Definition
The lifetime (also called mission time) is how long the SIS is planned to operate prior to being replaced or overhauled as new. This is a key decision made by the design team. This would be less than the useful life, which is a related concept.
This is used directly in the PFDavg calculations and listed as LT.
Key Points:
- Typically, 10-20 years for safety systems.
- Essentially it is the period between major upgrades or replacements.
- Do not confuse with Useful Life
- Different components of the SIS could have different lifetimes
Example:
A safety PLC certified for a 15-year lifetime under SIL 3 conditions.
See Also: useful life
Cited Source:
- IEC 61511-1:2016, Clause 11.2.
- Power Quality Blog – Lifetime and Reliability – related to power distribution but its the same concept