A measurement of the effectiveness of safety systems based on the probability of tolerable incidents that can occur within a number of processing demands. For example, SIL level 4 means that for every 10,000 demands on the system, there could be one incident. See safety instrumented system.


Probability Categories
SIL From To

1 1 in 10 1 in 100
2 1 in 100 1 in 1,000
3 1 in 1,000 1 in 10,000
4 1 in 10,000 1 in 100,000