Why Was Unintended Car Movement Protection (UCMP) Added to the Code?

Why Was Unintended Car Movement Protection (UCMP) Added to the Code?

UCMP Educational Series – Part 3 of 6

In the first two posts, we defined Unintended Car Movement (UCM) and explained Unintended Car Movement Protection (UCMP). Now, let’s look at why UCMP became a code requirement.

When an elevator arrives at a floor, it is designed to hold its position and not move. In traction elevators, this position is held by a brake that keeps the gears (if used) and sheave from turning, which in turn prevents the hoist ropes and cab from moving.

One of the most common causes of UCM happens when the brake does not work properly because a component is worn, damaged, or sticking. When that occurs, if unloaded, the weight of the counterweight (equal to the empty cab + 40% of its rated capacity) is greater than the weight of the cab itself. The result is that the cab can begin drifting upward after it has already stopped at a floor. This can create a very dangerous situation.

Some examples of this behavior in action:
🔗 Example: cab drifting while passenger exits
🔗 Example: elevator rising unexpectedly at landing

And in more publicized cases, UCM has led to serious injury and even death:
🔗 2006 Minato Ward elevator accident
🔗 2019 JPS Hospital investigation report
🔗 Brake failure example

Incidents like these of brakes, control systems, or other critical components failing illustrate why UCMP was added to the code: to help prevent accidents before they happen by limiting the distance the car can move away from the landing.

Although rare, accidents in which elevators moved while a passenger was still entering or exiting made headlines due to their severity. By requiring UCMP, ASME standards aim to ensure that these kinds of incidents do not happen in the future.

Since the machine brake is one of the components UCMP is specifically meant to back up, part of UCMP involves installing a separate emergency brake to safely stop the car if the primary brake fails or is unable to hold the elevator in position.

In the next post, we will explore the different types of emergency brakes and how they work with UCMP to keep elevators safe.