Pre-action sprinkler systems

In an ordinary wet system the pipework above the ceiling is full of water at all times, and a knocked-off head empties it onto whatever is below. A pre-action system holds that water back at a valve in the plant room and keeps the grid dry. Detection in the room opens the valve; the sprinkler head still has to reach its own temperature before anything discharges.

That double interlock is the whole point. A damaged head on a dry grid releases air, not water — the panel reports a low-air fault and someone goes and looks. It is the arrangement for rooms where the contents are the reason the building exists: data halls, record rooms, control rooms, museum stores and switchgear rooms.

The design is a sprinkler design plus a detection design, and the two have to be commissioned together. We prove the releasing sequence end to end — detector to panel to solenoid to valve — with the grid isolated, then again with air pressure restored, and hand over the cause-and-effect matrix that the release logic was set against.