Safety integrity level high demand mode

Safety integrity level High demand or continuous mode of operation (Probability of a dangerous failure per hour)... 

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

Results of the evaluation typically include a number of safety integrity and availability measurements. Most important, the average probability of failure on demand (PFDavg) and the safe failure fraction (SFF) is calculated for low demand mode. Probability of failure per hour is calculated for high demand mode. From charts, the SIL level that the... 

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). 

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. 

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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