This paper deals with the problems of the planning and the design of safety related control systems. Such systems are used not only for protection purposes, but first of all for control purposes, as the many operation functions, e.g. start, stop, emergency stop, restart are considered as safety related functions. The safety life cycle of the system is presented and its design phase is discussed ... [Show full abstract] in details. The measure of functional safety - integrity level - is defined and the corresponding requirements are referenced. On this basis, the organising actions, required by the functional safety methodology, are presented.