Definiton of “interlock” – to lock together; to connect so that the motion or operation of any part is constrained by another. Preaction systems are interlocked systems that require a supplemental detection system in the same area as the sprinkler system. Operation of the deluge (preaction) valve is dependent upon activation of a detector, or both a detector and a sprinkler. Single interlock preaction systems require that only a detector operate to cause the deluge valve to trip. Once the valve trips, water fills the system piping. Water will not be discharged until a sprinkler operates due to the heat of the fire. Given that water is present at the sprinkler at the time of sprinkler operation, single interlock preaction systems are treated as wet systems from a design standpoint. Double interlock preaction systems require that both a detector AND a sprinkler operate before causing the valve to trip. A good analogy is a door with two locks. You must open both locks before the door will