Average Probability of Failure of Aperiodically Operated Devices

The aperiodically operated devices are typically non-operated or stored, usually in a powered-down state. The duration of being operated is much shorter than that of storage. These devices have to perform extremely reliably during usage, while the operation usually occurs under circumstances worse than the average. This paper proposes a calculation procedure of value jumps of failure rate caused by operating condition shifts, to determine the average probability of failure, based on the standards IEC 61511 and ANSI/ISA-84. In the suggested calculation method, a proposal is also made for taking the failure caused by the human factor during operation into account. It is known that the probability of successful operation is increasable with periodic diagnostic tests. The circumstances of diagnostic tests which interrupt the non-operated storage of the aperiodically operated devices differ from those described in the standards IEC 61511 and ANSI/ISA-84. As the repair rate given for the continuous technologies cannot be interpreted for diagnostic tests which interrupt the powered-down storage of the aperiodically operated devices, this paper suggests the implementation of the effect of tests into the calculation procedure as correction of state probabilities, and gives the required formulae. At last the article provides an easy algorithm for the suggested calculation method.