In the paper, we will study the PFD and its connection with the probability of failure per hour and failure rates of equipment using very simple models. We describe the philosophies that are standing behind the PFD and the THR. A comparison shows, how the philosophies are connected and which connections between PFH and PFD are implied. Depending on additional parameters, there can be deviations between safety integrity levels that are derived on the basis of the PFD and the PFH. Problems are discussed, which can arise when working with the PFD. We describe, how PFD and PFH in IEC 61508 are connected with the THR defined in the standard EN 50129.
We discuss arguments that show, why care is needed when using the PFD. Moreover, we present a reasoning, why a probability of failure on demand (PFD) might be misleading.
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