In these systems, sprinklers are open at all times. They are connected to a dry pipe that is connected to a main water supply. A fire detection device controls the main valve. When it is activated, the valve opens, allowing large amounts of water to flow through all of the sprinklers. The purpose of a deluge system is to quickly wet down an entire hazard area to prevent a fire from spreading. They are usually used in facilities that contain hazardous materials such as: flammable liquids, chemicals, and explosives. Rooms with high ceilings sometimes use deluge systems, as it is difficult to direct water over the burning area from such a distance.