Iec 61508 pdf free download

Now try Exercise 4. Exercise 4: Given that a single operator at risk is usually in attendance and has little opportunity for speedy escape or alternative mitigation, and that there is a moderate release of hydrocarbon, then: Repeat Exercise 1 using the risk graph in Figure 2. Risk graph approaches will not always be appropriate but they can be useful for screening as a means of quickly assigning priorities. Thus if the risk graph method suggests that nine 36 Functional Safety 2. This may not, of course, be the case.

A safety function might consist of a flow transmitter, logic element and a solenoid valve to protect against high flow. The flow transmitter, on its own, does not have a SIL and to suggest such is nearly meaningless.

Its target SIL may vary from one application to another. The only way in which it can claim any SIL status in its own right is in respect of the life-cycle activities during its design, and this will be dealt with in Chapters 3, 4 and 5.

Assume that this programmable Distributed Control System say a DCS for a process plant causes various process shutdown functions to occur. In addition, let there be a hardwired Emergency Shutdown presumably safety-related system which can also independently bring about these shutdown conditions.

We are therefore justified in saying that the DCS is not safety-related. IEC and some other guidance also refers to severe environmental damage. It is not known how the Figure 2. Furthermore, although not directly relevant here, the same SIL approach can be applied to loss of production and, again, the UKOOA document provides a risk graph approach. Having established a SIL target it is not sufficient merely to assess that the design will meet the maximum tolerable risk target.

It is necessary to establish if improvements are justified and thus the principle of ALARP as low as reasonably practicable is called for. This is implied by paragraphs 7. In the UK this is also necessary in order to meet safety legislation. Cost comes into the picture in that any potential reduction in risk would be compared with the cost needed to achieve it. The concept is that all reasonable measures will be taken in respect of risks which lie in the tolerable ALARP zone to reduce them further until the cost of further risk reduction is grossly disproportionate to the benefit.

Perception of risk is certainly influenced by the circumstances. A far higher risk is tolerated from voluntary activities than involuntary ones people feel that they are more in control of the situation on roads than on a railway.

This explains the use of different targets for employee voluntary and public involuntary in Table 2. The plant life is 30 years. It should be noted that all the financial benefits of the proposed risk reduction measures should be included in the cost benefit calculation e. Cost benefit arguments should not be used to justify circumventing established good practice.

The plant life is 25 years. It does, however, give a quick indication of which region the scenario represents. In the event of its being in the tolerable ALARP region classifications 2 and 3 then further justification is required to show that the risks usually are ALARP, maybe using a combination, of approaches. These might include the quantification described earlier in this Chapter as well as demonstrating that approved codes of practice have been followed.

Sections 3. However, the following points should be noted first. The degree of variability which is used in the specification of techniques and measures is far greater than could ever be reasonably correlated with actual performance. In any case each technique and the degree of refinement e. The combination of text e. Their interpretation requires the simultaneous reading of textual paragraphs, A tables, B tables and Table B6 — all on different pages of the Standard.

The A tables are described as referring to measures for controlling i. Type 2. Chapter 5, involving the restricted subset of IEC , also caters for the Type 2 case.

The exact nature of the design-cycle model will depend on complexity and the type of system being designed. The IEC model in Part 1 of the Standard may well be suitable and the model in Chapter 1 of this book is very similar. A major point worth making is that the life-cycle activities should all be documented. Unless this is done, there is no visibility to the design process and an assessment cannot verify that the standard has been followed.

This should be a familiar discipline in as much as most readers will be operating within an ISO standard of practice. The design should be conducted under a project management regime and adequately documented to provide traceability. These requirements can be met by following a quality system such as specified in ISO The level and depth of the required project management and documentation will depend on the SIL level.

The use of checklists is desirable at all stages. IEC Part 2 as well as Part 3 for the software expects this to have been addressed. Irrespective of SIL there needs to be a basic project management structure which defines all the required actions and 44 Functional Safety 3. All documentation and procedures need to be well structured, for each design phase, and sufficiently clear that the recipient for the next phase can easily understand the inputs to that task.

SIL 3 and SIL 4 require, also, that the project management identify the additional procedures and activities required at these levels and that there is a robust reporting mechanism to confirm both the completion and correctness of each activity. The documentation used for these higher SIL systems should be generated based on standards which give guidance on consistency and layout and include checklists. In addition, for SIL 4 systems, computer aided configuration control and computer aided design documentation should be used.

Table B6 of the Standard elaborates a little on what constitutes a higher rigour of project management. We have attempted to avoid this repetition in this book. The need for validation planning is stressed in Section 7. In general this whole section will be met by implementing the Functional Safety Procedure described in Appendix 1. At the system application level the functional requirements i. All this can be suitable up to SIL 3. For SIL 4 applications structured methods should be used.

In the case of new product design rather than applications engineering i. In order to reduce the probability of common cause failures the specification should also cover the degree of separation required, both physically and electrically, between the EUC and the safety system s.

Any necessary data interchange between the two systems should also be tightly specified. These requirements need to be applied to any redundant elements of the safety-related system s. Achieving this separation may not always be possible since parts of the EUC may include a safety function that cannot be dissociated from the control of the equipment. This is more likely for the continuous mode of operation in which case the whole control system should be treated as safety-related pending target SIL calculations Section 2.

If the safety-related and non-safety-related system elements cannot be shown to be sufficiently independent then the complete system should be treated as safety-related. Physical separation should be considered. Physical separation of redundant parts of the safety system should be considered. These will address proven components and parts, preferred designs and configurations etc. In general, this requires that failure in a safety system having redundant paths should be repaired within the mean time to repair assumed in the hardware reliability calculations.

If this is not possible, then the procedure should be the same as for non-redundant paths as follows. On failure in the safety system with no redundant paths, either additional process monitoring should be provided to maintain adequate safety or the EUC should be shut down.

Many of these are contained in the documents listed in Chapter 9. Structured, in this context, implies clear partitioning of functions and a visible hierarchy of modules and their interconnection.

Such use of subjective descriptions e. In addition for SIL 3 systems, previous experience is needed in a relevant application and for a period of at least two years with ten devices or, alternatively, some third party certification. SIL 4 systems should be both proven in use, as mentioned above, and have third party certification. The coverage of these tests would need to be significantly increased for SIL 4 systems.

Thus the degree of testing of input and output modules, sensors and actuators would be substantially increased. Again, however, these are subjective statements and standards such as IEC do not and cannot give totally prescriptive guidance.

Nevertheless some guidance is given concerning diagnostic coverage. It should be noted that the minimum configuration table given in Section 3. This includes temperature and temperature cycling, EMC electromagnetic compatibility , vibration, electrostatic etc.

There needs to be feedback on operator actions, particularly when these involve keyboards, in order to assist the operator in detecting mistakes. As an example of this, for SIL 1 and SIL 2, all input operator actions should be repeated whereas, for SIL 3 and SIL 4, significant and consistent validation checks should be made on the operator action before acceptance of the commands.

The design should take into account human capabilities and limitations of operators and maintenance staff. Human factors are addressed in Chapter 6. The following section together with Appendix 4 discusses how to measure diagnostic capability and SFF. Should it then be necessary to enhance the diagnostic coverage, these tables can be used as a guide to techniques. This safe failure fraction, for each safety function, needs to be estimated as shown in Appendix 4. The higher the SFF percentage requirement the more onerous is the demonstration.

The tables provide the SIL number for each safe failure fraction case. Simplex infers no redundancy. At first this might seem a realistic range of safe fail fraction ranging from simple to comprehensive. However, it is worth considering how the diagnostic part of each of these coverage levels might be established.

There are two ways in which diagnostic coverage and safe failure fraction ratios can be assessed: 1. By test: where failures are simulated and the number of diagnosed failures, or those leading to a safe condition, are counted. By FMEA: where the circuit is examined to ascertain, for each potential component failure mode, whether it would be revealed by the diagnostic program or lead to a safe condition. This is illustrated in Appendix 4. In both cases the cost and time become more significant.

In order to take credit for diagnostic coverage, as described in the Standard i. For the case of a continuous system then the auto-test interval plus the time to put the system into a safe state should be within the time it takes for a failure to propagate to the hazard.

It involves specifying the reliability model, the failure rates to be assumed, the component down times, diagnostic intervals and coverage. Techniques such as FMEA failure mode and effect analysis and fault tree analysis are involved and Chapters 6 and 7 briefly describe how to carry these out.

The Standard refers to confidence levels in respect of failure rates and this will be dealt with later. In Chapter 1 we mentioned the anomaly concerning the allocation of the quantitative failure probability target to the random hardware failures alone.

There is yet another anomaly concerning judgement of whether the target is met. If the fully quantified approach described in Chapter 2 has been adopted then the failure target will be a PFD probability of failure on demand or a failure rate.

The reliability prediction might suggest that the target is not met although still remaining within the limits of the SIL in question. The rule here is that since we have chosen to adopt a fully quantitative approach we should meet the target set paragraphs 7. This is, of course, SIL 2. However, 52 Functional Safety 3.

Once again there is no right or wrong answer to the dilemma. The Standard does not address it and, as in all such matters, the judgement of the responsible engineer is needed. Both approaches are admissible and, in any case, the accuracy of quantification is not very high see Chapter 7. This is the type of testing which, for example, looks at the output responses to various combinations of inputs.

This applies to all SILs. For SIL 3 and SIL 4 the tests should be extended to include test cases that combine critical logic requirements at operation boundaries. These procedures, and the safety system interface with personnel, should be designed to be user, and maintenance, friendly. This applies to all SIL levels. There need to be records of the demand rate of the safety-related equipment, and furthermore failures also need to be recorded.

These records should be Meeting IEC Part 2 53 periodically reviewed, to verify that the target safety integrity level was indeed appropriate and that it has been achieved. This should include a study of the relationship between the safety-related system and the EUC.

It is necessary to ensure that any remedial action or additional testing arising from earlier tests has been carried out. This requirement applies to all SIL levels. This paragraph applies to all SILs.

Although Part 2 does not specify this for the hardware the authors consider this to be good practice. The random hardware failures prediction and safe failure fraction demonstrations are, however, still required. The previous field experience should be in an application and environment, which is very similar to the intended use. All failures experienced, whether due to hardware failures or systematic faults, should be recorded, along with total running hours.

Paragraphs 7. In Part 7 Annex D there are a number of pieces of statistical theory which purport to be appropriate to establishing confidence for software failures. The times for larger numbers of failures can be calculated accordingly i.

The following Conformance Demonstration Template is suggested as a possible format based on the layout of this chapter. The Standard, in Part 6, gives two examples of Part 3 assessments. It does not, however, provide these for the Part 2 requirements.

Conformance Demonstration Template IEC Part 2 For embedded software designs, with new hardware design, the demonstration might involve a reprint of all the tables from the Standard. The evidence for each item would then be entered in the right-hand column as in the simple tables below.

The following tables might be considered adequate for relatively simple designs, particularly with existing platforms and simple low variability code as in the case of PLCs. General Paras 7. Description of overall novelty, complexity, SILs, rigour needed etc.

Sections 4. Whereas the reliability prediction of hardware failures, as addressed in Section 3. All that can be reasonably claimed is that, given the state of the art, we believe the measures specified are appropriate for the integrity level in question and that therefore the systematic failures will probably be similar to and not exceed the hardware failure rate of that SIL.

The Annexes of Part 3 offer appropriate techniques, by SIL, in the form of tables followed by more detailed tables with cross-references. This chapter attempts to provide a simple and usable interpretation.

Chapter 5, using the restricted subset of IEC , also caters for the Type 2 case. Progressive testing of the system starts with the lowest level of software module, followed by integrating modules, and working up to testing the complete safety system. Normally, a level of testing for each level of design would be required.

The life-cycle should be described in writing as well as graphical figures such as are shown in Figures 4. System and hardware interfaces should be addressed and it should reflect the architectural design. At SIL 2 and above there needs to be evidence of positive justifications and reviews of departures from the life-cycle activities listed in the Standard. Figures 4.

Beneath each of the figures is a statement describing how they meet the activities specified in the Standard. Figure 4. The life-cycle model in Figure 4. Integration is a part of the functional test and validation is achieved by means of acceptance test and other activities listed in the Quality and Safety Plan. Validation is achieved by Requirements specification Quality and Safety plan Acceptance tests Functional specification Sub-system specifications Reviews Integration tests Module descriptions Functional tests Reviews Source code C and assembler maybe Static analysis semantic Figure 4.

The specification should extend down to the configuration control level. The document should be free from ambiguity and clear to those for whom it is intended. For SIL 1 and SIL 2 systems, this specification should use semiformal methods to describe the critical parts of the requirement e. For SIL 3 and SIL 4, semiformal methods should be used for all the requirements and, in addition, at SIL 4 there should be the use of computer support tools for the critical parts e.

In the event of detecting an error or fault the system should, if appropriate, be allowed to continue but with the faulty redundant element or complete part of the system isolated. Semi-formal methods should be applied, together with design and coding standards including structured programming, suitable for the application.

This is called for from SIL 2 upwards. There should also be limited use of interrupts, pointers, and recursion. Meeting IEC Part 3 4. At SIL 2 and above, dynamic objects and unconditional branches should be forbidden. At SIL 3 and SIL 4 more rigorous rules should be considered such as the limiting of interrupts and pointers, and the use of diverse functions to protect against errors which might arise from tools.

Version numbers of modules and of test instructions should be clearly indicated. Discrepancies from the anticipated results should be clearly visible. Any modifications or changes to the software, which are implemented after any phase of the testing, should be analysed to determine the full extent of retest that is required. As an example, for SIL 1 and SIL 2 systems the testing should include boundary value testing and partitioning testing and in addition, for SIL 3 and SIL 4, tests generated from cause consequence analysis of certain critical events.

The Functional Safety Management requirements Chapter 2 should cover the requirements for both validation and verification. It should cover the entire lifecycle activities and will show audit points.

At SILs 3 and 4 a more rigorous coverage of accuracy, consistency, conformance with standards e. This paragraph applies for all SIL levels. The modification records should make it clear which documents have been changed and the nature of the change.

There are packages available which carry out the procedures and, indeed, modern compilers frequently carry out some of the static analysis procedures such as data flow analysis. It should be remembered, however, that static analysis packages are only available for procedural high-level languages and require a translator that is language specific. It is, however, not trivial and might well involve several mandays of analysis effort for a line segment of code. It is not referred to in the Standard.

Static analysis, although powerful, is not a panacea for code quality. It only reflects the functionality in order for the analyst to review the code against the specification. Furthermore it is concerned only with logic and cannot address timing features. It is worth noting that, in Table B8, design review is treated as an element of static analysis. The term formal methods is much used and much abused.

In software engineering it covers a number of methodologies and techniques for specifying and designing systems, both non-programmable and programmable. These can be applied throughout the life-cycle including the specification stage and the software coding itself.

The term is often used to describe a range of mathematical notations and techniques applied to the rigorous definition of system requirements which can then be propagated into the subsequent design stages. The strength of formal methods is that they address the requirements at the beginning of the design cycle. One of the main benefits of this is that formalism applied at this early stage may lead to the prevention, or at least early detection, of incipient errors. The cost of errors revealed at this stage is dramatically less than if they are allowed to persist until commissioning or even field use.

This is because the longer they remain undetected the potentially more serious and far reaching are the changes required to correct them. The potential benefits may be considerable but they cannot be realised without properly trained people and appropriate tools. Formal methods are not easy to use. As with all languages, it is easier to read a piece of specification than it is to write it.

A further complication is the choice of method for a particular Meeting IEC Part 3 71 application. Unfortunately, there is not a universally suitable method for all situations. Ladder Logic which is a limited variability language usually having no branching statements. These earlier languages are suitable for use at all SILs with only minor restrictions on the instruction set.

Currently PLCs have wider instruction sets, involving branching instructions etc. With the advent of IEC there is a range of limited variability programming languages and the choice will be governed partly by the application. Again restricted subsets may be needed for safety-related applications. Some application specific languages are now available, as, for example, the facility to program plant shutdown systems directly by means of Cause and Effect Diagrams.

Inherently, this is a restricted sub-set created for safety-related applications. They are used in slightly different contexts but basically refer to the same concept of empirical evidence from use. It is frequently assumed that the reuse of software, including specifications, algorithms and code will, since the item is proven, lead to fewer failures than if the software were developed anew.

There are reasons for and against this assumption. The item has been subject to more than average test. The time saving can be used for more development or test.

The item has been tested in real applications environments. If the item has been designed for reuse it will be more likely to have stand-alone features such as less coupling. If the item has been designed for reuse it may contain facilities not required for a particular application therefore the item may not be ideal for the application and it may have to be modified.

Problems may arise from the internal operation of the item not being fully understood. In conclusion, provided that there is adequate control involving procedures to minimise the effects of the above then significant advantages can be gained by the reuse of software at all SILs.

An obvious example would be the number of branching statements in other words a measure of complexity which might be assumed to relate to error rate. There has been interest in this activity for many years but there are conflicting opinions as to its value. In the long term metrics, if collected extensively within a specific industry group or product application, might permit some correlation with field failure performance and safety-integrity.

The term metrics is also used to refer to statistics about test coverage, as called for in earlier paragraphs. It should also have satisfied suitable procedures, testing and verification for the SIL in question or have evidence to support its use from satisfactory previous use. Also, the operating experience should have exercised all the safety-related functions associated with the module. In Part 3, Paragraphs 7. In Part 7 Annex D there are a number of pieces of statistical theory which purport to be appropriate to the confidence in software.

Conformance Demonstration Template IEC Part 3 For embedded software designs, with new hardware design, the demonstration might involve a reprint of all the tables from the Standard.

Feature SIL 2 and above Alternative life-cycle models to be justified Configuration control to level of smallest compiled unit Feature SIL 3 and above Alternative life-cycle models to be justified and at least as rigorous Sample review of configuration status Feature SIL 4 Alternative measures to the life-cycle to be separately reviewed Evidence Evidence Evidence Evidence Specification Para. The standard was issued at the beginning of and is in three parts: Part 1 Part 2 Part 3 The normative standard Informative guidance on Part 1 Informative guidance on hazard and risk analysis Part 1 of the standard covers the life-cycle including: Management of Functional Safety Hazard and Risk Analysis Safety Instrumented Systems SIS Design through to SIS decommissioning The standard is intended for the activities of SIS system level designers, integrators and users in the process industry.

Component level product suppliers, such as field devices and logic solvers, are referred back to IEC as is everyone in the case of SIL 4. Part 2 gives general guidance to the use of Part 1 on a paragraph-by-paragraph basis.

Part 3 gives more detailed guidance on targeting the Safety Integrity Levels and has a number of appendixes covering both quantitative and qualitative methods. Since the standard is only aiming at the integration level of the SIS, rather than the individual elements, the requirements Meeting IEC 81 for design and development of the SIS covered by Parts 2 and 3 of IEC have been significantly simplified.

The software requirements are restricted to the applications software using either limited variability languages or fixed programs. For applications software using full variability languages the user is referred to IEC Unless specifically identified the same techniques and measures will be used for SILs 1, 2 and 3. Where a project involves the development and modification of a system architecture and application software for SIL 4, or the use of Full Variability Languages for applications software or the development of a sub-system product then IEC should be used.

Figure 5. The life-cycle is required to be included in the project Quality and Safety Plan. The assessment team should include at least one senior, competent person not involved in the project design. Part 1 of describes the typical layers of risk reduction namely Control and monitoring, Prevention, Mitigation, Plant emergency response and Community emergency response.

Part 3 gives examples of numerical approaches, a number of risk graphs and LOPA as mentioned in Section 2. Action taken on bad process variables e. The Standard gives guidance on the use of field devices and non-PE logic solvers for up to SIL 3 safety functions using proven-in-use justification. Unfortunately, both tables are formatted differently to the IEC table and assume type B sub-systems only i.

At any time the table in IEC can be used see Chapter 3. For interest the version is shown below. The complete system shall be validated by inspection and testing that the installed system meets all the requirements, that adequate testing and records have been completed for each stage of the life-cycle and that any deviations have been adequately addressed and closed out.

As part of this system validation the application software validation, if applicable, needs to be closed out. Change proposals will be positively identified, by the Project Safety Authority, as safety related or non-safety related. All safety-related change proposals will involve a design review, including an impact analysis, before approval. As a minimum the plan should include checking for completeness earthing, energy sources, instrument calibration, field devices Meeting IEC 89 operation, logic solver operation and all operational interfaces.

Records of all the testing results shall be kept and any deviations evaluated by a competent person. If there are any significant increases in hazard demand rate or decreases in the safety system availability between the design assumptions and those found in the operation of the plant which would compromise the plant safety targets then changes to the safety system will have to be made in order to maintain the plant safety. Chapters 3 and 4 provided Conformance Demonstration Templates comprising simplified tables for demonstrating conformance in the case of straightforward applications designs.

However, some of the items may not be applicable at this application type level. These two chapters largely concern random hardware failures. They go beyond IEC in providing a calibrated common cause failure model and a method of applying confidence limits to reliability predictions.

Since unavailability is the probability of being failed at a randomly chosen moment then it is the same as the probability of failure on demand.

The way in which failure is defined determines, to some extent, what is included in the down time. If the unavailability of a process is confined to failures whilst production is in progress then outage due to scheduled preventive maintenance is not included in the definition of failure. However, the definition of dormant failures of redundant units affects the overall unavailability as calculated by the equations in the next section. Furthermore, the component failure rates used will be those which lead to ignoring a genuine signal.

Now the component failure rates will be of the modes which lead to a spurious signal. The two most commonly used modelling methods are reliability block diagram analysis and fault tree analysis. Reliability modelling techniques 95 The mathematics of this arrangement is simple. We add the failure rates or unavailabilities of series items. However, the traditional results given in Reliability Maintainability and Risk and the majority of textbooks and standards have been challenged by K G L Simpson.

It is now generally acknowledged that the traditional Markov model does not correctly represent the normal repair activities for redundant systems. The results are summarised here to enable the failure rates and unavailabilities of redundant configurations to be calculated. The suitability of Markov modelling of redundant repairable systems has been questioned by a number of people. Ken Simpson has studied a range of redundant systems and applied various different techniques to calculate system failure rates and unavailabilities.

When comparing the results from the different techniques there is good agreement with the exception of conventional Markov modelling which shows a pessimistic difference of for a 1oo2 and up to for 1oo4 voted systems. For a redundant repairable system without a dedicated repair crew per equipment the transition from a multiple failure state does not depend on the repair of the last failure as it should for the process to be applicable to a Markov model but on the continued repair of the previous failure.

For this reason a Markov model of this system is pessimistic as it underestimates the transition rate from the failed state. It is as if the repair crew abandon the earlier repair to carry out the repair of the latest failure. With a dedicated repair crew per equipment the repair of the last failure is independent of preceding failures and the process is a Markov one.

The calculations give the correct answer, which in real life situations is not a practicable situation. For a redundant repairable system with faults detected at periodic inspection for failed items the process is also not a Markov one as the transition rate from the failed state multiple failures is a function of the time spent in the previous state only one item failed. The KGLS paper above should be consulted for a deeper understanding.

It is worth mentioning that, as with all redundant systems, the total system failure rate or PFD will be dominated by the effect of common cause failure dealt with later in this chapter. Tables 6. Table 6. Whether manually scheduled or automatically initiated e. Two examples are: a The presence of water vapour in gas causing two valves to seize due to icing.

In this case the interval between the two failures might be in the order of days. However, if the proof-test interval for this dormant failure is two months then the two failures will, to all intents and purposes, be simultaneous.

Typically, causes arise from a b c d Requirements: incomplete or conflicting. Design: common power supplies, software, emc, noise. Manufacturing: batch related component deficiencies. Defences against CCF involve design and operating features which form the assessment criteria given in Appendix 3. Common cause failures often dominate the unreliability of redundant systems by virtue of defeating the random coincident failure feature of redundant protection. Consider the duplicated system in Figure 6.

The failure rate of the redundant element in other words the coincident failures can be calculated using the formula developed in Table 6. However, if only one failure in 20 is of such a nature as to affect both channels and thus defeat the redundancy, it is necessary to add the series element, shown as 2 in Figure 6.

The effect is to swamp the redundant part of the prediction and it is thus important to include CCF in reliability models.

This sensitivity of system failure to CCF places emphasis on the credibility of CCF estimation and thus justifies efforts to improve the models. Thus the BETA multiplier is made up by adding together the contributions from each of a number of factors within each group. In other words the choice of checklist scores, when assessing the design, can be recorded and reviewed; it is possible for any user of the model to develop the checklists further to take account of any relevant failure causal factors that may be perceived; it is possible to calibrate the model against actual failure rates, albeit with very limited data; there is a credible relationship between the checklists and the system features being analysed.

The method is thus likely to be acceptable to the non-specialist; the additive scoring method allows the partial contributors to to be weighted separately; the method acknowledges a direct relationship between 2 and 1 as depicted in Figure 6. Reliability modelling techniques b SCORING: The maximum score for each question has been weighted by calibrating the results of assessments against known field operational data.

Column A contains the scores for those features of CCF protection which are perceived as being enhanced by an increase in diagnostic frequency. Column B , however, contains the scores for those features believed not to be enhanced by an improvement in diagnostic frequency. In some cases the score has been split between the two columns, where it is thought that some, but not all, aspects of the feature are affected see Appendix 3.

The A column scores are modified by multiplying by a factor C derived from diagnostic related considerations. Many safety-related systems that would have used electro- mechanical technology or solid-state electronics now use pro- grammable electronics instead.

Devices such as programmable controllers, programmable logic controllers PLCs and digital communication systems e. Furthermore, enabling technologies, such as application specific integrated circuits ASICs , micro- processors, and intelligent sensors, transmitters and actuators, are increasingly being integrated into products and systems.

Example applications include crane safe load indicators, vari- able speed motor drives used to restrict speed for protection, systems for interlocking and controlling the exposure dose of medical radiotherapy machines, or the indicator lights, anti- lock braking, and engine-management systems on automo- biles. Other examples are emergency shutdown systems in haz- ardous chemical plants, railway signalling systems and fly-by- wire operation of aircraft flight control surfaces.

An example is the remote monitoring, operation or programming of a net- work-enabled water treatment plant. For example an information-based decision support tool might be safety-related if erroneous results affect safety.

IEC is the basis for a published nuclear sector stan- dard. It is also currently being used as a basis for developing other sector standards e. Many requirements of IEC , particularly in parts 2 and 3, are not repeated in the application sector or product stan- dards but are referenced instead. The result is that most users will need IEC also. The market for any product, component or subsystem that complies with IEC is potentially very large since in principle they are capable of meeting the requirements of any sector standard based on IEC The worldwide use of IEC standards supports the transfer of electrotechnology, assists conformity assessment for example, certification and promotes international trade of uniform high- quality products and services.

International standards establish objective specifications that both buyer and seller can rely on. For buyers, they widen the range of choices and lower costs, primarily because they often increase the number of competitors.

In short, for everyone in society, international standards raise the overall efficiency and productivity of the economy. The IEC membership, which now comprises more than 60 countries, includes all the world's major trading nations. This membership collectively represents about 85 percent of the world's population and 95 percent of the world's electrical generating capacity.

Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. The scheme lists the referenced standards and specifies procedures which describes their test methods, surveillance audit policy, public documentation policies, and other specific aspects of their program.

The faster, easier way to work with standards. MISRA C has gone on to become the de facto standard for embedded C programming in the majority of safety-related industries, and also used to improve software quality even where safety is not the main consideration. IEC specifies techniques that should be used for each phase of the life-cycle.

Specific techniques ensure that mistakes and errors are avoided across the entire life-cycle. Search all products by. Each has defined jec own scheme based upon IEC and other functional safety standards.

Accept and continue Learn more about the cookies we use and how to change your settings. Retrieved from ic https: Articles needing additional references from March All articles needing additional references Use British English Oxford spelling from January Your basket is empty. Views Read Edit View history. The main requirement in Unit Testing is to ensure that the software is fully tested at the function level and that all possible branches and paths are taken through the software.

The process industry sector includes many types of manufacturing processes, such as refineries, petrochemical, chemical, pharmaceutical, pulp paper, and power. Take the smart route to manage medical device compliance. The requirements include appropriate quality control, management processes, validation and verification techniques, failure analysis etc. Central to the standard are the concepts of probabilistic risk for each safety function.

The risk is a function of frequency or likelihood of the hazardous event and the event consequence severity.