Concurrent maintainability
The ability to maintain or replace any component without shutting down IT load, the defining trait of Tier III.
Current numbers
Tier III / IVUptime Tier III/IV outcomes: concurrent maintainability vs fault tolerance under the standard's defined conditions
Tier III vs IVconcurrent maintainability (any path serviceable, no load impact) vs fault tolerance (survive any single unplanned fault)
~25–40% facilitywhole-facility capital premium for Tier IV fault tolerance over Tier III concurrent maintainability (single-source planning heuristic)