Quick answer
Tier ratings (from the Uptime Institute) describe redundancy. Tier 3 is concurrently maintainable — any component can be serviced without downtime. Tier 4 is fault tolerant — it keeps running through a single failure, using fully duplicated systems. Tier 4 needs more equipment, space and often dual power feeds.
Key takeaways
- Tier 3: concurrently maintainable
- Tier 4: fault tolerant
- Tier 4 needs more land and dual feeds
Tier 3
Redundant components and multiple distribution paths, so maintenance needs no shutdown.
Tier 4
Fully duplicated, independent systems that tolerate any single fault.
Impact on land
Tier 4 sites need more space for duplicated plant and ideally two independent grid feeds.
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