Down conductors must be connected at either the upper or lower extremities when they run on which types of structures?

The correct answer is that down conductors must be connected at either the upper or lower extremities when they run on reinforced concrete columns or structural steel columns. This requirement is rooted in the structural properties of these materials, which are conductive and can efficiently channel electrical energy in the event of a lightning strike.

Reinforced concrete and structural steel provide a much better pathway for lightning current due to their inherent conductivity compared to materials like wood, brick, or plastic. Properly connecting down conductors to these types of columns ensures that any electrical discharge from lightning is safely grounded and does not cause damage to the structure or pose a hazard to any occupants.

In contrast, wooden columns do not provide the same level of conductivity for lightning currents. Brick columns, while somewhat conductive, generally do not match the efficiency and effectiveness of reinforced concrete or steel in managing lightning strikes. Plastic columns, on the other hand, are non-conductive and cannot effectively channel electrical currents at all, making them unsuitable for down conductor connections.