Fault Tolerant

Fault Tolerant describes an infrastructure design in which the failure of any single component or distribution path continues to serve the critical load without interruption, because every capacity system is fully duplicated (2N or 2N+1) end to end. It is the specific attribute the Uptime Institute requires for its Tier IV Certification of Constructed Facility, alongside compartmentalization that contains any single fire, flood or physical event to one part of the facility. This is a stricter standard than Tier III's Concurrently Maintainable requirement, which guarantees planned maintenance without downtime but tolerates brief interruption from an unplanned single failure. Only a small share of Uptime-certified facilities worldwide hold Tier IV, since the doubled electrical and mechanical plant roughly adds 15-25% to construction cost over Tier III. Operators sometimes market a design as "fault tolerant" without holding the Uptime certificate, which is a self-declared claim rather than a verified rating.