This is the perimeter protection for the premises. The magnetic contact is usually installed in exterior opening doors and windows that are reachable from the ground (or from another part of a building which is accessible from the ground – such as a garage). The contacts operate on the principle that when the door or window moves more that 1/2″ from the frame, the magnetic field on the switch is removed and this breaks the alarm circuit (or zone) which trips off the system. Once tripped, the alarm signal cannot be stopped by just closing the door or window. Magnetic contacts are popular and widely used because they detect an intruder before he enters the protected premises.