What must be done with the rectifier before it can be deemed safe to work on?

Before any maintenance or work can be performed on a rectifier, it is essential to ensure that the AC disconnect is turned off. This action is critical for ensuring safety because when the AC disconnect is off, it cuts power to the rectifier, which prevents any risk of electrical shock or unintended energization during maintenance activities.

While grounding the rectifier may be an important safety measure in certain contexts, the primary step for ensuring a safe working environment relates directly to disconnecting the power source. Energizing the circuit or allowing for the rectifier's voltage to remain while working on it would create hazardous conditions, which must be avoided at all costs. Therefore, confirming that the AC disconnect is off is the appropriate and necessary action to ensure safety before proceeding with any work on the rectifier.