A fixture trap shall be protected against _______ and backpressure.

A fixture trap must be protected against siphonage and backpressure to ensure that it functions effectively in preventing sewer gas from entering the building and to maintain proper drainage. Siphonage occurs when there's a negative pressure in the drainage system that can pull water out of the fixture trap, creating a pathway for hazardous gases to escape into occupied spaces. By preventing siphonage, the trap maintains the water seal, which is crucial for the trap's purpose.

Backpressure, occurring when there is an increase in pressure in the drainage system that could push wastewater back into the fixture, can also compromise the trap's effectiveness. Thus, safeguarding against siphonage ensures that the integrity of the trap is maintained, preventing the potential health hazards associated with sewer gas exposure and ensuring the fixture operates correctly.

While blockage, backflow, and overflow are also important considerations in plumbing systems, they do not directly relate to the specific functions of a fixture trap as much as siphonage does. Blockage pertains to physical obstruction in piping, backflow deals with reverse flow of water that can occur due to pressure changes or other issues, and overflow refers to excess water spilling out of a fixture, neither of which specifically targets the critical issue of maintaining the water seal in traps.