Another sprinkler system type is the pre-action system which employs the basic concept of a dry pipe system in that water is not normally in the pipes. The difference is that water is held out of the piping by an electrically operated valve, know as a pre-action valve. Valve operation is controlled by an independent system of heat, flame or smoke detectors.
Two separate events must take place to initiate a water discharge. First the detection system must sense the heat, flame or smoke of a developing fire and then open the pre-action valve allowing water to flood the piping. Secondly, the individual sprinkler heads must be fused or activated by heat from the fire in order to allow the water to flow from the head.
The primary advantage of a pre-action system is that a minimum of two separate actions must take place for water to be discharged, thereby providing an added level of protection against accidental discharge. The disadvantages of a pre-action system are high installation and maintenance cost, which potentially decrease reliability due to the systems complexity and modification difficulties caused by specific size limitations.
A variation of the pre-action system is the deluge system. Basically a deluge system is a pre-action system with open sprinkler heads. Operation of the pre-action valve caused by the separation detection system allows water to immediately flow from all the sprinkler heads.