Tier Classification Myths and Misconceptions (Uptime Institute)

Overview

The page gathers myths and questions that Uptime Institute published in its Tier Program emails. The entries date from 2009 and 2010 and correct, one by one, common misunderstandings about what Tier classification measures.

The central message is that Tiers evaluate a facility's capability to allow maintenance and to withstand a fault, and are not available in checklist form. The page also says Tiers are disconnected from TIA-942. Tier II requires shutdown for planned maintenance, whereas Concurrent Maintainability defines Tier III. Which class a design falls into therefore depends on how its paths behave during maintenance and faults, not on an equipment list.

Many hardware myths are addressed too. Tiers do not mandate an emergency power off (EPO) switch; if one is installed it must meet the Concurrent Maintainability or Fault Tolerance objective. The engine-generator plant must be sized to carry the critical load without runtime limits, yet need not run at all times. Component counts such as N+1, N+2, S+S or 2(N+1) do not determine Tier, and the number of utility feeds neither predicts nor influences it.

The Q&A section describes rack-mounted transfer switches as a concession for equipment with an odd number of power cords. Details like this show how the classification is interpreted in practice. Raised floors and specific security arrangements are likewise not Tier requirements; they are owner choices. Limits: the page is a period collection and does not reflect later changes to the standard. For designers its value is as a reminder to avoid arguing over the wrong criteria and to focus on path-based analysis when classification is debated.

Key points

1. Tiers evaluate the capability to allow maintenance and to withstand a fault; they are not available as a checklist.
2. Tiers do not require an EPO; if installed it must meet the Concurrent Maintainability or Fault Tolerance objective.
3. The generator plant must carry critical load without runtime limits but need not run at all times.
4. N+1, N+2, S+S or 2(N+1) component counts do not determine Tier.
5. The number of utility feeds neither predicts nor influences Tier.
6. Rack-mounted transfer switches are a concession for equipment with odd numbers of cords.
7. Tier II requires shutdown for planned maintenance; Concurrent Maintainability defines Tier III.
8. Tiers are independent of TIA-942; raised floors and specific security measures are not Tier requirements.

Using it in design

• Avoid labelling designs by N-count; run a path-based concurrent-maintainability analysis in Tier advisory tools.
• Present EPO and raised floor as owner choices, not Tier requirements.
• Size the generator plant on the principle of carrying critical load without runtime limits.
• Do not justify a Tier claim by the number of utility feeds; examine how each path behaves under maintenance and fault.

Who should read it

Engineers and consultants designing to a Tier target, and owners who use Tier language in contracts.

Related standards

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

Publication details

Authors

Uptime Institute

Publisher

Uptime Institute

Year

2010

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