NFPA 13R: Sprinkler Systems for Low-Rise Residences Practice Test

Dry pipe underground means sprinklers are fed from piping that runs underground and is kept dry by keeping the distribution lines under pressurized air (or nitrogen). Water is held back behind a dry-pipe valve, so the underground piping isn’t filled with water until a sprinkler head is activated. When a head trips, the valve opens and water rapidly fills the underground piping to reach the sprinklers. This keeps underground runs protected from freezing and reduces water-filled piping before activation. The other options describe different configurations: underground pipes that are already water-filled would be a wet-pipe underground system, above-ground air-pressurized piping isn’t underground, and exterior open spaces aren’t a sprinkler-piping configuration.