A plumber is performing a mandatory annual inspection of a Reduced Pressure Principle (RP) backflow preventer in a commercial facility. During the test, the differential pressure relief valve opens to atmosphere when the inlet shutoff valve is closed and the outlet test cock is opened. What does this specific observation indicate?

Question: A plumber is performing a mandatory annual inspection of a Reduced Pressure Principle (RP) backflow preventer in a commercial facility. During the test, the differential pressure relief valve opens to atmosphere when the inlet shutoff valve is closed and the outlet test cock is opened. What does this specific observation indicate?

Answer options: ✅ The RP device is functioning correctly, indicating proper operation of the relief valve.

• The RP device has a failed check valve, allowing water to leak past the first check.
• The RP device is installed backward, causing the relief valve to react incorrectly.
• The test is invalid due to insufficient water pressure in the system.

Correct answer: The RP device is functioning correctly, indicating proper operation of the relief valve.

Explanation: During an RP backflow preventer test, the opening of the differential pressure relief valve (dprv) when the inlet shutoff is closed and the outlet test cock is opened is a critical step in verifying its proper operation. This action effectively reduces the pressure in the zone between the two check valves, and the dprv opening ensures that this pressure is maintained at least 14 kPa (2 psi) lower than the supply pressure, which is a key function of an RP device to prevent backflow in severe hazard situations.