SIF Testing Techniques

The safety and availability of a set of equipment used for a safety instrumented function may benefit from testing. However, that depends on redundancy and how often the demand occurs. Three modes of operation have been defined in lEC 61508 for equipment providing a safety instrumented function continuous demand mode, high demand mode and low demand mode. This book will use the lEC 61508 definitions to designate those three different situations. 

These three modes have been defined because SIF testing may or may not be given credit depending on the level of redundancy and the mode. The probability of failure on demand is calculated differently for each mode. The essential differences are due to the relationship between the dangerous condition (the demand) and the diagnostic testing. 

Three time intervals must be known to define what credit may be taken for automatic diagnostic testing and manual proof testing. These three time intervals are the average demand interval, the manual proof test interval and the automatic diagnostic test interval (usually the worst-case time is considered). The three modes and their relationsldps are shown in Table 7.2. 

Mode Demand Interval versus Automatic Diagnostic Interval Demand Interval versus Manual Proof Test Interval Probability Measure 

In continuous mode, the demand is effectively always present. Dangerous conditions always exist and a dangerous failure of the safety instrumented function will immediately result in an incident. There are no safety benefits that can be claimed for manual proof testing or even automatic on-line diagnostics in a single channel system (Tool). By the time the diagnostics detect the fault and initiate action, it is too late. Therefore, in continuous demand mode probability evaluation cannot take credit for any diagnostics except in redundant systems. 

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Given the design objectives in the SRS, the designer must choose equipment, determine if redundancy is needed, determine SIF testing techniques and perform a set of calculations to determine if various metrics are within the range for the desired SIL level. A diagram of the process is shown in Figure 7-1. 

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