Which aspect must be checked to ensure the rectifier is safe to touch?

To ensure the rectifier is safe to touch, measuring the case-to-ground voltage or using an AC voltage detector is essential. This is important because it helps determine if there is any unintended voltage present on the case of the rectifier that could pose a shock hazard. If the case is energized, touching it could lead to serious injury or even fatality.

Measuring the case-to-ground voltage specifically identifies whether the rectifier's casing has any leakage current or voltage that would indicate a failure of insulation or other protective measures. Using an AC voltage detector serves a similar purpose, as it can quickly indicate if there is any voltage present, providing a level of safety before maintenance or repair work is undertaken.

While visual inspection for damage, measuring DC output voltage, and confirming that the AC input is disconnected are all important safety checks in their own right, they do not directly address the immediate concern of electric shock risk associated with the rectifier casing. Thus, the direct measurement of voltage on the case ensures that all potential risks are assessed before any physical contact is made.