Data Center Design Guidelines ((link)) -

| Tier | Uptime % | Annual Downtime | Redundancy Path | Concurrent Maintainability | | :--- | :--- | :--- | :--- | :--- | | | 99.671% | 28.8 hrs | N | No | | Tier II | 99.741% | 22.0 hrs | N+1 | No | | Tier III | 99.982% | 1.6 hrs | N+1 | Yes | | Tier IV | 99.995% | 26.3 min | 2N or 2(N+1) | Yes (Fault Tolerant) |