Abstract: Method for detecting and controlling the concentration of a flammable gas in an enclosed space. The method includes providing a source of flammable gas to an enclosed space, wherein the enclosed space includes a device which consumes or stores the flammable gas, and wherein the enclosed space further includes unclassified electrical equipment and an electrical power supply to the unclassified electrical equipment. The atmosphere in the enclosed space is analyzed to determine the concentration of the flammable gas therein and this concentration is compared to a first reference concentration and a second reference concentration that is less than the first reference concentration. When the concentration of the flammable gas in the enclosed space is between the first and second reference concentrations, the source of flammable gas to the enclosed space is terminated and the enclosed space is ventilated.