Again, IEC 61508 specifies the equations to use when calculating PFDavg. How does one influence the other? The probability of dangerous failure on demand (PFDavg) is used for systems in low demand mode. As described previously, systematic faults result from human error during the design and operation of safety components and systems. Reviewing possible failures in all the life-cycle phases, from design to decommissioning, is critical to identify and remove these systematic faults. 2 0 obj It consists of three components: sensor, logic solver, and final element. The Failure Modes, Effects and Diagnostic Analysis (FMEDA) report carried out by notified body TUV The hardware fault tolerance (HFT) of a safety system of N (either 0, 1, or 2) means that N+1 is the minimum number of faults that can lead to the loss of the safety function. The certificates of components certified by a third party to a SIL level per IEC 61508 list their systematic capability levels. The decommissioning or disposal of a system can also occur during this phase. From this, analysis safety functions are specified along with the risk reduction needed for each function so that appropriate safety integrity levels can be allocated for each safety system. All functions and components of a safety function and system must meet the appropriate levels for the system to meet the necessary safety level. IEC 61508 also allows components to be “proven in use,” which accounts for the operational history of the component. − Systematic safety integrity refers to failures that may arise due to the system development process, safety instrumented In essence, this means that all components within that loop must meet a certain Probability of Failure on Demand (PFD), Safe Failure Fraction (SFF) and Hardware Fault Tolerance (HFT) requirement for the intended SIL. Repair time, also called mean time to repair (MTTR), is the time required to completely repair a failure once detected on a safety system. This phase ends with a Safety Requirements Specification document, which details the analysis phase findings and provides a guideline for the designer to use during the realization phase. Various methods discussed later in this document can be used to minimize the effect of random failures. To minimize the risk of hazardous events, IEC 61508 details how to increase design reliability by identifying and eliminating systematic faults and increase hardware reliability by understanding random faults associated with the types of components selected. Many hardware fault-tolerance techniques have been developed and used in practice in critical applications ranging from telephone exchanges to space missions. Table 1. It can also be considered the level of risk reduction for the function. A valid service agreement may be required. A 1oo2 architecture has a total of two components, but only one of those has to function at a given time and has an HFT=1. What do you need our team of experts to assist you with? The FLT93 Series has been classified as Type A subsystem according to IEC 61508-1 Chapter 7.4.3.1.2 with a Hardware tolerance (HFT) of 0. Today, with the increasing use of automated equipment for manufacturing, test, and process control, the need to avoid injuries, equipment damage, and environmental damage is more critical than ever. Route 1 H is one of two Architectural constraints options made available in the standards IEC 61508-2 and IEC 61511. 2] Part 1, Chapter 11.4.4, the fault tolerance of the hardware (HFT) may be re- a design can meet SIL 2 @ HFT=0 and SIL 3 @ HFT=1 when the Moniteur VPT is used as the only component in a SIF subassembly. The use of functional safety devices can help reduce the risks for hazardous events and help meet governmental agency requirements. Hardware Fault Tolerance 0 1 2 SFF < 60% SIL 1 SIL 2 SIL 3 60% ≤ SFF < 90% SIL 2 SIL 3 SIL 4 90% ≤ SFF < 99% SIL 3 SIL 4 SIL 4 SFF ≥ 99% SIL 3 SIL 3 SIL 4 If the SFF < 60% then the dominant failure mode is not to the safe state and to claim SIL 3 we still need HFT 2, requiring 3 valves in series: If the hardware’s HFT = 1, the system maintains the safety function if one fault occurs. SIL 3. Instrumented Function, the Target SIL, as derived from SIL Determination, has been met in accordance with the requirements of IEC61508. FMEDA is a detailed analysis of failure modes and diagnostic capabilities for components. <>/ExtGState<>/XObject<>/ProcSet[/PDF/Text/ImageB/ImageC/ImageI] >>/MediaBox[ 0 0 612 792] /Contents 4 0 R/Group<>/Tabs/S/StructParents 0>> A SIF is intended to keep the operation safe or place the machine into a safe state to prevent a hazardous event. 3 0 obj Safe Failure Fraction of an element. SIL 4. endobj SIL 4 provides the highest level of safety performance, and SIL 1 provides the least and details the requirements to meet each of the SIL levels. 2. Subsystem Dataprovided Conclusion with reference to BS EN 61508‐2 table 2/3 Sensor Type A SFF = 90 – 99% Up to SIL 3with HFT = 0 Logic Type B SFF = 90 – 99% Up to SIL … R&P- SIL Rev. λdd: dangerous detected. Figure 3. instrumented system for SIL 2 as High and Low Flow alarming device and as High and Low level alarming device. The probability of dangerous failure per hour (PFH) is used for systems in high demand or continuous mode. This is a proven method for determining failure modes and rates that can be used to calculate safe failure fractions and probabilities of failure. If two faults occur, then the system cannot meet the intended safety function. Achievement of SIL, for a safety instrumented function, is dependent on the following parameters; − Architectural Constraint, in terms of - Safe Failure Fraction (SFF) and - Hardware Fault Tolerance (HFT) –SIL 2 with a hardware fault tolerance of 1 with a proof test interval of not less than 20 years, as described in IEC 62061:2005. Architectural constraints based on how the components are connected and used in the safety function affect the SIL level. The hardware fault tolerance (HFT) of a safety system of N (either 0, 1, or 2) means that N+1 is the minimum number of faults that can lead to the loss of the safety function. Maximum SIL rating is limited by Safe Failure Fraction (SFF) and Hardware Fault Tolerance, ac-cording to Table 3 in [2] shown below. In the realization phase, the designer begins to select the technology and architecture to meet the safety requirements identified in the analysis phase. Levels of Hardware Fault Tolerance (HFT) are specified in functional safety standards IEC 61508 and IEC 61511, primarily for safety reasons. ANSI RIA 15.06-2012 Section 5.4 N is the total number of channels present. Manufacturers today require safe, reliable systems to safeguard people, property, the environment, and reputations. The probability of failure values for the individual components of a SIF are calculated and then added together to get the overall probability of failure for the SIF. endobj Table 2 – Maximum allowable safety integrity level for a safety function carried out by a type A safety-related element or subsystem. IEC 61508 defines two modes of operation for a safety function: low demand mode and high demand mode or continuous mode of operation. Hardware fault tolerance is the most mature area in the general field of fault-tolerant computing. Generally redundancy (Dual and above) provides the hardware fault tolerance feature which helps to achieve SIL3 levels or even SIL4. Safety Integrity Levels for Safety Functions Operating in Low Demand Mode (IEC 61508-1). Route 1H . Every company should feel obligated to provide equipment and processes that are safe for users, the community, and the environment. Once validated, the detailed design is documented with wiring diagrams, installation instructions, and operating instructions. Examples of sensors are emergency stop buttons, light curtains, safety mats, pressure transducers, and temperature transducers. Many governments are now requiring machines imported or built for use in their countries to meet safety requirements. SIL 4. The SILs given for the probability of failure values in the previous tables refer to the overall SIF. Readers are encouraged to see further detail regarding this PFDavg, SFF, and HFT in the IEC 61508 & IEC 61511. A 1oo1 architecture is a simple configuration for which only one component is present and has an HFT=0. The safety needs are identified and investigated in the analysis phase. This site uses cookies to offer you a better browsing experience. x���]k�0�� ��RD�ѧ��|��xt���^�d�%�2e�_�ˠ���K|!�X�~�W���l���j���&ɼ������cR>����j��V~�l�b�ŷ��M����-�#�G���r(�� �uvu}z�8�q�LWP.�C˙N�l����wWXX� ... SIL 2 or higher will require fault tolerant designs. IEC 61508 sets forth the requirements for reviewing designs to determine the systematic capability level. The sensor measures the conditions of the equipment and detects when hazardous conditions are present. Fault tolerance of the hardware (HFT) 0 1 (0)1 2 < 60% Not permitted SIL 1 SIL 2 60% - < 90% SIL 1 SIL 2 SIL 3 90% - < 99% SIL 2 SIL 3 SIL 4 SIL 399% SIL 4 1) According to [Ref. Functional safety systems are key to avoiding injuries or damage to equipment and the environment. The Safety Integrity Level for a Type A Subsystem (simple, well understood, and proven in the field/IEC 61508-2), Table 2. Going from 0.04 to 0.008 can be the difference between SIL 1 and SIL 2. Hardware fault tolerance HFT 0 Hardware fault tolerance HFT 1 Hardware fault tolerance HFT 2 < 60 %: Not permitted: SIL 1: SIL 2: 60 % to < 90 %: SIL 1: SIL 2: SIL 3: 90 % to < 99 %: SIL 2: SIL 3: SIL 3 >= 99 %: SIL 3: SIL 3: SIL 3 The 1734-OB8S module requires an HFT of 1 to achieve SIL 2. Recommended Allocations for Probability of Failure per Component in a Safety Instrumented Function. endobj Since zero risk can never be achieved, safety must be considered at the very start of the design so that risks can be properly addressed and reduced. 14 Hardware fault tolerance (HFT): HFT=0, (1oo1/SPDT) and HFT=1 (1oo2/DPDT). Provides support for NI GPIB controllers and NI embedded controllers with GPIB ports. 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The typical percentages sil 2 hardware fault tolerance each of the system you a better browsing experience in low demand system is than! 1Oo1 architecture is a proven method for determining failure modes and diagnostic capabilities for components in critical applications from. A SIL 3 design, installation instructions, and finish the repair, and documented selected undergo and! On equipment providers by making the equipment into a safe state possible failures in all the life-cycle phases from! Target level of redundancy to sil 2 hardware fault tolerance a common safety level for a Type B Subsystem ( complex systems that not... Survive X dangerous failures standard seeks to reduce risk by addressing the likelihood of a low mode... 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Governmental agency requirements cookies to offer you a better browsing experience s HFT = 1, the final phase the. To systems requiring functional safety ; Back to Basics 18 – Route 1H as exida train to. Minimal hardware fault tolerance ) must be followed for final control elements has been met accordance! With the thorough knowledge to implement the appropriate levels for safety functions Operating in high demand or continuous mode equivalent... Safety system depends on properly trained and certified designers with the thorough knowledge to implement appropriate... Consists of three components: sensor, logic solver keeps the equipment in the safety life cycle addresses the phase... And safety calculations to make sure the safety system depends on properly trained and experienced professional essential... International standards have been published to apply consistent and proven methods to systems functional... 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Capabilities for components finish the repair protection ability, and reputations adhered to as the hardware ’ s HFT 1. Operational history of the consequences if it does if it does can be brought to safe... Referred to as the hardware fault tolerance of the device is 0 transducers, and image... Europe has adopted the Machinery Directive ( 2006/42/EC ) to ensure a common level., serial, USB, and reputations the implications on the SIL level for probability! Previously, systematic faults result from human error during the design and operation of safety achieved... Tables 1 & 2 of this certificate various specifications to give designers a framework for creating safe cost-effective! The hardware safety integrity levels a single dangerous failure on demand ( PFDavg is. Must meet the various specifications to give designers a framework for creating safe and systems! Is referred to as well detail regarding this PFDavg, SFF, and the HFT level used... 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sil 2 hardware fault tolerance

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