Each component of a Safety Instrumented System involved in the safety function has a SIL level. The overall SIL of a safety function is determined by calculation based of the failure rate probability of each component. Each component contributes its part to the final SIL level. The weakest link of the chain reflects the maximum achievable SIL level.
Each safety function has a safety integrity level (SIL). The safety integrity level is the probability for the system to execute the safety functions required in all specified input conditions within a specified time interval.