In a fertilizer production plant, an ammonia leak injured 3 workers, one of whom died. On December 1st, 1994, the weekly cleaning of the dryer was scheduled. The tubular reactor of this dryer was shut off at 6:25 am then cooled and purged. At 9:30 am, the dryer’s ventilation was turned off, while at 9:45, the electrical unit undertook its semi-annual replacement of the inverter protecting the automated production system. This operation consisted of stopping all programmes, cutting the electrical power supply of the in-service inverter, connecting the backup inverter and then reinitialising the programmes. At 10 am, three maintenance workers entered the dryer. All production chains were shut down; two of the workers were cleaning the reactor supply impellers in front of the ammonia injection nozzle, while the third was inspecting the hot gas intake upstream of the injection nozzle. At 10:30, the automatic valve controlling ammonia injection into the dryer opened. The gas contained inside the pipe running between the automatic valve and a manual valve, which was closed at the time and located 10 m upstream, was released. One worker was able to escape via the dryer supply chute after passing underneath the injection nozzle, while the two others facing the nozzle tried to reach the dryer access hatch located 25 m away. Only one of these two was able to exit the dryer. After scrambling less than 10 m, the second man tripped. Wearing a mask, the maintenance foreman evacuated the crew member unable to advance (within 3 min). All three men complained of facial burns and breathing difficulties and were taken to hospital. One was slightly affected by the exposure, the second more seriously, while the worker who fell died 6 days later subsequent to the pulmonary burns he had sustained. The accident was due to poor coordination of these maintenance works, since the replacement of inverters and the cleaning, performed by two different teams, should never have been carried out simultaneously. Upon completion of the inverter replacement operation, the step of reinitialising the automation programmes caused the two automatic valves controlling ammonia and phosphoric acid injection inside the dryer to open. The workers were verifying that pipes had successfully drained using a manometer, which was only able to indicate the pressure drop but could not confirm the pipe had been fully drained. When the valve opened, the ammonia left in the pipe (5 litres) was released. Following this accident, an additional valve was installed at the entrance to the reactor upstream of the automatic valve. In addition, specific guidelines were introduced that mandated: prohibition of all simultaneous operations; a 1-hour purge of the pipe between the two valves; modification of computer programmes to ensure that no valve could remain open during the re-initialisation sequence; placement of a full seal after the manual valve during onsite work. A study of the plant’s SMS was planned.