An explosion occurred in the hydrogen washing tower of a chlorate plant caused by the breaking of the rupture diaphragms on the tower cover. The discharged hydrogen ignited, causing a minor leakage of hydrochloric acid in the hydrochloric acid top container above it and the related pipes.The explosion was preceded by an abrupt reduction in the electrolysis flow resulting from a rectifier malfunction and operation at low flow density. This caused a significant increase in the oxygen level in the hydrogen gas. For the temperature prevailing in the cubicle, the hydrogen gas contained an excessive amount of diluting water vapour which, however, was absent in the top section of the washing tower. The explosive gas ignited and broke the rupture diaphragm. The electrolysis flow in the unit was cut off with the emergency shut-off switch two minutes after the explosion, at which point nitrogen flushing to dilute the gas was also started. Burning hydrogen gas was discharged from the rupture disk opening until the connection to the main hydrogen pipe was closed.