Separately derived systems supplied by transformers that have a primary voltage rating less than 1,000 volts are required to be grounded.

The correct answer indicates that separately derived systems supplied by transformers with a primary voltage rating of less than 1,000 volts do not have a mandatory requirement to be grounded. This stems from the National Electrical Code (NEC), which outlines specific grounding instructions for different types of systems.

In the case of separately derived systems, if the system meets certain criteria, grounding may not be strictly necessary. For instance, if the transformer is a portion of a system that does not pose safety hazards or increase the risk of electrocution or equipment damage, grounding may be deemed unnecessary. The NEC has provisions that allow for grounding flexibility based on the operational environment, usage, and other technical details.

Although grounding provides important safety benefits, specifically preventing electric shock and equipment failure, not all systems with lower voltage ratings are universally required to be grounded if they do not pose identified risks. Therefore, the understanding of exceptions in the NEC allows the conclusion that grounding of these transformers is not always mandated.