The improvement proposal for PFD<sub>avg</sub> formula of IEC61508-6 edition 2.0

Average probability of dangerous failure on demand (PFD_avg ) formula is derived in IEC 61508 edition 2.0 part 6 issued in 2010. But this formula is not consistent with the reality applied in safety instrumented function (SIF) design concept of process industry. Nowadays the safe detected failure, safe undetected failure and dangerous detected failure of sensor, logic solver and final element are designed to immediately lead to shutdown and system alarm in accordance with fail safe concept. But the PFD_avg formula in IEC 61508 edition 2.0 is derived based on the condition that dangerous detected failure does not lead to shutdown for mean repair time (MRT) after failure. So the PFD _avg formula shall need to be improved to suit the reality of plant. In this research, the PFD_avg formula are derived by reliability block diagram (RBD) methodology considering that only dangerous undetected failure leads to accident. Common cause factor (β)and proof test coverage (PC) are included in PFD_avg . The remaining two methodologies to determine result safety integrity level (SIL) like architectural constraint and systematic capability (SC) are excluded in this research. The votings discussed in this research are 1oo1, 1oo2, 2oo2, 2oo3 and 1oo3. The lower PFD_avgwill be derived by more realistic PFD_avg formula than the formula in IEC 61508 edition 2.0 if new PFD_avg is applied to real plant design. This new formula make the safer design and operation possible with the lower instrument investment cost.