TIER IV Requirements

Tier I
99.671% Availability
Tier II Redundant Components 99.741% Availability Tier III Concurrently Maintainable 99.982% Availability Tier IV
Fault Tolerant
99.995% Availability
Active Capacity Components to Support the IT Lead N N+1 N+1 N After any Failure
Distribution Paths 1 1 1 Active
1 Alternative
2 Simultaneously Active
Concurrently Maintainable No No Yes Yes
Fault Tolerant No No No Yes
Compartmentalization No No No Yes
Continuous Cooling Load Density Dependent Load Density Dependent Load Density Dependent Class A
Fundamental Requirements for a Tier IV Site Infrastructure 
  • A Fault Tolerant data center must have multiple, independent, physically isolated systems that provide redundant capacity components and multiple, independent, diverse, active distribution paths simultaneously serving the computer equipment.  The redundant capacity components and diverse distribution paths must be configured such that “N” capacity is providing power and cooling to the computer equipment after any infrastructure failure.
  • Complimentary systems and distribution paths must be physically isolated from one another (compartmentalized) to prevent any single event from simultaneously impacting both systems and distribution paths.
  • Continuous cooling must be provided for continuous availability.
  • Twelve hours of on-site fuel storage must be available for “N” capacity.
Performance Confirmation Tests
  • A single failure of any capacity system, capacity component, or distribution element will not impact the computer equipment.
  • The system itself automatically responds (‘self heals’) to a failure to prevent further impact to the site.
  • Each and every capacity component and element in the distribution paths can be removed from service on a planned basis without impacting any of the computer equipment.
  • There is sufficient capacity to meet the needs of the site when redundant components or distribution paths are removed from service for any reason.
Operational Impacts
  • The site is not susceptible to disruption from a single unplanned event.
  • The site is not susceptible to disruption from any planned work activity.
  • The site infrastructure maintenance can be performed by using the redundant capacity components and distribution paths to safely work on the remaining equipment.