I have installed a few of them for gas/mini mart stations here in the city. Generally they mount on the entry door and are hooked up to a 12 volt switch behind the bullet proof glass cash register area. One click of a toogle switch and that door has a good 1200 pounds of magnetic pressure holding it closed.
Get the idea?
City inspectors will shut to down if one is found. Luckily they are easy to uninstall and reinstall. LOL
They are basicly ext Al with a electo magnet inside.