No, I have not found that in certain occupancies, the NFPA requires Class A as opposed to Class B. Most of the time, the NFPA leaves upgrading from Class B to Class A to the AHJs.
NFPA System Approval
The National Fire Protection Association, Inc. (NFPA, a non-profit publishing house) does not approve, inspect, or certify any installations, procedures, equipment, or materials. . . [Paraphrased from the definition of "Approved", located in the appendix in the back of the NFPA 72 Code, 2007 Edition.]
Instead, the NFPA 72 is more of a guideline showing the minimum needed for an adequate fire alarm system. The guideline also shows many of the ways to improve on the bare minimums.
For example, Class B circuits. If anything goes wrong with a Class B circuit (a wire comes loose or breaks), and a fire occurs before the circuit is fixed, Class A would have been an improvement that could have provided more protection.
Then again, if there's a wire-to-wire short in an addressable Class B or Class A circuit, and the short is not fixed before a fire occurs, Class X would have allowed the circuit to still work, and therefore Class X would have provided more protection.
The AHJ Decides
Described in the appendix at the back of the NFPA Code book, the NFPA shows many different AHJs (Authorities Having Jurisdiction).
If any AHJ has more stringent requirements for a Fire Detection and Alarm System than the NFPA's minimum needed, the fire alarm system will be set up according to the more stringent requirements. Class A would be more stringent than Class B.
AHJ - Fire Marshal: Has motivation for fire safety, has studied fire safety, and represents the government
AHJ - Electrical Inspector: Has motivation for electrical safety, has studied electrical safety, and represents the government
AHJ - Insurance Representative: Represents the insurance company that pays for damages, the insurance representative is motivated for safety
AHJ - Engineer Representing the Owner: Has motivation for a safe design for the fire alarm system
There are many more AHJs listed in the NFPA 72. All of them have motivations for a safe and effective Fire Detection and Alarm System.
Who's the Boss
We, as designers, installers, technicians, and testers of fire alarm systems, are bottom feeders; all the AHJs are our bosses.
We are required to provide the bare minimum for a Fire Detection and Alarm System shown in the NFPA, but if one of the many AHJs requires more, then we, who are doing the actual work, do what AHJ requires.
Of course, if there's something we don't understand, we can discuss the extra requirements with the AHJ. Maybe we can learn something, maybe the AHJ can learn something, or else change their mind. In the end, though, all of the AHJs are our boss, we just do what we're hired to do.
Douglas Krantz
