Abstract
The paper presents new approximate formulas for the Probability of Failure on Demand (PFD) for standard configurations discussed in the IEC 61508 standard. It is investigated how the diagnostic cover-age (DC) is affected when comparison of channels is applied as part of the diagnostic testing for multiple channel systems. In this case the DC may be different for independent and common cause failures. The new PFD formulas are particularly useful for treating the so-called 1oo2D configuration properly, and are also used for relating 1oo2D to the standard 1oo2 and 2oo2 votings.