The broke out at a hydrogen compressor of the reforming unit of the refinery. Following the safety procedures, the emergency stop of the unit was activated and the unit was decompressed using a safety flare. The emergency plan was kept for an hour. A security perimeter is in place. The isolation of the compressor was started automatically. The isolation of the discharge circuit was successful, but the suction circuit could only be partially closed due to a block of a valve. The operators then closed manually the suction valve, by removing the excess of grease that had been applied to it. Nitrogen was injected into the suction circuit, to replace the still burning hydrogen. The gas detection system set up did not reveal any explosive atmosphere near the equipment.The leak of the oil/hydrogen mixture is estimated at 500 kg.Shortly before the event, during planned work, an unplanned tripping of the 400 V low voltage electrical panel had caused a loss of electrical power to the reforming unit. The triggering was due to the presence of an electrical bypass which had been left in the test position. The event sequence occurred as follows: 1. The lack of power supply caused the stop of the oil pump of the compressor. Another turbine-driven pump with the same role did not take over because it had not been foreseen its start-up in the event of a loss of electrical power. 2. The very low oil level caused the compressor to stop. 3. Hydrogen migrated through the sealing into the oil circuit, causing a pressure build-up. The final consequence was then the loss of containment of the oil and hydrogen mixture at the level of the motor of the valves regulating the inlet steam to the turbine. The release hydrogen quickly ignited.