Comparing Availability of Various Rack Power Redundancy Configurations

Overview

The paper examines five configurations, from a single-corded load on one path to a dual-corded load on two independent paths. The options compared include static transfer switches (STS) upstream of transformers, an STS with redundant transformers, and a rack-level ATS for single-corded equipment. This makes it possible to see which investment adds most availability in rooms that contain many single-corded devices.

The method is a reliability block model that yields an availability percentage for each configuration. Results are: single-corded 99.985%, STS with a single transformer 99.986%, STS with redundant transformers 99.997%, single-corded with a rack ATS 99.99993%, and dual-corded 99.9999977%. The model assumes a mean time to repair of 4 hours for UPS repair. The main conclusion is that bringing the switching point closer to the load greatly improves availability. Feeding dual-corded loads from dual paths gives up to 10,000 times less downtime than a single path. The appendix notes that in the single-path case, transformer parts are the dominant contributor to unavailability, at about 99.989%.

A limit to keep in mind is that the results depend on the model's assumptions, in particular the repair time and component reliability values; change them and the figures change too. Even so, the comparison is a useful reference when judging Tier-level claims, because it shows quantitatively how single-corded devices can become the weak link even in a 2N topology.

Key points

1. Availability: single-corded 99.985%; STS with single transformer 99.986%; STS with redundant transformers 99.997%.
2. Single-corded with rack ATS reaches 99.99993%; dual-corded reaches 99.9999977%.
3. Dual-path to dual-corded loads gives up to 10,000x less downtime than a single path.
4. The model assumes a 4-hour mean time to repair for the UPS.
5. Transformer parts are the dominant unavailability contributor in single-path designs (about 99.989%).
6. The closer the switching point is to the load, the higher the availability.

Using it in design

• When topology is 2N but some loads are single-corded, require a rack ATS per single-corded device by default.
• Display the availability penalty for single-corded loads to the user.
• Do not show Tier IV compliance for single-corded loads without point-of-use switching.
• Build a per-rack reliability block model using these default availability values.

Who should read it

Electrical designers and reliability engineers evaluating rack-level power redundancy and availability calculations.

Related standards

• Uptime Tier Standard: Topology
• TIA-942
• IEEE 493 (Gold Book)

Publication details

Authors

Victor Avelar

Publisher

Schneider Electric

Year

2011

Document number

WP 48

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

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

Citation