SAE Gates and Order-Dependent Events
SAE ARP4761 describes approaches for accounting for order-dependent events. For example, consider a situation where a monitor is used to detect failures of functional circuitry that can cause the top event to occur.
• If the monitor fails first, the failure might remain latent until the monitor is next checked.
• If the Function X circuitry fails first, the top-level event does not occur because the monitor annunciates the failure.
Therefore, it is important to consider the order of occurrence of events in the analysis.
The high-level modeling supported in the FTA module automatically considers the order-dependent events associated with the monitor. In addition to monitor failures, order-dependent analysis might be required for other situations. In such cases, these events must be specified explicitly.
SAE ARP4761 provides appendices with fault trees that include basic events both with and without latency and required order factors. In Appendix D, SAE ARP4761 describes two different approaches to modeling order-dependent events.
• In Figure D5, order-dependent events are modeled using an AND gate with an additional undeveloped event. The undeveloped event adjusts the probability of the AND gate by using a required order factor as its probability.
• In Figure D16, order-dependent events are modeled using a special AND gate that is similar to a Priority AND gate. The required factor is then a gate level input or property.
Conceptually, these two approaches are the same. In the FTA module, you use a special Sequencing AND gate with two options for determining the required order factor to model order-dependent events. For more information, see
Sequencing AND Gate (SAE Fault Trees Only).