Mode (Low Demand)

(also demand mode per IEC 61511) when demands to activate the safety instrumented function (SIF) are infrequent compared to the test interval of the SIF. The process industry defines this mode when the demands to activate the SIF are less than once every two proof test intervals. The low demand mode of operation is the most common mode in the process industries. When defining safety integrity level for the low demand mode, a SIF’s performance is measured in terms of average Probability of Failure on Demand (PFDavg). In this demand mode, the frequency of the initiating event, modified by the SIF’s probability of failure on demand times the demand rate and any other downstream layers of protection determine the
frequency of unwanted accidents.

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