Both the single interlock and non-interlock systems require only one event to occur before water is admitted to the system. The single interlock system is activated by the release of the detection system. Sprinkler activation does not affect this function. The non-interlock system is activated if either the detection system or a sprinkler operates.
The double interlock preaction system requires two events to occur before water is admitted to the system. One event consists of the activation of a device installed on the supplemental detection system. The other event includes the operation of a sprinkler that causes the maintained air pressure in the system to fall to a predetermined level, which is similar to that of a dry pipe system. When one of these events occurs, the system activation valve goes into a preset position. When the second event occurs, the valve opens, and water enters the system. Water does not enter the system until both events occur. These two events can occur in any order and result in the same outcome.