Which of the following air terminals must be lead covered when installed on smokestacks?

The correct choice is copper because it is required in specific applications where corrosion is a concern, such as in installations on smokestacks. When copper air terminals are installed on smokestacks, they must be lead covered to protect against the corrosive gases and high-temperature conditions they encounter. Lead covering enhances the durability and longevity of the copper terminal, preventing deterioration that could compromise its effectiveness in lightning protection.

In contrast, aluminum terminals typically do not require lead covering in the same conditions, as aluminum has a natural oxide layer that provides some resistance to corrosion, while steel can be galvanized to improve its resistance but doesn’t need lead covering specifically. Plastic terminals are not suitable for high-temperature or corrosive environments due to their lower resistance to heat and potential degradation, which limits their use in smokestacks. Therefore, copper, when lead covered, is the most appropriate choice in this context for providing reliable protection against lightning strikes.