Sometimes, to understand what is meant in the legal-ease of the fire alarm code, we have to look at the meaning from another point of view: a fire alarm system is a Fire Detection and Alarm System (FDAS). Because the FDAS detects fires, it also does things like controlling smoke, capturing elevators, calling the fire department, and sometimes it suppresses fires. This is a holistic view of what a fire alarm system does.
The National Fire Protection Association (NFPA) isn't saying that the fire alarm system includes only the equipment provided by the fire alarm control panel's manufacturer; the NFPA is saying that the Fire Detection and Alarm System includes everything fire related that can initiate an alarm or supervisory. It also includes everything fire related that notifies people, controls smoke, calls the fire department for help, and suppresses fire.
If it's connected to the fire alarm system, it actually is part of the system that detects fire and sounds the alarm.
Much of the interface equipment is installed by others, like the elevators, the sprinkler system, the kitchen's range hood suppression and gas shunt, the HVAC air handlers and smoke dampers, etc. It isn't directly installed, maintained, or serviced by the fire alarm company, but the bottom line is that someone has to take some responsibility for the entire Fire Detection and Alarm System.
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That requirement by the state fire marshal seems to be a little extreme. However, even though other vendors install, maintain, and test peripherical fire alarm equipment like elevators, HVAC, sprinklers, smoke evacuation, etc., the NFPA is saying that someone has to be responsible for the whole Fire Detection and Alarm System.
In other words, the NFPA wants someone to oversee and make sure the Fire Detection and Alarm System, as a whole, will work.
The companies installing, maintaining, and testing the interface equipment to the fire alarm system have their own requirements to make sure their systems work. However, none of them really is qualified to oversee the whole Fire Detection and Alarm System.
The people installing the central fire alarm system are the most qualified to coordinate all the separate
systems together. It's the fire alarm people, being the most qualified, that are the ones picked to be in charge.
Yes, if the interface equipment can send alarms, supervisory signals, and trouble signals, it is part of the fire alarm system. Yes, if the interface equipment does things because the fire alarm system is sending signals, it is part of the fire alarm system.
If the interface equipment is connected to the fire alarm system, then it really is part of the overall Fire Detection and Alarm System.