An explosion of a compressed natural gas (CNG) tank should not be able to occur, as the tank is fitted with a pressure relief device (PRD). This PRD should release the CNG in the tank, prior to the pressure reaching the tank burst pressure.the PRD did not release in the subject Wisconsin police cruiser explosion.NHTSA has recovered the PRD valve from the tank that exploded and testing will be necessary to establish why this tank safety valve did not release in the very high temperature fire that occurred in the police cruiser.there is only the single known event, but due to the catastropic force of such an explosion and the potential for injury and loss of life, this investigation is being upgraded.
