What is fail-safe design?
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For example, in interlock logic: 1, does the level switch require a 0 (off) state to trigger the logic in the SIS? 2. Does the SIS system output of 0 trigger interlock (a relay-based interlock that disconnects when power is lost)? I wonder what failures are taken into account in this fail-safe design? Considering the possibility of failure of the on-site instruments, the level switch should be at 0 (off) to trigger the SIS logic ； Considering a failure of the SIS system, does the SIS output result in a 0 interlock, or is it only when both the field equipment and the SIS system are in a 0 interlock state that it is considered fail-safe?
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The last edit to this post was made by budaoweng123 on 2015-10-22 at 17:13; 1. Does the level switch need to be in the 0 (off) position to trigger the logic in the SIS? Yes, a value of 0 (off) is required to trigger the interlock. For example, in a container with high internal pressure, we use a pressure switch to interlock the vent valve. If you set it up as a 1 (closed) interlock, then if the wires in the circuit become loose (a fault that can only be detected during shutdown maintenance checks, unless it’s the Numur circuit), and the pressure inside the container has already reached the interlock threshold, the interlock will not be triggered, which could lead to an explosion. If you understand this, then you actually understand the concept of fault safety.