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10 For further information www.norgren.com/us 2. Safety Integritiy Level (SIL) A Safety Integrity Level (SIL) is a statistical representation of the Safety Integrated System (SIS) when a process demand occurs. It is used in both ANSI/ISA-S84.01 and IEC 61508 to measure the reliability of Safety Integrated System (SIS). Both ISA and IEC have agreed that there are three levels of safety integrity: SILs 1, 2 and 3. IEC also includes an additional level, SIL 4, that ISA does not. The higher the SIL is, the more reliable the system is. SIL is therefore a measure for risk reduction. The Risk graph evaluates the risk and determines what level of risk reduction is needed to reach an acceptable risk. SILs are correlated to the Probability of Failure on Demand (PFD) which is equivalent to the unavailability of a system at the time of a process demand. PFH is another measure for evaluating in how far a device is suitable for use in safety relevant plant parts, if used in High Demand Mode (HDM).This value indicates the probability of failure referred to a time interval. 3. Low Demand Mode & High Demand Mode Low Demand Mode (LDM) exists where the frequency of operation of the safety-related system is not greater than once a year and not greater than twice the test frequency of the Safety Integrated System (SIS). In all other cases a High Demand Mode (HDM) with a high switching frequency exists in the sense of IEC61508. For LDM the allocation of the SIL is carried out on the basis of the determined PFD value of the SIS according to following table. Correlation of SIL and PFD. SIL IEC 61508 ANSI S84.01 PFD Availabilty Required 1/pfd 4 YES NO 10 -5 to 10 -4 >99.99% 100,000 to 10.000 3 YES YES 10 -4 to 10 -3 99.90% to 99.99% 10,000 to 1,000 2 YES YES 10 -3 to 10 -2 99.00% to 99.90% 1.000 to 100 1 YES YES 10 -2 to 10 -1 90.00% to 99.00% 100 to 10 SIL (Safety Integrity Level) Low demand mode of operation (Average probability of failure to perform its design on demand) 4 greater or equal to 10 -5 to smaller than 10 -4 3 greater or equal to 10 -4 to smaller than 10 -3 2 greater or equal to 10 -3 to smaller than 10 -2 1 greater or equal to 10 -2 to smaller than 10 -1 SIL (Safety Integrity Level) High demand mode of operation (Probability of dangerous failure per hour) 4 greater or equal to 10 -9 to smaller than 10 -8 3 greater or equal to 10 -8 to smaller than 10 -7 2 greater or equal to 10 -7 to smaller than 10 -6 1 greater or equal to 10 -6 to smaller than 10 -5