Question 19 QMED01 - Junior Engineer

The Correct Answer is C **Explanation for Option C (Correct):** Option C, "Location of the potable water spigots above the rim of the galley sink," describes the principle of an **air gap**. An air gap is the most effective and reliable method recognized by plumbing codes (such as the Uniform Plumbing Code or International Plumbing Code) to prevent **back-siphonage** or **backflow** contamination. If the drain lines become clogged and sewage backs up into the sink basin (reaching the rim), and simultaneously the water pressure in the potable supply system drops (creating a vacuum or negative pressure), water from the sink basin could be sucked back into the clean water supply system. By ensuring the lowest opening of the potable water spout (the spigot) is physically located higher than the flood level rim (the top edge) of the sink, there is a permanent, physical separation (the air gap) between the clean water source and the potential contaminant source. This prevents any sewage backup from ever reaching the potable water outlet. **Explanation for Incorrect Options:** **A) Location of the potable water spigots below the rim of the galley sink.** This is incorrect. Placing the spigot below the rim (or even slightly above but too close to the rim) eliminates the necessary air gap, creating a potential cross-connection hazard where backflow or back-siphonage can easily occur, directly introducing sewage contaminants into the potable water system. **B) A "P" trap in the drain line draining sewage waste from the galley sink.** This is incorrect. A P-trap (or U-bend) holds a water seal to prevent sewer gases from entering the living space. While essential for hygiene and odor control, the P-trap does nothing to prevent **backflow** of contaminated water or sewage from the sink basin into the potable supply lines if negative pressure occurs in the supply lines (back-siphonage) or if the sewage level rises above the rim of the sink. **D) Check valves in the potable water supply lines delivering water to the galley sink.** This is incorrect, or at least insufficient and unreliable compared to an air gap. A check valve (a type of backflow prevention device) is designed to allow water flow in only one direction. However, mechanical devices can fail due to wear, fouling, or misalignment (e.g., if a piece of scale or debris lodges in the valve seat). Because the air gap (Option C) is a non-mechanical, fail-safe separation, it is the primary method required for ensuring protection against high-hazard backflow events in fixture connections like sinks. Check valves may be used as secondary protection (a backflow preventer) but cannot replace the air gap as the fundamental defense against contamination from an open fixture like a sink.