My best bet is that the 9 volt battery sounds the screamer if the power goes out.
Remember these are life dependent machines and kicking the cord out of the wall as you leave the room could very well kill someone.
And, of course, engineers being who they are, the alarm will also sound if the battery gets too low so you have to keep a good one in there even if you don't want to.
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The Zombie Apocalypse is Upon Us.
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