Discussion on SIS interlock design
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GBT50770-2013, Design Code for Safety Instrumented Systems in Petrochemical Industries, 4.3.1: Positive logic can be used for the logical design of non-safety interlock systems, while negative logic is suitable for the logical design of safety interlocks. HGT20511-2014, Design Code for Chemical Process Automation, 11.1.1: Negative logic is recommended for the logical design of application software. How do you understand the fact that one of these two codes recommends positive logic for SIS interlocks and the other recommends negative logic?
Non-safety interlock systems can use positive logic; non-safety interlock systems are not SIS systems.