What needs to be done before disconnecting the supply wires of a hydraulic elevator?

Before disconnecting the supply wires of a hydraulic elevator, it is essential to turn off the power unit. This step ensures that there is no electrical power flowing to the elevator system, minimizing the risk of accidental activation. Prioritizing safety is crucial in elevator maintenance and electrical work, as it protects both the technician and the equipment.

In the context of the other choices, while draining the oil in the tank may be a part of a maintenance procedure, it is not a prerequisite for safely disconnecting the supply wires. Similarly, locking the machine room and performing an inspection are good safety practices, but they do not specifically address the immediate safety concern of ensuring that the elevator is not operational while someone is working on the electrical aspects of the system. The correct action to take first in this scenario is to ensure that the power unit is switched off, providing a safe environment for further maintenance or repair activities.