System classifying data centres by their level of redundancy and availability, from Tier I to Tier IV.
The Uptime Institute Tier classification ranks data centres by their redundancy architecture and the resulting availability, from Tier I to Tier IV. Tier I offers basic infrastructure with no redundancy. Tier II adds redundant components. Tier III, called concurrently maintainable, allows maintenance without service interruption. Tier IV, called fault tolerant, withstands the failure of any single component without loss of availability, with theoretical annual downtime of a few minutes. For the insurer, the Tier level is a major determinant of business interruption risk: a properly certified Tier IV centre has a far lower interruption probability than a Tier II. Certification distinguishes the design level from the level actually built then operated, three distinct certifications that underwriting must read to avoid overstating a site's real resilience.
An operator claiming a Tier IV design but running a site without construction certification exposes the insurer to a gap between claimed and actual resilience.
Tier I à IV, Uptime Institute, Tier IV, data center tiers