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Mode (Continuous)

When demands to activate a safety function (SIF) are frequent compared to the test interval of the SIF. Note that other sectors define a separate high demand mode, based on whether diagnostics can reduce the accident rate. In either case, the continuous mode is where the frequency of an unwanted accident is essentially determined by the frequency of a dangerous SIF failure. When the SIF fails, the demand for its action will occur in a much shorter time frame than the function test, so speaking of its failure probability is not meaningful. Essentially all of the dangerous faults of a SIF in continuous mode service will be revealed by a process demand instead of a function test. See low demand mode, high demand mode, and SIL.


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