PLANT CONTROL SYSTEM AND INTERLOCK FACTOR SPECIFYING METHOD

To provide a plant control system that has 1) a function of creating and executing a uniform control program for diagnosing an interlock condition, 2) a function of displaying a ladder circuit including only plural factor signals when plant equipment cannot be started, and 3) a function of displaying a ladder circuit including only plural factor signals and a trigger signal of the factor signals when the plant equipment is abnormally stopped. A plant control system includes program editing means 10 for creating a control program including an interlock condition and diagnosis of interlock, plant control means CONT for executing a program object in the control program, and interlock diagnosing means 20 for updating and displaying satisfaction/dissatisfaction at a predetermined point before a present point of the interlock condition or latest time when the interlock condition is dissatisfied in the past, further specifying only a source factor variable of the dissatisfaction even if plural stages of interlock circuits are formed, and automatically displaying the source factor variable in a graphic language, the interlock diagnosing means having a logical operation unit and a display unit.