A control system (23) is arranged to control the operation of an apparatus (200) for extracorporeal blood treatment. The apparatus (200) comprises an extracorporeal blood circuit (20) and a connection system (C) for connecting the blood circuit (20) to the vascular system of a patient. The blood circuit (20) comprises a blood processing device (6), and at least one pumping device (3). The control system is operable to switch between a pre-treatment mode and a blood treatment mode. The blood treatment mode involves operating the blood circuit (20) to pump blood from the vascular system via the connection system (C) through the blood processing device (6) and back to the vascular system via the connection system (C). The control system (23) is operable to obtain measurement data from at least one energy transfer sensor (40) arranged to sense a transfer of energy between the patient and the connection system (C) or between the patient and the blood circuit (20). The control system (23) is configured to, in the pre-treatment mode, process the measurement data for identification of a characteristic change indicating a connection of the blood circuit (20) to the vascular system of the patient, and, upon such identification, take dedicated action. The action may involve activating at least part of a patient protection system and/or enabling entry into the blood treatment mode. The control system may be included in an apparatus (200) for blood treatment, such as a dialysis machine.