ABSTRACT
From time to time, SIL analysts are faced with the task of evaluating average PFD values of safety instrumented systems with higher-order redundancy. IEC 61508 (IEC 1998) and IEC 61511 (IEC 2003) indicate several methods for PFD calculations. The simplest one is that of the analytical equations. IEC 61508 provides analytical equations for only five of the most commonlyusedSISarchitectures, namely, 1oo1, 1oo2, 1oo2D, 2oo2 and 2oo3. There is a belief that the analytical equations would not give reasonable approximations for higher-order redundancies and that other methods such as fault tree analysis or markov analysis should be used instead (ISA 2002). It does happen that analytical equations aremuch simpler to use than those other two methods since, contrary to fault tree and markov, they do not need any special software. Any good programmable calculator or a simple spreadsheet program could be used to perform the calculations.
