Regarding the issue of SIL3 system degradation
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What does the downgrade mode mentioned in the figure mean? And why use a 2oo3D system instead of a 1oo3D system, when the latter offers even higher hardware margin? Please offer some guidance.
The downgrade mode means that originally it is 3 out of 2; when another detection circuit fails, it is isolated, resulting in a configuration of 2 out of 1. This represents a reduction in the safety level (a decrease from one safety level to another). 2oo3D and 1oo3D relate to issues of safety and availability – the higher the availability, the lower the safety level, and vice versa. A configuration of 3 out of 2 is the best as it balances both safety and availability
What exactly do 3-3-2-0 and 3-2-0 mean in that diagram?