According to standards, what should the circuit breaker protecting the ASD fan be?

The correct choice indicates that the circuit breaker protecting the Air Supply Distribution (ASD) fan should be labeled. Proper labeling is crucial for safety and maintenance reasons. It ensures that individuals can quickly identify the circuit breaker responsible for the fan, which is especially important during emergencies or maintenance work. A clearly labeled circuit helps prevent confusion and allows for rapid response if the system needs to be shut down, thereby enhancing safety for both the personnel and the equipment.

The other options—while they might seem reasonable—do not address the essential function of ensuring safety and operational clarity directly associated with the circuit breaker. For example, color-coding can assist in identification but does not replace the need for explicit labeling, which is the most direct means of conveying vital information. The requirement for double voltage rating is not standard for every application, and the location of the circuit breaker, while relevant, does not fundamentally ensure the safe operation of the ASD fan. Hence, having the circuit breaker labeled becomes paramount in adhering to safety standards and facilitating effective management of the electrical systems involved.