In SIL certification, we know that failure rate is a very core data. By obtaining the product failure rate, we can know whether the product meets the requirements of the corresponding SIL level, whether it needs to be modified, what the direction of modification is, and so on. Therefore, in the Chinese SIL certification standard GB/T 20438.4-2017, we can know the two methods for determining failure rate.
The first method can be called theoretical calculation. We can use a failure analysis tool such as FMEDA to obtain the total failure rate of the entire product based on the inherent failure rate of each component. In this process, we rely more on mathematical tools and the application of functional safety principles to obtain mathematical and theoretical data.
Another method we can call empirical analysis is based on statistical principles. Its specific method is to collect the number of failure events that occur throughout the entire product's service life cycle, and obtain a frequency of failure events. When the sample we collect is large enough, statistical principles tell us that the frequency of failure events will tend to a fixed value, which is the inherent failure rate of the product.
By analyzing these two methods, we can clearly understand that when a new product is launched, we often use the first method for SIL certification, while for a traditional product, using the second method for SIL certification is more accurate.
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