Definition
The lifetime (also called mission time) is how long the SIS will operate prior to be replaced or overhauled like new. It is a key decision made by the design team. This would be less than the useful life which is a related concept. It is a part of some of the more complex mathematical calculations.
This is used directly in the PFDavg calculations and listed as LT.
Key Points:
- Typically, 10-20 years for safety systems.
- Essentially 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.