This work deals with safe control of manufacturing systems controlled by Pro-grammable Logic Controller (PLC). The used approach is based on Boolean safety constraints, which act as a safety filter, in order to guarantee the safety on-line. To ensure the safety whatever the inputs and the control program, the safety constraints must be checked formally. In this paper we proposed a formal off-line approach to check and ensure the consistency. The approach uses graph representation of the safety constraints. Thanks to graph algorithm, it is possible to detect inconsistencies and to help designers to solve them. Previous works on safety filter have proposed necessary conditions but none sufficient conditions, in this paper we proposed a necessary and sufficient condition to check and ensure the consistency. The safety filter approach and the consistency problem are presented. Then the approach is illustrated and a discussion around the application to manufacturing system is done.