A design requirement that each safety function be achievable even with a single random failure anywhere in the system, on top of the initiating event and any equipment already out for maintenance where allowed. It drives the use of at least two independent trains for most safety systems and shapes how outages and maintenance are scheduled. Failures caused by a shared cause are handled separately through diversity.
If we pull train B for the pump overhaul we can't meet single failure criterion on that function, so we're in the LCO.
Also heard as
- SFC
Related terms
Redundancy
Providing more components or trains than needed, so a safety function survives if one fails.
Diversity
Using different designs, technologies or physical principles for the same safety function to guard against common-cause failure.
Defence in depth
The safety strategy of stacking independent layers of protection so no single failure can lead to harm.
LCO
Limiting condition for operation: a technical specification requirement for equipment or conditions needed to run the plant.