Cooperative Personnel Services Firefighter (CPS) Practice Exam

The relief device begins to open at the set pressure, which is a critical point that ensures safety within pressurized systems. This set pressure is predetermined during the design phase and is the point at which the device is calibrated to release excess pressure to prevent system failure.

When the pressure in the system reaches this specific level, the relief device activates to allow fluid or gas to escape, thus maintaining safe operational conditions and protecting the integrity of the system. This mechanism is essential for preventing overpressure situations that could lead to equipment damage or hazardous conditions.

The other choices relate to different contexts of operational safety: the designed pressure threshold refers to the theoretical limit of the system, while operational pressure indicates the normal working range, and maximum allowable temperature pertains to thermal limits rather than pressure management. Each serves an important purpose in system design and monitoring, but they do not directly correspond to the activation of the relief device.