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IEC 61508-Functional Safety Overview | Risk | Safety
IEC 61508-Functional Safety OverviewUploaded by Veera RagavanRelated InterestsRiskSafetyTechnologyComputing And Information TechnologyBusinessRating and Stats0.0 (0)Document ActionsDownloadShare or Embed DocumentEmbedDescription: safetyView MoresafetyCopyright: Attribution Non-Commercial (BY-NC)List price: $0.00Download as PDF, TXT or read online from ScribdFlag for inappropriate contentAn Overview of IEC 61508 on E/E/PE Functional SafetyPeter B. Ladkin Causalis Limited and University of Bielefeld
© Peter B. Ladkin 2008
What IEC 61508 is about, how it is standardised, how used
The International Electrotechnical Commission is the organisation which develops and sets international standards in electrotechnical engineering areas. In 1997 the IEC published the standard IEC 61508, Functional safety of electrical/electronic/programmable electronic safety-related systems. The phrase “electrical/electronic/programmable electronic” is cumbersome and is often shortened to E/E/PE, which some pronounce “ee-ee-pee-ee” and others such as myself “eepee”. The standard has been slowly making its way into the safety-critical and safety-related digital systems mainstream over the last decade. The British Health and Safety Executive (HSE) uses it as a measure of whether a safety-critical system has reduced risk “as low as reasonably practicable”, a requirement of English law, otherwise known as ALARP. The terminology ALARP refers back to a judgment by Lord Asquith of Bishopstone in the English case Edwards vs. National Coal Board in 1949 in which Lord Asquith also issued guidance as to the meaning of the phrase “reasonably practicable”. It is intended that there shall be sector-specific norms deriving from IEC 61508, which will take precedence in those sectors for which they exist. So, for example, the European Committee for Electrotechnical Standardisation (Europäisches Komitee für elektrotechnische Normung) CENELEC issues standards for railway automation, such as EN 50128, on software for railway control and monitoring systems which are based on, but which supercede, the requirements of IEC 61508.
Definitions devised by engineers are unfortunately not generally of the same quality as those used by mathematicians, logicians or philosophers. However, they do give some idea of what concepts are being addressed by a standard, and IEC 61508 is no exception.
Harm is physical injury or damage to the health of people either directly, or indirectly as a result of damage to property or to the environment
Harm is the basic notion of what you don't want, and the basis for explaining a notion of safety as, say, absence of harmful events, although, s we shall see, this is not how IEC 61508 does it. There is no notion of accident as a harmful event in IEC 61508, but rather
Hazardous event: a hazardous situation which results in harm
The standard does not explain what is meant by a situation. Given we have a definition of state, it is tempting to identify a situation as a state, since this is how the word is used in normal everyday language. However, for us an event is a pair of states, so an event logically cannot be a state (pairs 1
cannot be identical with their elements according to the usual set-theoretic logic) and it would follow that an event logically cannot be a situation, if situations were to be states. But the definition identifies hazardous event with a certain type of hazardous situation, so it seems as if situations are events here. The standard defines
Hazardous situation: a circumstance in which a person is exposed to hazard(s)
We are not much nearer, though, since the term circumstance is not defined. Let us therefore not set too much store by the word “situation” or “circumstance”, and take a hazardous event to be an event from which harm causally results. The question arises how long or convoluted the causal chain is to be between hazardous event and harm, and what other factors may come into play, and how much. For example, I ride my bicycle from my driveway onto the roadway, and as I do so a car comes around the corner on my side of the road and wipes me out. Riding my bicycle onto the roadway is certainly an event, and it did causally result in harm, by the Counterfactual Test, since if I hadn't done it I wouldn't have been wiped out. But do we really want to call it a hazardous event and make it the center of focus? Surely the event that should be at the center of focus as unwanted is the fact that car and bicycle met in the same physical place at the same time and harm directly resulted from the collision: what we would usually call an accident. And then you can ask how this might have happened causally, and trace precursor events. But we cannot do this with the IEC 61508 vocabulary, which allows us no accidents; only hazardous events, and provides no guidance as to how causally “near” an event must be to harm to count as a “hazardous event”. In fact, the state of the usage is that most engineers of my acquaintance who are conversant with the standard would use the phrase “hazardous event” as equivalent to “accident or a direct precursor of an accident”. So that is probably the best way to go. As we have seen, hazardous events are hazardous situations and these are circumstances in which one is exposed to a hazard, which is
We know what harm is, but not what it is to be a source of harm, let alone a potential source. As the philosopher and logician W.V.O. Quine once asked, exactly how many potential fat men are standing in that empty doorway? There are long-standing difficulties with the coherence of notions of potentialthis-and-thats. Let us look therefore at the notion of source. I guess a source is some kind of origin. One may guess further that what is meant is a causal origin, i.e., a cause. We can put this together with the common usage of the term “hazard” to refer to a state out of which sometimes harm occurs, but not inevitably, and guess that maybe this is what is meant by “potential”: not inevitable. We are familiar from Chapter XXXX with a notion of hazard as some state from which an accident will result if the environment is unfavorable, and our current notion looks to be similar. We can imagine that a hazard might be a state, or an event, which contains a significant set of causal factors of an accident, but which causal set is not necessarily sufficient for an accident to occur, in other words that other causes have to participate. The point of the term “significant” is to indicate something like “a lot”, or “almost all”; that is, that you have a lot of causal factors present in the hazard and it only requires one or two more, maybe under your control but maybe not, to “tip the scales” and result in an accident, that is, harm. That is the core set of concepts in IEC 61508 which talk about what we would call accidents. We have not get seen a definition of safety, and that is because the standard defines safety not through accidents, but through risk. 2
Risk: [the/a] combination of the probability of occurrence of harm and the severity of that harm
We have seen something like this before. The definition does not say how the probability and severity are “combined” but we have already discussed various proposals in technical detail and can imagine that something along those lines is meant here. Now, here comes a novelty of the IEC 61508 standard:
Tolerable risk: risk which is acceptable in a given context, based on the current values of society
“Acceptable” means something which is possible to accept, or which is accepted. It is not suggested who does the accepting of the risk. Lawmakers? Politicians? Individuals exposed to harm? Other “stakeholders”? Neither is it suggested how “current values of society” might be determined, or even to what this phrase refers. 5,000+ people die in accidents involving automobiles on the road in Germany every year; 3,000+ in Britain. Is the risk of 1 in 15,000 of dying in any given year on the road in Germany “acceptable”, based on the “current values of society”? The current value of road deaths is indeed 5000+, and people do drive, so it seems as if people in general in some sense do “accept” this risk. But consider the police and state campaigns to control errant driving and reduce accident totals. If 5000+ deaths per year represent the “current values of society” then it seems as if these campaigns are fighting against the “current values of society”. Indeed so, and who would say it wouldn't be a good thing in this case to change the “current values of society” so that 5000+ deaths per year are no longer “acceptable? So we can see there are lots of problems with this definition. But the basic idea is clear. The guidance as to what is an acceptable risk or not comes not from theory, but from social considerations, from asking the society at large. And however one does that, and whatever answer one receives, one works with that answer to determine “tolerable risk”. And then one can define
The concept “unacceptable risk” is not defined, but let us imagine that it means “risk that is not tolerable risk” (examples of such sloppiness should be trivial to correct, but it seems that in the process of deriving international standards such as this it can become almost impossibly hard). So safety is not defined in terms of absence of harm or other unwanted effects, but in terms of the absence of risk of a certain sort. Let us reconsider the 5000+ road-deaths-per-year figure above, and the argument that this is what seems to be acceptable based on German society's current values. It then follows from the definition of safety that German roads are safe. And, mutatis mutandis, British roads and Greek roads and Spanish roads and Indonesian roads and everybody else's roads as well. Because the current levels of road accidents, whatever they may be, are what is - obviously - accepted in those societies. The reader may conclude for hisherself whether this notion of safety makes much sense. But it is what we have to work with when we work to IEC 61508. Let us briefly compare with other definitions of unwanted events, say the definition of aircraft accident in the US Code of Federal Regulations, 49 CFR 830.2. This definition is similar to that in Annex 13 to the Convention on International Civil Aviation, usually known as ICAO Annex 13, which is longer, and 3
or in which the aircraft receives substantial damage. Since that definition has been around. a certain amount of harm (in the 61508 sense) occurs or substantial damage to the aircraft. And according to the definition. although there are commonalities. as happened to an Airbus A340 aircraft from Madrid to Quito. overrun the runway. suppose after a long transatlantic flight you arrive at an airport with a shortish runway and a difficult approach over obstacles. representing certainty (that scratch will occur). and damage the aircraft enough so that it has to be scrapped. and the catastrophic severity of my life being lost. since no harm resulted . You have had an accident according to 49 CFR 830.2. who knows but I hope not. In that specific event. It would suggest that riding a bicycle on the road is similar to using a screwdriver in risk. I may be run over by a car sometime and killed. whatever it may be. used and abused for 300 years and still going strong for financial people. What severity? There are at least two to choose from: the trivial severity of the scratch. Although we succeeded in interpreting the definition of “risk” in IEC 61508 in a manner consistent with other notions of risk.very luckily. Maybe we should combine it with the catastrophe of the deadly accident? But then that would indicate a similarity to. I am supposed to combine that probability 1 with “severity of that harm”. A good set of definitions 4
. So that can't be it either. of riding my bicycle doesn't seem to me at all like the risk of jumping off a tall building onto a concrete sidewalk. high probability “combined” with catastrophic consequence. And I want to say the risk. Ecuador on 11 November. We need something like the modified de Moivre definition to make sense of the notion. comprising the landing gear so that some braking systems no longer function as intended. land hard. jumping off a tall building on to a concrete sidewalk. We leave it to the reader to decide whether this seems to be an appropriate selection of concepts. It is certain that I shall suffer some small degree of harm from using my bicycle regularly. or think we know. but not a “hazardous event” according to IEC 61508. it makes sense to continue to use it. were we to have been ignorant of other notions of risk and have tried to derive our understanding from the IEC 61508 definition directly. 2007. For example. So the probability of occurrence of harm is 1. This latter phenomenon is not covered by IEC 61508. and in which any person suffers death or serious injury. what is meant. this definition thus covers outcomes which may be fundamentally different from those covered by IEC 61508. and you well might on a bicycle. say. 49 CFR 830.2 reads as follows:
Aircraft accident means an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked. The conclusion is that we cannot take the IEC 61508 definition of risk as a good guide to what risk is. And. we were able to do so because we know. then my calculation of risk would lose the significant fact that I stand some chance of being killed. which suggest the harm to which the probability attaches). But you don't die if you stop paying attention to the surroundings when using the screwdriver. Let us briefly see what kind of difficulties we might have had. even if one has to approximate it sometimes. So combining the probability 1 with the trivial severity doesn't seem right. If I combine the probability 1 with the trivial severity (following the indication of the words “that harm”. Once every couple of months I scratch myself on something (that is harm) and that is going to continue happening.
There is here a specific definition of an event which is called an accident.also accounts for missing aircraft.
The control system of the equipment which is under control. This follows from the observation that two specific subsystems are always present. or SIL. The EUCCS is a “system which responds to input signals from the process and/or from an operator and generates output signals causing the EUC to operate in the desired manner. The EUC is not an SRS. A detailed system life-cycle model. Besides this.” The standard says further that “the EUC control system is separate and distinct from the EUC. The standard implicitly assumes that there is no such thing as zero risk . The standard requires the provision of so-called safety functions for the mitigation of risk associated with functions of the system in the cases in which this risk is deemed to be too high 3. ranging from moderately stringent to very stringent integrity 5
. of a required function. This is the subsystem consisting of the equipment providing some or all of the functions for which the system was designed. It is important to understand that all systems are considered by the standard to be control systems. there may be Safety-Related Systems (SRS). sometimes complex.) the EUC Control System (EUCCS). 5. This classification may not be exclusive: the EUCCS may or may not be a SRS. SIL 1 through SIL 4. concepts. namely
the Equipment under Control (EUC). The safety functions are to perform the risk reduction where it is needed 4. A focus on risk reduction. of which there is only one of each).risk cannot be entirely eliminated. (The definition of EUC in IEC 61508 is somewhat unhelpful. A four-way classification of (sub)system types. The notion of functional safety.● ● ● ● ●
defines terms before they are used tries to be precise tries to reduce or eliminate ambiguity tries to limit the number of concepts left undefined tries to be clear to the intended audience as to what is meant
From the discussion above. There are four degrees of SIL. the reader might well conclude that IEC 61508 does not do very well on any of these criteria. Besides the EUC and EUCCS (the first two subsystem types. The notion of Safety Integrity Level. there may be subsystems which are none of these.”
The fundamental concepts of IEC 61508 are: 1.
The Major Concepts of IEC 61508
IEC 61508 bases its approach on a number of fundamental. from initial development through decommissioning 2.
I take such lifecycle models seriously. The task graph associates 16 steps with the safety life cycle. but how it can go dangerously wrong. or to the sunroof. functionally. the hubs. the axles and associated subsystems. or risk being judged as not having followed the standard. Then the engine may cut out on starting. the Safety Requirements Allocation of requirements to subsystems to which they apply. This is Stage 5. And so on. one may then begin to consider to which parts of the system each safety requirement must apply. The safety requirement that the wheels not fall off applies to the wheels themselves. as would a failure of the brakes to work when applied. This is Stage 4 and these constraints form the Overall Safety Requirements. in the sense that they are partial specifications of processes. 6
. The wheels must be attached firmly enough that they cannot come loose. Say we are designing a car. When such a model appears in a standard such as IEC 61508. you have some idea of how the system can go dangerously wrong. The life cycle does not claim to be the only appropriate life cycle. with Hazard and Risk Analysis. They are fairly simple in that they subdivide development and give a precedence ordering to the parts. or the driver must be warned in good time of a problem. Having performed the preliminary hazard analysis. which in some cases in some countries can be a criminal offence. called by some in other standards the Preliminary Hazard Analysis. The safety life cycle is that part of the life cycle of a system during which activities related to assuring the safety of the system take place. But if a wheel falls off at high speed. At this point you can start formulating constraints that it not go dangerously wrong in these identified ways. At this point. The brakes must be so designed that they cannot fail to operate moderately effectively when required. this would be a dangerous failure. Given the Overall Safety Requirements. Not how it can go wrong. It is generally agreed amongst critical-system engineers that at this point it makes sense to start figuring out all the ways in which things can go dangerously wrong. and we may not want this. as far as I know. you have some idea from concept and scope definition what the system should do. The first safety-assurance activity occurs at Stage 3.
The System Life-Cycle
The safety life cycle of a safety-related system is defined in a task flow graph. It does not concern the fuel tank. but this is not a dangerous failure. or the driver must be warned appropriately in advance.requirements (we shall see later exactly what integrity may be). albeit abstractly. Other tasks appear if they are prerequisites for tasks associated with assuring the safety of the system. and thus be audited to ensure that every necessary activity or measure has been undertaken. then anyone adhering to the norm must be able to subdivide hisher development process in exactly the way specified by the Lifecycle model and exhibit it in documentation. but it is important to have precisely one defined such model to which all can adhere. Various features of the life cycle model are worth noting. The point of the safety life cycle flow graph and its tasks is to define a fixed structure within which safety-assurance activities associated with the system may fit. Identifying all these types of dangerous failure is the activity at Stage 3.
I said “verify”. software verification specialists.At this point. and others use the terms exactly opposite to these meanings. and indeed it has been a concern of some that IEC 61508 was heavily influenced by concerns from the process industries rather than. Here. say. which are of two types: those involving E/E/PE components (Stage 9) and those which do not (Stage 10). The system begins at this point to be designed in detail. The standard acknowledges that there may be ways to reduce risk that do not involve the provision of safety functions. and there are safety activities required during operation and maintenance. is called a Safety-Related System (SRS). The further stages. This is Stage 12. for a car this may simply mean “sent out of the factory to a dealer”). Stages 6-8 concern the overall planning. and the wheel attachments and brakes have to cope). And so on. in our car example. How you assure that the wheel nuts will not work loose between inspections. say. and so these modifications need to be incorporated). Some have it one way. How you assure that the brake lines are sufficiently robust to resist rupture between the inspection intervals. say. that the standard assures safety through provision of safety functions in (sub)systems contributing to an activity which is insufficiently safe (whatever this might be. in Stage 13. Stages 9-11 form the nub of the safety-design activities. as we shall see. say. in the Spiral Model of system development. which corresponds rather better to actual practice it will be revisited many times during system development). Stage 8 concerns how the system is to be installed and commissioned. Then the system must be checked to verify that all the required safety attributes have been identified and appropriately handled during building and installation. operated and maintained. analysis and implementation of these SRSs. I don't propose to resolve this issue of meaning here.) Then the system may be put into operation. So be it. the motor needed to be more powerful. we have yet to discuss how this is determined). This is then Stage 15. to address the competitor's new model. and how will the various continuing safety requirements be assured during installation. the roads are more bumpy and slippery than they were at design time. Stages 12-16 are performed when the system has been built. by restricting the speed at which the car may travel. when the 7
. The standard does not have anything further to say about Stages 10 and 11. operation and maintenance of the system? Are the wheel nuts to be installed with a locking pin? How often does the tightness of the wheel nuts need to be checked? Whether there is metal fatigue around the holes for the locking pins? How often the brake lines need to be checked for fluid leaks. It must be installed and commissioned (if it is a processing plant. Stage 6: how will the system be installed. It is also foreseen that the system will be modified during operation. the auto industry. say to cope with an operating environment with different characteristics than foreseen at system-design time (say. Stages 9 and 10 concern the design. or which contributes to the achievement of a required SIL. These form Stage 14. Stage 7 concerns how the safety of the system is to be checked (validated). We have briefly seen. or the brakes more powerful. A (sub)system required to implement a safety function. it is understood that the requirements and requirements analysis part of system development is finished (in the very simple and abstract so-called Waterfall model of system development. so Stage 9 will be decomposed further. Such ways are pursued in Stage 11. (There is some systematic confusion amongst engineers as to what activities the terms “verify” and “validate” refer. IEC 61508 is concerned primarily with the E/E/PE components. Finally. an example of a chemical processing plant would be more appropriate than considering a car. or to cope with extended functional requirements (say. so that the wheel attachments are not subject to the kinds of forces that will work them loose. above. IEC 61508 says this is “validation”. telecommunications system engineers.
and recycle the metal in some of the parts. Stage 16. you need to dispose of it. And maybe you want to reuse rubber tires. You need to remove the battery and other toxic items and dispose of them separately.
. There may be activities here concerning safety. This is the final stage. you cannot just put the complete car in a landfill.system is all clapped out. For example.
Safety-related systems: E/E/PES
OveralI operation and maintenance planning
OveralI installation and commissioning planning
Safety-related systems: other technology
(see E/E/PES safety lifecycle)
Back to appropriate overall safety lifecycle phase modification 15 Overall and retrofit
14 maintenance and repair 16
Overall operation. E/E/PES and software safety lifecycle phases. with the programmable electronic (hardware and software) aspects of boxes 13. NOTE 3 Parts 2 and 3 deal with box 9 (realisation) but they also deal.
NOTE 1 Activities relating to verification. 14 and 15. management of functional safety and functional safety assessment are not shown for reasons of clarity but are relevent to all overall. where relevant. NOTE 2 The phases represented by boxes 10 and 11 are outside the scope of this standard.
2 Safety integrity requirements specification
9. that is. the schema plays a role in system justification. They emphasise that along with development come specific tasks to plan for and execute the validation of the safety properties of the system or subsystem. when one prepares what is called the safety case for the system. In the case of software.2
E/E/PES safety validation planning
9.The E/E/PE and Software Safety Lifecycles
IEC 61508 defines more detailed. These mimic the overall life cycle tasks in miniature. To repeat.1.1
E/E/PES safety requirements specification
9. of getting the code running on the target HW. the integration of the subsystem in with other system parts is explicitly represented.3
E/E/PES design and development
9.6 One E/E/PES safety lifecycle for each E/E/PE safety-related system
E/E/PES safety validation
To box 14 in figure 2 To box 12 in figure 2
. the E/E/PE (sub)system development and the development of its software. the integration activity includes the activity of “integrating” the software with the hardware. these life cycle schemas are intended as a means of book keeping.5
E/E/PES operation and maintenance procedures
9. but still general. Because this life cycle task structure is intended to apply to subsystems as well as the overall system. Apart from guiding the development to ensure that no important safety-relevant tasks are inadvertently omitted.1. When one documents the safety activities undertaken and untertakes to demonstrate that they are sufficient to ensure the required level of safety. that is.
Box 9 in figure 2
E/E/PES safety lifecycle
9. then it is convenient to have such a standard task decomposition to help structure the safety case document.4 E/E/PES integration
9. lifecycle stages for Stage 9 in the overall life cycle.1 Safety functions requirements specification
PE integration (hardware/software)
9. And these requirements are analysed abstractly for completeness. consistency. and unit testing. Integration. and the hardware embedded in the larger system. called integration testing 5. When the system is built and operating.1. what functions it has to perform.6
Common Software Lifecycle Stages and the IEC 61508 Lifecycle
Those developing software usually organise the activities into stages. Commissioning and Operational “Maintenance”.2
9. It is also shown in this phase how the design is to fulfil the Requirements Specification. The detailed design of the system is performed.2 Safety integrity requirements specification
E/E/PES safety lifecycle (see figure 3)
9. Implementation. This phase will usually also include verification.5 Software operation and modification procedures
9. modifications will usually need to be made. although “maintenance” is an activity that consumes most of the budget of any large project.Software safety lifecycle
9. 3.1 Safety functions requirements specification
Software safety validation planning
9. Requirements Specification and Analysis Phase. showing that the code indeed fulfils the Design Specification. In this phase is determined what the system has to do.1. Design Phase. This phase usually includes a testing task. These are often referred to as “maintenance”. The design is coded.3
9. The code is loaded on to the hardware. feasibility and other desirable properties. A typical Waterfall Model might have 1. which follow one another. yielding a so-called Waterfall Model of development. The result is a Design Specification. 4. 11
of course). Integration is not quite as easy a task as it might appear. until one is finally done. in which stages are explicitly revisited. To date. However.
Program Coding & Verification
HW/SW & System Integration
Maintenance & Modification A Simple “Waterfall” Model of Software Development The Waterfall Model is an idealisation. one hopes with decreasing modifications. Typically. there is no explicit integration of the IEC 61508 safety lifecycle model with even crude software development models such as a Waterfall or a simple Spiral Model. phenomena encountered in later stages will result in revisions undertaken at earlier stages. There is thus no guidance available for software developers how they might then integrate the IEC 61508 lifecycle model into their development processes. For example. in typical system safety processes. The need for revisiting earlier stages led to the so-called Spiral Model of development. the more costly it is to fix in earlier stages (people may suggest a factor of 10 cost per backward stage. the later in the “Waterfall” a phenomenon is discovered. a hazard analysis performed at requirements-specification time will be a preliminary hazard 12
.indeed may consume some 60-80% of it in largish projects. but this is only a very crude estimate.
4.. The standard does recognise the potential need for more than one hazard analysis:
7. Does the code generator always produce code which is deterministic and whose meaning exactly matches the meaning of the state machine specification from which it was generated? At the HW/SW integration state. yet further hazards arise which were not expressible at earlier stages. As the system becomes more concrete during the design phase. or any that lead to non-deterministic behavior? ● The source code language is C.. Hazards beyond those specified in the preliminary analysis almost inevitably crop up during design. Does the source code contain any constructs that are ambiguous. for example
A train approaches and the barrier stays up The barriers come down before the warning lights and bells activate A barrier comes half-way but not fully down
At the Design stage.3).
. Are we sure that our source code will be compiled in such a way as to preserve the meaning of our program? The target hardware is Y. However. suppose we are building a rail-road level-crossing (see the next section for a picture).1: A hazard and risk analysis shall be undertaken which shall take into account information from the overall scope definition phase (see 7. Stage 3..analysis. often a further hazard analysis will be performed as the interaction of the components of the design is specified. This has known weaknesses which we believe will not affect our code.2.
We are using the compiler from company X. The source code language is C. the IEC 61508 life cycle only allows for one hazard analysis stage. The C code is synthesised by a code generator from a statemachine-like “specification” language. The hazard analysis at the Requirements stage will concern functional deficits. E/E/PES or software safety lifecycle phases which may change the basis on which the earlier decisions were taken. Can the compiled code exhibit run-time errors?
So we see that at each stage of the Waterfall SW lifecycle we can identify hazards that could not be expressed in the language of the earlier stages. What happens when the cable is severed or partially severed? Is the processor cabinet sufficiently protected against the ingress of rainwater?
At the Program Coding and Verification stage. new hazards may be identified that are not present at the Requirements stage.. If decisions are taken at later stages in the overall. NOTE 2: It may be necessary for more than one hazard and risk analysis to be carried out.. For example. The need for many many hazard analyses as the design progresses into code and object code fits ill with this sole designated stage in the standard.. ..
The train-sensors and the logic processor are physically separated and joined by a cable. then a further hazard and risk analysis shall be undertaken.. further hazards may be identified that are not present at either Requirements or Design stages.
But then how do we determine which tasks we may skip on a path with a loop. ● within the “inside” of Stage 9.5.1. Both of these steps involve modification of the Safety Lifecycle diagram. before Stage 9. presenting some difficulties. ● after Stage 9.3 could be addressed by appending a note to the effect that between all recognised software development lifecycle stages within Stage 9. even one as simplistic as the Waterfall model.3 itself.4 and back to Stage 3. and 9. E/E/PES integration.3 and 9. the other passing from Stage 3 down to Stage 9. respectively 9. So when one has finished Stage 9. back to Stage 3. and. The hazard and risk analyses within Stage 9. Let us indicate this with simple arrows. but this is not the Waterfall model. This would involve inserting a hazard and risk analysis step ● after Stage 9. We may conclude that the current diagram in the standard is an inaccurate portrayal of what must be performed. and performs the hazard analysis (passing to Stage 3 again). one may expect a hazard and risk analysis to be performed. So now it seems as if the tasks in the flow down the Safety Lifecycle may no longer be compulsory: one may skip certain tasks if one is. and when? There is no guidance. respectively 9. one may skip Stage 9. An alternative would be expressly to insert the additional necessary hazard and risk analyses in the Safety Lifecycle model at the points at which they must be performed. Hazard and risk analyses must be performed after Stages 9. as we have just seen. And the meaning of the arrows is no longer clear. Now. the standard does not explicitly recognise that such analyses are inevitable at each stage of the software Waterfall lifecycle (Note 2 above says “may be necessary”). These loops are nested. To show that integrating the Safety Lifecycle with a software development model. then a further iteration of Stage 9.3. before Stage 9.4. there is no guidance on how the Safety Lifecycle model with its 13 stages needs to be modified to account for them.3.
. Can this be right? Well. will not be trivial. lies before us. Both the Waterfall model and the Safety Lifecycle model were task-precedence diagrams without loops. say.4. E/E/PES design and development. on the second time along a path which contains a loop. E/E/PES design and development.4.4.3.However.4. E/E/PES integration. There is one stage for hazard and risk analysis. Hence there must be shown a path from Stage 9.3. It follows that in an integrated Safety Lifecycle/Waterfall development model an instance of Stage 3 must follow both Stages 9.3. maybe if one is using the so-called Spiral Model of software development. namely Stage 3. back to Stage 3. And given that they will occur. from Stage 9. ● after Stage 9. for (let us assume) one has completed this stage successfully.3. whatever the software development lifecycle model used. There are now two loops in the Safety Lifecycle graph: one loop passing from Stage 3 down to Stage 9.4. in which the various stages are revisited again and again.3 the second time through.2 and 9. for it seems that on looping back to Stage 3 from Stage 9.3 and back to Stage 3. consider the following.4. there is a loop in the Safety Lifecycle model. before the confluence of the arrows from Stages 9.4.3.
These aspects are usually noted by the term non-functional safety. U. a grade crossing in Montana. Is it possible for the barriers to remain raised when a train approaches? Do the lights and bells always operate when a train approaches? How do operators know when a light or bell or barrier is no longer functional? Are the barriers always visible to road traffic when lowered? IEC 61508 concerns these aspects of a system.A. Whether the lubrication oil or grease is toxic. cross left-right between the barriers. whether the paint on the barriers is lead-based and whether this poses any danger to humans and animals in the vicinity. oil. whether it is guaranteed to be contained or whether some can leak out. and.. “Level Crossing”.S. On moderately well-used roads.) at which a road intersects a railway line. grease and paint are essential to the correct 15
. Consider for example a level crossing (called a grade crossing in the U. There will also usually be an assortment of visual and aural warnings used when a train approaches. Non-functional safety aspects might concern. The barriers will be raised when no train is approaching. the toxicity of materials used in the construction. say. In other words.. say. This phrase refers to aspects of safety concerning the function of the system. and thus trains. U. http://en.
A level crossing in Montana. A typical level crossing. is shown below.org/wiki/Level_crossing) Functional safety aspects of this system would concern. the operation of the barriers and lights. if so. allow road traffic across the rails.S. The rail lines.wikipedia. There will typically be safety aspects of a system which do not concern the function of the system directly.A. and lowered to halt road traffic when a train is approaching the crossing.Functional Safety
The IEC 61508 standard concerns itself with functional safety. Say. When a train approaches.S. and the red warning lights and the bells operate. when raised. there will typically be movable barriers which. when lowered. the barriers are lowered. safety aspects of the system which are secondary to its intended function. The barriers are shown raised. The view is taken from along the road. viewed from along the road. and.A. Of course. act as a barrier to passage of road vehicles. (From Wikipedia.
Another way. But safety is not necessarily solely concerned with states. which is intended to achieve or maintain a safe state for the EUC. One way to do this is to redesign the system such that the specific risky aspect is no longer present. at a road-rail crossing.
Assuring Functional Safety and Safety Functions
If it is found during the hazard and risk analysis of the system functions that a particular aspect is too risky (we discuss below how this is determined). it would be reasonable to think that the airbags execute a safety function.. the mitigation of the risk is phrased in terms of a “safe state” for the Equipment under Control. The airbag deployment serves to reduce the severity of this hazardous event considerably. IEC 61508 is not concerned with the non-functional aspects such as toxicity of system components. if it cannot be determined to a satisfactory degree that barriers will always come down and lights flash and bells sound when a train approaches a level crossing.. Call this Case 1.. Although. maybe one would build a road bridge over the railroad instead. This hazardous event has a certain risk associated with it. So it looks as if airbag deployment is not a safety function in the sense of IEC 61508. is to provide additional functions whose purpose is to intervene to mitigate the identified risk so that it becomes acceptable. but also concerned with behavior. because it was a hazardous situation which resulted in harm: people were injured. This function has also. so this still counts as harm). a lot to do with avoiding harm. Call this Case 2. but the function of the system concerns solely when barriers and warnings operate..functioning of the system. and the same dynamics. But the airbag deployment does not return the EUC to a safe state. for example. emphasised by IEC 61508. Indeed. indeed in Case 1 to the point at which it is barely still a hazardous event according to IEC 61508 (let us suppose the occupants are badly shaken. We can certainly say that Case 2 is a hazardous event. indeed in theoretical computer science a “safety property” of a computational system is a partition of the states of the system into two (which may be called “safe” and “unsafe”). Having functioning 16
. Consider as well the exact same collision. divided into “safe” and “non-safe”. Consider. a car collision in which airbags are deployed and save the car occupants from injury. and when they do not. since the human body cannot tolerate decelerations of the sort that occur in substantial car collisions without airbags. the EUC remains just as thoroughly totalled in Case 1 as is does in Case 2. Now. They certainly have a function: to deploy and cushion in case of a sudden deceleration.. and the occupants are seriously injured. and when they should (and maybe when they should not). For example. The definition is somewhat convoluted:
Safety function: Function to be implemented . then the specific risk must be mitigated or eliminated. but this time without airbag deployment. obviously. Such a function is called a safety function in the standard.. Let us look again at my above explanation of a safety function as one whose purpose is to intervene to mitigate the identified risk so that it becomes acceptable. with the same car and people at the same place. namely its likelihood of occurrence combined with its severity. in respect of a specific hazardous event
5. In the hazard and risk analysis. and does so through affecting the second component of the risk calculation rather than the first. the de-Moivre-type risk. Apart from the definition of “risk” which we have discussed. for each determined hazardous event Given that the EUC risk has been determined for a specific hazardous event. Thus it is a risk reduction measure (one factor in the risk calculation remains the same. This suggests that the overall risk of using a system. is not a concept to which IEC 61508 explicitly gives much credence.2. Paragraph A5.7 is that
● 7. Indeed.4. we would say that EUC risk is a measure of some features of a specific hazardous event.airbags thus serves not to alter the likelihood of any hazardous events that are collisions. and not aggregate as in a de-Moivre-type risk assessment.general concepts.7 The EUC risk shall be evaluated. I discuss here how this is envisaged to happen. Annex A for “discussion”. The necessary risk reduction may be determined in a quantitative and/or qualitative manner. IEC 61508 says that “[r]isk is a measure of the probability and consequence of a specified hazardous event occurring” (Part 5. or estimated. other technology safety-related systems and external risk-reduction facilities in order to ensure that the tolerable risk is not exceeded
This process of determining risk reduction is summarised in the following diagram:
. So as a function to reduce risk to an acceptable level.4.2. It mitigates the risk so that it becomes acceptable.2 The necessary risk reduction shall be determined for each determined hazardous event. the other is reduced). so we may assume this is intended to be definitive). Assuming there will be many sorts of hazardous events. this does suggest that we are being led to consider each of them individually. the requirement in Part 1. but it serves to alter the severity of each of those events. The definitions explicitly refer to Part 5. Consider the definition of “EUC risk”:
EUC risk: risk arising from the EUC or its interaction with the EUC control system
If we are to put together EUC risk with its explanation as a measure of the “probability and consequence of a specific hazardous event”. there is now a requirement that ●
This refers to the notion of “necessary risk reduction”:
Necessary risk reduction: risk reduction to be achieved by the E/E/PE safety-related systems.2. the airbag deployment works.
The entire assessment of safety in IEC 61508 occurs through risk assessment and risk reduction. hazardous events are identified and the necessary risk reduction for these events determined. Paragraph 7. Annex A: Risk and safety integrity . It may make sense to consider it and its brethren an “honorary safety function” in an IEC 61508-conforming development.
2 Consideration should be given to elimination of the hazards 18
. Indeed. This phrase has no formal definition. where
S' = EUC enhanced with the introduced functions
is at or below the tolerable risk of E. as required in 7. The risk of E in the operation of S' is called
Residual risk: risk remaining after protective measures have been taken
The difference between the EUC risk and the residual risk is denoted in the diagram as the “actual risk reduction”. IEC 61508 requires
7.4. what risk “society accepts” of E). One of the ways in which one can deal with an identified hazard is to redesign the system to eliminate it. E. This analysis and prophylaxis is supposed to take place for each specific hazardous event. probably because it is clear what it means. Then one must take steps to ensure that the risk of E in the overall system S is reduced to at most the tolerable risk of E. Suppose further than the EUC risk of E is higher than the tolerable risk of E. The means envisaged by IEC 61508 for the risk reduction in the E/E/PE part is the introduction of functions which specifically reduce the risk of E.4.2.Residual risk
Partial risk covered by other technology safety-related systems Partial risk covered by E/E/PE safety-related systems Partial risk covered by external risk reduction facilities
Let us consider a specific hazardous event. and suppose one has determined the EUC risk of E and the tolerable risk of E (in other words.2.
Determination of Risk Reduction: Balancing the Options
Suppose one has performed a hazard analysis and identified a set of hazardous events and their likelihoods and severities. so that the risk of E in the operation of the system S'.
Suppose indeed that development of AMK had been continued. IEC 61508 would suggest to increase control-system reliability through moving to electrics and would not necessarily recognise the increased wiring-fire hazard which it might engender. and from aircraft fuel tanks to engines. A test was performed using a remote-piloted Boeing B720. is “invisible” to IEC 61508 and therefore so is any trade-off of risk. suppose one had decided to eliminate the hazard of immediate conflagration in an aircraft accident by using AMK. would have been required. increases the chances of insulation faults and arcing within and between electrical wires. putting more electrical wires into airplanes. but burned at a lower temperature than otherwise expected. Cambridge University Press. The Golem at Large: what you should know about technology. The most trivial would be to say that IEC 61508 does not cover commercial aviation. a couple of decades ago. The idea of the kerosene was to inhibit ignition of the fuel in the case of an aircraft accident. a phenomenon given increased prominence after the accident to TWA 800 in 1996 and to Swissair 111 in 1998. The fuel ignited. So in this example the second risk. nor indeed is there any explicit recognition in the standard that such circumstances could arise. that
1 For a discussion of the experiments around AMK. So. 1998. it has also been argued that electrical/digital-technology control systems are actually more reliable than their forbears. inter alia because of the added ease of implementing fault-tolerance in such systems. and less of it burned. that of a wiring fire. so called “arc faults”. the television coverage showed a fireball and developments of AMK were effectively discontinued. and new means of delivery. and the ensuing investigations into wiring quality on commercial aircraft. IEC 61508 gives no guidance on how one may proceed in such cases. and see what happened. experiments were performed on an anti-misting kerosene (AMK) to be used in jet aircraft. which would have been engendered by the change of delivery technology required? Consider another example. Indeed. How does one balance this against the increased risk of engine problems during flight. The fuel had different physical characteristics from the usual jet fuels. bound together in wire bundles. the FAA believed it had identified 32 accidents between 1960 and 1984 in which AMK could have saved lives1.But suppose eliminating one hazard increases the risk of another hazardous event not associated with that hazard? For example. which was landed in the desert into a set of large steel “knives”. under the definition of functional/non-functional we have seen above). There are a number of possible ways in which the convenors of IEC 61508 could reply to the above queries. thus giving potential passengers more time to evacuate away from the aircraft.
. Nevertheless. see Chapter 3 of Harry Collins and Trevor Pinch. One of the reasons for this has been to reduce overall aircraft weight and to increase the ease of building the aircraft. However. whose purpose was to slice into the tanks and release fuel into the atmosphere. However. How may one balance the potentially reduced risk of functional faults with electrical control systems against the increased risk of arc faults and ensuing aircraft fires due to the increase in required electrical wiring? This example is particularly poignant because it involves exchanging a risk in function (operational reliability of a control system) with a non-functional risk (a wiring fire is a non-functional safety hazard. both from fuel tankers to aircraft. There has been a general trend over the last thirty years in new commercial aircraft design to replace mechanical control systems with electrical/electronic control systems. placed vertically in the ground.
This seems then to be an approach which is contra-indicated by IEC 61508 as written. H1 is the chance of immediate. maybe the “tolerable level” associated with H2 events is such that the risk of (H2events with AMK) is higher than the joint risk of (H1-events without AMK) together with (H2-events without AMK). while IEC 61508 concerns itself with reducing risk of infividual events. in Part 5 Annex B. and then work harder to reduce the increased risk of others down to their defined “tolerable level”. Say.
IEC 61508 considers ALARP explicitly. The IEC 61508 tells us we must look at hazardous events arising from H2. Suppose we do so. deadly conflagration of jet fuel in the case of a tank rupture. The core of this issue lies with the focus in IEC 61508 on risks of individual “hazardous events”. but this author has nevertheless heard such kinds of reply.would just be avoiding the question. and that the guidance it does offer is not necessarily appropriate! As follows. It may be the case that there is no generally-agreed way to proceed. the ALARP principle encompasses overall risk. IEC 61508 says to give consideration to eliminating the hazard. and thereby increase the risk associated with H2 (engines failing during cruise flight because of interactions because the fuel delivery systems and the physical properties of the fuel argue with each other).
. but this is so with lots of other issues. rather than on overall risk. H1 and H2. but neither explicitly recognises the situation nor offers guidance because there is no generally agreed way to proceed. such as when and how to apply so-called formal methods in development and analysis. such as the deMoivre type. but implicitly. Let us suppose we have two hazards. on which the standard does offer guidance: at least an explicit recognition of the situation seems appropriate. But what about considering the joint risk of H1-events and H2-events? Although one has eliminated the risk of H1-events. The first observation to make is that. and eliminate hazard H1 (say. A risk reduction approach associated with a de-Moivre-type risk would say that the joint risk is what is most important and advise to reduce the joint risk as far as possible. ALARP and tolerable risk concepts. A second reply would be to say that the standard allows recognition of the situation. So it seems as if IEC 61508 might be telling us to eliminate hazards where possible. as considered by a de-Moivre-type calculation. A third answer would be that the standard in fact does offer guidance. by introducing AMK into daily commercial flight operations). suggests that explicit guidance in IEC 61508 is necessary. And it is not there at the moment. and H2 is all engines cutting out in flight. In the quandary over how jointly to handle H1 and H2 above. and reduce the risk of those hazardous events to a tolerable level (to the “tolerable risk” associated with the event). The possibility of reading IEC 61508 contrary to established reasonable approaches. whose risk has been increased (through increasing the probability of occurrence of H2). ALARP would say to reduce the joint risk (if it is “reasonably practicable” so to do) rather than eliminated H1 and incurring thereby an increased risk of H2-events above that of the previous joint risk.
therefore. The legal principle of ALARP seems to require that you do so. 2. As an engineer. It may be that engineering principles can be devised that. There are cases in which the risk is (socially) acceptable anyway.either it is impracticable to do so. to reiterate that ALARP is a principle of English law. A lawyer might well advise 21
. would have avoided the cases in which English law decided that ALARP had been violated. Suppose the risk is acceptable.There have been various attempts to turn ALARP into a principle of engineering.the cost of doing so is “grossly disproportionate” to the “improvement gained”. the less. But suppose for a certain amount of available resources you could have reduced the risk further. at best. not an engineering principle. You have no need to reduce risk further than you have if: . if you are in this region then you don't need to perform “detailed working” to demonstrate adherence to ALARP. it is necessary to spend to reduce it further to satisfy ALARP. There are cases in which the risk is unacceptable. The concept of diminishing proportion is shown by the triangle. the IEC 61508 guidance says “no need to perform detailed working to demonstrate” this. but there is no a priori guarantee that these principles exactly encompass all that case law has and will decide about ALARP. suggests the following 1. It is with this caveat that we may consider how ALARP can guide engineers. The engineering advice is: don't go there except in “extraordinary circumstances”. on the vertical axis. This seems to be. questionable advice. or . There is a region in which ALARP affects your work as an engineer. say. Let us consider first Case 2. no matter what you do.
(No need for detailed working to demonstrate ALARP)
This diagram. The guidance given in IEC 61508 Part 5 Annex B uses a diagram:
Risk cannot be justified except in extraordinary circumstances Tolerable only if further risk reduction is impracticable or if its cost is grossly disproportionate to the improvement gained
Intolerable region The ALARP or tolerability region
(Risk is undertaken only if a benefit is desired)
As the risk is reduced. 3. It seems wise. proportionately. and IEC 61508 Part 5 Annex B is just one of them. However.
york. that “as the risk is reduced. at the end of the post. This suggests that “detailed working” might indeed be helpful. But note there is also further advice in the diagram.” Readers may also want to refer to HSE's enforcement policy statement at http://www. This advice relies explicitly upon the finding of Lord Asquith of Bishopstone in Edwards vs.K.hse. If it is impracticable to reduce risk further (and you can demonstrate to the satisfaction of the court that it is impracticable!) then indeed you have no need to reduce risk further. for suggesting this way of explaining HSE's deliberations (personal communication. for example http://www. in contrast to Part 5 Annex B's suggestion that there is “no need”. one factor HSE considers is. Case 3 is an interpretation of case law as it presently is in England.
.K. on its own or
2 Thanks to Mark Bowell. through regulatory behavior.” It was set up as a regulatory body by the Health and Safety at Work Act of 1974. Thus is a strong connection made between ALARP and IEC 61508 in the U. It acts as the de facto regulator for complex safety-critical systems that are not otherwise regulated (for example. HSE brings legal cases against companies deemed to have violated HSW. HSE. and HSE often uses IEC 61508 as a reference standard to test this duty2. of course. Mark has explained HSE's policies more thoroughly in notes to a mailing-list on safety-critical systems. National Coal Board 1949.uk/pubns/hsc15. Health and Safety Executive explains its role thus: “HSE's job is to protect people against risks to health or safety arising out of work activities.ac. Mark suggests that “the most important part of this [note] from HSE's perspective is the reproduction of the statement from the HSE board. the less. trouble or money) is placed in the other and that. in which he said “Reasonably practicable is a narrower term than ‘Physically possible’ and implies that a computation must be made […] in which the quantum of risk is placed in one scale and the sacrifice involved in the measures necessary for averting the risk (whether in time. make sure your documents prove you've done everything reasonable practicable to reduce it”.” This may be taken broadly to substantiate the advice given in the diagram above. EUC control system (EUCCS). In deciding whether to bring such cases. 11. A system which “both implements the required safety functions needed to achieve or maintain a safe state for the EUC and is intended to achieve. The U.pdf . proportionately. All kinds of random assertions are made in court trials: my view is that it is best to be well prepared. August 2008).html . neither does it follow from the finding of Lord Asquith.uk/hise/safety-critical-archive/2004/0473. it is necessary to spend to reduce it further to satisfy ALARP”. commercial aviation is otherwise regulated).gov.cs. I do not know of a legal basis for this advice.“whatever you think of the magnitude of the risk. if it be shown that there is a great disproportion between them – the risk being insignificant in relation to the sacrifice – the person upon whom the obligation is imposed discharges the onus which is upon him. Safety-related system (SRS).
There are three types of system and subsystem to which IEC 61508 continuously refers:
Equipment under control (EUC). whether a company has fulfilled its duty to reduce risk ALARP.
There is likewise no definition of what a “fail-to-function state” is.with other E/E/PE safety-related systems.
The safety integrity refers to not to failure but to dangerous failure:
dangerous failure: [a] failure which has the potential to put the safety-related system in a hazardous or fail-to-function state
We recall that there is no definition of what a “hazardous state” might be. There is just one of EUC and EUCCS. The SRS is required to implement one or more safety functions. but it might well be a SRS state as a consequence of which hazardous situation ensues. Besides these three subsystem types.
The four safety integrity levels for so-called high demand or continuous-mode systems are logarithmically related.. and a safety function is specified because it has been determined that the EUC risk of a specific 23
. and here it is harder to guess. there is the fourth type. according to the following table:
Safety integrity High demand or continuous mode of operation level (Probability of a dangerous failure per hour) 4  10-9 to  10-8 3  10-8 to  10-7 2  10-7 to  10-6 1  10-6 to  10-5 NOTE See notes 3 to 9 below for details on interpreting this table. the necessary safety integrity for the required safety functions.. . An SRS is required to mitigate any hazardous event for which the tolerable risk is lower than the EUC risk. other technology safety-related systems or external risk reduction facilities. There may be many SRSs. of
Subsystems which are none of the above
Safety integrity is a key concept in IEC 61508 and is defined as the reliability of an SRS: Safety integrity: probability of a safety-related system satisfactorily performing the required safety functions under all the state conditions within a stated period of time E/E/PE SRSs are assigned a safety integrity level:
safety integrity level (SIL): discrete level (one out of a possible four) for specifying the safety integrity requirements of the safety functions to be allocated to the E/E/PE safety-related systems. and there may be many such hazardous events.. then it is easy to see how hazards arise.” So an SRS implements the safety functions and is which includes amongst its purposes not only implementing safety functions but also achieving the required safety integrity. Is a fail-to-function state one in which the SRS engages in no dynamic behavior at all. or can it be a state in which the behavior of the SRS is different from that specified or required? If it is a state in which the SRS engages in no behavior (it is stopped).
Table E. The safety function is there to reduce this risk to a tolerable risk. but also that more than or equal to one dangerous failure must occur every 10^9 hours of operation. but yet it accrues to no SIL. which is how the categories are indeed defined in the table. thousand LOC = 24
. for example. a SIL is an attempt to designate the criticality of an SRS implementing a safety function. so that it is not necessary to try to estimate the exact reliability with which a safety function is performed. However. There may also appear to be an incompleteness at the other end. so it is easy to understand how a failure which might lead to a SRS failure to function (under this interpretation) could be termed “dangerous”. It is an attempt to do so which retains a certain amount of simplicity. then the EUC risk of this specific hazardous event is not tolerable. IEC 61508 proffers guidance on what techniques are appropriate for developing an SRS with a specified SIL. but “only exceptionally. which seems perverse (as well as likely indemonstrable). one is drawn most obviously to some kind of logarithmic scale. No failure rate lower than one dangerous failure in 10^9 hours of operation occurs in the SIL table. But there is no reason to think that the safety function F is any less critical to safety than any other safety function. there is correspondingly no guidance. OBJ. but since the requirement for F corresponds to no SIL. A fundamental disquiet with the notion of SIL used in the standard is the association of a SIL with a set of recommended development techniques. between one in ten million and one in one hundred million hours of operation. and one that is not necessarily easy to achieve. for example. Now. for some very basic components only” for SIL 3 (Part 6. the categories are incomplete. VDM and Z is “recommended”. the table suggests one has to show not only that a failure rate is lower than one dangerous failure in 10^8 hours of operation. One can query the wisdom of this sort of advice. First. CSP. but it only needs to be reliable to the point of one dangerous failure in ten thousand operational hours? That is a critical figure. LOTOS. We may hope this anomaly will be corrected in future revisions of the standard. as Martyn Thomas has pointed out. the use of formal methods such as CCS. The standard sets a lower limit of one dangerous failure in 10^9 hours of operation on the any dangerous-failure measure that can be claimed. On the other hand.14). There has been considerable discussion amongst safety-critical digital system engineers about the adequacy of this notion as defined in IEC 61508. It would have been preferable simply to specify a probability of dangerous failure per hour of “< 10^(-8)”. So it does not allow anyone to claim lower than this in any circumstance. to achieve SIL 4. very low dangerous failure rate. If the safety function is no longer active.hazardous event is not tolerable. and many have considerable reservations about it. The simplicity is advantageous in that the categories are necessarily broad. typical safety-critical code quality is about one error per thousand lines of source code (per KLOC: “Lines of code” = LOC. HOL. namely there are just a few “boxes” into which any SRS function can be put. To have few categories while covering all the possibilities. I emphasise that the notion of SIL applies to the E/E/PE parts of a system. temporal logic. What if you need a safety function F. There are other notions of SIL used in other standards which do not necessarily suffer from the same disadvantages. So. as follows. A SIL of 3 requires a system attain a very. but just to fit it into a general region. One reason for this is explained in Note 6 accompanying the table. whether the use of formal methods is or is not recommended.
this runs up against the difficulty of what might constitute good evidence. In question here are two aspects of the association in IEC 61508 of development methods (as “not recommended”.K. Even if such an association were to be shown between. say. no association has ever been shown between the use of a specific development technique and a specific quality level of the resulting software. what reason would we have to expect that such a boundary would lie at 10^(-7) rather than. as the standard suggests. Others are able to attain one error per 4 KLOC on smallish systems. say. from SIL 0 (if it existed) through SIL 4. 12^(-7)? More obviously reasonable advice on association between development techniques and SILs would surely be that one be free to use whatever development techniques one thinks suitable. It follows that a worst-case assumption is that each error in SRS software may result in a dangerous failure. and Butler and Finelli for a 25
. personal communication). let alone for a range of development techniques and the entire SIL quality-specification range. state-of-the-art quality-attainment methods in every critical software development. what could be a reason for not wishing software to be as reliable as possible.KLOC) (John McDermid. one may observe that certain companies use formal methods in order to produce software of higher quality with lower resource consumption. Indeed. indeed require. Safer C. must be demonstrated to a high degree of confidence based on objectively good evidence. As is well-known. “recommended” or “highly recommended”) with software quality as represented in the SILs. the SPARK Ada system of the U. even using integrated formal methods throughout development. The first aspect is that. requirements analysis and specification in Z and rates of dangerous failure in the range of less than 10^(-7) per operational hour. Littlewood and Strigini pointed out. but that the results. and it is rare to see how to do better than this worst-case assumption. However. The second aspect is that any association of specific methods with reliability ranges that are powers of ten must be somewhat arbitrary. for a Bayesian approach to evidence. In 1993. it sets the state of the art and there is no reason not to recommend. Against. even if there were such evidence. the consequences of these errors in software can hardly be functionally bounded in advance. in terms of the quality of delivered software. company Praxis High-Integrity Systems). but there is no known evidence that one can achieve SIL 3 reliability without using rigorous development techniques. even exceeding its SIL requirement? One suspects that behind such advice lies the old canard about formal methods consume resources in great quantity when compared with the gain in quality that they enable. although one may reasonably expect particularly careful development processes to result in software of higher quality. It is usually not possible to say with any degree of confidence how the system will ultimately behave if a certain type of error occurs. and a requirement for dangerous failure such as contained in SIL 3. Some organisations are able to attain measured quality of one error in 25 KLOC by using highly-integrated formal methods while writing the programs (for example. Use of formal methods such as by Praxis seem to improve the quality of delivered software by one to two orders of magnitude. So there is a wide gap between measured software quality. say a few tens of KLOC (see for example Les Hatton. Since this can be done. Addison-Wesley 1989).
It seems likely. one may expect 10^9 lifetime operational hours on just this one model year.frequentist approach.
Relationship between SILs and ALARP
Felix Redmill has elucidated the relationship between SILs and the ALARP principle for U. And if you have that good reason in any case. So if a component is used on three popular models over three model years. we are in the realm of 10^(10) operational hours. and popular model cars sell of the order of a million examples per year. critical subsystems must be free of any single failure that causes a catastrophic event (loss of the airplane. Redmill pointed out that a SIL is an a priori requirement. there would still be of the order of 10 critical events during the system life. In the automobile industry. why do all that testing? There are serious issues here of evidence and achievement that cannot satisfactorily be summarised in a few paragraphs. the lowest level recognised by IEC 61508. you had to go into testing with a very good reason to claim (a prior probability in Bayesian-theory terms) that you had achieved that rate already. it is well possible that some critical electronic components attain of the order of 10^(10) lifetime operational hours3. In commercial airplane certification. 2000).K. 1. In just over three years.
Lowest Recognised Risk Reduction
We have noted that the standard sets a lower limit of one dangerous failure in 10^9 hours of operation on the any dangerous-failure measure that can be claimed.K. which is defined and allocated in the Safety3 A car is said to travel on average 300 hours per year. There are at least two areas in which this lower limite may or must be exceeded. For these reasons and others. that any anchoring of evidence to failure rates will have to take place at much higher failure rates than those envisaged in the SIL table in IEC 61508. though. SpingerVerlag London. but constitute rather a research program. 2.
.. U. and critical components especially on popular cars are also supplied to competitors. as far as one could tell. or more generally discriminate this case from that of ten dangerous failures. Southampton.Then. One may well query whether it is wise to have a standard that does not allow us even to express a measure of one dangerous failure. It seems certain that that table will have to be significantly modified. in the operational life of a particular system. Littlewood and Strigini observed that if you needed the result of your testing to confirm a failure rate of 10^(-6) per hour or below. Suppose they could be analysed and guaranteed down to one dangerous failure 10^9 operational hours. But a device may be used on cars over many model years. engineers (Proceedings of the 8th Safety-Critical Systems Symposium. Indeed. loss of life) to a level of at most one in 10^9 operational hours. many thoughtful engineers consider the requirements concerning SILs in IEC 61508 to be irrecoverably flawed. that assuring any failure rate (dangerous failure or not) of below 10^(-5) per hour through statistical testing of a software system was infeasible.
we may now look at some possible applications and see how and whether the standard fits those applications conceptually. Reverse thrust is a mechanism to help deceleration on the runway during landing. One may also further observe that. An appropriate application An example of an appropriate application of a safety function is the interlock on Boeing B767 aircraft (as well as many others) which prevents in-flight deployment of reverse thrust. “clamshell” or “cascade”
A deployed clamshell thrust reverser
We have seen that achieving risk reduction through safety functions may not always be the most appropriate way to reduce risk. The normal thrust on a jet aircraft comes from the air and other gases expelled out of the rear of the engine.Requirements-Analysis task during development. ALARP is a dynamic requirement that is assigned and handled in the task of Design. when designing and building the system is underway. However. A thrust reverse mechanism can be one of two types.
Appropriate and Inappropriate Applications of the IEC 61508 Approach
With some understanding of the basic concepts behind IEC 61508. since the HSE consider ALARP to be satisfied if IEC 61508 has been followed during system development. He concludes that in some particular case the ALARP principle could well require a reduction in risk beyond that required by the SIL for a function. but to date no examples of this phenomenon have appeared in the literature. that no further evidence would be required by the regulator that ALARP has been followed than by exhibiting the derived SIL and the evidence for its attainment. This observation is astute.
and thus inhibiting the interlock function. it was thereby known that the interlock could fail and thus that in-flight thrust reverse. and could partly disintegrate. Deployment of the left engine thrust reverser in flight. was designated as the probable cause by the accident investigators. So such an event would count as a hazard. This forwards-directed component of thrust helps brake the aircraft on the runway during landing. and it can be eliminated through inhibitory mechanisms. along with wheel brakes. most of which involve sensing main landing gear strut movement as the aircraft weight compresses the shock absorbers.
A deployed cascade reverser On many if not most modern jet aircraft. Subsequent tests discovered such a failure mode. the safety function as IEC 61508 would have called it. An inappropriate application The ground spoilers and thrust reversers on the Airbus A320 aircraft are used. It was determined that thrust reverse had been electrically commanded (the crew spoke of the deployment warning light illuminating on the cockpit voice recorder) and the subsequent radar track showed a loss-of-control descent consistent with thrust reverse having actually been deployed on the affected engine. However. leaving hard rubber particles in the hydraulic fluid. We have discussed thrust reverse.Two parts of the cowling rotate into the thrust stream and mechanically deflect it upwards/downwards and. which is defined as a “catastrophic” event in commercial aviation certification terms. since the thrust stream is reversed this time inside the cowling and exits midway. These particles could become lodged in a valve in the interlock mechanism. In May of 1991. Since at least one failure mode of the interlock mechanism had been demonstrated. there was no failure mode known of the interlock mechanism. which is actually a minor braking 28
. preventing it from closing fully when it needed to. thrust reverser deployment in flight almost inevitably leads to loss of control of the aircraft. A thrust reverse mechanism of the “cascade” type is not quite as visible. A cascade reverser works on the thrust stream essentially the same way as a cascade reverser. leading to loss of control. to brake the aircraft on the runway. while unlikely. The rubber-compound seals on the interlock hydraulics became brittle with time. a Lauda Air Boeing B767 crashed over Thailand. most importantly. was possible. On the Boeing B767 aircraft. somewhat forwards. WoW may be determined in a number of ways. there is a hydromechanical mechanism which physically prevents the thrust reverse mechanism from operating when the landing gear is not deployed and there is not weight on the main landing gear (known as a “weight on wheels” or WoW criterion).
on a road car.wikipedia. It seems inappropriate to speak of implementing a “safety function” here. The inhibition of ground spoilers and reverse thrust during flight has the same justification on the A320 as on the Boeing B767. The aircraft ran off the end of the runway. escaped without injury. an Airbus A320 landed fast on a very wet runway in Warsaw just after passage of a thunderstorm with accompanying wind shear (which the previously landing aircraft had experienced a minute or so earlier). Two people lost their lives. An appropriate application Consider an anti-lock braking system. However.
Ground spoilers deployed on an Airbus A321 (from http://en. Ground spoilers are deployed and thrust reverse enabled on landing through digital logic involving the WoW criterion. and burned. and reverse thrust was also commanded. Rather. putting the aircraft weight on the wheels and enabling effective wheel braking. reverse-thrust inhibition proceeds on the B767 through an explicit safety function. it seems preferable to speak of a safety requirement being satisfied through the design of the digital logic. ground spoilers should have deployed automatically.org/wiki/Spoiler_(aeronautics) ) In September 1993. passengers and crew. namely an interlock mechanism. On the Airbus A320 family. neither functioned for some 9 seconds (and the wheel brakes not for 13 seconds). In terms of IEC 61508 concepts. Upon touchdown. but all others. which signals also have other functions and effects. As we have seen. this inhibition is accomplished through digital logical conditions. an ABS. say when very wet or icy). Ground spoilers dump the lift on the wings. the system components are as follows. hit an earth bank.
the EUC is the brakes the EUCCS is the brake activation mechanism.effect (but which becomes significant when the runway is slippery.
We have seen that risk reduction is the main mechanism through which IEC 61508 enhances safety. from pedal to brake pads the SRS is the wheel-rotation sensors and the responsive brake-release-and-reapply mechanism 29
Designers wish to allow the car to be driven also when the ABS does not function. The controllers modify the planned flights of the participating aircraft dynamically. the procedures contained in the laws of flight plus the centralised planning that results in the Flight Plan for each participating aircraft the SRS is the dynamic control exercised by the controllers with whom the aircraft crews talk when under way: Air Traffic Control (ATC). The IEC 61508 subsystems in air traffic control would be as follows. Proceedings of the Eighth Safety Critical Systems Symposium.The EUC risk we can take as known. certainly under all modern traffic and ATM conditions. Consider air traffic control.
So it seems as if air traffic management and control is not a suitable area in which to apply the riskreduction concepts of IEC 61508 without considerable alteration in the foreseen approach. indeed it may not seem appropriate even to ask it. so one can calculate the required risk reduction and transform this requirement into a SIL according to the SIL table. a certain probability of collision. An inappropriate application This section owes a considerable debt to the discussion in Derek Fowler's paper Application of IEC 61508 to Air Traffic Management and Similar Complex Critical Systems. But ATC historically has been present since there has been a significant risk of collision amongst air traffic.
But now we run into some difficulty concerning the necessary estimates of risk according to IEC 61508. So there are no statistics at all by which to judge EUC risk ● Tolerable risk would be given by the Target Level of Safety (TLS). published in Lessons in System Safety. The purpose of air traffic control is to avoid mid-air collisions between participating aircraft by maintaining a specified amount separation between the aircraft. London 2000. ed. EUC risk. Note that IEC 61508 is not applied in this way. One has no idea of the relative contributions of ATM and ATC to the TLS. to ensure that separation is maintained. I chose it because it is rather clear. but without ATC. Springer-Verlag. Then one demonstrates that the ABS fulfils the SIL.) This question likely cannot be answered. Felix Redmill and Tom Anderson. while enabling each aircraft to continue towards its destination and landing.
the EUC is the air traffic itself: all participating aircraft the EUCCS is Air Traffic Management (ATM). This would be the probability of collision with ATM.
. This can be and has been set by ATC specialist organisations when needed The question then arises of how much risk reduction needs to be achieved by ATC over that provided by ATM? (The “required risk reduction” of IEC 61508. similarly the tolerable risk. So this example is conceptual. So ABS is not designated as an SRS (which must always be active) but rather as a functional enhancement which is not formally safety-related. that is.
This is a device which must be continually activated by a train driver. That is obviously unnecessarily stringent. if the driver becomes incapacitated for any reason (stroke or heart attack. If it is released.000 years. The device is there to ensure that. A designation of SIL 4 for an on-demand function requires that it be more reliable than one failure in 10.000 applications (the SIL table for on-demand safety functions is different from that for continuous functions which we discussed above). obviously.wikipedia.org/wiki/Dead_man's_switch An inappropriate application 31
. or “dead man's handle”.org/wiki/Waterfall_train_disaster
The statistics on train drivers being incapacitated are. The train derailed. say). The required risk reduction can therefore be determined. killing seven people on board including the driver. New South Wales. to a condition that the safety function provided could fail once every 10. or Target Levels of Safety. known in detail by every rail authority. The device could be happily designed to an on-demand SIL 2 or SIL 3 requirement. Indeed.Safety Integrity Levels
An appropriate application Consider what is called on British trains the Driver's Safety Device. and may be a pedal or a lever. More details on the device may be found at http://en. near Waterfall. system-wide. In January 2003. a train driver suffered a heart attack but the “dead man's brake” did not activate. after a second or two emergency brakes are automatically applied. Australia. or some other such device
A “dead man's handle” on a suburban train The device on British trains is a lever with a red knob. the train will be brought immediately to a stop. A brief description of the accident may be found at
http://en. These authorities also set tolerable risk. and from this the SIL of the Driver Safety Device may be read off.wikipedia. then. which must be kept continually depressed by the driver's hand when the train is running. This would translate. the dead man's handle implements an on-demand function which is triggered less than once a year.
Its functional-dangerous-failure probability requirement lies in the region of one every 10. by deploying the take-over command button and flying the aircraft himself. typical TLS's for air traffic control are in the range of a probability of dangerous failure per hour of 10^(-8) or lower. so he was able to recover. he exacerbated the left wing dip and the wing came within a couple of meters of hitting the ground. for this observation. consisting of 100 or more independent subsystems. that is.760 hours. Again.000 hours is fulfilled. There are many independent ATC subsystems. at SIL 4. whereas the most stringent SIL allowed by the IEC 61508 standard is a failure probability of between 10^(-8) and 10^(-9) per hour.
Difficulties with Applying ALARP
To show that IEC 61508 is not alone in its conceptual difficulties.784 hours in a leap year). more or less. and Bayesian statistical calculations from this statistical testing allow one to form a high degree of confidence that the dangerous-failure requirement of one failure every 10. without a failure. An inappropriate application Consider now a digital-logic-based flight control system (DFCS) for an aircraft. the entire system. can then only be shown to satisfy at most SIL 2 (100 x 10^(-9) = 10^(-7)). But there is no corresponding requirement on maintenance procedures.An inappropriate application is represented somewhat trivially by the thrust-reverser interlock mechanism discussed above. Since failure of flight controls can lead to loss of control. Another inappropriate application is represented by the air traffic control system discussed above. an Airbus A320 aircraft departing Frankfurt was found to be cross-controlled..
. or 8. An appropriate application Consider a SIL 1 programmable electronic subsystem. It is actually some 8. this system must be required to be reliable to the order of one failure in 10^9 operating hours. say implemented on a Field Programmable Gate Array (FPGA). In March 2001. Certification requires a failure probability of less than 10^(-9) per operating hour. we can consider how ALARP may fare. Luckily. cit. I am indebted to Derek Fowler. However. Then one has accumulated failure-free experience of the order of some ten million operating hours. say of the order of some hundred. a failure probability of 10^(-9) per operating hour. at the highest possible. Say one took 4. so this says that such a failure may not be expected in the fleet life of the aircraft. However. which is about once per year of continual operation (a year is typically taken by system engineers to be 10.000 hours.000 operating hours. when the pilot flying applied right bank to correct a left wing dip on take-off.000 of these FPGAs and ran them continuously with inputs spanning the entire projected operating range for some 3 months. The quantity of 10^9 operating hours has never been exceeded by any fleet of commercial aircraft of a single model. op. the non-flying pilot noticed what was happening and his control was not crossed. These subsystems can be classified. considered a “catastrophic” event.
In other words. that would apply should an accident occur through maintenance mistakes and should it be decided to process such an event through the legal system. it had certainly not been reduced as far as practicable on the maintenance. although risk had been reduced as far as reasonably practicable on the design of the DFCS. and not necessarily to maintenance. and failed to detect the error during post-maintenance control checks.
. which cross-wired the captain's control for bank. although of course there are other legal principles. such as duty of care which occur under the various concepts of negligence. the ALARP principle applies to design.
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