Approaching the rectifier case one should first:

The primary reason for checking the rectifier for AC case to ground voltage first is to ensure safety before any hands-on work begins. Rectifiers are often associated with high voltage systems, and there is a risk of electric shock if AC voltage is present. By verifying there is no AC voltage on the case, a technician can confirm that it is safe to proceed with further actions, such as unlocking the padlock or physically touching the case.

This initial check is a critical step in safe practice, as electrical equipment can potentially pose serious hazards if proper precautions are not taken. This approach is consistent with standard safety protocols in the industry, where verifying the absence of potentially dangerous voltage is paramount before performing any maintenance or investigations on electrical equipment.