Which sprinkler system requires two separate events for water discharge?

The correct answer is the pre-action system because it is specifically designed to require two distinct events for water discharge. In a pre-action system, the first event is typically the activation of a fire detection device, such as a smoke or heat detector, which sends a signal to the system's control panel. Once this signal is received, the system fills the pipes with water, but the water does not discharge from the sprinkler heads until a second event occurs, typically the actual activation of a sprinkler head due to heat or flames. This design helps reduce the risk of accidental discharge, providing an additional safety measure that differentiates it from other types of sprinkler systems.

In contrast, a wet pipe system automatically releases water when a sprinkler head is activated by heat, with no separate triggering event needed. Similarly, a dry pipe system, which contains pressurized air in its pipes, releases water through the activation of the sprinkler heads without requiring a separate triggering event; the system fills with water once the air pressure drops when a head opens. A deluge system releases water simultaneously from all open sprinkler heads as soon as the system is activated, also lacking the two-event mechanism of the pre-action system.