The on-duty field technician noticed a self-sustained ignition in the hydrogen pipes of a sodium chlorate plant. The flare-like flame of less than one metre in height was located in the space above the plant control room right above the exit door. The technician signalled an alarm to the plant control room where the field operator immediately informed the shift supervisor and also the regional emergency response centre.The other people on the site took fire extinguishers from the control room and rushed to the fire scene. When opening, from the outside, the door leading to the space above the control room, they noticed that the leak was located right at the door. Even though they discharged two extinguishers to suppress the fire, it did not go out. After that, they shut down Plant-1 using the emergency stop function. Plant-3 and Plant-4 were similarly shut down. During the shutdown, fire fighters dragged a fire hose up onto the landing on which the fire, now dying down, was fully extinguished.The post-accident analysis of the components detected pulverised rust dust inside the removed pipe section. The metal of the corroded section had completely softened, and the connector piece could be removed by pulling by hand.To determine the cause of the hydrogen ignition, the potential differences across the pipe were measured using a voltage meter. No potential differences were detected, so the only conceivable cause of ignition was charged particles, e.g. snowfall early in the morning. Hydrogen appears to have leaked out from the leak site for an extended period of time, until an igniting factor was introduced.