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IEC 61508 – IEC 61511 Presentation G.M. International s.r.l Via San Fiorano, 70 20058 Villasanta (Milano) ITALY Document.

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Presentation on theme: "IEC 61508 – IEC 61511 Presentation G.M. International s.r.l Via San Fiorano, 70 20058 Villasanta (Milano) ITALY Document."— Presentation transcript:

1 IEC – IEC Presentation G.M. International s.r.l Via San Fiorano, Villasanta (Milano) ITALY Document last revised 20 May 2005

2 Standard Definitions Title: Standard for Functional Safety of Electrical / Electronic / Programmable Electronic Safety-Related System IEC has been developed as a Process Sector of IEC Title: Safety Instrumented Systems for the Process Industry

3 Standard History The IEC was conceived to define and harmonize a method to reduce risks of human and/or valuable harms in all environments. The IEC integrates and extends American Standard ISA-S84.01 (1996) and German DIN (1994).

4 Standard Requirements

5 Other related standards DIN (1994) Title: Fundamental Safety aspects to be considered for measuring and control equipment Deals with Quantitative Risk Analysis used for Part 5 of IEC 61508, classification in AK classes 1-8 similar to SIL levels ISA-S84.01 (1996) Title: Application of Safety Instrumented Systems (SIS) for the process industry Defines Safety Lifecycles assuming Risk analysis and SIL been carried out.

6 Fundamental Concepts Risk Reduction and Risk Reduction Factor (RRF) Safety Integrity Level (SIL) Independence Levels and consequences Probability of Failure on Demand (PFD) Reliability Availability Failure Rate (λ) Proof Test Interval between two proof tests (T[Proof]) Failure In Time (FIT) Mean Time To Failure (MTTF) Mean Time Between Failure (MTBF) Mean Time To Repair (MTTR) Safe Failure Fraction (SFF) Safety Lifecycle Safety Instrumented System (SIS)

7 Risk Reduction As Low As Reasonably Practicable or Tollerable Risk (ALARP ZONE) Fundamental Concepts

8 Risk Reduction Fundamental Concepts

9 Safety Integrity Level (SIL) SIL levels (Safety Integrity Level) RRF (Risk Reduction Factor) PFD avg (Average Probability of Failure on Demand) SIL Table for Demand and Continuous mode of Operation Fundamental Concepts

10 Independence Levels Assessement Independence Level as a function of consequences Fundamental Concepts

11 PFDavg / RRF Correlation between Probability of Failure on Demand and Risk Reduction Factor Fundamental Concepts

12 Reliability Reliability is a function of operating time. All reliability functions start from reliability one and decrease to reliability zero. The device must be successful for an entire time interval. The statement: Reliability = 0.76 for a time of hs makes perfect sense. R(t) = P(T>t) Fundamental Concepts

13 Reliability Reliability is the probability that a device will perform its intended function when required to do so, if operated within its specified design limits. –The device intended function must be known. –When the device is required to function must be judged. –Satisfactory performance must be determined. –The specified design limits must be known. Mathematically reliability is the probability that a device will be successful in the time interval from zero to t in term of a random variable T. Fundamental Concepts

14 Availability Availability is the probability that a device is successful at time t. No time interval is involved. A device is available if its operating. The measure of success is MTTF (Mean Time To Failure) Fundamental Concepts

15 MTTF MTTF is an indication of the average successful operating time of a device (system) before a failure in any mode. MTBF (Mean Time Between Failures) MTBF = MTTF + MTTR MTTF = MTBF - MTTR MTTR (Mean Time To Repair) Since (MTBF >> MTTR) MTBF is very near to MTTF in value. Fundamental Concepts

16 MTBF and Failure Rate Relation between MTBF and Failure Rate λ Failure per unit time 1 λ = = Quantity Exposed MTBF 1 Quantity Exposed MTBF = = λ Failure per unit time Fundamental Concepts

17 MTBF - Example Instantaneous failure rate is commonly used as measure of reliability. Eg. 300 Isolators have been operating for 10 years. 3 failures have occurred. The average failure rate of the isolators is: Failure per unit time 3 λ = = = Quantity Exposed 300*10*8760 = per hour = = 38 FIT (Failure per billion hours) = = 38 probabilities of failure in one billion hours. MTBF = 1 / λ = 303 years (for constant failure rate) Fundamental Concepts

18 Failure Rate Categories λ tot = λ safe + λ dangerous λ s = λ sd + λ su λ d = λ dd + λ du λ tot = λ sd + λ su + λ dd + λ du Where: sd = Safe detected su = Safe undetected dd = Dangerous detected du = Dangerous undetected Fundamental Concepts

19 FIT Failure In Time is the number of failures per one billion devices hours. 1 FIT = 1 Failure in 10 9 hours = = Failures per hour Fundamental Concepts

20 SFF (Safe Failure Fraction) Fundamental Concepts SFF summarizes the fraction of failures, which lead to a safe state and the fraction of failure which will be detected by diagnostic measure and lead to a defined safety action

21 Type A SFF Chart Type A components are described as simple devices with well-known failure modes and a solid history of operation Fundamental Concepts

22 Type B SFF Chart Type B: Complex component (using micro controllers or programmable logic); according of IEC Fundamental Concepts

23 HSE Study Results of system failure cause study done by English Health and Safety Executive (HSE) Fundamental Concepts

24 Safety Lifecycle Origin Fundamental Concepts

25 Safety Lifecycle 1/5 Fundamental Concepts

26 Safety Lifecycle 2/5 First portion of the overall safety lifecycle ANALYSIS (End user / Consultant) Fundamental Concepts

27 Safety Lifecycle 3/5 Realisation activities in the overall safety lifecycle Fundamental Concepts

28 Safety Lifecycle 4/5 Safety lifecycle for the E/E/PES (Electrical / Electronic / Programmable Electronic) Safety - Related System (IEC 61508, Part 2) Fundamental Concepts

29 Safety Lifecycle 5/5 Last portion of the overall safety lifecycle OPERATION (End User / Contractor) Fundamental Concepts

30 SIS SIS (Safety Instrumented System) according to IEC and IEC Fundamental Concepts

31 IEC Safety Instrumented Systems for Process Industry IEC has been developed as a Process Sector implementation of the IEC The Safety Lifecycle forms the central framework which links together most of the concepts in this standard, and evaluates process risks and SIS performance requirements (availability and risk reduction). Layers of protection are designed and analysed. A SIS, if needed, is optimally designed to meet particular process risk.

32 Process sector system standard IEC 61511

33 IEC Parts The Standard is divided into three Parts Part 1: Framework, Definitions, Systems, Hardware and Software Requirements Part 2: Guidelines in the application of IEC Part 3: Guidelines in the application of hazard and risk analysis IEC 61511

34 IEC Part 3 Guidelines in the application of hazard and risk analysis IEC 61511

35 FMEDA Failure Modes and Effects Diagnostic Analysis (FMEDA) Is one of the steps taken to achieve functional safety assessement of a device per IEC and is considered to be a systematic way to: identify and evaluate the effects of each potential component failure mode; classify failure severity; determine what could eliminate or reduce the chance of failure; document the system (or sub-system) under analysis.

36 FMEDA The following assumptions are usually made during the FMEDA Constant Failure Rates (wear out mechanisms not included) Propagation of failures is not relevant Repair Time = 8 hours Stress levels according IEC , Class C (sheltered location), with temperature limits within the manufacturers rating and an average temperature over a long period of time of 40°C

37 FMEDA

38 1oo1 Architecture PFD avg (T1) = λ dd * RT + λ du * T1/2 because RT (avg. repair time) is << T1 PFD avg = λ du * T1/2 λ du = λ du (sensor) + λ du (isolator) + λ du (controller) + λ du (final element) SIL level is the lowest in the loop.

39 1oo2 Architecture PFD avg = λ duc * (T1/2) + λ ddc * RT+(λ ddn * RT) 2 + (λ ddn * RT * λ dun * T1) 2 /2 + (λ dun * T1) 2 /3 PFD avg = (λ dun * T1) 2 /2 + (λ dun * T1) 2 /3

40 2oo3 Architecture PFDavg = λduc * (T1/2) + 3[λddc * RT+(λddn* RT)2 + (λddn* RT * λdun* T1)2/2 + (λdun* T1)2 /3]

41 SIL3 using SIL2 subsystem SIL3 Control Loop or Safety Function using SIL2 SubSystems in 1oo2 Architecture

42 Safety Manual A Safety Manual is a document provided to users of a product that specifies their responsabilities for installation and operation in order to maintain the design safety level. The following information shall be available for each safety- related sub-system..

43 Safety Manual Requirements 1.Functional specification and safety function 2.Estimated rate of failure in any mode which would cause both undetected and detected safety function dangerous failures 3.Environment and lifetime limits for the sub-system 4.Periodic Proof Tests and/or maintainance requirements 5.T proof test time interval 6.Information necessary for PFD avg, MTTR, MTBF, SFF, λ du, λ total 7.Hardware fault tolerance and failure categories 8.Highest SIL that can be claimed (not required for proven in use sub-systems) 9.Documentary evidence for sub-systems validation (EXIDA) Procedures 10.Proof Test Procedures to reveal dangerous faults which are undetected by diagnostic tests.

44 Standard references Remembering that: SIL (Safety Integrity Level) RRF (Risk Reduction Factor) PFD avg (Average Probability of Failure on Demand) SIL Table for operative modes high and low demand Using the Safety Manual

45 Standard references Remembring definitions given for type A and B components, sub-systems, and related SFF values Using the Safety Manual

46 Loop PFD avg calculation 1oo1 typical control loop PFDavg(sys) = PFDavg(tx) + PFDavg(i) + PFDavg(c) + PFDavg(fe) Using the Safety Manual

47 Loop PFD avg calculation For calculating the entire loops reliability (Loop PFD avg ), PFD avg values for each sub-systems must first be found and be given a proportional value (weight) compared to the total 100%. This duty is usually assigned to personnel in charge of plants safety, process and maintainance. Using the Safety Manual

48 Loop PFD avg calculation Equation for 1oo1 loop Where: RT = repair time in hours (conventionally 8 hours) T1 = T proof test, time between circuit functional tests ( years) λ dd = failure rate for detected dangerous failures λ du = failure rate for undetected dangerous failures Using the Safety Manual

49 Loop PFD avg calculation If T1 = 1 year then but being λ dd * 8 far smaller than λ du * 4380 Using the Safety Manual

50 Example 1 PFDavg = λ du * T1/2 For D1014 λ du is equal to 34 FIT (see manual) Therefore PFDavg = 34 * * 4380 = = 0, = FIT Using the Safety Manual

51 Example 2 Weights of each sub-system in the loop must be verified in relation with expected SIL level PFDavg and data from the devices safety manual. For example, supposing SIL 2 level to be achieved by the loop on the right in a low demand mode: PFDavg(sys) is between and per year Weight of D1014 Isolator is 10% Therefore PFDavg(i) should be between and per year. Using the Safety Manual

52 Example 2 Given the table above (in the safety manual) conclusions are: 1.Being D1014 a type A component with SFF = 90%, it can be used both in SIL 2 and SIL 3 applications. 2.PFDavg with T proof = 1yr allows SIL3 applications 3.PFDavg with T proof = 5yr allows SIL2 applications 4.PFDavg with T proof = 10yr allows SIL1 applications Using the Safety Manual

53 1oo2 architecture What happens if the total PFDavg does not reach the wanted SIL 2 level, or the end user requires to reach a higher SIL 3 level? The solution is to use a 1oo2 architecture which offers very low PFDavg, thus increasing fail-safe failure probabilites. Using the Safety Manual

54 1oo2 architecture For D1014S (1oo1): PFDavg = λdu* T1/2 PFDavg = FIT For D1014D (1oo2): PFDavg = (λdun* T1)2/2 + (λdun* T1)2 /3 PFDavg = 75 FIT In this case a 1oo2 architecture gives a 2000 times smaller PFDavg for the sub-system Using the Safety Manual

55 Final considerations Always check that the Safety Manual contains information necessary for the calculation of SFF and PFDavg values. Between alternative suppliers, choose the one that offers: highest SIL level, highest SFF value, longest T[proof] time interval for the same SIL level, lowest value of PFDavg for the same T[proof]. When in presence of units with more than one channel and only one power supply circuit, the safety function allows the use of only one channel. Using both of the channels is allowed only when supply is given by two independent power circuits (like D1014D). Check that the Safety Manual provides all proof tests procedures to detect dangerous undetected faults. Using the Safety Manual

56 Credits and Contacts G.M. International s.r.l Via San Fiorano, Villasanta (Milan) ITALY Document last revised 20 May 2005 TR Automatyka Sp. z o.o. ul. Lechicka Warszawa POLAND


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