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I’ve read many posts recently and found that many people confuse fail-safe design with logical expression descriptions; it’s truly a mistake! In simple terms, 1) for analog sensing elements, fail-safe operation generally means that the state output by the instrument after detecting a fault should tend to be a safe state. For example, if high interlocking is achieved through temperature changes, then the output after a temperature-related failure should lean in the direction of interlocking. (Interlocking is safe), and so on. 2. For switch-type sensing elements, the fault should occur in a de-energized state, without electricity present. With the bypass switch as an exception, it is normally open and should be closed when bypassing is required. 3. For physical circuits, they should be connected (closed) under normal conditions, and disconnected in the event of a fault. 4. For logic controllers, in the event of a fault, both the inputs and outputs should be de-energized, disconnected, and passive. 5. For control components, in the event of a fault, they should be de-energized, disconnected, and passive ； The valve is either open or closed, thus being in a safe position for industrial production. 6. For logical design, in accordance with the requirements of Clause 11.1.1 of the \"GB/T 50770-2013 Code for Design of Safety Instrumented Systems in Petrochemical Industries\", positive logic shall be employed. In other words, when the event described by the variable occurs, the value of the variable is “true”. For example, in the case of a high-temperature interlock (where the system shuts down if TIHH1001 is greater than 500 degrees), during logical design, when the temperature exceeds 500 degrees, TIHH1001 will be “TRUE” or “1”!