A control device for controlling a technical installation includes a guard module. The control device has a modular structure with an input module, which receives an input signal representing a state of the technical installation, and an output module, which outputs an output signal controlling the technical installation based on the input signal. A guard module is integrated into the modular structure to monitor operability of the input module and the output module. The guard module confirms the output signal of the output module based on its monitoring of the input module and the output module. Within the modular structure, the input module and the output module form a first potential group. The guard module forms a second potential group within in the modular structure. The second potential group is independent of the first potential group.