N+1 Redundancy
Running one more unit than the load requires lets an installation lose any single unit without losing power.
The principle
N+1 means installing one more generating unit than the peak critical load needs. If N units carry the load, the +1 covers the next outage — planned maintenance or an unplanned trip — without dropping the mission. N+2 extends this to two concurrent losses for the most demanding sites.
Why it matters for Aegis specifically
Because a single Aegis unit's availability is only 0.86–0.995, redundancy is the mechanism that turns moderate unit reliability into installation-grade continuity. With independent units, the probability that enough of them are simultaneously down shrinks quickly as spares are added. The maintenance program is then scheduled against the +1 so that planned outages never coincide with reduced margin.
- N units sized to peak critical load
- +1 covers the worst single outage; +2 for two concurrent
- Units kept independent so failures do not share a cause
- Maintenance scheduled into the spare capacity
Independence is the catch
Redundancy only helps if failures are independent. Shared cooling, shared control, shared fuel handling, or physical proximity can defeat it by taking multiple units at once. Aegis siting therefore separates units physically and gives each independent auxiliaries where practical, so the redundancy math holds under real fault trees.
This is the direct engineering consequence of the availability gate: not a single heroic machine, but a small, independent, well-separated set. In practice the +1 unit is not idle spare metal: it is rotated through the duty so all units age evenly, and its presence is what lets the maintenance program run at all without exposing the mission to a coincident second failure.