As you have probably surmised, there are numerous 'types' of water based sprinkler systems. Generally, you can divide them into two groups, then subdivide beyond that: wet and dry systems.
A wet system is one in which there is always water present in the piping network of the system. When any given sprinkler head is burst (heats to a point where the thermal element is compromised), water will be discharged. Note that each sprinkler head acts independently in a sprinkler system; so all those movies and commercials you've seen where all the sprinklers go off at the same time?! Not accurate.
A dry system is one in which water is NOT present in the piping network all the time. A dry system includes a valve near the primary feed pipe of the water which is 'closed' until such time as it is required by the system.
A dry system requires some sort of action (thus the term pre-action) prior to actually flooding the pipe with water, and potentially discharging into the protected space. These actions typically come in a variety of ways and / or combinations:
- Smoke detection. Spot type smoke detectors are used to provide an alarm or signal to a central control panel that can then tell the valve connected to the water line to 'open'.
- Pressure switch. In some cases, the 'dry' or empty piping is actually pressurized with air or nitrogen. In a fire scenario, a given sprinkler head bursts, the pressure in the piping dissipates and the pressure switch is tripped. The tripped switch sends a signal to the control panel, and can then tell the valve to be opened, sending water throughout the piping network.
When BOTH the above actions are required for water to be discharged into the pipe network, that is generally referred to as a 'double interlock' or 'dual interlock' system. This type of strategy is common for sprinkler systems in high value asset areas like data centers.
You may want to use common web-based search tools for more information on this topic. There is plenty of data and text available 'out there'.
This was first published in November 2006