Inherent reliability is probably the single most important characteristic of any system or piece of equipment in terms of determining overall reliability performance. The inherent reliability of a system or device is determined by its configuration and component selection. For instance, if a plant has a redundant feed pumps or recycle compressors, that fact will profoundly affect the inherent reliability. Also, if the components were chosen based in lifecycle cost rather than just first cost, the inherent reliability will be enhanced. In performing this analysis, the lifecycle costs includes first cost, all forms of maintenance costs, the costs associated with unreliability (e.g. lost profit associated with unplanned outages) and costs associated with unavailability (e.g. costs associated with necessary planned outages).
The inherent reliability is a measure of the overall "robustness" of a system or piece of equipment. It provides an upper limit to the reliability and availability that can be achieved. In other words, no matter how much inspection or maintenance you perform, you will never exceed the inherent reliability. If you operate, maintain, and inspect a device as well as possible, you will be able to harvest all of the inherent reliability. On the other hand, if there are gaps in your operating, maintenance or inspection practices, you will harvest only some of the inherent reliability.
Tip from Reliability Assessment: A Guide to Aligning Expectations, Practices and Performance by Daniel T. Daley (Industrial Press)