A reduction of ______ percent of pressure during a main drain test must be identified and corrected.

• A. 5 percent
• B. 10 percent
• C. 15 percent
• D. 20 percent

Answer: (B)

During a main drain test, the system is pressurized and the main drain is opened to see how much pressure is lost. A small, expected drop happens as water is released, but a larger drop points to leaks, valve seating problems, or damaged piping. The standard acceptance criterion is that the pressure should not fall more than ten percent of the operating pressure. If the drop reaches ten percent or more, you must identify the source of the leak and correct it before the test passes. That’s why ten percent is the threshold used. Smaller drops are typically acceptable, while larger drops indicate more serious issues that need repair.