Topology & maintainability
Pick a Tier, then take blocks out. The sweep tests every element under planned maintenance and under failure.
Maintenance is planned, so an alternate path may be switched in first. Failure is not, so only paths already carrying load count. Switch between the power and cooling paths; both face the same two tests. Compartmentalisation (Tier IV) is not modelled.
| Tier | Objective | Capacity components | Distribution paths | Test that proves it |
|---|---|---|---|---|
| I | Basic capacity | N | 1 | Planned work on any component shuts the site |
| II | Redundant capacity components | N+1 | 1 | A redundant unit can be removed; the path cannot |
| III | Concurrently maintainable | N+1 | 1 active + 1 alternate | Every component and path removable for planned work |
| IV | Fault tolerant | N after any failure | 2 active, compartmentalised | Any single failure tolerated; continuous cooling |
| Notation | Meaning | Units for N = 3 |
|---|---|---|
| N | Exactly the capacity the load needs | 3 |
| N+1 | One spare unit in the same system | 4 |
| 2N | Two complete, independent systems | 6 |
| 2(N+1) | Two independent systems, each with a spare | 8 |
On percentages. The figures often quoted per Tier (99.671 %, 99.741 %, 99.982 %, 99.995 %) are not part of the Tier Standard: Topology, which defines Tiers by the objectives above. Availability depends on operations as much as topology — compute it for your own blocks in the Availability tab.
Fuel and engines. Every Tier requires at least 12 hours of on-site fuel. For Tier III and IV, on-site engine generators are the primary source and must be rated to carry the site without a runtime limit at the rating used.
Certification. Tier Certification of Design Documents (TCDD), of Constructed Facility (TCCF) and of Operational Sustainability are issued by Uptime Institute only; a self-declared “Tier III design” is not a certification.