Tom Horne wrote:


The 115 Volt AC type of smoke detector are manufactured both with and
without a backup battery compartment. Were detectors are required to
meet code the regulations often, but not always, require the kind with
back up battery.
..
The 2008 NEC, under AFCIs (210.12-B), has a fine print note referring to
NFPA 72, the standard for smoke alarms. IIRC the discussion for the 2008
code change on AFCIs was that NFPA 72 requires a "secondary power
supply" for smoke alarms on AFCI circuits. AFCIs in the 2008 NEC are
required just about everywhere in a residence a GFCI is not required.
(It would help if the FPN was more explicit.)

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