This part of IEC 61400 specifies a method to calculate the design reliability of wind turbines gearboxes covered by IEC 61400‑4, based upon failure modes where standardized calculation methods are publicly available.

Currently, not all failure mechanisms that occur in the field have accepted theoretical models. Therefore, the method only provides a quantitative assessment method of the failure mechanisms that can be described with accepted mathematical models for the complete gearbox, stages (functional units), field replaceable units, and individual components.

For the calculable failure mechanisms, it is possible to compare the reliability between different gearbox designs within the limitations of the theoretical models. The use of field-based statistical parameters can improve the accuracy of the calculated reliability.

The calculated design reliability can provide information for the lifecycle management strategy. However, this document does not provide trade-off decisions between higher design reliability and maintenance strategies (e.g. preventive or predictive maintenance). This document does not consider repairable system analysis.

Due to the lack of accepted theoretical models for some failure modes, the model can currently not predict the apparent failure probability in the field.

Neither this document nor IEC 61400-4 specify a minimum value of design reliability.