Are the terms "Mean Time Between Failures" (MTBF) and "Safety Integrity Level" (SIL) related to each other? Some would think not, but read on if you'd like to discover if that's true.
What is MTBF in relation to SIL?
Equipment suppliers have been providing mean time between failure - MTBF - data for electrical and electronic equipment for decades. The information usually comes from study of failures in service, or for new products, from an analysis of failures during accelerated life testing.
MTBF is a statistical indication that an equipment item will operate within its defined specification for a stated time. The logical assumption is that repair and restoration get included in the MTBF model. If repair and restoration are not included, the term Mean Time To Failure (MTTF) is the more correct term to use.
Safety Integrity Level - SIL, is a more recent metric defined in standards like IEC 61508 and IEC 61511 to specify the safety performance of a Safety Instrumented Function. The integrity targets apply to collections of equipment that perform an end-to-end function, including a sensor to detect a hazardous condition, a logic solver to decide on the action to take, and a final element such as a valve to take an action.
When a safety instrumented function includes a SIL requirement, there is a need to calculate the predicted performance of the devices included in the design. This calculation can be relatively simple, or quite complex, depending on the system architecture. Read about PFD and PFH calculations if you want to know more about those terms.
What many do not realise is that failure rates, MTBF and SIL are all somewhat related.
Rule-of-thumb relationship
With the underpinning failure rates, some "rules of thumb" can be derived, which may prove useful when trying to estimate a failure rate for an equipment item.
MTBF is approximately equal to 1/failure rate (lambda)
Therefore, a failure rate can be approximated from the MTBF by taking the inverse: 1/MTBF.
Once the failure rate is known, it is typically conservative to assume a 50/50 distribution of safe and dangerous failure.
Using the 50% dangerous failure rate, both PFD (low demand) and PFH (high demand) can be calculated, and these can be related back to SIL
Conclusion
In conclusion, MTBF can be related to the underpinning calculations required for SIL conformance, even if no other supporting data is available. The rule of thumb is conservative, provided the MTBF data itself is not flawed.
Don't fall into the trap of thinking that SIL relates only to such calculations. Many accidents occur because of poor procedures, lack of understanding and systematic errors. The achievement of a safe design requires much more than calculations that are