The door zone is the region above and below floor level within which the car doors are permitted to open. EN 81-20 limits this unlocking zone to a maximum of 0.35 metres above and below the landing, and the doors may only be released when the car is inside it. Position is detected by magnetic vanes or photoelectric sensors in the shaft feeding the door zone signal to the controller. Any fault that could allow the doors to open outside this zone is treated as a safety-critical failure.