This incident occurred at the electrolyzer of a refuelling station. An employee of the electrolyzers manufacturer was on-site to conduct a reset of the plant power breaker system and to restart gas generation. Whilst putting the system into generation, a loud bang was heard. By reviewing the station CCTV evidence, a cloud of water vapour was seen being emitted from the oxygen vent line of the electrolyzer container. The gas generation system was safely shut down, and an investigation was started to understand the cause and potential impacts. The on-site investigation revealed no visible damage to electrolyzer equipment, including the water purification system, the oxygen exhaust pipe. On the contrary, damage was observed in the stack: the stack initially developed a crack in the first foil on the bottom end-plate (cause unknown). It is likely that this was causing a leak and an increase in the pressure drop. But very probably this drop was not big enough to be detected during the pressure drop test. The stack could still generate at pressure as the rate of generation would be greater than the leak rate. From analysis of the data, set-points designed to detect this type of failure mode where shown to have been adjusted from those initially set. Therefore the plant did not automatically alarm at the appropriate time to stop all operations.An important part of the study was to understand if the cloud was water vapour or smoke. Since no evidence of a flame and no heat damage to pipework were found, it can be concluded that the cloud was water vapour and not a result of a fire. The water vapour was released due to combining small amounts of hydrogen and oxygen gas within the stack system.