What prevents high-pressure oxygen from entering the downstream system if the pressure reducer fails?

The correct answer is the pressure relief valve. The primary function of a pressure relief valve is to ensure that the pressure within a system remains within safe limits. In the context of a failure in the pressure reducer, if high-pressure oxygen were to enter the downstream system, it could lead to dangerous conditions, including over-pressurization and potential ruptures.

The pressure relief valve acts as a safeguard by venting excess pressure when it exceeds a predetermined threshold. This prevents high-pressure oxygen from flowing downstream, thereby protecting sensitive equipment and maintaining safe operational conditions within the cabin atmosphere control system. The pressure relief valve effectively serves as a safety mechanism that opens to release gas when necessary, ensuring that the downstream system does not encounter pressures above safe operating levels.

In contrast, the other options do not provide the same level of protection against high-pressure entries. An emergency shutoff valve would typically be used to stop the flow of gas entirely but is not specifically designed to manage overpressure situations. The check valve allows flow in one direction but does not prevent backflow or manage pressure levels. A pressure gauge simply indicates the pressure in the system but does not actively control or mitigate pressure levels.