Which device allows you to lower a hydraulic elevator manually in the event of a power failure?

The manual lowering valve is designed specifically for situations where it is necessary to lower a hydraulic elevator manually, such as during a power failure. When activated, this valve allows the operator to control the flow of hydraulic fluid, facilitating a safe descent of the elevator car to the nearest landing. This is critical for ensuring passenger safety and minimizing potential hazards associated with being trapped in an elevator during an outage.

Other devices listed do play significant roles in elevator operations; however, they do not specifically provide the capability for manual descent in emergencies. The hydraulic control unit primarily manages the overall hydraulic system's operations and pressures, while the emergency brake system is focused on stopping the elevator during normal or emergency conditions, and the override switch might enable certain functions or bypass controls, but does not directly facilitate manual lowering.