A tier 3 data center is the most common of the four standardized data center tiers developed and marketed by the Uptime Institute as a gauge of reliability for data center design and construction. In order for a data center to be qualified and marketed as a tier 3 data center, it must be certified by the Uptime Institute. Most colocation data centers are not certified by the Uptime Institue and therefore cannot be marketed to customers as being any of the Uptime Institute's data center tiers. A major gating factor to determine if a data center meets or exceeds the requirements for a tier 3 data center is if it is Concurrently Maintainable - that each and every component of its electrical and mechanical infrastructure has resiliency (and/or redundancy) built into the the components to enable any portion of the system to be taken offline for maintenance purposes without affecting the power or cooling of the data center. Customers evaluating data centers may often see data centers marketed as "Tier 3 Equivalent" or "Tier 3 Compatible" but must critically evaluate these data centers to determine if the design truly exceeds or conforms to the Uptime Institute Tier Ratings.