How common cause ruins the safety rating of a fault tolerant PES

Programmable Electronic Systems (PES) are being used in many industrial safety applications because they can supply important benefits including calculation capability, extensive online diagnostics and automated documentation. However, a conventional PLC or DCS (general purpose PES) may fail dangerously and, therefore, should not be used for safety. A specially designed PES should be used. These specially designed PESs typically use dual and triple architectures to provide safety and availability. Specially designed PESs have been quantitatively analyzed for safety and availability. However, most of the analysis work did not account for the effects of common cause. This paper explains the common cause phenomenon and shows how the safety ratings of the 1oo2D and 2oo3 architectures are degraded when common cause is considered. The rules for avoiding this degradation are provide