Tier IV is a designation given to data centers with equipment that meets at least N+1 redundancy with no single points of failure.  Also these data centers must be able to undergo normal maintenance without disrupting service to the computer systems.  According to the Uptime Institute and worldwide averages, a Tier IV data center should provide 99.995% availability.

The Uptime Institute developed a four tiered classification approach to site infrastructure functionality that addresses the need for a common benchmarking standard.  The Institute's system has been in use since 1995 and has become the default standard for uninterruptible uptime industry.

Over the last 40 years, data center infrastructure designs have evolved through at least four distinct stages, which are captured in the Institute's classification system.  Historically, Tier I first appeared in the early 1960's, Tier II in the 1970's, Tier III in the late 1980's and Tier IV in 1994.

 

As you can see from the following definitions, the act of installing a UPS and generator does not get a data center beyond the Tier I classification.

 

A Tier I basic data center has non-redundant capacity components and single non-redundant path distribution paths serving the site's computer equipment.  An example would be a computer room with a single UPS, generator and HVAC system.

 

A Tier II data center has redundant capacity components and single non-redundant distribution paths serving the site's computer equipment.  An example would be a computer room with a single UPS and generator, but a redundant HVAC system.

 

A Tier III data center redundant capacity components and multiple distribution paths serving the site's computer equipment.  Generally, only one distribution path serves the computer equipment at any time.  An example would be a computer room with a single UPS that has a maintenance bypass switch wrapped around the system and a generator.  Also, the data center would have redundant HVAC systems.

 

A Tier IV data center has redundant capacity systems and multiple distribution paths simultaneously serving the site's computer equipment.  A minimum example of this configuration would be multiple UPS's serving equipment via multiple paths with no single point of failure.  The UPS's would be backed up by generators that are redundant and have no single point of failure.  Finally, the data center would have at least N+1 HVAC systems.