Uptime Institute tier classification [/ˈuptime ˈɪnstɪˌtut ˈtir ˌklæsəfəˈkeɪʃən/] n - The Uptime Institute Tier Standard is the industry’s shared vocabulary for how resilient a facility is. It grades the whole of The Engines of the Hall (and cooling) upon a four-level scale, each tier subsuming the one beneath it.

• Tier I - Basic capacity. [/ˈtɪɛr ˈɪ ˈbæsɪk ˈkæpækɪti/] n A single path for power and cooling, no redundancy. Any failure or maintenance means downtime. Roughly N.
• Tier II - Redundant capacity components. [/ˈtɪɛr ˈɪɪ ˈrɛdʌndænt ˈkæpækɪti ˈkɒmpɒnɛnts/] n Spare components (an extra Uninterruptible power supply or The Standby Engine) but still a single distribution path. Roughly N+1 components.
• Tier III - Concurrently maintainable. [/ˈtɪɛr ˈɪɪɪ ˈkɒnkʌrrɛntli ˈmæɪntæɪnæblɛ/] n Multiple paths, though only one active at a time, so any component can be maintained without downtime. This is the “concurrently maintainable” property from Redundancy topologies.
• Tier IV - Fault tolerant. [/ˈtɪɛr ˈɪv ˈfæʌlt ˈtɒlɛrænt/] n Multiple active independent paths, so any single unexpected failure is invisible to the load: the “fault tolerant” property, generally requiring 2N.

A higher tier is not automatically “better”; it is dearer, and warranted only by workloads that justify it. The rating turns upon the weakest link in the chain, which is why a single un-duplicated Automatic transfer switch or Power distribution unit can cap an otherwise excellent design at a lower tier.

Tier ratings say nothing of efficiency; that is the office of Power usage effectiveness.

Part of The Engines of the Hall.