Definition:
“Five Nines” of Reliability refers to achieving 99.999% system uptime or availability, meaning that total unplanned downtime is less than approximately 5.26 minutes per year. Not directly addressed in IEC 61508 or 61511, though high availability concepts apply to continuous-mode operations
Key Points:
- Commonly used in IT, telecommunications, and critical network infrastructure.
- Represents extremely high system availability but does not specifically address system safety.
- Not directly referenced in IEC 61508 or 61511, but high availability concepts are often discussed in continuous process industry applications. Perhaps it could be discussed in the same context as spurious trip rate (STR).
- Focused on minimizing service interruptions, not necessarily on preventing hazardous failures.
Example:
A cloud data center designed to achieve Five Nines of Reliability would target no more than 5.26 minutes of downtime across an entire year.
See Also: STR
Cited Source:
- ITU-T E.800: Definitions of Availability and Reliability Terms (International Telecommunication Union)
- IBM article – What is High Availability?