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. IEC 61508 specifies two types of subsystems (components), Type A and Type B, and requires certain SFF and HFT conditions that depend on these subsystems. Type B Device: A … Systems or functions with ZERO hardware fault tolerance (HFT = 0) cannot tolerate a single dangerous failure. Safety Life Cycle Defined by IEC 61508. You can request repair, schedule calibration, or get technical support. 1 0 obj
Again, IEC 61508 specifies the equations to use when calculating PFDavg. SIL Explained Safe Failure Fraction Architectural constraints on Type A safety-related subsystems Safe Failure Fraction Hardware Fault Tolerance* 0 1 2 < 60% SIL1 SIL2 SIL3 < 60% - < 90% SIL2 SIL3 SIL4 90% - < 99% SIL3 SIL4 SIL4 > 99% SIL3 SIL4 SIL4 * A hardware fault tolerance of N means that N + 1 faults could cause a loss of the safety function. Examples of final elements are relays and valves. To help understand the risks and likelihood of failures caused by random hardware faults, techniques such as failure mode effects and diagnostics analysis (FMEDA) are conducted. Running in continuous mode is equivalent to running in very high demand mode. − Systematic safety integrity refers to failures that may arise due to the system development process, safety instrumented 60% ≤ 90%. Learn more about the LOPA functionality in BowTieXP. Safe failure Hardware fault tolerance (see note) fraction SFF 0 1 2 < 60 % Not allowed SIL 1 SIL 2 60 % - < 90 % SIL 1 SIL 2 SIL 3 90 % - < 99 % SIL 2 SIL 3 SIL 4 99 % SIL 3 SIL 4 SIL 4 Note 2: A hardware fault tolerance of N means that N + 1 undetected faults could cause a loss of the safety function The requirements of minimum hardware fault tolerance (HFT) according to Tab.6 of IEC 61511-1 have to be observed but, as long as has been performed an assessment report Layers of Protection Analysis (LOPA) is presented in the IEC 61511 standard, and many of our users may not have yet discovered the industry-verified LOPA Plugin tool for BowTieXP that integrates LOPA in the BowTie model. 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: Table 3 shows the required PFH values for high demand or continuous mode systems to meet the various SIL levels. Architectural constraints based on how the components are connected and used in the safety function affect the SIL level. Generally redundancy (Dual and above) provides the hardware fault tolerance feature which helps to achieve SIL3 levels or even SIL4. 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. 13 Safe Failure Fraction: See tables 1 &2 of this certificate. A defined life cycle addresses the analysis, design, installation, operation, and maintenance of equipment. Very generally speaking, the higher the safety integrity Level (SIL) required, the more hardware fault tolerance is expected in the design. Provides support for Ethernet, GPIB, serial, USB, and other types of instruments. Figure 3. ANSI RIA 15.06-2012 Section 5.4 In this post we explain the differences. Table 4 shows the required PFDavg values for low demand systems to meet the various SIL levels: Probability of Dangerous Failure on Demand (PFDavg), Table 4. <>
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. 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. The likelihood of a malfunction or failure of a system due to hardware faults, known as the probability of failure, depends on the mode of operation. Companies can calculate the probability of failure for a component and use it to determine the amount of risk associated with the component and system. Any failures detected in proof tests are repaired so the system is in a like-new state. It can also be considered the level of risk reduction for the function. An example of a low demand system is a high integrity pressure protection system (HIPPS) in a processing plant. Systematic faults result from human error during the design and operation of safety components and systems. Safety Integrity Levels for Safety Functions Operating in Low Demand Mode (IEC 61508-1). 2. IEC 61508 sets forth the requirements for reviewing designs to determine the systematic capability level. All hardware used in this safety function, except the 1734-OB8S digital safety output module, is capable of achieving SIL 2 with a hardware fault tolerance (HFT) equal to 0. 1. The FLT93 Series has been classified as Type A subsystem according to IEC 61508-1 Chapter 188.8.131.52.2 with a Hardware tolerance (HFT) of 0. Dutyholders have the obligation to keep record of all incidents, process deviations, and non-conformities. Many governments are now requiring machines imported or built for use in their countries to meet safety requirements. The safety integrity level (SIL) is a measure of the safety performance for a safety function. By increasing the frequency of proof tests, designers can reach higher SIL levels, but they must consider the cost and complexity of the test. SILs depend on many different factors such as systematic capability level for the design and the component suppliers, architectural constraints, hardware fault tolerance and safe failure fraction, and the probability of failure. Route 1 H is one of two Architectural constraints options made available in the standards IEC 61508-2 and IEC 61511. Safety system designs account for random failures using statistical information produced from test and historical data. In the realization phase, the designer begins to select the technology and architecture to meet the safety requirements identified in the analysis phase. For a SIL 3 design, an HFT = 1 must be followed for final control elements. The financial impact due to liability claims, equipment loss, business interruption, and company image can severely affect businesses of all sizes. FMEDA is a detailed analysis of failure modes and diagnostic capabilities for components. endobj
IEC 61508 defines two modes of operation for a safety function: low demand mode and high demand mode or continuous mode of operation. SIL 4. <>/ExtGState<>/XObject<>/ProcSet[/PDF/Text/ImageB/ImageC/ImageI] >>/MediaBox[ 0 0 612 792] /Contents 4 0 R/Group<>/Tabs/S/StructParents 0>>
Safe Failure Fraction of an element. This means there must be at least 1 level of redundancy to ensure the system can be brought to its safe state. 15 Highest SIL (architecture/type A/B): Type A. endobj
It then sends an output signal to a final element to place the equipment into a nonhazardous/safe state. At this point, the system can be installed and commissioned so that a factory acceptance test can be completed. 2 A hardware fault tolerance of X means that X + 1 dangerous failures would cause a loss of the safety function. The Failure Modes, Effects and Diagnostic Analysis (FMEDA) report carried out by notified body TUV IEC 61508 defines four SIL levels. Levels of Hardware Fault Tolerance (HFT) are specified in functional safety standards IEC 61508 and IEC 61511, primarily for safety reasons. If two faults occur, then the system cannot meet the intended safety function. 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. A 1oo1 architecture is a simple configuration for which only one component is present and has an HFT=0. The standard seeks to reduce risk by addressing the likelihood of a hazardous event occurring and the severity of the consequences if it does. Every company should feel obligated to provide equipment and processes that are safe for users, the community, and the environment. A trained and experienced professional is essential to make sure the safety life cycle is properly followed, validated, and documented. A valid service agreement may be required. IEC 61511 Part 1: 3.2.72 says a safety instrumented system (SIS) is an “instrumented system used to implement one or more safety instrumented functions. The SILs given for the probability of failure values in the previous tables refer to the overall SIF. What do you need our team of experts to assist you with? Provides support for NI GPIB controllers and NI embedded controllers with GPIB ports. R&P- SIL Rev. When a system runs in high demand mode, the frequency for safety demands on the system is less than a year. 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 is exactly redundancy, HFT and voting? 90% ≤ 99%. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s).” A SIS is used to prevent or minimize the risk associated with possible hazardous conditions in process and equipment. The level of safety is achieved by avoiding or controlling faults. Potential hazards and associated risks must be considered from the very beginning of the design, during the deployment and operation, and through the system decommissioning. 3 0 obj
Many hardware fault-tolerance techniques have been developed and used in practice in critical applications ranging from telephone exchanges to space missions. Table 2 – Maximum allowable safety integrity level for a safety function carried out by a type A safety-related element or subsystem. If two faults occur, then the system cannot meet the intended safety function. N is the total number of channels present. The probability of dangerous failure on demand (PFDavg) is used for systems in low demand mode. The probability of dangerous failure per hour (PFH) is used for systems in high demand or continuous mode. %����
The development process and quality system are evaluated during certification to determine the systematic capability level. SIL 1 SIL 2 1 SIL 1 SIL 2 SIL 3 2 SIL 2 SIL 3 Hardware Fault Tolerance (HFT) for Type B Device Safety Failure Fraction (SFF): The ratio of the average rate of safe failures plus dangerous detected failures of the subsystem to the total average failure of the subsystem.