Robert11 wrote:

Hello:

About to buy a few Kidde P12000 Smoke alarms.
The AC wired type, with both ionization and photocell.

Looked at their on-line instructions, and was surprised
to see that they say to install on circuits that are NOT
GFCI protected.

Seems surprising.

Anyone know the reason why ?

GFCI's can have nuisance trips. Even a thunderstorm nearby can sometimes
trip a GFCI. This could greatly shorten backup battery time for the smoke
detector, or even cause complete failure if the circuit isn't promptly reset
before the battery dies. The smoke detectors should also be on the same
circuit as often used lighting to make it very obvious if the circuit is open.

For similar reasons, it's not a great idea to put a refrigerator on a GFCI
circuit.