Fault tolerance
The ability to withstand any single equipment failure without disrupting IT load, the defining trait of Tier IV.
Current numbers
Tier III vs IVconcurrent maintainability (any path serviceable, no load impact) vs fault tolerance (survive any single unplanned fault)
III: concurrent maintainability / IV: fault tolerance + continuous coolingUptime Tier III/IV outcomes: concurrent maintainability vs fault tolerance under the standard's defined conditions