Safety integrity level continuous mode

The outcome of the hazard and risk assessment and allocation process should be a clear description of the functions to be carried out by the safety systems, including potential safety instrumented systems together with safety integrity level requirements (along with mode of operation, continuous or demand) for any safety instrumented function. This forms the basis for the SIS safety requirements specification. The description of the functions should be clear as to what needs to be done to ensure that safety is maintained. 

Safety Integrity Level Probability of dangerous failure per hour (Continuous mode of operation)... 

SIL 1 is the lowest safety integrity level and probably describes many basic and rudementary control systems (not safety systems). In continuous demand mode, where the safety function is required continuously (apphcable for mine hoist brake systems), SIL 1 can have no more than... 

SIL 2 is the next safety integrity level. In continuous demand mode, SIL 2 can have no more than one dangerous failure every 1,000,000 hours (114.1 years). 

NOTE 2 In the case of a safety instrumented function operating in the continuous mode of operation, the target failure measure should be expressed in terms of the frequency of a dangerous failure per hour, as determined by the safety integrity level of the safety instrumented function (see Table 4). 

ANSI/ISA-84.00.01-2004-1, Clause 9.2.3, provides two tables for defining the SIL requirements. Table 3 provides the SIL requirements in terms of PFDavg- Table 4 provides the SIL requirements in terms of Frequency of Failure (e.g., failures per hour) and defines the acceptable hazard rate for the high-demand/continuous SIF. ANSI/ISA-84.00.01-2004-1, Clause 9.2.3, states that when Table 4 is used, neither the proof-test interval nor the demand rate is used in the determination of the safety integrity level. This means that the Table 4 requirements should not be converted into PFDavg requirements, using the proof-test interval or the demand rate. Erroneous results can easily occur if a high-demand mode SIF is treated as a low-demand mode SIF, followed by incorrect use of Tables 3 and 4 (ANSI/ISA-84.00.01-2004-1 Clause 9.2.3). 

Safety instrumented functions (SIF) are assigned a safety integrity level (SIL) and a mode of operation (continuous or demand) during the H RA. These are recent requirements and application guidance is limited at best. 

This paper examines the approach of lEC 61508 towards the determination of safety integrity levels. It examines the distinction made by the standard between low demand and high demand or continuous modes of operation and the possible changes being considered by the lEC working group. 

Refer to lEC 61508-1, table 2 (for low demand mode operation) or table 3 (for continuous or high demand mode operation) to determine the safety integrity level (SIL). The SIL then guides the selection of the techniques used for the avoidance of systematic faults in both hardware and software, so that as the risk reduction increases, or the hazard rate decreases, there is a reduction in the likelihood that systematic failures (including those resulting from incorrect specification) will result in a hazard. 

Table 3. Safety integrity levels target failure measures for a safety function operating in a high demand or continuous mode of operation...

Safety Integrity Level (SIL) Probability of Dangerous Failure (per HR)— Continuous Mode Probability of Failure on Demand—On-demand Mode... 

Table 13.2 Safety integrity levels - high demand/continuous mode.

The different requirements of the altered Safety Integrity Levels (SIL) are in dependent on the probability of failure and are shown in Table 1. The probability values are defined as a PFD-value (probability of failure on demand), if the system is in a low demand mode and has to execute a safety function. However, if a system is operating in a high demand mode or continuous mode and a safety function has to be executed, then the probability of failure is specified with the PFH (probability of failure per hour). Its dimension or unit is (1/h). (Carsten 2006), (Kainka 1996)... 

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