The quest for 99.999%: Increasing levels of resilience
Siemens
This six-page 2025 Siemens brochure sets the ISO/IEC 22237 (EN 50600) availability classes beside the Uptime Tier I-IV classification and explains N, N+1 and 2N redundancy. It collects the annual downtime figures commonly attributed to each Tier in one table.
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Overview
The brochure compares two frameworks. EN 50600 / ISO 22237 defines classes 1-4 as low, medium, high and very high resilience, while Uptime uses Tiers I-IV. The approximate mapping reads I/1, II/2, III/3 and IV/4, but the two systems come from different bodies with different methods and are not strictly equivalent. For power paths and cooling it explains N, N+1 and 2N, and the key distinction is that class 3 is concurrently maintainable while class 4 is fault tolerant: in class 4 a single fault or a maintenance action does not cause a loss of service.
The definition puts weight on how the paths are separated rather than on how many redundant units exist. It reproduces the commonly cited Tier availability figures: Tier I 99.67% (28.8 h/yr), Tier II 99.75% (22 h), Tier III 99.98% (1.6 h), Tier IV 99.99% (0.8 h). Five nines, 99.999%, equals about 5.3 minutes a year. For example, in class 3 taking one path out for maintenance does not cut service, but a second fault during that work could; class 4 covers that case too.
The role of BMS and EPMS is presented as the operational contribution needed to reach five-nines operation. These numbers are not official Uptime values; Uptime no longer publishes availability percentages, and the brochure presents them only as commonly cited figures. As a six-page vendor brochure it offers a decision-maker summary rather than a worked calculation. The gap between Tier IV's 0.8 h/yr and five nines' 5.3 minutes shows the classification alone does not guarantee five nines.
Key points
- EN 50600/ISO 22237 availability classes 1-4: low, medium, high, very high resilience.
- Commonly cited Tier availability: I 99.67% (28.8 h/yr), II 99.75% (22 h), III 99.98% (1.6 h), IV 99.99% (0.8 h).
- Five nines = 99.999%, about 5.3 min/yr.
- Class 3 = concurrently maintainable; class 4 = fault tolerant (single fault or maintenance without loss).
- Standards covered: ISO/IEC 22237, EN 50600-1, EN 50600-2-2, EN 50600-2-3.
Using it in design
- In the rating module, map Uptime Tier to EN 50600 class (I/1, II/2, III/3, IV/4) as an approximate equivalence with a caveat tooltip.
- Show per-Tier availability numbers only labelled as commonly cited, not official Uptime values.
- Run single-point-of-failure checks on the maintenance path for class 3 and on the failure path for class 4.
- Translate a target availability into annual downtime (for example 99.999% is about 5.3 min) when presenting it to users.
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Who should read it
Engineers and managers choosing between Tier and EN 50600 classes or explaining resilience targets to leadership.
Related standards
- ISO/IEC 22237
- EN 50600-1
- EN 50600-2-2
- EN 50600-2-3
- Uptime Tier Standard: Topology
Publication details
- Authors
- Siemens
- Publisher
- Siemens
- Year
- 2025
- 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
Siemens (2025). The quest for 99.999%: Increasing levels of resilience. Siemens. https://assets.new.siemens.com/siemens/assets/api/uuid:34aace9c-ac2a-4090-bd6f-db92566b5c92/Data-Centers-RefArch-Quest-99_original.pdf
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