For example, a wet-based sprinkler system may be mandatory, but can't it be complemented with other systems that are activated first when there is a fire?
The short answer here is, yes. In the vast majority of cases, local code (which is frequently driven from NFPA or IBC or other standardized codes) will dictate the need for water based systems, and in most cases, do require them, throughout the building.
In some cases, although rare, the AHJ (Authority Having Jurisdiction) will allow a waterless, clean agent suppression system in lieu of a water based system. NFPA 101, the Life Safety Code, section 9.7.3 contains an example of verbiage that justifies this position.
90 percent of the time or more, waterless, clean agent systems are selected for use by the owner of the building and/or the owner of the assets to be protected. Systems like FM-200 are employed to protect the operational value and the contents of the facility, not necessarily for structural protection of the building... thats what the sprinklers are there for!
Also, your conclusion regarding sequence of events is correct. In almost every case where both a sprinkler system and a waterless system is employed, the clean agent will discharge prior to a sprinkler discharge. Fundamentally, this is because the waterless system works from smoke detection (fast acting) while the sprinkler works from heat detection (slightly slower acting).
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