Myths and Misconceptions Regarding the Uptime Institute's Tier Certification System

Overview

This article is a true/false list correcting common misunderstandings of Uptime Tiers, published in the Uptime Institute Journal in 2017. Its core message is that Tiers are outcome-based, not component counts. Component count does not determine Tier; the system is performance-based, not a checklist, and N-counting shorthand is misleading. Expected downtime per year was removed from the Tier Standard in 2009 and was never part of the Tier definitions.

It also clarifies electrical sources: only the engine-generator plant is treated as a reliable source, the number of utility feeds does not change Tier, and generators need not run continuously but must be rated to carry the load. Tier III requires active/active distribution from UPS output downward, with both rack feeds live, while mechanical loops may be active/inactive. Tier II allows concurrent maintenance of capacity components (gensets, UPS, chillers, pumps) but not of paths (switchboards, panels, ATS, transformers, cables, pipes).

Rack-mounted ATSs are an accepted concession for single- or odd-corded IT, but the rack must still have two independent feeds, and ductwork must also meet the Tier objective. EPOs mandated by code do not prevent compliance, but Tier III needs a maintainable EPO and Tier IV a Fault Tolerant EPO; DRUPS (typically 10-30 s ride-through) can be used. The article is a freely accessible expert piece; only the record's summary and key facts were used here and the full text was not read.

Key points

1. Component count does not determine Tier; Tiers are performance-based, not a checklist, and N-counting shorthand is misleading.
2. Expected downtime per year was removed from the Tier Standard in 2009 and was never part of the Tier definitions.
3. Only the engine-generator plant is treated as a reliable source; the number of utility feeds does not change Tier; generators need not run continuously but must be rated to carry the load.
4. Tier III requires active/active distribution from UPS output downward (both rack feeds live); mechanical loops may be active/inactive.
5. Tier II allows concurrent maintenance of capacity components (gensets, UPS, chillers, pumps) but not of paths (switchboards, panels, ATS, transformers, cables, pipes).
6. Rack-mounted ATSs are an accepted concession for single- or odd-corded IT, but the rack must still have two independent feeds; ductwork must also meet the Tier objective.
7. EPOs mandated by code do not prevent compliance; Tier III needs a maintainable EPO and Tier IV a Fault Tolerant EPO; DRUPS (typically 10-30 s ride-through) can be used.

Using it in design

• Evaluate the Tier checker on topology, with paths and components removed one at a time, not on component counts.
• For Tier III and IV, require both rack feeds to be active below the UPS.
• Allow rack ATS only for single-corded loads with dual upstream feeds.
• Never display downtime minutes per year as a Tier attribute.

Who should read it

Electrical and reliability engineers who design to Uptime Tiers, evaluate Tier claims or prepare for Tier certification.

Related standards

• Uptime Institute Tier Standard: Topology
• ANSI/TIA-942

Publication details

Authors

Uptime Institute Journal

Publisher

Uptime Institute Journal

Year

2017

Document language

English

Source type

Industry body

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation