Concurrently maintainable

Concurrently maintainable describes a data center design — the defining requirement of Uptime Institute Tier III — in which any single capacity component or distribution path for power or cooling can be shut down for planned maintenance or replacement without disrupting IT load. It is achieved by adding a redundant delivery path on top of the redundant components already required at Tier II, so every element in the chain, from switchgear to CRAH units, has an alternate route that keeps running while its twin is serviced. Tier III facilities target 99.982% availability and are the de facto minimum standard most enterprise and hyperscale colocation buyers require in RFPs, since it eliminates most planned-maintenance downtime without the cost of full fault tolerance. It is distinct from Tier IV's "fault tolerant" requirement, which additionally guarantees continuous operation through an unplanned single failure, not just planned maintenance.