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Fuel cell vehicles that use high pressure hydrogen gas as a fuel should be able to immediately release hydrogen gas from the cylinder through pressure relief devices (PRDs) in the event of a vehicle fire. The release through PRDs prevents the cylinder from exploding due to the increased pressure of hydrogen gas, but the method of releasing the gas needs to be specified in order to avoid secondary disaster due to the spread of fire. Since hydrogen cylinders for fuel cell buses are different in terms of installation location and size from those for ordinary vehicles, the location of PRDs and the release direction of hydrogen gas should be separately examined. For example, the improper locations of PRDs would raise the possibility of explosion because of a delay in temperature rise, and the direct release of hydrogen gas from a cylinder installed on the rooftop of the bus may disperse the flame over a wide area.

At the time of a vehicle fire, high pressure hydrogen gas in a tank (a high pressure hydrogen gas cylinder) of a fuel cell vehicle (FCV), which is a passenger vehicle, must exhaust through a pressure relief device (PRD) as quickly as possible in order to prevent any accidental bursts by a temperature rise of hydrogen gas in the cylinder. The high temperature region surrounding a vehicle develops when the hydrogen gas is released through a small nozzle to the air directly. Therefore, to suppress the high temperature region, the effectiveness of a diffusion box is considered further. A pressure relief device (PRD) detects differences in temperature of the environment surrounding an FCV on fire and releases hydrogen gas in a tank to the air by which the valve opens when the temperature in the environment becomes high. The PRD also releases hydrogen gas through a nozzle, e.g. installed upward or downward, to the outside of the vehicle. The PRD is required to be installed in an FCV.