Which condition exempts non-current-carrying metal parts of cord-and-plug-connected equipment from needing grounding?

Non-current-carrying metal parts of cord-and-plug-connected equipment are exempt from needing grounding when they are listed equipment with distinct markings. This means that the equipment has been tested and certified by a recognized testing laboratory and meets specific safety standards. The distinct markings indicate that these metal parts do not pose a risk of electric shock under normal operating conditions, thereby alleviating the requirement for grounding.

This exemption is particularly important because it reflects an understanding of the overall safety design of the equipment. The certification process ensures that the parts are safe to touch and are not subject to electrical hazards if they do not carry current during regular use.

In contrast, other conditions presented do not provide sufficient rationale for exemption. For example, being subject to high voltage implies potential hazards, while having a plastic housing does not affect the need for grounding in other contexts. Lastly, relying solely on circuit breakers for protection does not meet the safety standards that the listed equipment exemption does, as circuit breakers alone do not prevent electric shock in every scenario. Thus, the correct answer ensures a standard of safety that is thoroughly validated through appropriate certification.