At around 4:20 p.m., alerted by several faults in the supervision centre, an agent noticed the deterioration of several cables in the field. After an unsuccessful attempt to repair them, operations were set up in the control room to monitor the situation overnight. At around 7:40 p.m., an alarm signalled a connection failure between the Cusset hydroelectric power plant and the Jonage dam. The dam could no longer be controlled and closed automatically. A difference in the water levels occurred both upstream and downstream of the dam. The drop of the water level in the tailrace of the Jonage Canal resulted in the collapse (over a length of 45 metres) but no breach of the sheet piling undergoing repairs, separating the canal and the Grand Large reservoir. Starting at 3 a.m., the facility operator was able to restore normal operating conditions by switching to manual mode. Monitoring was in place until the cables were repaired on 17/07. The area where the sheet piles had collapsed was marked out for safety reasons.

The cause of the event was attributed to the accidental severing of a connecting cable during dyke reinforcement operations. During the cable search phase, before construction had begun, the worksite manager had been informed of the presence of 2 cable routes. However, the plans did not indicate that each route contained 2 cables. Only 2 of the 4 cables had been secured before the work was carried out. In addition, the equipment used for network cable detection was not suitable for very low voltage components.