Influence of the control system structure with safety PLC on its reliability and safety

Control of safety critical applications requires using of control systems with defined safety level. It is necessary to fulfil requirements not only on safety but in some cases also on reliability of the control system, too. Achievement of these properties depends on the choice of an appropriate structure of the control system. Safety programmable logic controllers (PLC) are modular systems and allow construction of a large number of different structures. This paper deals with effects of some structures of control system with safety PLC on its reliability and safety properties.