Which of the following can cause a backflow condition?

A decrease in water pressure in the distribution system can lead to a backflow condition because it creates a scenario where the normal flow of water is disrupted. In systems where a significant drop in pressure occurs, such as during high demand or a mainline break, the available pressure to push water through the system decreases. This allows the potential for backflow, as water from service lines or lower pressure areas can be drawn back into the potable water supply, carrying contaminants with it.

In contrast, clogged drainage pipes primarily create blockages that prevent proper drainage but do not inherently create a backflow risk into potable water systems. Improper installation of fixtures can result in various plumbing issues, but the link to backflow is more about how the system is maintained rather than pressure-related changes. Excessive water flow rates can lead to issues with system capacity but are typically not a direct cause of backflow under standard conditions. The connection between pressure drops and backflow is well-documented in plumbing codes and regulations, making the understanding of pressure dynamics crucial for preventing contamination in water systems.