hierarchy . 24
19 6.2.2 A system’s requirements and characteristics . 25
20 6.2.3 Defining a system . 26
21 6.3 Railway system overview . 26
22 6.3.1 Introduction. 26
23 6.3.2 Bodies/entities involved in a railway system . 27
24 6.3.3 Railway system environment and the balance of requirements . 27
25 6.3.4 Railway system structure and apportionment of RAMS requirements . 27
26 6.4 Railway RAMS and quality of service . 28
27 6.5 Elements of railway RAMS . 28
28 6.6 Factors influencing railway RAMS . 29
29 6.6.1 General . 29
30 6.6.2 Classes of failures . 30
31 6.6.3 Derivation of detailed railway specific influencing factors . 30
32 6.6.4 Evaluation of factors . 34
33 6.7 Specification of railway RAMS requirements . 34
34 6.7.1 General . 34
35 6.7.2 RAMS specification . 34
36 6.8 Risk based approach . 35
37 6.9 Risk reduction strategy . 36
38 6.9.1 Introduction. 36
39 6.9.2 Reduction of risks related to safety . 36
40 6.10 Safety integrity . 37
41 6.10.1 Safety integrity concept . 37
42 7 Management of railway RAMS . 38
43 7.1 RAMS process . 38

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44 7.1.1 General . 38
45 7.1.2 Safety management within the RAMS Process . 38
46 7.1.3 Independence of roles . 39
47 7.2 System life-cycle . 39
48 7.3 Application and tailoring of this standard . 46
49 7.4 General requirements on RAMS documentation . 47
50 8 RAMS life-cycle . 48
51 8.1 General . 48
52 8.2 Phase 1: concept . 48
53 8.2.1 Objectives . 48
54 8.2.2 Activities . 48
55 8.2.3 Deliverables . 49
56 8.2.4 Specific verification tasks. 49
57 8.2.5 Specific validation tasks . 49
58 8.3 Phase 2: system definition and operational context . 49
59 8.3.1 Objectives . 49
60 8.3.2 Activities . 49
61 8.3.3 Deliverables . 53
62 8.3.4 Specific verification tasks. 53
63 8.3.5 Specific validation tasks . 53
64 8.4 Phase 3: risk analysis and evaluation . 53
65 8.4.1 Objectives . 53
66 8.4.2 Activities . 54
67 8.4.3 Deliverables . 55
68 8.4.4 Specific verification tasks. 55
69 8.4.5 Specific validation tasks . 55
70 8.5 Phase 4: specification of system requirements . 56
71 8.5.1 Objectives . 56
72 8.5.2 Activities . 56
73 8.5.3 Deliverables . 57
74 8.5.4 Specific verification tasks. 57
75 8.5.5 Specific validation tasks . 57
76 8.6 Phase 5: architecture and apportionment of system requirements. 57
77 8.6.1 Objectives . 57
78 8.6.2 Activities . 58
79 8.6.3 Deliverables . 58
80 8.6.4 Specific verification tasks. 59
81 8.6.5 Specific validation tasks . 59
82 8.7 Phase 6: design and implementation . 59
83 8.7.1 Objectives . 59
84 8.7.2 Activities . 59
85 8.7.3 Deliverables . 60
86 8.7.4 Specific verification tasks. 60
87 8.7.5 Specific validation tasks . 60
88 8.8 Phase 7: manufacture . 60
89 8.8.1 Objectives . 60
90 8.8.2 Activities . 60
91 8.8.3 Deliverables . 61
92 8.8.4 Specific verification tasks. 61

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93 8.8.5 Specific validation tasks . 61
94 8.9 Phase 8: integration. 61
95 8.9.1 Objectives . 61
96 8.9.2 Activities . 61
97 8.9.3 Deliverables . 62
98 8.9.4 Specific verification tasks. 62
99 8.9.5 Specific validation tasks . 62
100 8.10 Phase 9: system validation . 62
101 8.10.1 Objectives . 62
102 8.10.2 Activities . 62
103 8.10.3 Deliverables . 63
104 8.10.4 Specific verification tasks. 63
105 8.10.5 Specific validation tasks . 63
106 8.11 Phase 10: system acceptance . 63
107 8.11.1 Objectives . 63
108 8.11.2 Activities . 63
109 8.11.3 Deliverables . 64
110 8.11.4 Specific verification tasks. 64
111 8.11.5 Specific validation tasks . 64
112 8.12 Phase 11: operation, maintenance and performance monitoring . 64
113 8.12.1 Objectives . 64
114 8.12.2 Activities . 64
115 8.12.3 Deliverables . 65
116 8.12.4 Specific verification tasks. 65
117 8.12.5 Specific validation tasks . 65
118 8.13 Phase 12: decommissioning . 65
119 8.13.1 Objectives . 65
120 8.13.2 Activities . 66
121 8.13.3 Deliverables . 66
122 8.13.4 Specific verification tasks. 66
123 8.13.5 Specific validation tasks . 66
124 9 Risk assessment . 66
125 9.1 Scope . 66
126 9.2 Risk analysis methodology . 66
127 9.3 Risk evaluation and acceptance . 69
128 9.3.1 Acceptance principles . 69
129 9.3.2 Methods for determining risk acceptance criteria . 69
130 10 Deliverables and structure of a safety case . 70
131 10.1 Purpose of a safety case . 70
132 10.2 Types of safety case . 70
133 10.3 Safety case structure . 71
134 10.3.1 General . 71
135 10.3.2 Definition of system . 72
136 10.3.3 Quality management report . 73
137 10.3.4 Safety management report . 73
138 10.3.5 Technical safety report . 74
139 10.3.6 Related safety cases . 75
140 10.3.7 Co nc lus i o n . 75
141 10.3.8 References . 75

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142 Annex A (informative) RAMS plan . 76
143 Annex B (informative) Examples of parameters for railway . 81
144 B.1 Reliability parameters . 81
145 B.2 Maintainability parameters . 81
146 B.3 Availability parameters . 82
147 B.4 Logistic support parameters . 83
148 B.5 Safety parameters . 84
149 Annex C (informative) Hazards at railway system level – example structured list. 85
150 C.1 Example list . 86
151 Annex D (informative) Risk matrix calibration and risk acceptance categories . 91
152 D.1 Frequency of occurrence levels . 91
153 D.2 Severity levels . 92
154 D.3 Risk acceptance categories . 93
155 Bibliograph y . 95
157 Figure 1 – Interrelation of RAMS-management process and system life-cycle . 22
158 Figure 2 − Illustration of system hierarchy . 25
159 Figure 3 − Example of deriving cause/effect relations in a diagrammatic approach . 32
160 Figure 4 − Relationship of cause, hazard and accident . 36
161 Figure 5 − The "V" representation drawing . 41
162 Figure 6 – Verification . 42
163 Figure 7 – Validation . 42
164 Figure 8 − Structure of a safety case . 72
165 Figure B.1 – Availability concept and related terms . 83
167 Table 1 – System phase related tasks (informative) . 43
168 Table 2 – Frequency – Severity matrix . 69
169 Table A.1 – Example of a basic RAMS plan outline . 77
170 Table B.1 – Examples of reliability parameters . 81
171 Table B.2 – Examples of maintainability parameters . 81
172 Table B.3 – Examples of availability parameters . 82
173 Table B.4 – Examples of logistic support parameters . 83
174 Table B.5 – Examples of safety performance parameters . 84
175 Table C.1 – Example for a hazard list . 87
176 Table C.2 – Example scenarios . 89
177 Table D.1 – Frequency of occurrence of events with examples for quantification (time based) . 91
178 Table D.2 – Frequency of occurrence of events with examples for quantification (distance based) . 92
179 Table D.3 – Severity categories (example related to RAM) . 92
180 Table D.4 – Severity categories (example 1 related to RAMS) . 93
181 Table D.5 – Categories (example 2 related to Safety) . 93
182 Table D.6 – Financial severity levels (example) . 93
183 Table D.7 – Risk acceptance categories (example 1 for binary decisions) . 93
184 Table D.8 – Risk acceptance categories (example 2) . 94
185 Table D.9 – Risk acceptance categories (example related to safety) . 94
oSIST prEN 50126-1:2013
187 Foreword
188 This document [prEN 50126-1:2012] has been prepared by CLC/TC 9X "Electrical and electronic
189 applications for railways".
190 This document is currently submitted to the Enquiry.
191 EN 50126 "Railway applications – The specification and demonstration of Reliability, Availability,
192 Maintainability and Safety (RAMS)" consists of the following parts:
193 – Part 1: Generic RAMS process;
194 – Part 2: Systems approach to safety;
195 – Part 4: Functional safety – Electrical/Electronic/Programmable electronic systems;
196 – Part 5: Functional safety – Software.
197 This new edition of EN 50126 (all parts) will supersede EN 50126-1:1999,
198 CLC/TR 50126-2:2007, CLC/TR 50126-3:2008, EN 50128:2011 and EN 50129:2003.
199 This part of EN 50126 covers the RAMS process. It is mainly based on EN 50126-1:1999.
200 This part of EN 50126 will supersede EN 50126-1:1999 (together with prEN 50126-2:2012),
201 CLC/TR 50126-2:2007 and CLC/TR 50126-3:2008.
202 This document has been prepared under a mandate given to CENELEC by the European
203 Commission and the European Free Trade Association, and supports essential requirements of
204 EU Directive(s).
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205 Introduction
206 EN 50126-1:1999 was produced to introduce the application of a systematic RAMS management
207 process in the railway sector. For safety-related electronic systems for signalling, EN 50128 and
208 EN 50129 were produced. Through the application of these standards and the experiences
209 gained over the last years, the need for revision and restructuring became apparent with a need
210 to deliver a systematic and coherent approach to RAMS applicable to all the railway application
211 fields Signalling, Rolling Stock and Electric power supply for Railways (Fixed Installations).
212 The revision work improved the coherency and consistency of the standards, the concept of
213 safety management and the practical usage of EN 50126 and took into consideration the existing
214 and related Technical Reports as well.
215 This European Standard provides railway duty holders and the railway suppliers, throughout the
216 European Union, with a process which will enable the implementation of a consistent approach to
217 the management of reliability, availability, maintainability and safety, denoted by the acronym
218 RAMS.
219 Processes for the specification and demonstration of RAMS requirements are cornerstones of
220 this standard. This European Standard promotes a common understanding and approach to the
221 management of RAMS.
222 EN 50126 is the railway sector specific application of IEC 61508. Meeting the requirements in
223 this European Standard is sufficient to ensure that additional compliance to IEC 61508 does not
224 need to be demonstrated.
225 With regard to safety EN 50126-1 provides a Safety Management Process which is supported by
226 guidance and methods described in EN 50126-2.
227 EN 50126-1 and EN 50126-2 are independent from the technology used. EN 50126-4 and
228 EN 50126-5 provide guidance specific to safety-related E/E/PE technology of railway
229 applications. Their application is determined through the application of the general RAMS
230 process of EN 50126-1 and through the outcome of the safety-related methods described in
231 EN 50126-2. As far as safety is concerned, EN 50126 takes the perspective of functional safety.
232 This does not exclude other aspects of safety. However, these are not the focus.
233 The aims set for revision of EN 50126 required a better understanding of the systems approach
234 and improved methods for applying the safety management process described in EN 50126-1.
235 EN 50126-2 provides this guidance.
236 The application of this standard should be adapted to the specific requirements of the system
237 under consideration.
238 This European Standard can be applied systematically by the railway duty holders and railway
239 suppliers, throughout all phases of the life-cycle of a railway application, to develop railway
240 specific RAMS requirements and to achieve compliance with these requirements. The systems-
241 level approach developed by this European Standard facilitates assessment of the RAMS
242 interactions between elements of railway applications even if they are of complex nature.
243 This European Standard promotes co-operation between the stakeholders of Railways in the
244 achievement of an optimal combination of RAMS and cost for railway applications. Adoption of
245 this European Standard will support the principles of the European Single Market and facilitate
246 European railway inter-operability.
247 The process defined by this European Standard assumes that railway duty holders and railway
248 suppliers have business-level policies addressing Quality, Performance and Safety. The
249 approach defined in this standard is consistent with the application of quality management
250 requirements contained within the ISO 9001.

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1)
251 In accordance with CENELEC editing rules , mandatory requirements in this standard are
252 indicated with the modal verb “shall”. Where justifiable, the standard permits process tailoring.
253 Specific guidance on the application of this standard in the case of process tailoring is provided
254 in 7.3 of EN 50126-1. EN 50126-2 provides various methods for use in the safety management
255 process. Where a particular method is selected for the system under consideration, the
256 mandatory requirements of this method are by consequence mandatory for the safety
257 management of the system under consideration.
—————————
1) CENELEC “Internal Regulations Part 3: Rules for the structure and drafting of CEN/CENELEC Publications (2009-
08), Annex H
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258 1 Scope
259 This part 1 of EN 50126
260  considers RAMS, understood as reliability, availability, maintainability and safety and their
261 interaction;
262  considers the generic aspects of the RAMS life-cycle. The guidance in this part is still
263 applicable in the application of specific standards;
264  defines
265 – a process, based on the system life-cycle and tasks within it, for managing RAMS;
266 – a systematic process, tailorable to the type and size of system under consideration, for
267 specifying requirements for RAMS and demonstrating that these requirements are
268 achieved;
269  addresses railway specifics;
270  enables conflicts between RAMS elements to be controlled and managed effectively;
271  does not define
272 – RAMS targets, quantities, requirements or solutions for specific railway applications;
273 – rules or processes pertaining to the certification of railway products against the
274 requirements of this standard;
275 – an approval process by the safety authority;
276  does not specify requirements for ensuring system security.
277 This part 1 of EN 50126 is applicable
278  to the specification and demonstration of RAMS for all railway applications and at all levels
279 of such an application, as appropriate, from complete railway systems to major systems and
280 to individual and combined sub-systems and components within these major systems,
281 including those containing software; in particular:
282 – to new systems;
283 – to new systems integrated into existing systems in operation prior to the creation of this
284 standard, although it is not generally applicable to other aspects of the existing system;
285 – to modifications of existing systems in operation prior to the creation of this standard,
286 although it is not generally applicable to other aspects of the existing system;
287  at all relevant phases of the life-cycle of an application;
288  for use by railway duty holders and the railway suppliers.
289 It is not required to apply this standard to existing systems including those systems already
290 compliant with any version of former EN 50126, EN 50128 or EN 50129, which remain
291 unmodified. Railway applications mean Command, Control & Signalling, Rolling Stock and
292 Electric Power Supply for Railways (Fixed Installations).
293 In this standard the term hardware refers to E/E/PE components or systems. If non E/E/PE
294 hardware is meant, this is specifically mentioned.

oSIST prEN 50126-1:2013
295 2 Normative references
296 The following documents, in whole or in part, are normatively referenced in this document and
297 are indispensable for its application. For dated references, only the edition cited applies. For
298 undated references, the latest edition of the referenced document (including any amendments)
299 applies.
300 ISO 9001, Quality management systems – Requirements
301 ISO/IEC GUIDE 51, Safety aspects – Guidelines for their inclusion in standards
302 3 Terms and definitions
303 For the purposes of this document, the following terms and definitions apply.
304 3.1
305 acceptance
306 the status achieved by a product, system or process once it has been agreed that it is suitable
307 for its intended purpose
308 3.2
309 accident
310 an unintended event or series of events resulting in loss of human health or life, damage to
311 property or environmental damage
312 Note 1 to entry: The term includes losses from accidents arising within a short time scale (e.g. collision, explosion)
313 and also those incurred over the long-term (e.g. release of a toxic substance).
314 3.3
315 application conditions
316 those conditions which need to be met in order for a system to be safely integrated and safely
317 operated.
318 Note 1 to entry: Application conditions can for example be: operational restrictions (e.g. speed limit, maximum
319 duration of use) operational rules, maintenance restrictions (e.g. requested maintenance intervals) or environmental
320 conditions.
321 3.4
322 ap p r o v al
323 the legal act, often focused on safety, to allow a product, system or process to be placed into
324 service
325 Note 1 to entry: A legal act can be performed by an authorised entity (i.e. a NOBO)
326 3.5
327 assessment
328 process to form a judgement on whether a product, system or process meets the specified
329 requirements, based on evidence
330 Note 1 to entry: Independence of assessment is only necessary where explicitly specified.
331 3.6
332 assessor
333 an entity that carries out an assessment
334 Note 1 to entry: Independence of the assessor is only necessary where explicitly specified.
335 3.7
336 assurance
337 confidence in achieving a goal being pursued. Declaration intended to give confidence
338 3.8
339 audit
340 a documented, systematic and independent examination to determine whether the procedures
341 specific to the requirements
342 • comply with the planned arrangements,
343 • are implemented effectively and
344 • are suitable to achieve the specified objectives

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345 3.9
346 availability
347 the ability of a product to be in a state to perform a required function under given conditions at a
348 given instant of time or over a given time interval assuming that the required external resources
349 are provided
350 Note 1 to entry: Figure B.1 (Annex B) illustrates the concept of availability and clarifies the correct use of contributory
351 terms.
352 3.10
353 collective risk
354 a risk, resulting from e.g. a product, process or system, to which a population or group of people
355 is exposed
356 Note 1 to entry: Collective risk is not to be confused with multiple victim accidents.
357 Note 2 to entry: Collective risk is the sum of the individual risks to those individuals in the population or group.
358 However, the collective risk divided by the number of individuals will only provide the average individual risk.
359 Note 3 to entry: A group of people could be, for example, rail staff working in a restaurant car or all passengers using
360 a particular network.
361 3.11
362 commercial off-the-shelf software
363 software defined by market-driven need, commercially available and whose fitness for purpose
364 has been deemed acceptable by a broad spectrum of commercial users
365 3.12
366 common cause failure
367 failures of different items resulting from the same cause and where these failures are not
368 consequences of each other
369 3.13
370 compliance
371 a state where a characteristic or property of a product, system or process satisfies the specified
372 requirements
373 3.14
374 configuration management
375 a discipline applying technical and administrative direction and surveillance to identify and
376 document the functional and physical characteristics of a configuration item, to control changes
377 to those characteristics, to record and report change processing and implementation status and
378 to verify compliance with specified requirements
379 3.15
380 consequence analysis
381 to analyze the consequences of each hazard up to accidents and losses
382 3.16
383 corrective maintenance
384 the maintenance carried out after fault recognition and intended to put a product into a state in
385 which it can perform a required function
386 3.17
387 data-driven software
388 software configured by data and/or algorithms for producing the executable software for an
389 application by making use of an existing generic software
390 3.18
391 de s i g n e r
392 an entity that analyses and transforms specified requirements into acceptable design solutions
393 which have the required safety integrity
394 3.19
395 d e te rm in is t i c
396 expresses that a behaviour can be predicted with certainty
397 Note 1 to entry: A deterministic event in a system can be predicted with certainty from preceding events which are
398 either known or are the same as for a proven equivalent system.
399 3.20
400 diversity
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401 a means of achieving all or part of the specified requirements in more than one independent and
402 dissimilar manner
403 Note 1 to entry: Diversity may be achieved by different physical methods or different design approaches.
404 3.21
405 entity
406 a person, group or organisation who fulfil a role as defined in this standard
407 3.22
408 equivalent fatality
409 an expression of fatalities and weighted injuries and a convention for combining injuries and
410 fatalities into one figure for ease of evaluation and comparison of risks
411 3.23
412 error
413 a discrepancy between a computed, observed or measured value or condition and the true,
414 specified or theoretically correct value or condition
415 Note 1 to entry: An error can be caused by a faulty item, e.g. a computing error made by faulty computer equipment.
416 Note 2 to entry: A human error can be seen as a human action or inaction that can produce an unintended result.
417 3.24
418 fail-safe
419 a concept which is incorporated into the design of a product such that, in the event of a failure, it
420 enters or remains in a safe state
421 3.25
422 failure
423 the termination of the ability of an item to perform a required function
424 Note 1 to entry: After failure the item has a fault.
425 Note 2 to entry: "Failure" is an event, as distinguished from "fault", which is a state.
426 3.26
427 failure mode
428 a predicted or observed manner in which the product, system or process under consideration can
429 fail
430 3.27
431 failure rate
432 the limit, if this exists, of the ratio of the conditional probability that the instant of time, T, of a
433 failure of a product falls within a given time interval (t, t+∆t) and the length of this interval, ∆t,
434 when ∆t tends towards zero, given that the item is in an up state at the start of the time interval
435 Note 1 to entry: Failure rates are often assumed as constant. This is not always valid, e.g. for components subject to
436 wear out (mechanical, pneumatic, electromechanical, etc.).
437 3.28
438 fault
439 the state of an item characterized by inability to perform a required function, excluding the
440 inability during preventive maintenance or other planned actions
441 Note 1 to entry: A fault is often the result of a failure of the item itself, but may exist without prior failure (e.g. in case
442 of a design fault).
443 3.29
444 fault detection time
445 time span which begins at the instant when a failure occurs and ends when the existence of the
446 fault is detected
447 3.30
448 fault tolerance
449 ability of a functional unit to continue to perform a required function in the presence of faults or
450 errors
451 3.31
452 fi rm w a re
453 an ordered set of instructions and associated data stored in a way that is functionally
454 independent of main storage

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455 3.32
456 function
457 a specified action or activity which may be performed by tec
...