Glossary · Automation fundamentals, platforms and components
Availability
Also known as: Operational availability
German: Verfügbarkeit
In dependability engineering, availability is the ability of an item to be in a state to perform as required when needed. It is often expressed as the fraction of time an item is operable, for example from mean time between failures and mean time to repair.
- Automation components
- Standards
In one sentence
Availability is the ability of an item to perform as required when needed, often expressed as the share of time it is operable.
Example
A redundant controller pair raises the availability of a water treatment plant because a single CPU failure no longer stops the process.
How it applies
- Engineering: Availability targets drive decisions on Redundancy, hot-swappable modules, diagnostics and spare parts strategies.
- Operation: Availability is one factor of Overall equipment effectiveness (OEE) and is tracked via Downtime records.
- Maintenance: Good Maintainability shortens repair time and so improves availability.
- Documentation: Clear troubleshooting and maintenance instructions directly shorten repair times. Document which faults allow continued operation and which require a stop.
Availability vs. safety
A highly available system keeps running; a safe system reaches a Safe state when a dangerous fault occurs, which may mean stopping. These goals can conflict, and design choices such as fail-safe or fail-operational behavior must be justified. High availability is never evidence of safety. In OT security, availability is also one of the protection goals, typically ranked higher than in office IT.