What safety measure is required before opening a boiler's access door?

Before opening a boiler's access door, it is essential that the boiler is depressurized and cool. This is a critical safety measure to protect personnel from potential burns or injuries caused by the high temperature and pressure of the boiler’s internal environment.

When a boiler is under pressure, any sudden release of that pressure upon opening the access door could create a dangerous situation, including scalding steam or hot water escaping. Additionally, if the boiler is still hot, the risk of burns increases significantly. Ensuring that the boiler is cool and depressurized allows for safe maintenance or inspection activities to be carried out without endangering the operator or technician.

Other possible conditions mentioned in the choices, such as having the boiler pressurized and hot, would directly contradict safety protocols and present severe risks. A boiler being filled with water is not a primary concern in relation to accessing the door, as the state of pressure and temperature takes precedence. Lastly, the boiler being in operation would pose additional hazards, making it unsafe to access the interior. Therefore, maintaining a depressurized and cool state is the vital safety requirement for safely opening a boiler's access door.