A standard dry-pipe system is designed for use in unheated environments where the piping may be subjected to freezing temperatures. The piping network downstream of the dry-pipe valve is filled with pressurized air or nitrogen. This pressure holds the clapper in the dry-pipe valve closed, preventing water from entering the system. When a fire activates a sprinkler head, the air pressure is released, causing the valve to open and allowing water to flow into the piping and discharge through the open sprinkler. This operational principle requires the piping to contain pressurized gas at all times under normal conditions.

B. A wet-piping system is filled with water under pressure at all times, not air or nitrogen.

C. A preaction system's piping contains air that may or may not be under pressure; its primary activation relies on a supplemental detection system, not solely pressure loss.

D. A deluge system uses open sprinklers, and its piping is at atmospheric pressure, not pressurized with air or nitrogen.

1. NFPA 13, Standard for the Installation of Sprinkler Systems, 2022 Edition.

Section 3.3.21.5 Dry Pipe System: "A sprinkler system employing automatic sprinklers that are attached to a piping system containing air or nitrogen under pressure, the release of which (as from the opening of a sprinkler) permits the water pressure to open a valve known as a dry pipe valve, and the water then flows into the piping system and out the opened sprinklers." This definition directly supports the correct answer.

Section 3.3.21.11 Wet Pipe System: "A sprinkler system employing automatic sprinklers that are attached to a piping system containing water and connected to a water supply so that water discharges immediately from sprinklers opened by heat from a fire." This confirms option B is incorrect.

Section 3.3.21.8 Preaction System: "A sprinkler system employing automatic sprinklers that are attached to a piping system that contains air that might or might not be under pressure..." This confirms that pressurized air is not a defining requirement as it is for a dry-pipe system.

Section 3.3.21.4 Deluge System: "A sprinkler system employing open sprinklers that are attached to a piping system that is connected to a water supply through a valve that is opened by the operation of a detection system..." This confirms the piping is not pressurized.