If two different water supplies are connected and one is not potable, a plumber should install a

• A. Globe valve
• B. Relief valve
• C. Back flow prevention device
• D. Pressure reducing valve

Answer: (C)

When two water supplies are linked and one may be non-potable, you must protect the clean potable supply from contamination. A backflow prevention device does exactly that by stopping water from flowing backward into the potable system during pressure changes or if the non-potable side pushes water back toward the clean supply. The other options don’t provide this protective barrier: a globe valve is just a shutoff valve and doesn’t prevent backflow; a relief valve releases excess pressure but won’t stop backflow into the potable line; a pressure reducing valve lowers supply pressure but doesn’t prevent backflow. Installing a backflow preventer at the connection point preserves the safety of the drinking water by preventing cross-connection contamination.