Safety integrity levels model for IEC 61508 - Examination of modes of operation

The present paper modifies the algorithm to estimate harmful event frequenc ies and examines the definition of modes of operation in IEC 61508. As far as the continuous mode concerns, the calculated results coincide with those obtained based on the standard. However, for the intermediate region of me dium demand frequencies and/or medium demand durations, the standard gives much higher harmful event frequencies than the real values. In order to avo id this difficulty, a new definition of modes of operation and a shortcut m ethod for allocation of SILs are presented.