Glossary
Unintended Car Movement (UCM)
A hazardous condition in which the elevator car moves away from a landing while the doors are open, or the doors open while the car is between floors, addressed by dedicated detection and prevention systems required by modern codes.
Unintended car movement is one of the most serious hazards in elevator operation. It can occur due to brake failure, control system malfunction, or a combination of factors that allow the car to move when the doors are open at a landing. Passengers stepping into or out of the car can be caught between the car sill and the landing sill, leading to severe injury or death.
Modern elevator codes (ASME A17.1 since the 2007 edition, with mandatory compliance phased in through the 2012 and subsequent code cycles) require a dedicated UCM protection system independent of the main controller. This system continuously monitors car position and door state using independent sensors. If it detects car movement while the doors are open, it applies the car brake (or a dedicated UCM brake) and prevents further travel. UCM protection has been one of the most significant safety improvements in elevator codes over the past two decades.