[NOTE: the attribution to hydrogen formation as cause of the explosion is 'probable 'but not fully demonstrated].Two explosions in two different sprinkler systems occurred, wit the same assumed cause.The first occurred at the premises of a company, where a service engineer was injured, even though the explosion actually moved out into the open air. The second occurred in the department store, where an installation contractor was burned because an explosion occurred in a large pressure storage tank in a small room with the result that the explosion was particularly powerful. On both occasions, the sprinkler system ignited and exploded due to a flammable gas in the system following the draining of water.It is believed that hydrogen formed by reaction of water with zinc in the inner coating layer of the pipes. The gas escaped during operational and maintenance work, whereby the pipes were opened in order to discharge the water. In the open air, the gas mixed with the oxygen, reaching a critical concentration, which was then ignited by sparks from tools.