What must metallic mounting structures used for grounding purposes be?

Metallic mounting structures used for grounding purposes must be identified as equipment grounding conductors because this classification ensures that they are properly recognized and utilized within an electrical system for effective grounding. Equipment grounding conductors play a crucial role in enhancing safety by providing a low-resistance path for fault currents to return to the ground. This helps prevent electrical shock hazards and equipment damage.

Identifying these structures as equipment grounding conductors ensures they meet the necessary standards and requirements set by electrical codes. It implies they are specifically designed and intended for grounding applications, thus maintaining system integrity and safety.

The other options suggest either non-conductive materials, which would not be useful for grounding, or a general description of structures that do not emphasize their grounding function. Additionally, being grounded to the main panel is an important aspect, but without the proper identification of these metallic structures as grounding conductors, they are not guaranteed to fulfill their intended purpose effectively.