What is a common safety feature in hydraulic elevators that prevents unintended movement?

The correct answer is emergency brakes. Emergency brakes are a crucial safety feature in hydraulic elevators designed to prevent unintended movement, especially in scenarios where there is a sudden loss of power or system failure. These brakes engage automatically to hold the elevator car in place, ensuring the safety of passengers and preventing dangerous falls. The emergency brake system is activated by mechanisms that detect anomalies, such as excessive speed or possible free-fall conditions, effectively locking the elevator in a safe position.

Other safety features, while important, serve different functions. A dead man's switch requires constant pressure or activation from the operator to function, ensuring that the elevator won't move unless actively controlled. Safety locks are designed to secure the elevator doors and prevent them from opening when the car is not aligned with the landing. A velocity sensor monitors the speed of the elevator and helps detect abnormalities, but it does not directly prevent movement like emergency brakes do. This makes emergency brakes a critical and common component for safety in hydraulic elevators.