Protecting against unintended car movement (UCM)

In the early 1980s, BODE premiered unintended car movement protection. From then on, it was possible to ensure the safety of people and objects when loading and unloading the car. The prevention of uncontrolled movement was adopted in standard EN 81-1/2 in the form of Amendment 3 in response to the growing number of increasingly fatal accidents.

Since then, “UCM” has become the shorthand for this in the lift world.
The current version of EN 81-20/50:2014 regulates protection against unintended car movement in section 5.6.7 and in the type testing of our overspeed governors under 1.2 through 1.4.

The prerequisite is that the overspeed governor is fitted with UCM protection. In normal operation, during regular car movement, the operating voltage is present at the magnet coil of the UCM protection device, which retains the bolt in its end position in its energised state. The catch pendulum on the overspeed governor can swing freely.

When the car stops, the protection device is de-energised, and the spring-driven bolt is released. The switch on the protection device signals the current position of the bolt. Depending on the position of the wheel, the bolt of the protection device either strikes the switching cam or slides clear over it. The protection device does not need to be retracted when adjusting the position of the lift in the stop.

If an unintended movement of the car occurs, the bolt of the UCM protection mechanism holds the switching cam with the catch pendulum in the release position. The wheel of the overspeed governor is arrested and engages the safety catch. The safety switch on the overspeed governor interrupts the safety circuit.

For an overspeed governor wheel diameter of 200 mm, there is a minimum distance of 50 mm before the mechanism is engaged; for a wheel diameter of 300 mm, this distance is 70 mm. Within this minimum distance, there is scope to adjust the car position in the secured state. The bolt on the UCM protector does not need to be retracted for this.

The lift controller verifies this function every time the UCM protection system is de-energised. The switch on the UCM protector is fitted with a second contact for this purpose. In the event of a malfunction, the lift must be shut down.

The fall protection system should be battery-buffered, as otherwise it would block the overspeed governor in the event of a power failure.