Using a safety PLC to Implement the Safety Function

Nowadays almost every PLC manufacturer offer a so-called safety PLC.It is a specific category of PLC, which in recent years have become a commonly used means of performing safety functions, especially in industrial applications.In this area of specific applications, a maximum of SIL 3 is normally required.However, the guaranteed safety features of the PLC lead to the consideration or discussion, whether they could be used in applications with higher safety requirements.This paper deals with the possibility of using the safety PLC to implement safety functions with SIL 4. The paper presents the long-term experience of the authors in the development of control systems for railway applications with the required level of SIL4.