In a thermal power plant, a fire broke out at around 8.15 pm on the main transformer of one unit during its multi-annual shutdown. The upper part of the device, containing 65 t of mineral oil (without PCB), was damaged and the transformer was automatically isolated by opening the circuit breakers. The automatic sprinkler system was also activated. The operator activated the internal emergency plan. The emergency services stopped the traffic on the nearest road. Fire-fighters from outside the site controlled the fire at around 9 pm and then sprayed the device until 11:30 pm to cool it. The fire extinguishing water contained in the power plant’s watertight stormwater basin was discharged into the River RHONE according to the results of the analyses carried out. The transformer was inspected by experts. The accident had not affected the operation of the power plant’s other unit. An electrical fault caused the fire which resulted in a loss of tightness of the upper part of the device (ejection of the connection bar) and ignition of the dielectric medium.