You know those secured doors with smart card or magnetic card access? where you have to flash or swipe to get in?
if by battery? what happen if the battery died? will dismounting the thing trigger any alarm?
Most such buildings have a back-up generator that kicks in if there is a black-out. These generators usually only run emergency and essential things (such as security doors and emergency lighting) and can run for 12 -24 hours usually.
After this period, they doors will be deactivated (unlocked) as the electromagnet will be turned off.
The doors themself are powered through the AC mains, with the back-up generator in parallel, so neither battery or plug powered (a plug comes from the AC mains though, I am only say this as you cannot grab a plug an pull in out for the door to turn off).
Interfering with the power supply to the door will most likely trigger an alarm, a testing circuit would probably be in the circuit to test whether the door has been unlocked without autherisation.
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