Before conducting work on a transformer, what should be confirmed about the system?

When preparing to work on a transformer, it is critical to ensure that the system has been properly powered down. This step is essential for safety reasons. Working on energized equipment poses significant risks, including electric shock or arc flash, which can lead to serious injury or even fatality. By confirming that the transformer is completely de-energized, along with implementing lockout/tagout procedures, you can protect yourself and others from unexpected energization or accidental contact.

Although the other options may pertain to operational considerations, they do not directly address the immediate safety protocols necessary before commencing work on electrical equipment. For example, ensuring that no one is currently using it does not guarantee that it is safe to work on, and the service history of the transformer does not imply that power is disabled. Likewise, while it's important for the transformer to operate under normal voltage ratings, this condition alone does not preclude the necessity of making sure that it is safely powered down before any maintenance or installation work.