Effects of diagnostic on the safety of a control system realised by safety PLC

Safety programmable logic controller (S-PLC) is primarily being utilised to perform safety functions of safety-related control systems (SRCS) on the process level. The SRCS also consists of sensors output circuits (including actuators), which can take part on execution of the individual safety functions. In this case the detection of their failures and an early reaction on the detected failure can be an unavoidable mean to achieve the required safety integrity level (SIL) of the performed safety function. In SRCS with S-PLC the failure detection of sensors and output circuits has to be implemented in the application program while considering some specific properties of S-PLC. The paper deals with the new problem of some relations between S-PLC parameters, diagnostics options and mathematical expression of safety integrity of a SRCS with S-PLC. In the practical part implementation of application diagnostics in S-PLC is mentioned and results of diagnostics parameters influence on safety integrity of SRCS for selected example connected to practical applying are introduced.